Real-time X-ray photoelectron spectroscopy of surface reactions

Real-time X-ray photoelectron spectroscopy of surface reactions

Alessandro Baraldia,b,c, Giovanni Comellib,c, Silvano Lizzita,Maya Kiskinovaa,*, Giorgio Paoluccia

aSincrotrone Trieste S.C.p.A., s.s. 14 Km 163.5, 34012 Trieste, ItalybDipartimento di Fisica, Universita di Trieste, Via Valerio 2, 34127 Trieste, Italy

cLaboratorio TASC-INFM, s.s. 14 Km 163.5, 34012 Trieste, Italy

Received in final from 14 November 2002

Abstract

The experimental determination of the composition and structure of gas–solid surface interface at different stages

of surface reactions is a crucial point in elucidating the reaction mechanism. This requires a quantitative surface

sensitive technique, providing information on how the substrate surface, adsorbed species and their bonding con-

figuration evolve at the time scale of the surface processes. The present review illustrates how the high performance

levels achieved in X-ray photoelectron spectroscopy at the third generation synchrotron facilities, in particular the

reduced data acquisition time down to a second range, have made possible studies of surface processes in real time.

The article summarizes the wealth of knowledge that has been gained using representative examples of adsorption

systems, where the relation between adsorption–desorption rate, adsorbate coverage, bonding configuration and

interconversion between adsorption sites was established, and simple reaction systems, where the effects of the

substrate structure and of the changes in the adsorbate layer under non-linear reaction conditions were probed.

# 2003 Elsevier Science B.V. All rights reserved.

PACS: 82.65.þr; 79.60.-i; 68.43.-h; 68.43.Mn

Keywords: Chemisorption; Surface reactions; Catalysis; X-ray photoelectron spectroscopy

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

2. The SuperESCA beamline of ELETTRA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

3. Molecular adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

3.1. Carbon monoxide adsorption on Rh(1 1 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

3.2. Carbon monoxide adsorption on Rh(1 0 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

Surface Science Reports 49 (2003) 169–224

0167-5729/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0167-5729(03)00013-X

* Corresponding author. Tel.: þ39-040-375-8549; fax: þ39-040-375-8565.

E-mail address: [email protected] (M. Kiskinova).

4. Dissociative adsorption: the adsorbates core levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

4.1. Oxygen dissociative adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

4.1.1. Oxygen adsorption on Rh(1 1 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

4.1.2. Oxygen adsorption on Rh(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

4.1.3. Oxygen adsorption on Ru(1 0 �1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

4.2. Nitric oxide adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

4.2.1. Nitric oxide adsorption on Rh(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

4.2.2. Nitric oxide adsorption on Ir(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

4.2.3. Nitric oxide adsorption on stepped surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

5. Adsorbate bonding configuration probed by the substrate core-level shifts . . . . . . . . . . . . . . . . . . . 194

5.1. Oxygen adsorption on Rh(1 1 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

5.2. Oxygen adsorption on Ru(1 0 �1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197

6. Molecular desorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

6.1. Carbon monoxide desorption from Rh(1 1 0). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

6.2. Carbon monoxide desorption from Rh(1 1 1). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202

6.3. C2N2 desorption from Pd(1 1 0). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

6.4. SO2 interaction with Cu(1 0 0) and Ni(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206

7. Surface reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

7.1. COþO reaction on Rh(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

7.2. NOþH2 reaction on Rh(5 3 3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214

7.3. C2H2 trimerization on Pd(1 1 1). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

7.4. Alkene epoxidation on Cu(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217

8. Future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222

1. Introduction

The interest of the scientific community in understanding the mechanism of surface reactions datesfrom the beginning of the 20th century. In 1922, Langmuir suggested two possible mechanisms forexplaining chemical catalytic reactions, both involving adsorbed species on the catalyst surface [1].According to the first mechanism, known as Langmuir–Hinshelwood mechanism, the reaction takesplace only between species adsorbed on the surface. According to the second mechanism, known as theEley–Rideal mechanism, the reaction involves the interaction between an adsorbed species with amolecule in the gas-phase. In the last 20 years, systematic studies of catalytic reactions using thesurface science approach have provided direct evidence that the mechanisms of surface reactions andsurface reactivity are controlled exclusively by the properties of the interface. These properties aredetermined by the composition of the chemisorbed layer and the chemical state and structure ofsubstrate layers, which can change in the various reaction stages. A great deal of understanding in thisrespect has been obtained by the investigations of model systems under UHV conditions. Pioneeringstudies tackled the dependence of the reaction rate on the partial pressure of the reacting gases andprovided information on the reaction products, usually monitored by means of mass spectroscopytechniques, such as temperature programmed desorption (TPD) and molecular beam reactive scattering(MBRS). Reaction products result from elementary reaction steps, which involve species adsorbed at

170 A. Baraldi et al. / Surface Science Reports 49 (2003) 169–224

the catalyst surface. The evolution of each of these steps is characterized by a specific time constant andis part of the complex mechanism leading to the formation of the final products. Since the concentrationof adsorbed species is relatively small (usually less than 1015 atoms/cm2) and the lifetime of some ofthe intermediate species is short, the scientific community has developed a large number of surfacesensitive techniques to measure in situ the coverage of the different reacting species. Among them wemention static secondary ion mass spectroscopy (SSIMS) and electron stimulated desorption ionangular distribution (ESDIAD). SSIMS has proved to be a very useful method to study reaction kineticsat the second time scale [2], its high speed being mainly due to the high ion detection efficiency.However, the method has serious limitations such as the irreversible surface destruction, theimpossibility of distinguishing ions formed by secondary processes, and the difficulties to calibrate thesecondary ion yield in the case of multiphase interfaces, as it depends strongly on the chemicalenvironment. ESDIAD combined with time of flight (TOF) techniques has also been applied toinvestigate surface reactions in real time. In their studies Sasaki et al. [3] were able to identify the massof the desorbing ions, thus obtaining interesting information on the temporal evolution of coadsorbedlayers on single crystal surfaces as a function of temperature. A notable progress in surface reactionstudies was the development of high-resolution electron energy loss spectroscopy (HREELS), a methodthat provides chemical information, to work at sub-second time scales. In the late 1980s three groups,lead by Ho [4] at Cornell University, Kevan and co-workers [5] at the AT-T Bell Labs and Froitzheimet al. [6] at the University of Erlangen, successfully developed experimental set-ups that allowedHREELS spectra to be acquired on the millisecond time scale. The new technique was therefore namedtime resolved electron energy loss spectroscopy (TREELS). Since the method is very powerful in theidentification of molecular species and radicals, it has been extensively used for the studies of theadsorption and desorption of simple molecules and surface reactions in real time. The TREELS hasprovided fundamental information regarding interactions in model chemisorption and reaction systems.Examples of systems studied by TREELS are: oxygen adsorption on Pt(1 1 1) [7], methanoldecomposition on Ni(1 1 0) [8], the effect of adsorbate proximity on surface reactions on Rh(1 0 0) [9],the desorption of water from Ag(0 0 1) [10], the two-dimensional gas–liquid phase transition ofmethane on Ag(1 1 0) [11] and the carbon monoxide diffusion on Pt(1 1 1) [12]. Despite the promisingresults obtained using TREELS, this method has also strong limitations, which can be summarized asfollows:

(i) very low sensitivity to atomic adsorbates, such as oxygen, nitrogen, carbon etc., imposing seriouslimitation for quantitative measurements of atomic species, which are the most commonparticipants in many industrially important catalytic reactions;

(ii) low sensitivity to the modifications of the substrate layer induced by molecular and/or atomicadsorption, which are of fundamental importance in determining the reaction mechanisms;

(iii) no straightforward relationship between the relative intensity of the spectroscopic features and thecoverage of the probed species, mainly due to changes in the dynamic dipole moments at highcoverage caused by dipole–dipole interactions [13].

A milestone in studies of interfacial phenomena and catalysis related surface science studies is thedevelopment of the X-ray photoelectron spectroscopy (XPS) in the 1960s [14]. Thanks to the highsurface sensitivity, basic knowledge about the factors controlling the composition, structure, chemicalstate and reactivity of the interfaces has been obtained for model chemisorption and reaction systems.However, since surface reactions are evolving systems, a serious drawback of the XPS technique was its

A. Baraldi et al. / Surface Science Reports 49 (2003) 169–224 171

relatively slow acquisition rate, usually not sufficient to follow the kinetics of surface reactions.Providing element and bonding specificity at time scales (milliseconds to seconds) typical of surfaceprocesses such as diffusion and macroscopic reaction kinetics, is a key for elucidation of the reactionmechanisms of a broad range of surface reactions. In recent years, increasing the time resolution ofXPS experiments has become possible thanks to the high brightness and intensity of the X-raysprovided by the third generation synchrotron radiation sources. Although XPS has not broken yet thefrontiers in time resolution required for monitoring electron excitation–de-excitation processes, its greatadvantage is the high spectral resolution, which permits detailed determination of the changes in thechemical states of the adsorbed species and catalyst surface in real time. This motivated theconstruction of dedicated XPS beamlines and measurement stations which can be used for studies ofsurface reactions in real time at the third generation synchrotron light sources, such as ELETTRA [15],MAXLab [16] and Advanced Light Source [17].

The present paper reviews recent results, which illustrate the power of the real-time XPS method inunderstanding the impact of the geometric and electronic structure of the interface on chemicalreactivity. The discussed case studies concern interaction of atomic, molecular and fragment specieswith metal substrates, one of the traditional fields of surface science. The organization of the review isas follows. Section 2 describes the methodology and instrumentation used for real-time XPSmeasurements, outlining the advantages of using X-rays produced by undulators at the third generationsynchrotron radiation sources. Sections 3–7 address some recent real-time XPS research efforts instudies of (i) molecular adsorption, (ii) dissociative adsorption, (iii) molecular desorption and (iv)surface chemical reactions. They illustrate how the availability of high spectral resolution and intensityhas made it possible to monitor the evolution of the core levels of the adsorbates and of the substratesurface in real time. The selected examples reveal the potential of real-time XPS in shedding light on:

(i) the relation between adsorption rate, adsorbate coverage, ordered structures and bondingconfigurations during molecular adsorption (Section 3);

(ii) the effect of surface structure and composition on the dissociation probability, and on thepathways and transient species formed during dissociative adsorption of homo- and hetero-nuclearmolecules (Section 4);

(iii) the molecular desorption processes, involving evolution of the coverage of the different surfacespecies, changes in the bonding configurations and interconversion between different adsorptionsites as a function of surface temperature (Section 6);

(iv) the correlation between interfacial chemical composition, reaction mechanism and kinetics ofmodel catalytic reactions (Section 7).

In the concluding remarks, we attempt to outline the future developments of this technique.

2. The SuperESCA beamline of ELETTRA

All the experimental results presented in this review have been obtained at the SuperESCA beamlineof ELETTRA [15,18,19], the third generation synchrotron light source built in Trieste, Italy. Thephoton source of the SuperESCA beamline is a 5.6 cm, 81 period undulator [20] which providesphotons in the energy range between 80 and 1200 eV. The storage ring is operated at a maximumbeam current of 300 and 150 mA in multibunch mode at 2 and 2.4 GeV electron energy, respectively.


The electromagnetic radiation emitted by the undulator is very bright (up to 7 � 1012 photons/(s mm2 mrad2 0.1%bw) at 100 mA beam current) and is concentrated in bands around the multiplewavelength of the first harmonics of the insertion device. The wavelength can be tuned by changing theintensity of the magnetic field, i.e. by varying the gap between the two arrays of magnets. However, therelative full-width-at-half-maximum is DE=E ¼ 1=N, where N is the number of periods in theundulator [21] and the energy band emission is not narrow enough to perform high-energy resolutionexperiments. Therefore the radiation has to be monochromatized. The SuperESCA beamline uses amodified version of the SX-700 monochromator, the so-called stigmatic version [22]. The instrumentprovides a maximum resolving power above 10,000 in the photon energy range between 80 and 400 eV,which decreases to 4000 at a photon energy of 1000 eV. The combination of mirrors allows the photonbeam to be focussed onto the sample in the experimental chamber. The spot dimension is 100 � d mm2

(where d is the dimension of the exit slit, ranging from 5 to 1000 mm). The photon flux at the sample is1011 photons/s at 400 eV with a resolving power of �104 at a ring current of 100 mA. This value isabout three orders of magnitude higher than the photon flux of the best conventional X-ray tube.

The experimental station, shown in Fig. 1, was initially equipped with a VSW 150 mm mean radius,16 channel hemispherical electron energy analyzer, which has recently been replaced by a new double-pass hemispherical analyzer of the same radius [23], with a 96 channel detector [24].

Accurate sample positioning is achieved by means of a manipulator with 5 degrees of freedom(a modified CTPO-Fisons). An alternative manipulator with x; y; z and polar angle movement is used for

Fig. 1. Photo of the experimental station of the SuperESCA beamline of ELETTRA.


the experiments where temperatures in the range 15–100 K are needed. All the instruments arecontrolled by a computer, using LabVIEW programs, which allow simultaneous control of all thedevices (undulator, monochromator, electron energy analyzer, ion gauges, mass spectrometer, samplemanipulator, etc.) involved in a specific experiment [25].

Another important advantage of the synchrotron light is the possibility to tune the photon energy inorder to optimize the photoemission cross-section of the core level under investigation. For example, atphotoelectron kinetic energies of about 100 eV, the cross-section of the carbon and oxygen K-edge isincreased by a factor of �21.5 and 5.5, respectively, with respect to conventional X-ray source (e.g. aMg Ka source at 1253.6 eV) [26,27].

