Upload
mh-zhang
View
213
Download
0
Embed Size (px)
Citation preview
Decomposition pathways of NO on carbide andoxycarbide-modified W(111) surfaces
M.H. Zhang a, H.H. Hwu a, M.T. Buelow a, J.G. Chen a,*, T.H. Ballinger b,P.J. Andersen b, D.R. Mullins c
a Department of Materials Science and Engineering, Center for Catalytic Science and Technology, University of Delaware,
Newark, DE 19716, USAb Johnson Matthey, Wayne, PA 19087, USA
c Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
Received 11 June 2002; accepted for publication 13 September 2002
Abstract
The decomposition of NO has been studied on clean W(1 1 1), carbide-modified W(1 1 1), and oxycarbide-modified
W(