Optimized measurement parameters and the light brightness of the source reduce significantly thedata acquisition time. With a conventional Al or Mg Ka X-ray source a typical core-level spectrumfrom an adsorbate requires about 10 min to be acquired with an energy resolution slightly better than1 eV, so that quantitative measurements of surface phenomena were typically limited to the study ofstatic surface phenomena, such as frozen reactions. By using an X-Ray monochromator the spectralresolution can be improved (�0.3 eV), but at the expenses of the photon flux, further reducing theacquisition rate. At the SuperESCA beamline the C 1s, N 1s and O 1s core-level spectra from adsorbatesystems are acquired in about 10 s per spectrum with an energy resolution ranging from 0.3 to 0.5 eV.The limitation of the energy resolution in these cases is due to the broad instrinsic width of the 1s corelevel of the low atomic number elements, determined by the short lifetime of the 1s core-hole created inthe photoemission process. The surface sensitivity to adsorbate species at this data acquisition rate isbetter than 0.01 monolayer.

Because of the higher photoemission cross-section, core-level spectra of the substrate atoms (e.g. 3dand 4f emission of transition metals) can be acquired more rapidly (down to 100 ms/spectrum), with anoverall energy resolution of about 100 meV. The highest aquisition rate of 100 ms/spectrum has beenachieved with the 96 channel detector which allows data aquisition in snap-shot mode, i.e. withoutchanging the voltages of the electron energy analyzer electrostatic lenses.

The experimental station is equipped with two differential pumping systems, both on the electronenergy analyzer and the beamline piping. This allows data acquisition with a local pressure on thesample up to 10�6 mbar, which makes the system suitable for the real-time study of surface reactions.

All the photoemission spectra presented in this paper have been fitted using Doniach–Sunijc (DS)[28] functions, which take into account the characteristic asymmetric tail towards lower kinetic energy,due to the electron-hole pairs excitations and/or shake-up states. The DS, characterized by a Lorentzianwidth and an asymmetry parameter, are convoluted with a Gaussian which accounts for theinstrumental resolution and phonon broadening/vibrational fine structure.

3. Molecular adsorption

Adsorption of CO on single crystal metal surfaces represents one of the most widely investigatedmodel systems. Over the past few decades it has provided a wealth of information about gas–surfaceinteraction processes. The main reasons for the interest in the interaction of CO with metal surfacesare its relative simplicity and the technological importance of CO chemistry on transition metals.CO is used in catalytic chemistry for synthesis of methane, liquid hydrocarbons (synthetic fuel), foroxo-synthesis, etc. [29]. On the other hand, it is an undesired exhaust gas and its oxidation to CO2 on


transition metal catalysts is of particular importance for reducing the ambient pollution [30]. Theextensive studies of CO chemisorption on transition metals are a valuable resource in the developmentof the surface chemistry knowledge. By inspecting the data about CO interaction with transition metalsat room temperature, one can divide the CO/transition metal systems in two main groups. The firstincludes metals from the left side of the periodic table, such as Fe, W, Mo, which adsorb COdissociatively, while the second is composed by the elements from the right side of the periodic table,such as Ru, Rh, Ir, Pd, Pt, which tend to adsorb CO molecularly. However, CO dissociation can alsooccur on the latter metals under conditions (temperature and pressure) which overcome the activationbarrier for CO dissociation. The bonding of a CO molecule to a transition metal is now well understoodand can be described as result of attractive p-interactions (between metal d2p� band and hybridized1p�2p� CO orbitals) and repulsive s-interactions (between metal ds band and hybridized 4s�5s COorbitals). The p-interactions destabilize the C–O bond. Since both p- and s-interactions becomestronger as the number of the coordinating metal atoms increases, the variations in adsorption energy ofCO on transition metals are relatively small, although the CO electronic structure changes substantiallyin different adsorption sites [31,32].

XPS has been extensively used to probe the CO bonding configuration in adsorbed layers on metalsurfaces. Among the first high-resolution core-level photoemission applications is the determination ofthe CO adsorption site on Ni(1 0 0) in the three different long-range ordered structures formed withincreasing CO coverage [33,34]. In these structures, CO can occupy on-top or bridge sites, which wereclearly distinguished by measuring the C 1s and O 1s core-level energies. The core-level shifts betweenthe two bonding configurations are 0.5 and 0.9 eV for C 1s and O 1s, respectively. A general trend isthat the core-level binding energy decreases when the coordination to the first layer atoms increases.For example, the O 1s binding energy varies from 531.5 to 532.6 eV for on-top bonded CO and from530.7 to 531.5 eV for bridge-bonded CO on different single crystal surfaces [35]. As pointed out byMartensson and Nilsson [36], the core-level binding energy shifts can be understood from total energyconsiderations. The major contribution comes from the changes in the energy of the core-ionized state,because the difference of the CO adsorption energies between different adsorption sites for the neutralinitial state is very small (�100 meV).

3.1. Carbon monoxide adsorption on Rh(1 1 1)

Because of the great interest in Rh as a catalyst in the exhaust gas converters, CO adsorption onRh has been studied using almost all the available surface science methods. Low energy electrondiffraction (LEED) and HREELS have revealed the formation of different long-range orderedstructures, where CO occupies different adsorption sites. Our real-time XPS investigations have clearlyevidenced the dependence of the adsorption site on CO coverage and surface temperature [37]. Fig. 2shows O 1s spectra recorded during CO adsorption at 150, 300 and 400 K. The presence of two peaks,one at 531.5 eV, growing with the onset of the uptake, and the second at 530.1 eV, appearing in a laterstage, reflects subsequent occupation of two different adsorption sites. The higher binding energycomponent (531.5 eV) is assigned to CO adsorbed in on-top sites, while the lower binding energy peak(530.1 eV) is related to CO chemisorbed in threefold hollow sites, in agreement with previous XPSstudies [38,39]. By applying the fitting procedure described in Section 2 we were able to obtain thekinetic plots shown in Fig. 3. These plots illustrate the population of each adsorption site as a functionof CO exposure at three different temperatures. It is evident that the population of the different sites


decreases with increasing adsorption temperature, the effect being more pronounced for CO adsorbedin hollow sites. This temperature effect is mainly due to the competition between adsorption anddesorption, the desorption rate becoming significant at higher temperature. As a result, the CO speciesoccupying the energetically less favorable hollow sites are desorbing, so that at 400 K the dominantspecies is on-top CO. An interesting feature of the uptake curves at 150 and 300 K is that the on-top CO

Fig. 2. Evolution of the O 1s core-level spectra during CO uptake on Rh(1 1 1) at (a) T ¼ 150 K, (b) T ¼ 300 K and (c)

T ¼ 400 K. hn ¼650 eV. CO pressure ranging from 1 � 10�9 to 1 � 10�7 mbar.


coverage reaches a maximum in the range 0.25–0.35 ML and than slightly declines. This is due to thestrong repulsive interactions between CO molecules at short distances. In fact, between 0.25 and0.33 ML, the ð

ffiffiffi3

p�

ffiffiffi3

pÞR30� structure develops. It consists of CO molecules in next-nearest on-top

sites. Further population of on-top sites creates a locally highly compressed structure characterized bystrong CO–CO repulsive interactions, which push up the potential energy levels of the adsorption system.

Fig. 3. CO uptake on Rh(1 1 1) in on-top (filled circles) and in threefold hollow sites (empty circles) obtained from the

intensity of the corresponding components in the deconvoluted CO–O 1s spectra. The total CO uptake is shown with a solid

line.


As a result, the CO adlayer rearranges by moving some of the CO molecules from on-top to hollowsites in order to attain a thermodynamically more favorable lower potential energy state. Suchrearrangement is a quite common phenomenon, observed also in the desorption experiments (seeSections 6.1 and 6.2). The first derivative of the CO uptake curves reflects the dependence of thesticking coefficient on the CO coverage. Since the evaluation of the absolute value of the initial stickingcoefficient S0 requires a very precise calibration of the ion gauge we assume that the initial stickingcoefficient S0 is 1 for 150 K. Fig. 4 shows the dependence of the normalized sticking coefficient on theCO coverage, obtained from the uptake plots measured at 150 and 300 K. The 400 K set has not beenconsidered because at this temperature the CO desorption strongly competes with CO adsorption.Notable features of the sticking coefficient plots in Fig. 4 are:

(i) at low coverage (y � 0:35 ML) the sticking coefficient decreases linearly with CO coverage, theslope becoming steeper with increasing adsorption temperature;

(ii) the slope changes at high coverage and the sticking coefficient becomes zero at saturation.

The measured SðyCOÞ plots can be fitted satisfactorily using the following equation, based on theKisliuk precursor kinetic model [40]:

SðyÞ ¼ S0 1 � ky1 � y

� �; (1)

where k ¼ Pd=Pc is the ratio of the probability for desorption of the precursor over an occupied site, Pd,and the probability of chemisorption from the precursor state, Pc. The k values of 1.02 and 0.97, whichgive the best fit of our data, are perfectly compatible within the error bars with the value of 1. For thisvalue, Eq. (1) corresponds to a Langmuir-like dependence of the sticking coefficient. This indicatesthat, in the range between 150 and 300 K, the probability for adsorption of CO on Rh(1 1 1) isdominated by a site blocking effect and that the role of the precursor is negligible. This implies a veryshort lifetime of the precursor above the already occupied sites. Such behavior is rather unusual, sincefor most CO/transition metal systems the adsorption rate of CO changes negligibly up to temperatures

Fig. 4. Plots of CO sticking coefficient on Rh(1 1 1) versus CO coverage at 150 and 300 K.


near the onset of CO desorption [41], i.e. the precursor lifetime on the occupied sites remainsreasonably high up to high CO coverage.

3.2. Carbon monoxide adsorption on Rh(1 0 0)

Similarly to the Rh(1 1 1) case, CO adsorption on Rh(1 0 0) also occurs in two different adsorptionsites [42,43]. In this case, we used the C 1s spectra as a fingerprint of adsorption site occupation.Fig. 5(a) shows the C 1s spectra acquired while dosing CO at 173 K. The on-top and bridge adsorptionsites are characterized by C 1s binding energies of 285.73 and 285.35 eV, respectively, in agreementwith the previous studies [44]. The main difference compared to CO adsorption on Rh(1 1 1) is therelative occupation of the two sites at low coverages. During CO adsorption on Rh(1 1 1) the moleculesoccupy only the on-top sites below �0.25 ML (see the uptake plots in Fig. 3), whereas on Rh(1 0 0)both on-top and bridge sites are simultaneously occupied from the onset of CO adsorption.

The plots in Fig. 5(b) clearly show the coexistence of the two species already at CO coverages below0.1 ML. The on-top population grows faster, reaches a maximum at �0.5 ML and decreases withfurther CO exposure to �0.4 ML at saturation. The maximum in the on-top CO coverage correspondsto the break in the uptake plot of bridge CO. The break marks the onset of faster increase of the bridgeCO coverage accompanied by a decline in the on-top CO population. The observed de-population of theon-top CO sites on Rh(1 0 0) above 0.5 ML is similar to the process occurring on Rh(1 1 1) at COcoverage above (1/3) ML, attributed to rearrangement in the adlayer caused by the onset of therepulsive CO–CO interactions. Apparently, in the CO/Rh(1 0 0) system this occurs above the ‘critical’coverage of 0.5 ML, when the cð2 � 2Þ structure is completed. As shown in Fig. 6, further adsorption of

Fig. 5. (a) Evolution of the C 1s core-level spectra during carbon monoxide uptake on Rh(1 0 0) at T ¼ 173 K. hn ¼ 400 eV.

(b) Uptake plots of CO in on-top (filled circles) and in bridge site (empty circles) evaluated from the deconvoluted CO–C 1s

spectra. The total CO uptake is plotted with empty squares, where the straight line indicates that the CO sticking coefficient is

constant up to �0.6 ML.


CO molecules in an on-top site inside the cð2 � 2Þ unit cell, leads to a dense configuration with strongrepulsions between the adjacent on-top CO molecules. Thus, at least two on-top CO molecules shouldhop to bridge sites in order to relax the system (steps (c) and (d) in Fig. 6). This rearrangementrepresents the first step towards the formation of the dense pð4

ffiffiffi2

p�

ffiffiffi2

pÞR45� structure, which is

completed at 0.75 ML. Another distinct feature that characterizes the CO coverage range above0.5 ML, i.e. the phase transition from cð2 � 2Þ to pð4

ffiffiffi2

p�

ffiffiffi2

pÞR45� structure, is that the C 1s binding

energy difference for molecules in the two adsorption sites, which is initially 0.38 eV, decreasesbecause of an energy shift of the C 1s peak corresponding to the on-top CO. This shift can betentatively attributed to a change in the local geometry of the on-top molecules, induced by therepulsions arising in the compressed layers formed above 0.5 ML. The presence of on-top COmolecules, slightly shifted with respect to the high-symmetric position, was also suggested by de Jongand Niemantsverdriet [45] in order to explain the HREELS results obtained for the same system at highCO coverage. The sticking coefficient of CO on Rh(1 0 0) determined using the XPS data shows acoverage dependence, different from that observed for CO on Rh(1 1 1). As discussed above, theinvariance in the sticking coeffficient with increasing CO coverage indicates precursor-dominatedadsorption kinetics for CO adsorption on Rh(1 0 0), in contrast to the ‘non-precursor’ CO kineticsobserved on Rh(1 1 1). This prompts us to suggest that the lifetime of the precursor is determined bythe intrinsic electronic structure probably related to the actual geometric arrangements of surfaceatoms.

4. Dissociative adsorption: the adsorbates core levels

4.1. Oxygen dissociative adsorption

Molecular oxygen adsorption on transition metal surfaces takes place only at very low temperatures.The reason for the low activation barrier for O2 dissociation on transition metal surfaces is the presence

Fig. 6. Structural models for the cð2 � 2Þ (y ¼ 0:5 ML) to pð4ffiffiffiffi2

p�

ffiffiffi2

pÞR45� (y ¼ 0:75 ML) transition during CO

adsorption on Rh(1 0 0).


of a pair of electrons in the antibonding 2p molecular orbital, which makes the O–O intramolecularbond much weaker than the CO bond. Thus, the interaction with a metal surface, which involves chargebackdonation to the antibonding O2 orbitals, leads to dissociation. The lifetime of the molecular oxygenstate decreases with increasing adsorption temperature and it is the natural precursor state in thedissociative oxygen adsorption. Apart from the fundamental importance in understanding thedissociative chemisorption processes, the interaction of O2 with transition metal surfaces is also ofpractical interest because of its involvement in the oxidation catalytic reactions and in the undesirablecorrosion of transition metals exposed to oxygen containing atmosphere. The reactive state of oxygen isusually an adsorbed atomic state or oxygen in a metastable metal oxide phase. In this respect, Rh andRu are illustrative examples for CO oxidation. On Rh surfaces, the CO oxidation involves adsorbedoxygen atoms, whereas on Ru surfaces oxygen species in the Ru oxide phase are involved in theoxidation reaction [46]. In the present review, we will illustrate the potential of core-level spectroscopyin determining the kinetics, adsorption sites and the coordination of the oxygen and surface metalatoms. The reported examples will refer to case studies of oxygen interaction with Rh and Ru singlecrystal surfaces.

4.1.1. Oxygen adsorption on Rh(1 1 1)

Previous studies of O2 dissociative adsorption on a Rh(1 1 1) surface have shown that at 300 K anordered pð2 � 2Þ structure is formed at 0.25 ML, with oxygen adatoms sitting in threefold hollow sites.At 0.5 ML, the ordered structure converts into a pð2 � 1Þ with three domains that give the same LEEDpattern of a single pð2 � 2Þ domain [47]. Also in the pð2 � 1Þ structure oxygen atoms occupy threefoldsites.

Fig. 7(a) shows the O 1s spectra collected during the oxygen uptake on a Rh(1 1 1) surface at 300 K.The sequence of spectra can be fitted using a single peak. As expected, the intensity of the O 1s peakincreases with increasing oxygen coverage. At the same time, its binding energy changes, as illustratedby the plot in Fig. 7(b). The O 1s binding energy position, initially located at 530:25 0:03 eV, starts to

Fig. 7. (a) Evolution of the O 1s core-level spectra during oxygen uptake on Rh(1 1 1) at T ¼ 300 K. hn ¼ 650 eV.

(b) Dependence of the O 1s binding energy on the oxygen coverage.


shift almost linearly towards lower binding energies above 0.3 ML reaching 530:17 0:03 eV at thesaturation coverage of 0.5 ML. The total shift of the O 1s binding energy is rather small, less than�80 30 meV, and cannot be assigned to a change of the oxygen adsorption site. The fact that the shiftis continuous also excludes a simultaneous occupation of two adsorption sites. Apparently, the oxygenpreserves the same adsorption site in agreement with the experimental [47] and density functional

Fig. 8. Oxygen induced ordered structures on Rh(1 1 0). (Top) Disordered and ð2 � 1Þp2mg structures formed at room or

lower temperature. (Bottom) ð2 � 2Þp2mg, cð2 � 6Þ, cð2 � 8Þ and cð2 � 10Þ structures formed at adsorption temperatures

higher than 400 K, when the substrate undergoes ð1 � nÞ missing-row reconstruction.


theory (DFT) [48] structural results. The XPS results are also in fair agreement with other theoreticalcalculations [49], which have predicted a rather small difference (�34 meV) in the O 1s binding energybetween the pð2 � 2Þ and pð2 � 1Þ structures, due to the small initial state effects.

4.1.2. Oxygen adsorption on Rh(1 1 0)Oxygen adsorption induces a rich variety of ordered structures on Rh(1 1 0) (see Fig. 8), involving

also surface reconstruction when the adsorption is performed at elevated temperature. The actualstructure is determined by the oxygen coverage and by the adsorption temperature [50]. The onlyordered structure that develops at room or lower temperature is the ð2 � 1Þp2mg structure with 1 MLoxygen coverage. The adsorption at temperatures higher than �400 K results in the formation of wellordered ð2 � 2Þp2mg and cð2 � 2nÞ structures, where n ¼ 3; 4 and 5 and the coverage corresponds to0.25, 0.5 and 0.6–0.8 ML, respectively [51–57]. The oxygen adsorption site is always threefold, ageneral trend observed for different rhodium surfaces [58]. All these structures, illustrated in the bottompanel of Fig. 8, involve ð1 � nÞ missing row substrate reconstructions, where every nth closed-packed[1 �1 0] row is missing. If oxygen is removed by reduction with hydrogen at 370 K, these reconstructionsremain metastable and convert into a ð1 � 1Þ unreconstructed surface upon annealing above 400 K.

In spite of the detailed structural information about the arrangement of oxygen in the ordered phaseson the Rh(1 1 0) surface, the kinetics of oxygen dissociative adsorption and the configuration of oxygenadatoms in the disordered phases at low coverage were verified by means of real-time XPS at twoadsorption temperatures, 270 and 570 K, i.e. below and above the reconstruction temperature [59].

Fig. 9(a) shows a set of O 1s spectra taken during exposure of the Rh(1 1 0) surface to O2 at 270 K.The binding energy and the shape of the O 1s spectra change when the oxygen coverage exceeds

Fig. 9. (a) Evolution of the O 1s core-level spectra during oxygen uptake on Rh(1 1 0) at T ¼ 270 K. hn ¼ 650 eV. O2

pressure ranges from 2:5 � 10�9 to 1 � 10�7 mbar. (b) Plots of the total (line), bridge-bonded (filled circles) and threefold

(open circles) oxygen coverage versus oxygen exposure.


�0.35 ML, due to the growth of a second component, indicating occupation of more than oneadsorption site. The analysis of the O 1s spectra confirmed that below 0.35 ML the oxygen adsorptionresults in a single peak at 529.65 eV, whereas above 0.35 ML a second peak at 530.25 eV develops.This second component gains intensity at the expenses of the first one, which disappears at �0.8 ML.The population of the two different adsorption sites, evidenced for the first time by the O 1s spectra,was confirmed lately by our STM study [60]. In accordance with the STM results the O 1s peaks at529.65 and 530.25 eV are assigned to oxygen located in asymmetric bridge and threefold sites,respectively. A close inspection of the total oxygen uptake and the uptake in each of the two adsorptionsites, illustrated by the plots in Fig. 9(b), shows striking correlation between the discontinuities in theslope of the total oxygen uptake and the changes in adsorption site occupation. As one can expect, theadsorption rate is highest at low coverage when oxygen adsorbs only in bridge sites in a disorderedmanner. The slope of the total uptake remains constant up to �0.3 ML and abruptly decreases,indicating a slower adsorption rate between 0.3 and 0.35 ML. The second change in the slope of thetotal uptake is above 0.35 ML. It coincides with the onset of adsorption in the threefold sites,accompanied by a conversion of oxygen from bridge to threefold sites. Above 0.8 ML all bridge-bonded oxygen has moved to threefold sites and the total uptake and the threefold uptake overlap,because further oxygen adsorption occurs only in threefold sites.

The initially constant adsorption rate is indicative of a negligible effect of the already adsorbedoxygen on the molecular precursor lifetime. At higher coverage, one should consider the repulsive O–Ointeractions (negligible at low coverage), which reduce the probability to find appropriate bridge sitepairs and result in a lower adsorption rate. The onset of threefold site occupation and of bridge tothreefold site conversion above a critical coverage indicates that at low coverages the Rh–O bonding inthe bridge sites is more favorable. In the denser adlayer the zig-zag threefold configuration offers moreappropriate adsorption pairs for dissociation of the molecular precursor, which accounts for the secondchange in the total uptake curve. However, compared to the initial adsorption rate the adsorption rateabove 0.35 ML is slower. Among the factors that might control the adsorption probabilities thefollowing three are the most important: (i) repulsive interactions between adsorbed species; (ii) theexistence of multiple precursors and (iii) the structural differences in the adlayer.

Oxygen dissociative adsorption at 570 K is accompanied by a ð1 � nÞ missing row reconstruction. Ithas been supposed that the initial step in the rearrangement of Rh atoms involves oxygen incorporationbelow the top layer, which distorts the substrate lattice and facilitates the displacement of the Rh atoms.This means that a critical adsorbate coverage is required to initiate the reconstruction process. The real-time XPS experiments have supported this scenario.

In fact the O 1s spectra in Fig. 10(a) and the uptake plots in Fig. 10(b), obtained at 570 K, show thefollowing two distinct differences compared to oxygen adsorption at 270 K: (i) the O 1s peak at529.65 eV due to the bonding in bridge sites is absent and (ii) the oxygen uptake curves havecompletely different shape. The O 1s data below 0.1 ML can be fitted with a peak at 529.95 eV, whichat 0.1 ML abruptly shifts to 530.25 eV, the same binding energy measured for the threefold oxygen onthe unreconstructed surface. The total uptake curve in Fig. 10(b) shows a short induction periodfollowed by two discontinuities in the slope. The initial slope is flatter than the slope above 0.15 ML.The steeper slope above 0.15 ML remains constant up to �0.5 ML, followed by a fast decrease,leveling off when the coverage approaches �0.8 ML.

The slower initial rate and the slightly different O 1s binding energy of the first adsorbed oxygenspecies correlate with the mechanism of surface reconstruction, which involves a supply of substrate


adatoms which are trapped by oxygen to form the structural units of the new phase, Rh–O chains in the½1 �1 0� direction. The idea is that the apparent slower initial uptake is due to the lower signal of theburied oxygen which facilitates the displacements of the Rh atoms and initiates the nucleation andgrowth of the ½1 �1 0� Rh–O rows. Alternatively, as suggested by a recent STM study [61] the lowbinding energy peak could be associated to Rh–O units detached from the steps and diffusing on thesurface, until they condense in the Rh–O rows. In these rows, the oxygen occupies f.c.c. threefold sites,accounting for the abrupt shift of the O 1s binding energy above 0.1 ML. The substantial slowing downof the adsorption rate above 0.5 ML can be attributed to strong reduction of the adsorption probabilitybecause in the ð2 � 2Þp2mg structure all favorable f.c.c. threefold sites are occupied. Undoubtedly theRh atom mass transport involved in the transition from a ð1 � 2Þ at 0.5 ML to a ð1 � 4Þ at 0.75 MLreconstructed surface is an activated process which slows down the oxygen uptake.

4.1.3. Oxygen adsorption on Ru(1 0 �1 0)

Using as a representative example the dissociative oxygen adsorption on Ru(1 0 �1 0), we willillustrate the advantage of performing real-time XPS measurements in combination with spot-profileanalysis LEED (SPA-LEED) [62]. The two techniques are complementary: LEED is a structural tech-nique, probing the long-range order of adsorbed layers, while XPS identifies their surface chemistry andelectronic structure.

SPA-LEED scans of the diffraction beams along the [1 �2 1 0] and [0 0 0 1] crystallographicdirections, taken while dosing oxygen at 270 K on a Ru(1 0 �1 0) surface, showed the development of aseries of distinguishable LEED patterns (see Fig. 11). The first pattern, characterized by the (1/2, 3/4)and (0, 1/2) diffraction spots, appears after exposure of about 0.5 L. Its development is illustrated by

Fig. 10. (a) Evolution of the O 1s core-level spectra during oxygen uptake on Rh(1 1 0) at T ¼ 570 K. hn ¼ 650 eV. Oxygen

pressure ranging from 2:5 � 10�9 to 1 � 10�7 mbar. (b) Adsorbed oxygen coverage versus oxygen exposure. The insert

illustrates the coverage dependence of the population of bridge and threefold sites observed in the initial stage.


the behavior of the diffraction spot intensities, Ið1=2;3=4Þ and widths versus exposure plotted in Fig. 11(a)and (b). The spots appear sharp along the [1 �2 1 0] direction and rather broad along the [0 0 0 1] direc-tion, indicating an anisotropic ordering process. They become sharper and narrower with increasingoxygen coverage when long-range ordered islands are formed. For an exposure of 1 L, when thecð2 � 4Þ structure is completed, the two spots reach their maximum intensity and minimum width, inagreement with previous investigations [63]. Upon further increasing the exposure, the (1/2, 3/4) spotloses intensity and coalesces into a (1/2, 1) spot at saturation. The intensities Ið1=2;1Þ and the FWHM ofthe (1/2, 1) spots along the [0 0 0 1] and [1 �2 1 0] directions are also reported in Fig. 11(c) and (d).Fig. 12(a) shows the O 1s spectra collected during the oxygen uptake performed at the sametemperature used in the LEED experiment. The results of the spectral analysis are shown in Fig. 12(b).

They show that the O 1s binding energy (open markers) changes slightly, from 529:84 0:01 eV atlow coverage to 529:75 0:01 eV in the coverage range 0.5–1.0 ML, i.e. when the transition from thecð2 � 4Þ to the ð2 � 1Þp2mg phase occurs. As discussed above for the O–Rh(1 1 1) system, such asmall O 1s binding energy shift, DEB ¼ ð85 30Þ meV, reflects the interactions arising between the

Fig. 11. Plots of the intensity (filled markers) and full-width-at-half-maximum (empty markers) versus oxygen exposure on

Ru(1 0 �1 0): (left) the (1/2, 3/4) spot characteristic of the cð2 � 4Þ ordered structure, measured along the (a) [0 0 0 1] and (b)

[1 �2 1 0] direction; (right) the (1/2, 1) spot characteristic of the ð2 � 1Þp2mg ordered structure, measured along the (c)

[0 0 0 1] and (d) [1 �2 1 0] direction. T ¼ 270 K.


oxygen atoms occupying the same adsorption site. The preservation of the same adsorption siteindependently of the oxygen coverage is also in accordance with the results of Schwegmann et al. [63].The insert in Fig. 12(b) shows the dependence of the oxygen sticking coefficient on the total oxygencoverage. Considering the fact that the molecular oxygen precursor needs two adjacent adsorption sitesto dissociate, we tried to elucidate the adsorption mechanism using the theoretical Kisliuk stickingprobability model for a double-site adsorption [40]. However, weak linear initial decay of S observeduntil completion of the cð2 � 4Þ structure indicates that the trapping into a molecular precursor state issomewhat affected by the already adsorbed oxygen. As judged from the LEED data, the oxygen pairsshould be adsorbed in a disordered manner up to �0.34 ML, when the cð2 � 4Þ diffraction spots appear.Apparently, the gradual decrease of the available adsorption site pairs with advancement of adsorptionaffects the adsorption rate from the very beginning. The break in the slope at coverages above 0.5 MLmarks a faster decrease of the sticking coefficient. According to the structural model of the cð2 � 4Þordered phase, proposed in Ref. [63], in order to create two nearby empty sites for further dissociationof oxygen molecules, the oxygen atoms must be displaced from the sites occupied in the cð2 � 4Þadsorption phase. The need of rearrangement of the adlayer above 0.5 ML sensibly lowers theadsorption probability, which determines the faster drop of the sticking coefficient.

4.2. Nitric oxide adsorption

NO, a toxic molecule contained in the automobile exhaust gases, is also an object of conversion viacatalytic reduction. This, along with the basic interest in understanding the mechanisms involved indissociative adsorption of heteroatomic molecules, has placed NO in the list of the most extensivelystudied adsorbed molecules. The electronic structure of NO is similar to that of CO and practically the

Fig. 12. (a) Evolution of the O 1s core-level spectra during oxygen uptake on Ru(1 0 �1 0) at T ¼ 270 K. hn ¼ 650 eV. (b)

Oxygen uptake and O 1s binding energy dependence as a function of the oxygen exposure. The insert shows the dependence of

the sticking coefficient on oxygen coverage.


same type of molecular orbitals are involved in the formation of the chemisorption bond. However, itssurface chemistry is more complex: NO exhibits a number of possible bonding configurations, linear,bend and side-on, and much lower barrier for dissociation compared to CO. This is due to the fact thatin the CO molecule the antibonding 2p� orbitals, which couple strongly with the dp metal electronicstates during adsorption, are empty, whereas NO has one electron in the antibonding 2p� orbital. SinceNO catalytic reduction involves NO dissociation as an intermediate step, the dissociation probability ofthe different bonding configurations of adsorbed NO molecules has been extensively studied.Theoretical and experimental results have suggested that a highly inclined NO (with an axis nearlyparallel to the surface), where both O and N are bonded to the surface, is the configuration of theprecursor to dissociation [64]. Here we illustrate with some examples how real-time XPS has providedthe missing links between the kinetics of molecular and dissociative NO adsorption and the precursor todissociation.

4.2.1. Nitric oxide adsorption on Rh(1 1 0)Since Rh is one of the active ingredients in automobile catalytic converters, the interaction of NO

with different single crystal Rh surfaces has gained a particular interest. Our real-time XPS studies ofNO adsorption on a Rh(1 1 0) surface were based on already available basic information about theadsorption sites and bonding geometry of the NO molecules [65]. The chemisorption of NO onRh(1 1 0) was studied at several temperatures, ranging from 210 to 370 K [66]. The possibility toacquire O 1s and N 1s spectra with an acquisition time of 14 s per spectrum, while the surface wasexposed to a constant pressure of NO has allowed us to evaluate the variation in the bondingconfigurations of the NO molecules and the dissociation products, O and N, with increasing NOcoverage.

For temperatures below 200 K only molecular NO adsorption occurs, in agreement with our earlierHREELS and XPS data [65,67]. At temperatures above the onset of NO dissociation, all three species,NO, O and N can easily be distinguished in the O 1s and N 1s spectra because of the large differences inthe corresponding binding energies.

Fig. 13 shows the evolution of the O 1s and N 1s spectra with NO exposure. The N 1s spectra can beanalyzed using two fitting components, corresponding to atomic nitrogen (in the range 397.2–397.4 eV)and molecular NO (in the range 399.7–400 eV), respectively. The O 1s spectra at low coverage requirea third component, in addition to the components assigned to atomic oxygen (in the range 529.7–530.2 eV) and NO (�531 eV). This component appears at an O 1s binding energy of 530.7 eV and itsrelative weight changes with adsorption temperature. The N 1s and O 1s spectra clearly show that attemperatures higher than 210 K the first adsorbed NO dissociates (only atomic nitrogen and oxygencomponents are present at low coverage), followed by simultaneous molecular and dissociativeadsorption up to a moderate coverage and pure molecular NO adsorption at high coverage. Thefraction of dissociated NO increases with temperature and in fact only dissociative adsorption occursabove 400 K. The plots of the O, N and NO uptakes, obtained from the decomposed N 1s and O 1sspectra measured at three adsorption temperatures, are illustrated in Fig. 14. There is an apparentdifference between the uptake of atomic oxygen and atomic nitrogen. The initial slope of N uptake isalmost constant and the intensity of the N 1s signal at 240 and 270 K indicates somewhat highercoverage than the oxygen signal. This reflects the different possible locations of atomic oxygen, in thesub-surface layers or in the troughs of the ðn � 1Þ reconstruction induced upon nitrogen adsorption[68–71].


The most interesting result is the behavior of the third O 1s component at 530.7 eV, which is alsotemperature dependent. In the temperature range, 210–270 K, characterized by low and moderatedissociation probability, its contribution represents up to 10% of the total intensity and its decline startsaround the onset of molecular adsorption. At higher temperature, e.g. 370 K, when the dissociativeadsorption dominates, the contribution of this component becomes very small and is observed only in avery narrow range of NO exposure. This O 1s component has been attributed to the presence of lying-down or highly inclined NO species, which are considered as a precursor to NO dissociation. This NObonding configuration has already been identified on Rh(1 0 0) [72] and on Ni(1 0 0) [73], by means ofHREELS and XPS, respectively. Also in the latter case the N 1s binding energy of the lying-downspecies is similar to that of the atomic nitrogen, while in the O 1s spectra the ‘precursor’ NO ischaracterized by a third component at a binding energy between those of molecular NO and atomicoxygen.

Our real-time XPS data have clearly manifested that the lying-down NO can be observed onlyin a narrow temperature and coverage range, i.e. under conditions when the N-induced ðn � 1Þreconstruction of Rh(1 1 0) is dominating [68–70]. This surface reconstruction results in formation of(1 0 0) microfacets along the [0 0 1] direction, which are the favorable surface geometry for lying-down

NO bonding, as revealed by HREELS data [72]. Judging from the relative weight of the third O 1scomponent, the lifetime of the lying-down NO precursor decreases with increasing temperature. Thefast decline in the concentration of the lying-down NO species with increasing N þ O coverage can beattributed not only to blocking effects, but also correlates with switching from a ðn � 1Þ N-inducedreconstruction to a ð1 � nÞ O-induced reconstruction, which exposes the (1 1 1) microfacets [70].

Fig. 13. Evolution of the O 1s (a) and N 1s (b) core-level spectra during NO uptake on Rh(1 1 0). hn ¼ 650 and 500 eV,

respectively. T ¼ 270 K.


In fact, the complete disappearance of the lying-down NO O 1s component marks the state when NOcontinues to adsorb only molecularly.

4.2.2. Nitric oxide adsorption on Ir(1 1 0)The importance of substrate structure in favoring a particular NO bonding configuration has been

further manifested by the studies of NO chemisorption on the Ir(1 1 0) surface, which does not undergothe ðn � 1Þ N-induced reconstruction occurring on the Rh(1 1 0) surface. The NO adsorption anddissociation on the Ir(1 1 0) surface were studied at temperatures ranging from 200 to 500 K [74]. Thedifference with respect to the Rh(1 1 0) case is that, apart from atomic oxygen (O 1s binding energyranging from 527.75 to 528.90 eV) and nitrogen (N 1s binding energy of 398.15 eV) the molecularNO species give only one component in the N 1s and O 1s spectra, at 401.25 eV and in the range530.10–531.20 eV, respectively. Typical N 1s and O 1s spectra measured during NO adsorption onthe Ir(1 1 0) surface are shown in Fig. 15. The uptake plots at different reaction temperature, alsoshown in Fig. 15, indicate that NO is the dominant species at 300 K, the fraction of dissociated NO

Fig. 14. Plots of the normalized O 1s (left) and N 1s (right) intensities versus NO exposure on Rh(1 1 0) for: upright NO (a),

lying-down NO (b) and atomic oxygen (c). Adsorption temperatures are: T ¼ 210 K (filled circles), T ¼ 270 K (filled squares)

and 370 K (open squares).


increasing with increasing temperature. The different regions indicated in the panels mark thefollowing stages of the NO–Ir(1 1 0) interaction: (i) predominant dissociative adsorption with gradualincrease of N and O coverage; (ii) mixed dissociative and molecular adsorption, accompanied bypartial or complete desorption of atomic N at temperatures higher than 300 K; (iii) predominant

Fig. 15. (Left) N 1s and O 1s core-level spectra showing the components corresponding to molecular NO, atomic nitrogen

and atomic oxygen in the NO/Ir(1 1 0) system. (Right) Total and species-resolved uptake plots (NO, nitrogen and oxygen),

measured at 300, 350, 375, 400, 450 and 500 K.


molecular adsorption reaching the saturation coverage of coadsorbed O and NO in stages (i) and (ii).It should be noted that at temperatures above 450 K NO adsorbs molecularly only during stage (iii).In the final stage, the saturation coverage ranging from 1 to 0.8 ML at the highest temperatures, isreached. The features that distinguish the NO uptake on Ir(1 1 0), from those on Rh(1 1 0), can besummarized as follows:

(i) absence of a lying-down NO;(ii) stronger destabilization effect of coadsorbed O and NO on the N adsorption state;

(iii) continuous dissociative NO adsorption until saturation favored by the liberation of adsorption sitesvia N desorption;

(iv) coadsorption of molecular NO up to temperatures as high as 500 K, compared to 400 K forRh(1 1 0).

This results clearly manifest that the energy barrier for NO dissociation on Ir(1 1 0) is higher than onRh(1 1 0), but less affected by the coverage. The failure to detect a precursor state suggests that theactivation barrier for transition to a molecular precursor state, which has a very short lifetime,determines the dissociation probability. Along with the specific differences in the electronic structure ofIr and Rh, we believe that the structural factor, i.e. the ðn � 1Þ reconstruction of Rh(1 1 0), providingmore favorable adsorption sites for lying-down NO, also plays an important role.

Finally, we would like to point out that the real-time XPS data are in agreement with themathematical model suggested to describe the oscillatory rate observed for the NO–H2 reaction overIr(1 1 0) [75].

4.2.3. Nitric oxide adsorption on stepped surfacesAlong with surface reconstructions of the substrate, the defects and irregularities of the surface

structure can also affect the adsorption rate and the bonding of the adsorbed species. Among the mostcommon irregularities are the monoatomic steps, which can also be introduced in a controlled manner.As discussed in Refs. [76,77], the steps are an important class of defects which can drastically modifythe reaction path on solid surfaces. We selected as a model system for our real-time XPS studies on therole of steps, NO dissociative adsorption on flat (1 1 1) and on stepped (5 3 3) and (3 1 1) Rh surfaces[78]. The (5 3 3) and (3 1 1) Rh surfaces are composed by (1 0 0) steps and (1 1 1) terraces, with a ratioof 3 and 1, respectively. In order to simplify the picture, the investigations were carried out attemperatures above 400 K, where only dissociated species are present on the surface. The surfacecoverage was calibrated assuming that NO dissociative adsorption on Rh(1 1 1) at 430 K leads to asaturation oxygen coverage of 0.5 ML. Similarly to the case of NO on Ir(1 1 0), also during NOadsorption on Rh(1 1 1) atomic N is destabilized and desorbs, so that only oxygen remains at saturation.This similar behavior of the Ir(1 1 0) and Rh(1 1 1) surfaces with respect to N stability confirms one ofthe conclusions of the previous paragraph, i.e. the important role of the N-induced structural changes ofRh(1 1 0), resulting in more effective NO dissociative adsorption on that plane. Fig. 16 reports the O 1s(a) and N 1s (b) core-level spectra measured after low, intermediate and high NO exposures (0.2, 0.8and 6 L, respectively) on the three Rh surfaces. A notable feature is that for all three surfaces the O 1sspectra, used for coverage calibration, are peaked at the same energy, which stays constant at allcoverages. On the contrary, with the exception of Rh(1 1 1), the N 1s spectra for Rh(3 1 1) andRh(5 3 3) clearly show the presence of two components. The dominant component at 397.4 eV is theonly N 1s peak on the Rh(1 1 1) surface, whereas the second one at 397.8 eV emerges on the two


Fig. 16. O 1s (a) and N 1s (b) core-level spectra measured at different coverages on the (3 1 1), (5 3 3) and (1 1 1) Rh

surfaces. hn ¼ 650 and 500 eV, respectively. (c) Uptake plots of the total O þ N coverage, the N coverage at steps and terraces,

NS and NT, and O coverage for the (3 1 1), (5 3 3) and (1 1 1) Rh surfaces.


stepped surfaces at intermediate exposures. Apparently the peak at 397.4 eV originates from nitrogenatoms located on the (1 1 1) terraces, while the higher energy one from nitrogen adsorbed at the steps.As expected, the weight of the 397.8 eV component is higher on the Rh(3 1 1) surface, which has ahigher step density.

The uptake plots obtained after data processing, shown in Fig. 16(c), and in particular the two Nuptake curves relative to the stepped surfaces, clearly manifest that below 0.25 ML the NO dissociationoccurs only at terraces. The higher activity of the (1 1 1) terraces in the initial stage of NO dissociativeadsorption is manifested by the steeper slope of the oxygen uptake measured on the flat (1 1 1) surface.However, this trend reverses when the O þ N coverage exceeds 0.25 ML, i.e. the dissociation rate onthe flat Rh(1 1 1) surface declines, whereas negligible changes occur on the stepped surfaces. This hasbeen attributed to preservation of the adsorption probability supposing two contributions: diffusion ofthe dissociated products to the steps, liberating space on the (1 1 1) terraces for further NO dissociation,or opening of a new channel for NO dissociation at the steps, activated at coverages larger than0.25 ML. We should point out that the role played by the steps during NO dissociation on Rh surfaces isnot a general trend for all stepped transition metal surfaces. For example, the STM studies have shownthat NO dissociation on Ru(0 0 0 1) takes place preferentially near the steps at low coverage, while onlyat high coverage the NO dissociation occurs also on the terraces [79].

5. Adsorbate bonding configuration probed by the substrate core-level shifts

The use of the adsorbate core levels for distinction of the adsorption site is not always veryinformative with respect to the rearrangements in the adsorbed phase occurring with increasingcoverage. For example, this is the case for oxygen adsorption on Rh(1 1 1) and Ru(1 0 �1 0), whereoxygen sits in the same adsorption site, independently of the coverage, and the O 1s core-level signal isnot sensitive to small modifications of the O–metal bond. The occupation of a single adsorption site is ageneral trend for the oxygen adsorption on transition metals (Rh, Ru, Pd, Pt, etc.) [80]. One can arguethat in denser layers the repulsive O–O interactions should exert some effect on the O 1s bindingenergy, but, as mentioned in the previous sections, these changes are usually very small. High-resolution photoelectron spectroscopy, probing the shifts of the substrate core levels induced by theadsorbing species with increasing coverage, offers the unique possibility of identifying the changes inthe local environment of the substrate atoms. It is well known that the core-level binding energy of thefirst layer atoms is different from that of bulk atoms, even for clean surfaces, and this difference iscalled surface core-level shift (SCLS) [36,81,82]. The SCLSs, which usually range from a fewmillielectron volts to more than 1 eV are a natural result of the different electronic and geometricalstructure of the first substrate layer. In fact an atom at the surface has a charge density which is moreatomic-like, because of the reduced number of nearest neighbour bonds. This results in a narrowing ofthe surface d band around its centroid. The band is than electrostatically shifted in order to keep thesurface and bulk atoms at the same chemical potential. In this framework, the SCLS is simplyproportional to the narrowing of the d band caused by the bond breaking at the surface, which is aninitial state effect, described as

SCLS /ffiffiffiffiffiffiCs

Cb

r� 1

� �; (2)


where Cs and Cb are the coordination of the surface and bulk atoms, respectively. This approachneglects the final state effects due to the different possible response of the valence electrons at thesurface and in the bulk to the core-hole created by the photoemission process. These final state effectsare believed to be quite small in transition metals, but even though their effect on the core-level shiftscan be distinguished with a high spectral resolution [83,84].

The presence of adsorbates usually changes the core-level binding energy of the surface atoms,because the coupling between the adsorbate and metal orbitals changes locally the electronic structure.From these SCLSs one can elucidate the local environment, e.g. the number of adsorbates bonded to asubstrate atom. For example, Andersen et al. [85] demonstrated that the Pd 3d5=2 core-level spectra arevery sensitive to the structure of the CO adlayer on Pd(1 0 0), the Pd 3d5=2 binding energy increasing byabout 0.5 eV per bond to a CO molecule. From the Pd 3d5=2 SCLSs they also elucidated the relativenumber of the first layer Pd atoms, single- and double-bonded to CO.

The determination of the SCLSs requires for the 4d and 5d transition metals high-energy resolutionphotoemission measurements with synchrotron radiation, because the variations in the core-levelbinding energy induced by atomic and/or molecular adsorption are relatively small. Moreover, a highsurface sensitivity, obtained by tuning the photon energy, is needed in order to enhance the weight ofthe core-level components of the surface atoms. The great advantage of having a high brightness photonsource is the possibility to follow the time-evolution of surface core levels and to obtain specificinformation about the kinetics of atomic and/or molecular adsorption on solid surfaces complementaryto that provided by the core-level spectroscopy of the adsorbate. The following two examplesdemonstrate the potential of real-time SCLS measurements to provide direct information how therelative number of surface atoms bonded to one or more adsorbed species changes with coverage and toidentify the geometrical structure of the adlayer.

5.1. Oxygen adsorption on Rh(1 1 1)

The real-time SCLS method has been applied to probe the evolution of the Rh 3d5=2 core level duringdissociative oxygen adsorption on Rh(1 1 1). This study provides complementary informationnecessary to verify the results obtained by following the evolution of the O 1s spectra, describedbriefly in Section 4.1.1. Fig. 17 shows a series of Rh 3d5=2 spectra acquired during dissociative oxygenadsorption at 300 K. The spectrum of the O-free surface consists of two well resolved peaks at 307.15(Rhb) and 306.66 (Rhs) eV, representing the emission from the bulk and the surface atoms, respectively.The SCLS of ð�485 20Þ meV is in good agreement with the previously determined value [86]. Uponoxygen adsorption the surface core-level peak changes and new components grow on the right and leftside of the Rhb component. The relative weight of these new components depends on the oxygencoverage and determines the shape of the Rh 3d5=2 peak. In order to identify correctly these componentswe performed deconvolution of the Rh 3d5=2 peak considering the structures of the pð2 � 2Þ andpð2 � 1Þ ordered phases, formed at 0.25 and 0.5 ML, respectively. According to the pð2 � 2Þ model(see left panel in Fig. 18), there are two inequivalent surface Rh atoms, not bonded to oxygen (Rhs) andone bonded to one oxygen atom (Rh–O). In the denser pð2 � 1Þ structure all Rh atoms are bonded tooxygen, but half of them are bonded to one oxygen, single-bonded Rh–O, and the other half to twooxygen atoms, double-bonded (Rh–2O). The best fit was obtained with components for Rh–O and Rh–2O atoms, shifted with respect to the bulk peak by ð�140 20Þ and ðþ295 20Þ meV, respectively.These experimental values are in very good agreement with the theoretical predictions of the core-level


shift values for clean (�459 meV), single-bonded (�100 meV) and double-bonded (390 meV), Rhsurface atoms obtained by DFT calculations, including both initial- and final-state effects [87].

The results of the data analysis are plotted in the right panel of Fig. 18. The plots illustrate thechanges in the relative number of surface Rh atoms with different coordination, noted as Rhs, Rh–O andRh–2O, with increasing oxygen coverage. In the early stage of oxygen adsorption the number of theRh–O atoms linearly increases with oxygen uptake at the expenses of the Rhs atoms. The Rh–2Ocomponent appears and grows later, at coverages above 0.25 ML, when the pð2 � 2Þ phase develops.The growth of the Rh–2O component correlates with the observed gradual shift of the O 1s peak(see Fig. 7(b)) attributed to the repulsive O–O interactions. In a rather wide coverage range around0.25 ML the slopes of the Rh–O and Rhs become flatter. In fact the Rh–2O plot passes through aplateau, starts to decline above 0.3 ML, due to the transformation to Rh–2O, and reaches the secondplateau above 0.4 ML. The Rhs plot after a break and a region of a very slow decline, decreases anddisappears at about 0.35 ML. The Rh–2O population reaches the maximum just at saturation (0.5 ML).At this coverage, the intensity of the single-bonded Rh component remains significant, in agreementwith the pð2 � 1Þ structural model.

Fig. 17. Evolution of the Rh 3d5=2 core-level spectra during oxygen adsorption on Rh(1 1 1) at T ¼ 300 K. The bulk Rhb and

surface Rhs components are indicated. Oxygen pressure ranges from 1 � 10�9 to 1 � 10�7 mbar. hn ¼390 eV.


5.2. Oxygen adsorption on Ru(1 0 �1 0)

Real-time SCLS studies have also been successful in identification of the adsorption site duringoxygen uptake on a Ru(1 0 �1 0) surface [88]. Since the Ru(1 0 �1 0) surface is a low-packed opensurface, not only the atoms of the first layer, but also the atoms of the second layer have an environmentdifferent from that of the bulk Ru atoms. This results in the presence of a third component in the Ru3d5=2 core-level spectra of the clean Ru(1 0 �1 0), as shown in Fig. 19(a) [89]. The components of thefirst (Ru1) and the second (Ru2) layer are shifted with respect to the bulk position by �480 20 and�240 20 meV, respectively. As already reported in Section 4.1.3, oxygen adsorption on Ru(1 0 �1 0) ischaracterized by two long-range ordered structures, cð2 � 4Þ at 0.5 ML and ð2 � 1Þp2mg at thesaturation coverage of 1 ML [62,63,90].

Similarly to the case of Rh(1 1 1), oxygen adsorption on Ru(1 0 �1 0) leads to replacement of theclean surface peak by new peaks due to the core-level shifts induced by the adsorbed oxygen (seeFig. 19(b)). By fitting the Ru 3d5=2 spectra, measured at increasing oxygen coverage, we obtained theplots shown in Fig. 19(c), which illustrate the changes in the intensity of the components assigned tofirst and second layer Ru atoms (see figure caption) with increasing oxygen coverage. Up to 0.5 ML theRu1 peak decreases fast and nearly disappears on completion of the cð2 � 4Þ structure. This indicatesthat in the cð2 � 4Þ structure all Ru atoms in the first layer are already bonded to oxygen. Thesecond layer component, Ru2, also loses intensity above 0.2 ML but disappears only when thesaturation is reached. In the 0–0.5 ML coverage range two new components appear, shifted by�85 20 and 465 20 meV with respect to the bulk peak. Their behavior with increasing oxygencoverage is completely different, as one reaches a maximum at �0.4 ML and disappears at 1 ML,whereas the second emerges later and gains intensity up to saturation. The third new component, shifted

Fig. 18. Evolution of the three different surface component Rhs, Rh–O and Rh–2O versus oxygen coverage on Rh(1 1 1).

The clean, pð2 � 2Þ and pð2 � 1Þ ordered structure, showing the different configuration of the first layer atoms, are reported in

the left panel.


Fig. 19. (a) Evolution of the Ru 3d5=2 core-level spectra during oxygen uptake on Ru(1 0 �1 0) at T ¼ 300 K. hn ¼ 380 eV. (b) Ru 3d5=2 core-level spectra

corresponding to the ð2 � 1Þp2mg-O, cð2 � 4Þ-O and ð1 � 1Þ structures, illustrating the components for bulk Ru, Rub, first layer Ru, Ru1, second layer Ru,

Ru2, single-bonded Ru atoms, Ru1–O and Ru2–O, and double-bonded Ru, Ru1–2O. (c) Plots illustrating the changes in the different Ru components with

increasing O coverage.

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by 215 20 meV with respect to the bulk peak, grows at coverages higher than 0.5 ML until saturationof the ð2 � 1Þp2mg structure. The most important information provided by the plots in Fig. 19(c) is thatextinction of the Ru1 component requires completion of the cð2 � 4Þ structure. This automaticallyexcludes many of the possible structural models for the cð2 � 4Þ structure, illustrated in Fig. 20,namely:

(i) all structures where oxygen is in a threefold f.c.c. site (models D, E and F);(ii) the models G and H with oxygen in an on-top site;

(iii) model I with oxygen in a long-bridge site;(iv) model B with oxygen in an hpc site.

Among the rest, the model L can also be excluded, because our experimental data show that in thecð2 � 4Þ structure also Ru atoms of the second layer should be bonded to oxygen (two new componentsare present). This indicates that the oxygen adsorption site is the threefold hcp (model A or C), in goodagreement with the results of the LEED I–V analysis (model A) [63]. The formation of the ð2 � 1Þp2mgstructure leads to the growth of the third component, shifted by 215 meV, which correlates with thedouble-bonded first layer Ru atoms (Ru1–2O).

6. Molecular desorption

Real-time XPS has also proved to be an excellent probe of the molecular desorption processes. Thisapproach, which we have called temperature programmed-XPS (TP-XPS), probes the changes in the

Fig. 20. Possible structural models for the cð2 � 4Þ structure, with oxygen sitting in threefold hcp, threefold f.c.c., on-top,

short-bridge and long-bridge adsorption sites of a Ru(1 0 �1 0) surface.


core-level spectra of the adsorbate and/or substrate induced by a continuous increase of the temperature[91]. The TP-XPS yields quantitative information on the evolution of the different surface species andon the relative population of the different adsorption sites, which are inaccessible for the conventionalTPD mass spectrometry. Moreover, as will be shown in the next paragraph, desorption rate curves canbe obtained as well by differentiating the adsorbate coverage versus temperature plots.

6.1. Carbon monoxide desorption from Rh(1 1 0)

As for the cases of CO adsorption on the (1 1 1) and (1 0 0) rhodium surfaces, reported in Sections 3.1and 3.2, CO adsorption on Rh(1 1 0) also leads to the formation of two different ordered structures.Up to 0.5 ML, when the cð2 � 2Þ structure is completed, CO occupies only on-top sites, characterizedby O 1s binding energy of 531.9 eV. At higher coverage phase transitions to other ordered structures,ð4 � 2Þ and ð2 � 1Þp2mg, occur, where CO occupies short-bridge sites as well (O 1s binding energyof 530.8 eV). In the ideal case of a perfect ð2 � 1Þp2mg structure all CO molecules should occupyshort-bridge sites. In real systems, due to the presence of structural imperfections, some CO remainsin on-top sites as evidenced by the O 1s spectra for saturation CO coverage. Fig. 21(a) presents the

Fig. 21. (a) Evolution of the O 1s core-level spectra while a linear temperature ramp of 0.3 K/s is applied to the CO

precovered Rh(1 1 0) sample. The peaks at 531.9 and 530.8 eV correspond to CO in on-top and bridge sites, respectively. The

initial saturated layer is a ð2 � 1Þp2mg structure. (b) Plots illustrating the evolution in the on-top (filled circles) and bridge CO

(empty circles) coverage with increasing desorption temperature. The open squares represent the total CO coverage. This plot

was used to obtain the TP-XPS curve shown in the insert together with the conventional TPD spectrum.


evolution of the O 1s spectra measured while a linear temperature ramp of 0.3 K/s was applied to thesample covered with a saturated CO-ð2 � 1Þp2mg layer. The O 1s spectra drastically change withincreasing temperature: they lose intensity due to CO desorption and undergo a shift to a lower bindingenergy. By fitting the spectra, using the O 1s components corresponding to on-top and bridge CO, theplots shown in Fig. 21(b) are obtained. These plots show the evolution of the on-top and bridge COcoverage during CO desorption. Up to �380 K the decrease of the total CO coverage is due to thedepopulation of the bridge-sites. However, only a fraction of the bridge CO molecules desorb. Anotherfraction converts into on-top CO, as evidenced by the increase of the on-top CO related component. Thechanges in the relative population of the on-top and bridge sites is in accordance with the predominantoccupation of on-top sites at CO coverage �0.5 ML. With advancement of CO desorption (above400 K) also the on-top sites are gradually depopulated.

The TP-XPS can also provide the same information obtained by TPD, i.e. the variation in thedesorption rate, �dy=dt, as a function of temperature. The TPD plots can be easily obtained from theTP-XPS data by a numerical differentiation of the total CO coverage versus temperature plot shown inFig. 21. The remarkable agreement with the conventional TPD data, plotted in the insert of Fig. 21(b),proves the reliability of the TP-XPS method, excluding any photon-induced effect.

The TP-XPS data can be analyzed in the framework of the thermodynamics of a two energy levelsystem. The site occupation represents a quasi-equilibrium situation at each temperature, as assumedfor other systems [92,93]. The equilibrium condition of the CO/Rh(1 1 0) system can be describedusing the equation:

lnytopy

StopðyS

bridge � ybridgeÞybridgey

SbridgeðyS

top � ytopÞ

" #¼ ln

nbridge

ntop

� �þ DE

KT; (3)

where nbridge; ntop and DE ¼ Etop � Ebridge are the pre-exponentials and the difference of adsorptionenthalpies between the top and bridge site, ytop (ybridge) and yS

top (ySbridge) are the actual and saturation

coverage of the on-top (bridge) species.By assuming the same pre-exponential values [93,94], it is possible to calculate the dependence of

DE on CO coverage. The plot in Fig. 22 shows that DE is constant up to a total CO coverage of

Fig. 22. Dependence of the difference in CO adsorption enthalpies between the on-top and the bridge adsorption sites,

DE ¼ Et � Eb, on the total CO coverage on Rh(1 1 0).


0.75 ML, and than suddenly drops, becoming negative above 0.9 ML. The changes of DE with COcoverage can be correlated to the observed changes in the adsorbed layer as follows:

(i) no energetically favorable bridge site is available for CO in the cð2 � 2Þ structure at 0.5 ML,because the strong repulsions prevent bridge-bonded sites next to on-top-bonded CO to beoccupied;

(ii) the addition of one bridge CO per unit cell at a low energy cost can be easily accomplished bysimply moving on-top CO molecules in adjacent on-top sites.

The resulting structure is formed by a sequence of two on-top and one bridge CO adsorbed along the[1 �1 0] direction.

Fig. 23 reports two models of the structure at 0.75 ML ((b) and (c)), the highest coverage achievableif occupation of only two adjacent on-top sites is allowed. Further adsorption of bridge-bonded CO isforbidden, because this requires the occupation of three adjacent on-top sites. Therefore, the surfacelayer rearranges above 0.75 ML moving all CO molecules to bridge sites in a zig-zag arrangement. Thisgeneral rearrangement is reflected by the sudden drop of DE at coverages higher than 0.75 ML.

6.2. Carbon monoxide desorption from Rh(1 1 1)

The capability of TP-XPS to yield information about the depopulation of different adsorption sitesand to evaluate the difference in the enthalpy between them has also been manifested in the study of COdesorption from the Rh(1 1 1) surface [37].

Fig. 24(a) and (b) shows the evolution of the O 1s spectra during TP-XPS of CO from a saturatedlayer and the changes in the total and relative CO coverage of the two bonding configurations. The twocomponents, corresponding to the CO molecules in on-top (filled circles) and hollow (empty circles)sites behave similarly to the CO/Rh(1 1 0) system, i.e. they show simultaneous CO desorption andconversion of hollow-site CO to on-top CO in the temperature range 300–360 K. This TP-XPS resultsare in good agreement with conventional TPD data [95,96] and with the general trend observed in theother TPD data for CO desorption on Ru, Pd and Pt single crystal metal surfaces.

Fig. 23. Structural models for the cð2 � 2Þ, ð4 � 2Þ and cð4 � 2Þ CO adsorbed layers on Rh(1 1 0).


Using the same assumption applied for the CO/Rh(1 1 0), i.e. considering the system in thermo-dynamic quasi-equilibrium, we found the coverage difference in the adsorption enthalpy. The plot,reported in Fig. 25, clearly manifests the preference for on-top sites up to �0.36 ML. Its behaviourcorrelates very well with the formed ordered structures with CO in on-top site (pð2 � 2Þ at 0.25 ML andðffiffiffi3

p�

ffiffiffi3

pÞR30� at 0.33 ML) and with CO in on-top and hollow sites at higher coverage.

Fig. 24. (a) Evolution of the O 1s core-level spectra while a linear temperature ramp of 0.3 K/s is applied to the CO

precovered Rh(1 1 1) sample. The peaks at 531.5 and 530.1 eV correspond to CO in on-top and threefold hollow site,

respectively. hn ¼ 645 eV. (b) Plots illustrating the changes in the population of on-top (filled circles) and threefold hollow

(empty circles) CO, and of the total CO coverage (open squares), with increasing desorption temperature.

Fig. 25. Difference in adsorption enthalpies, DE ¼ Et � Eh on Rh(1 1 1) as a function of the CO coverage.


6.3. C2N2 desorption from Pd(1 1 0)

The C2N2 chemistry on transition metals involves many complicated and competing reaction steps.For this reason, TP-XPS has been applied in order to identify the various surface species on Pd(1 1 0)and their respective coverage as a function of the substrate temperature [97].

We found that C2N2 adsorbs molecularly below 100 K, and is characterized by C 1s and N 1s bindingenergies of 284.80 and 397.55 eV, respectively (see Fig. 26). The other components (C 1s at 285.35 eVand N 1s at 398.30 eV), developing at high exposures, are assigned to the multilayer. Upon heating themultilayer desorbs below 200 K. The N 1s and C 1s peaks of the remaining monolayer undergo energyshift with further increase of the temperature. The shifts result from partial depopulation of the denseadlayer, which opens space for C2N2 dissociation. The resulting products are the CN species,characterized by N 1s and C 1s binding energies of 397 and 284.2 eV, respectively. The CN adlayerremains stable up to �600 K. Above this temperature, the CN species start to recombine and desorb asC2N2. The plots of the C2N2 and CN coverage as a function of temperature, obtained from the TP-XPSstarting with different C2N2 initial coverage, are shown in Fig. 27. The reduction of the C2N2 coveragewith temperature, reported in Fig. 27(b), is the result of two competing processes: C2N2 dissociation toadsorbed CN and C2N2 desorption. For the kinetic evaluation, it was assumed that at a giventemperature C2N2 and CN are in equilibrium. Under these conditions the rates of C2N2 dissociation andCN recombination are equal and follow the expression:

kCN½C2N2� 1 � ½C2N2� þ ½CN�½C2N2 þ CN�sat

� �¼ kC2N2

½CN�2 (4)

with ½C2N2� and ½CN� representing the surface concentrations of C2N2 and CN, respectively, andwith kCN and kC2N2

denoting the rate constants of CN formation via dissociation and C2N2 formationvia CN recombination. kCN and kC2N2

obey the standard relationship k ¼ n expð�E=RTÞ. In Eq. (4), thedissociation rate depends on the concentration of C2N2 and of the empty sites, whereas therecombination rate is a function of the CN coverage. The difference DE between the activation energyfor C2N2 dissociation and for C2N2 formation, ECN � EC2N2

, can be estimated from the plot of

ln½C2N2�ð1 � ½C2N2� � ½CN�Þ

½CN�2

!versus

1

T; (5)

shown in Fig. 28(a).The slope gives DE of 0:32 0:05 kcal/mol. Such evaluation becomes meaningful for low C2N2

coverage, when the dissociation dominates.It is also possible to estimate from the slope of the C2N2 coverage versus temperature plot, the

activation energy of dissociation, ECN, using the approximation of a first-order reaction:

� d½C2N2�dt

¼ k½C2N2� ¼ n½C2N2� exp �ECN

RT

� �: (6)

By using the Readhead’s formula [98] and assuming a pre-exponential factor of 1013 s�1, an activationenergy of 11 kcal/mol is obtained for C2N2 coverage �0.1 L (see Fig. 29).

At higher C2N2 coverage, the Readhead’s formula gives only an effective value for the removal ofC2N2 from the surface. Nevertheless, the obtained value of 14.3 kcal/mol at C2N2 ¼ 0:5 ML iscomparable with the value of 14.0 kcal/mol determined from standard TPD measurements [99].


Fig. 26. (a) C 1s and (b) N 1s core-level spectra measured during C2N2 adsorption on Pd(1 1 0) at T ¼ 80 K. hn ¼ 650 and 500 eV, respectively.

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The schematic energy diagram, drawn using the experimental values for activation energies ofdissociation and desorption of cyanogen on Pd(1 1 0), illustrates how the two processes compete in thetemperature range between 100 and 250 K.

6.4. SO2 interaction with Cu(1 0 0) and Ni(1 1 0)

SO2 interaction with Cu(1 0 0) [100] and Ni(1 1 0) [101] surfaces is another representative exampleof the potential of real-time XPS in probing dissociation and recombination of large moleculesinvolving the formation of more than one molecular fragment. The interest in SO2 interaction withtransition metals is mainly related to the fact that SO2 is a hazardous air pollutant and may also act as a

Fig. 27. (a) CN and (b) C2N2 coverages versus temperature on Pd(1 1 0), as derived from the TP-XPS experiments. (c) Rate

of the C2N2 coverage decrease obtained by differentiating the data reported in (b).


poison in a great number of catalytic reactions. In general, SO2 adsorbs molecularly on metals at lowtemperatures and decomposes upon heating.

According to the previous TPD data, the desorption of SO2 deposited on the Cu(1 0 0) surfaceat 180 K results in two desorption peaks at about 300 and 400 K leaving a small amount of sulfuratoms on the surface [102,103]. The X-ray absorption near edge spectroscopy (XANES) suggestedthe presence of SOx (x 6¼ 2) species on the surface after annealing of a saturated adlayer at roomtemperature [104]. The exact identity of these species has been clarified by TP-XPS measurements[100].

Fig. 30(a) shows S 2p spectra acquired while heating the Cu(1 0 0) surface saturated with SO2 at180 K. The S 2p spectra clearly present three components in the different stages of heating, with 2p3=2

binding energies of 160.2, 164.3 and 165.3 eV. The component at 164.3 eV, which dominates below250 K, is identified as SO2 in accordance with the XANES data [104]. The other two componentsappear and grow at the expense of the SO2 peak, which gradually disappears. The lowest bindingenergy peak corresponds to the S adatoms, while the component at 165.3 eV is tentatively assigned toSOx intermediate species. The latter gradually disappears with increasing temperature until only atomicS remains on the surface.

Fig. 28. (a) Arrhenius plot for the C2N2 decomposition on Pd(1 1 0) derived from the C 1s and N 1s coverages. (b) A sketch

of the C2N2 $ 2CN activation energy barrier.


Fig. 29. Activation energy for C2N2 dissociation and desorption as a function of the C2N2 coverage on Pd(1 1 0), obtained

by using Eq. (6).

Fig. 30. (a) Evolution of the S 2p3=2 core-level spectra during a TP-XPS experiment on Cu(1 0 0) on a SO2 precovered

Ni(1 1 0) surface. The first spectrum at 178 K corresponds to a SO2 saturated layer. (b) and (c) Temperature dependence of

SO2, SOx, atomic oxygen and atomic sulfur surface coverage, evaluated from S 2p and O 1s intensities.


The O 1s data were less informative, because the O 1s peaks of adsorbed SO2 and SOx are very closein energy and cannot be resolved. However, the intensity variation of the (SO2þSOx) O 1s component,along with the other two components related to atomic oxygen sitting in bridge and hollow adsorptionsites, has allowed elucidation of the intermediate steps of the SO2 decomposition [105].

Fitting of the evolving S 2p and O 1s spectra as a function of temperature resulted in the plots shownin Fig. 30(b) and (c), which illustrate the changes in the coverage of SO2, SOx, S and O adspecies withincreasing temperature. The coverages are calibrated using as a reference the Cu(1 0 0)–ð2 � 2Þ-Sstructure with sulfur coverage of 0.22 ML and the Cu(1 0 0)–ð

ffiffiffi2

p� 2

ffiffiffi2

pÞR45�-O structure with

oxygen coverage of 0.48 ML. This calibration procedure gives for temperature lower than 250 K a ratiobetween oxygen and sulphur of ð2:1 0:2Þ, confirming the XANES identification of SO2 as theadsorbed species. Around room temperature, the ratio becomes ð1:2 0:4Þ, clearly identifying the SOx

species as SO. These results indicate that at 300 K all SO2 molecules have already decomposed,releasing oxygen and sulfur in accordance with the O and S plots in Fig. 30.

Above 350 K, when the second desorption peak develops in the SO2 TPD spectra, the amount of SOand O decreases, while the S coverage remains constant. This means that the recombination to SO2

follows the reaction path

SOadsorbed þ Oadsorbed ! SO2;gas;

instead of

2SOadsorbed ! SO2;gas þ Sadsorbed: (7)

This explains the origin of the second, higher temperature SO2 desorption peak in the TPD desorptionspectra from Cu(1 0 0), which alone cannot identify the recombination pathway.

In the case of SO2/Ni(1 1 0) the real-time XPS studies solved the controversy concerning theadsorption state of SO2, which was alternatively identified as molecular or dissociative [106–108]. TheS 2p and O 1s spectra identified the different species present on the surface at low temperature and uponthermal-induced dissociation [101].

Fig. 31 shows the S 2p (a) and O 1s (b) spectra acquired while heating a saturated SO2 layerdeposited on Ni(1 1 0) at 160 K. In this system the S 2p spectra consist of more than three spin-orbitsplit components. Three S 2p3=2 peaks at 163.4, 164.1 and 165.2 eV were identified and assigned to twodifferent bonding configurations of SO2 and to SO3, respectively. In addition there are also a broadshoulder at the high binding energy side and a small peak at 161 eV. The latter is assigned to atomic sulfurin accordance with the S 2p spectrum measured for the cð2 � 2Þ-S structure on Ni(1 1 0) formed at0.5 ML [109]. Upon heating to �260 K, the S 2p3=2 components of the SO2 at 164.1 and 163.4 eVdisappear at about 210 and 255 K, respectively, the decay of the 163.4 eV component being accompaniedby a fast growth of the atomic S signal, which is the only species remaining above �350 K.

The SO2 and SO3 species in the O 1s spectra are not as well resolved as in the S 2p spectra, becauseof the intrinsic lifetime of the 1s oxygen core-hole, and appear as a broad peak at �531 eV. This peakslightly shifts to higher binding energy after annealing at 210 K and disappears at room temperature. Atthe same time, a new component growing at 530.4 eV is assigned to atomic oxygen, in accordance withthe Ni(1 1 0)–ð2 � 1Þ-O structure at 0.5 ML [110]. The atomic oxygen desorbs only at temperatureshigher than 800 K. The large shoulder at 533 eV seems to be coupled with the S 2p peak at the highestbinding energy and is assigned to non-stoichiometric SOx species.


The intensity of the different sulfur and oxygen components after conversion to coverage using forcalibration the known coverages of 0.5 ML for the Ni(1 1 0)–cð2 � 2Þ-S and the Ni(1 1 0)–ð2 � 1Þ-Olayers are plotted in Fig. 32. The following scenario for the temperature-induced processes can besuggested. The 164.1 S 2p3=2 and 530.9 eV O 1s components behave in similar manner with increasingcoverage and from the O 1s/S 2p ratio of ð2:31 0:5Þ can be ascribed to SO2. The S 2p3=2 peak at165.2 eV corresponds to the O 1s component at 531.2 eV and the O 1s/S 2p coverage ratio indicates thatthis species is SO3. Finally, by comparing the combined coverages of the 165.2 and 163.4 eVcomponents with that of the 531.2 eV O 1s peak, the 163.4 eV peak is assigned to a second SO2

species. In fact the two SO2 components correspond to molecules adsorbed in long-bridge and short-bridge adsorption sites, in accordance with the surface extended X-ray absorption fine structure(SEXAFS) data [101]. The broad structure at �167 eV in the S 2p spectra can be tentatively assigned toSO4 species, even though its relation to contaminants cannot be ruled out.

The result of the data analysis permits to univocally determine the decomposition and recombinationpathways of sulfur dioxide on Ni(1 10 ). At low temperature partial dissociation of SO2 leads to theformation of atomic S and SO3 according to the reaction:

3SO2;adsorbed ! 2SO3;adsorbed þ Sadsorbed: (8)

Fig. 31. Evolution of the S 2p3=2 (a) and O 1s (b) core-level spectra measured in a TP-XPS experiment. Linear temperature

ramp is 0.1 K/s. Reference spectra corresponding to the cð2 � 2Þ-S and ð2 � 1Þ-O structures are shown as well at the bottom.


A small amount of SOx species can form. The characteristic changes in the adlayer upon heating occurin the following three temperature ranges.

(i) Between 160 and 210 K the higher binding energy SO2 species is transformed into the more stableSO2 species. Part of the SO2 dissociates, producing SO3 and S.

(ii) In the range 210–255 K, the more stable SO2 also undergoes dissociation, leading to furtherincrease of SO3 and S and appearance of atomic oxygen as well. This process can be described bythe pathway:

2SO2;adsorbed ! SO3;adsorbed þ Sadsorbed þ Oadsorbed: (9)

The SOx species also disappears in this temperature range.(iii) In the range between 255 and 350 K, the SO3 species undergoes dissociation and only atomic

sulfur and oxygen remain on the surface. The dramatic increase of atomic oxygen on the surface,accompanied by desorption of SO2, as revealed by TPD experiments, is consistent with thefollowing reaction path:

2SO3;adsorbed ! SO2;gas þ Sadsorbed þ 4Oadsorbed: (10)

7. Surface reactions

Once proving its success in studies of chemisorption systems, including also dissociative adsorption andassociative desorption, real-time XPS studies have been expanded to characterization of surface reactions.

Fig. 32. Sulfur (a) and oxygen (b) coverage determined by the analysis of the core-level spectra reported in Fig. 31, as a

function of the substrate temperature.


The difficulty in the studies of reactions systems is introduced by the fact that many reaction channelsmay be accessible and a distribution of products may be produced. Apparently this requires excellentbasic knowledge of the behaviour of single adsorbate and coadsorbed systems in order to rationalisethe catalytic behaviour in terms of structure and chemistry of the substrate and reacting adlayer. Thefollowing examples illustrate the first achievements of real-time XPS in this respect.

7.1. CO þ O reaction on Rh(1 1 0)

Among the factors which govern the surface reactivity the bonding configuration of the adsorbatesinvolved in the reaction plays a very important role. Often the bonding configuration involves thereconstruction of the substrate in order to provide better accommodation of the adsorbed species. One ofour first real-time XPS reaction studies aimed to measuring the reactivity of oxygen adsorbed on theunreconstructed ð1 � 1Þ and reconstructed (1 � n) Rh(1 1 0) surfaces in the CO oxidation reaction [111].

In order to evaluate the influence of the substrate geometry we performed the titration reaction of O-ð2 � 1Þp2mg and O-cð2 � 8Þ surfaces at three different temperatures: 200, 290 and 350 K. In theoxygen ordered adlayers the atoms reside in threefold adsorption sites but in the O-cð2 � 8Þ structureone third of the atoms are located along the missing rows [51,112,113]. The real-time O 1s spectra werean excellent fingerprint of the changes in the oxygen and CO bonding configurations and coverage ofthe ongoing reaction. The decay of the O 1s signal corresponding to the O adatoms was used as ameasure of the titration rate.

Fig. 33 shows selected O 1s spectra recorded at 200 K during the titration of the ð2 � 1Þp2mg (a) andcð2 � 8Þ (b) oxygen adlayers. The O 1s peak growing at the higher binding energy side is due to thecoadsorption of CO. The two-dimensional plots (left) outline the intensity changes corresponding to theO–O 1s and CO–O 1s. Two distinct features are visible: (i) the peak corresponding to atomic oxygendecreases in intensity and shifts to lower binding energy; (ii) the CO related peak grows at �533 eV,which is 1 eV higher than the binding energy of CO sitting in on-top site on a clean Rh(1 1 0) surface[35]. In order to correlate the changes in the adsorbate coverage with the modifications in the surfacestructure, the LEED pattern was also monitored during the titration process as well.

Fig. 34 reports the results of the analysis of the spectra measured at 200, 290 and 350 K. The plotsclearly show the differences in the titration rate on unreconstructed and reconstructed oxygen-coveredRh(1 1 0) surfaces. This observation can be tentatively correlated with the reported structural sensitivityof the CO oxidation at high temperature and high oxygen coverage [114].

Let us first consider the difference in the oxygen titration rates as judged by the initial slope of theoxygen decay curves for the ð2 � 1Þp2mg and cð2 � 8Þ surfaces. The initial rate can be described by theexpression:

r ¼ n exp � E

RT

� �SPCOyO; (11)

where S is the CO sticking coefficient, n the pre-exponential and E the activation energy for CO2

formation. At the lowest temperature, the initial slope is two times steeper for the ð2 � 1Þp2mg

structure. This difference is easily explained if we consider as reactive only the oxygen species locatedalong the ð1 � 1Þ troughs. In the ð2 � 1Þp2mg structure, all oxygen, which equals 1 ML, sits along theð1 � 1Þ troughs, whereas in the cð2 � 8Þ structure it is only 0.5 ML, because the residual 0.25 ML sitsalong the ð1 � 2Þ troughs. Assuming that the latter, being most deeply embedded in the f.c.c. threefold


sites on the ð1 � 2Þ troughs of the ð1 � 4Þ reconstructed surface, is non-reactive, the factor of two in thereaction rate can be explained.

With advancement of the titration process the reaction rate becomes strongly temperature dependent,which we tentatively attribute to the following. In the first stage, the reaction takes place between theCO molecules which coadsorb weakly on the surface covered with the oxygen atoms in threefold sites[50,51]. The decrease of the oxygen coverage is accompanied by the conversion from threefold tobridge-bonded oxygen, which is the preferred adsorption configuration at low oxygen coverage, andcoadsorption of CO on the now available sites. Apparently the change of oxygen bonding configuration

Fig. 33. Evolution of the O 1s core-level spectra during CO titration reaction on Rh(1 1 0) at 200 K starting with: (a)

ð2 � 1Þp2mg oxygen overlayer of 1.0 ML and (b) cð2 � 8Þ oxygen overlayer of 0.8 ML. Left images are two-dimensional

plots showing the evolution with time of the O 1s components corresponding to CO and atomic O.


affects the reaction energetics and even on the ð1 � 1Þ surface the reaction is ceased at 200 K, whenstill �0.5 ML of oxygen remains on the surface. The activation barrier for CO2 formation on theð1 � 1Þ surface is apparently overcome at 290 K, whereas only at 350 K the O adatoms embeddedalong the ð1 � 2Þ troughs are activated.

Another interesting feature is the discontinuity of the reaction rate observed at 350 K at 0.5 L COexposure (see the insert in Fig. 34). Distinct changes can also be observed monitoring the CO2

production using mass spectroscopy. They can be correlated with the transformation from the ð1 � 4Þreconstructed surface to the ð1 � 1Þ unreconstructed surface, as evidenced by the LEED images. Thisresult clearly illustrates the close correlation between surface reactivity and adsorbate–substratestructure. It demonstrates the potential of real-time XPS to probe structural modifications occurringduring catalytic reactions, elucidating this impact on the reaction mechanisms. Our results prompted arecent combined STM and DFT study of the CO þ O reaction on Rh(1 1 0) [115] which has revealedthe details of the reaction mechanism at low temperature, confirming the existence of a short lived COstate on the terraces.

7.2. NO þ H2 reaction on Rh(5 3 3)

The ability to monitor the reaction products in gas-phase has been an important step in understandingphenomena-like hysteresis and oscillations in surface reactivity. However, the gas-phase data areinsufficient for modelling the surface reactions, because it is essential to know which species arepresent on the surface during the reaction.

Fig. 34. Evolution of the oxygen coverage on Rh(1 1 0) at 200, 290 and 350 K during titration reaction with CO:

ð2 � 1Þp2mg (filled circles) and cð2 � 8Þ (open circles).


According to gas-phase experiments, the rate of N2 formation during the NO þ H2 reaction on a(5 3 3) rhodium surface, composed by (1 1 1) terraces and (1 0 0) steps, undergoes a large hysteresisduring the heating–cooling cycle [116,117]. This peculiar behavior motivated a real-time XPS studyprobing the evolution of the N 1s and O 1s spectra during the NO þ H2 reaction on the Rh(5 3 3)surface [119].

N 1s and O 1s spectra were measured during the hysteresis cycle using NO:H2 gas pressure ratiosranging from 1:5 to 1:20. The changes in the relative coverage of NO, atomic N and atomic O specieswere evaluated by fitting the N 1s and O 1s spectra. In accordance with previous adsorption studies[118], the atomic nitrogen was identified by its two N 1s components at 397.4 (NT) and 397.85 eV (NS),originating from atomic nitrogen adsorbed on terraces and steps, respectively. The atomic oxygenappeared always as a single O 1s component. Fig. 35(a) and (b) summarizes the evolution of the surfacespecies during the hysteresis in the case of NO:H2 ratio of 1:20, the most interesting case because of thelarge modulation in the N2 production. Mass spectrometry data, acquired in parallel, are also shown.The starting condition is a coadsorbed layer containing NO, N (at the steps) and O. With increasingtemperature the molecular NO undergoes desorption and dissociation, but the reactivity remains very

Fig. 35. Evolution of the different surface species during NO þ H2 hysteresis reaction on the Rh(5 3 3) surface. Open and

filled markers indicate the coverage of the surface species during the heating and the cooling branch of the hysteresis,

respectively. H2/NO pressure ratio is 20. The N2 production, measured by mass spectroscopy, is shown for the sake of

comparison.


low up to 450 K, because the adsorbed species exert a blocking effect on the H2 adsorption. Opening offree surface space above the desorption temperature of nitrogen allows hydrogen adsorption. Thisimmediately provokes a fast H2O formation, as can be judged from the steep decrease of the oxygencoverage. The reaction rate is apparently controlled by the recombination rate of the nitrogen. Closerinspection of the N 1s spectra also reveals variations in the N coverage at the steps and on the terraces,the former leaving first the surface, whereas the last remaining the dominant species at the highesttemperature.

On the cooling branch the N2 production is reversed down to �470 K with the exception of thegrowing N adsorbed on the terraces. Apparently, this N suppresses the hydrogen adsorption aroundthe critical temperature of 470 K. Below this temperature the oxygen adsorbs on the surface and thereaction rate increases. There is a direct correlation between oxygen coverage and reaction rate, asmanifested by the in-phase oscillating traces of N2 production and oxygen coverage. The two differentnitrogen species behave also in concert with increasing oxygen coverage, the drop in intensity of thenitrogen on the terraces being accompanied by an increase of the nitrogen at the steps. This can betentatively attributed to diffusion of some oxygen destabilized terrace-NI to the steps, where apparentlythe oxygen destabilization effect is weaker.

On the basis of these findings, the following microscopic mechanism for the oscillation in thereaction rate on the cooling branch has been suggested: (i) nitrogen accumulates on the terraces untilthis results in suppression of hydrogen adsorption; (ii) oxygen begins to accumulate on the surface,destabilizing N on the terraces, which results in an increase of the rate of N2 production and indisplacement of nitrogen to the steps; (iii) the decrease of nitrogen on the terraces also leads to anincrease of the adsorption rate of NO and hydrogen; (iv) hydrogen titrates oxygen and nitrogen startsagain to accumulate on the terraces free of oxygen, i.e. the system is back to its initial conditions.

7.3. C2H2 trimerization on Pd(1 1 1)

Following C2H2 adsorption–desorption studies, the real-time XPS has also been applied toinvestigate ethyne trimerization on a Pd(1 1 1) single crystal surface [120]. This reaction represents aprototypical metal-catalyzed alkyne coupling reactions, and its real-time XPS study provides a valuabletest for fundamental concepts in surface science.

The C 1s signal was used as a fingerprint of the C2H2 coverage, adsorption state and modificationsduring adsorption and heating. At low temperature (100 K), C2H2 adsorbs on Pd(1 1 1) molecularly in asingle ethyne adsorption state (C 1s binding energy of 283.88 eV) with the C–C molecular axis lyingparallel to the surface in the whole coverage range up to saturation at 0.43 ML [121]. The trimerisation,which occurs after heating of the saturated C2H2 layer, causes a large variation in the intensity andshape of the C 1s spectra, as shown in Fig. 36(a). During the linear temperature ramp the C 1s spectraevolve in the following manner: the initial peak corresponding to the C2H2 species loses intensity andshifts towards higher binding energy and other two C 1s components develop at 284.55 and 285.0 eV.The two new species are assigned to strongly chemisorbed flat-lying benzene and weakly bound tilted-benzene, in accordance with previous results [122,123]. As reported in Fig. 36(b) in the first reactionstage, occurring at about 150 K, the formed benzene preserves the orientation of the C2H2 molecules,i.e. it is adsorbed flat on the surface. At higher temperatures, when a large part of C2H2 has reacted andthe flat-benzene reaches an intermediate coverage, the tilted-benzene appears as well. Above 330 K,when all ethyne species have trimerised, desorption and partial conversion of the tilted-benzene into


flat-benzene occurs. In fact, the increase of the C 1s intensity at �285 eV in the temperature range450–600 K is likely due to accumulation of dissociated phenyl species, which have a C 1s bindingenergy in the same range as tilted-benzene. As can be expected this is accompanied by a decrease in thecoverage of the flat-benzene. Above 600 K both the flat-benzene molecules and the phenyl speciesdisappear, one fraction desorbs as benzene and the other undergoes disproportionation, leaving someatomic carbon on the surface.

The observed evolution in the composition of the adlayer with increasing temperature indicates thatthe formation of the more stable flat-benzene configuration via trimerization of ethyne can be hinderedby steric factors in the denser layer. This blocking effect is less pronounced at elevated temperatures,probably due to the increased mobility of the adspecies, which favors some rearrangement and partialdesorption of the weakly bound bent-benzene. These results indicate that the configurational changes,as probed by real-time XPS, occurring in the adsorbed species when a surface chemical reaction takesplace, are extremely important in the determination of the overall reaction kinetics.

7.4. Alkene epoxidation on Cu(1 1 0)

Styrene oxidation on Cu(1 1 0) [124] is an example of a real-time XPS study of a rather complicatedreaction, involving a large molecule. This reaction is part of the heterogeneous alkene epoxidationreaction, which is among the important industrial syntheses. For ethyne, the epoxidation reaction has a

Fig. 36. (a) Evolution of the C 1s core-level spectra measured in a TP-XPS experiment, starting with the Pd(1 1 1) surface

covered with a saturated layer of C2H2. hn ¼ 400 eV. (b) Temperature dependence of the intensity of C 1s components,

corresponding to the different surface species: C2H2 (line), tilted-benzene (filled markers) and flat-benzene (open markers).


rather simple mechanism described as

C2H4 þ O ! C2H4O (12)

This reaction is usually carried on silver catalysts supported by alumina. However recent studies of thealkene chemisorption on copper surfaces [125,126] have suggested that an appropriately oxygenatedcopper surface can also epoxide alkenes.

In the case of styrene (C6H5CH¼¼CH2), the phenyl group serves for strengthening the bonding to thesilver surface via the terminal alkene functional group. Styrene adsorbs molecularly on Cu(1 1 0) at130 K with a very high sticking probability. Upon heating, the chemisorbed molecules desorb intact at�385 K. When the Cu(1 1 0) surface is pre-exposed to oxygen, epoxidation of styrene to styrene oxidemight occur, but as shown previously the reaction rate depends on the oxygen coverage. TPDexperiments have identified production of styrene oxide only when the oxygen coverage is very low.The high coverage pð2 � 1Þ-O structure at 0.5 ML is inert and, as manifested by the real-time XPSstudies, all styrene deposited on such surface desorbs intact. The activity of the Cu(1 1 0) surface withlow oxygen coverage towards styrene oxidation is elucidated by the evolution of the C 1s spectrameasured during a linear increase of the temperature of the sample after styrene saturation at 300 K.The spectra and the plot in Fig. 37 shows that even when the oxygen coverage is low the major amount(�75% ) of styrene (C 1s binding energy of 285.6 eV) desorbs intact in the temperature range 350–390 K. However, in this case a small fraction of styrene dissociates leaving a carbonaceous residue,identified by a C 1s peak at �285 eV [127]. This residual carbon reacts with coadsorbed oxygen above600 K forming CO, which immediately leaves the surface.

Fig. 37. (a) Evolution of the C 1s core-level spectra measured in a TP-XPS experiment, starting from the Cu(1 1 0) surface

with low oxygen coverage and a saturated layer of styrene at 500 K. The components at 285.6 and 285.0 eV correspond to

adsorbed styrene and atomic carbon, respectively. (b) Dependence of the styrene C 1s intensity on substrate temperature.


The quantitative evaluation of the XPS data, along with the TPD data have indicated that about 15%of adsorbed styrene desorbs from the surface producing epoxide, while 60% is not reacting. Theremaining part of styrene undergoes hydrogenation and oxidation to carbon monoxide.

Further investigation by means of NEXAFS indicated that styrene is always adsorbed in a flat-lyinggeometry. Therefore, the very different reactivity is not due to a difference in the hydrocarbonsadsorption geometry.

In conclusion, it seems that the epoxidation behavior found on Cu is consistent with the modeldeveloped for the Ag surface [128], namely the oxygen adatoms are effective, whereas the oxidic

oxygen is not. Moreover, the epoxidation temperature of 380 K found for copper is much lowerthan 500 K, the temperature at which epoxidation takes place on the Ag surfaces. These resultsindicate that a Cu-based catalysts may be more active and selective than the Ag catalysts used up tonow.

8. Future perspectives

The real-time XPS studies described in the present paper were performed introducing the gasmolecules in the background atmosphere of the UHV system. These molecules have a typicalBoltzmann energy distribution and hit the surface at random angles of incidence. However, it is wellknown that the angle of incidence, the translational and the vibrational energy of the adsorbingmolecules are important factors controlling not only the adsorption rate, but also the adsorptionmechanism. Another limitation, especially when time-modulated exposures are requested, is thelong response time of the pumping system. The MBRS technique, which uses the highly collimated,highly intense and energy controlled molecular beam produced during the supersonic expansionfrom a nozzle source overcomes all the above noted limitations. Combined with real-time XPS andwith the 96 channels detector working in snap-shot mode, it opens opportunity to probe fast dynamicprocesses changing the energy and the angle of incidence of the species hitting the surface, andmodulating the exposure. Fig. 38 illustrates the first results, obtained during commissioning of thesupersonic molecular beam system which has been designed to operate at the SuperESCA beamline[129].

We studied the reaction between oxygen and hydrogen on the Pt50Rh50 (1 0 0) single crystal surfaceby monitoring the Pt 4f7=2 core level, which exhibits a pronounced SCLS of 0.69 eV. Thanks to the 96channel detector we were able to measure the Pt 4f7=2 core level in a snap-shot mode, i.e. withoutscanning the analyzer voltages. Such a spectrum is shown in the left panel of Fig. 38. A photon energyof 185 eV has been used in order to enhance the signal from the top layer atoms. We used an O2

molecular beam, modulating the flux by opening and closing the shutter with different periods and dutycycles (with a rising constant below 100 ms), while the sample was exposed to a continuous hydrogenflux produced by a multichannel array doser. The advantage of this approach is that the rising time ofthe chopped supersonic molecular beam is much lower than that of an effusion system and permits tocontrol in a very precise way the oxygen exposure. The evolution of the Pt surface and bulkcomposition under oxidation (O2 beam ON) and reduction conditions (O2 beam OFF) is shown inthe right part of Fig. 38. It reflects the variation in the alloy composition of the top and sub-surfacelayers, respectively. The modulation of the Pt bulk component indicates that the depletion and enrich-ment of the sub-surface alloy layers with Pt are in concert with the oxidation and reduction cycles.


The results are in good agreement with previous experimental findings, but the advantage of thisapproach is that the kinetics of both, surface and sub-surface composition, can be simultaneouslyfollowed in a controlled way with a time resolution of 100 ms. In the future, we plan to performstroboscopic measurements by chopping the molecular beam. This should allow to reach data acquisitiontime shorter than 1 ms.

9. Conclusions

We hope that the examples selected in the present review have illustrated the strength of the real-timeXPS in studies of surface reactions. It is easy to forecast that still many challenges exist in this field ofresearch. One is to expand the investigations to oxide surfaces and microstructured surfaces, mimickingsupported catalysts. The advent of very bright X-ray beams also should make possible real-time micro(m)-XPS with high lateral resolution, probing the dynamic processes, such as directional diffusionand pattern formation, at their natural length and time scales. Some examples of m-XPS at a minutetime scale are already in the literature and they have provided important parameters for realisticreaction modeling, such as mobility of the different surface species and pattern formation [130,131].We foresee that real-time m-XPS at a second time scale is a realistic goal. Combined with supersonicmolecular beam it opens up unique opportunities to establish the factors that control the propertiesand reaction mechanisms of the oscillatory reactions on solid surfaces, which include a great number oftechnologically important catalytic systems.

Fig. 38. First results from the supersonic molecular beam. (Left) Pt 4f7=2 core level measured in snap-shot mode

on PtRh(1 0 0), with acquisition time of 100 ms. The bulk and surface components are shown. (Right) Evolution of the bulk

and surface components during the H2 þ O2 reaction. The modulated intensity of the molecular flux is reported in the

background.


Acknowledgements

The authors express their special gratitude to Renzo Rosei, who has inspired the SuperESCA projectand since the very beginning he has followed all the progress, always providing encouragement andscientific enlightening. We also wish to thank our colleagues D. Cocco, M. Barnaba, F. Esch, L. Rumiz,A. Tamai and B. Brena who actively participated in the research activities and developments indifferent stages of the SuperESCA project. The authors are very grateful and indepted to the externalresearch groups with whom we established fruitful collaborations. In particular, we want to thankR.M. Lambert, A.F. Lee, A.K. Santra, B.E. Nieuwenhuys, J.W. Bakker, F.P. Netzer, M. Ramsey,M. Polcik, J. Haase, F. Buatier de Mongeot and U. Valbusa, with whom we have collaborated in theexperiments described in this review, for their creative and invaluable contributions to the real-timeXPS.

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Documents

Real-time X-ray photoelectron spectroscopy of surface reactions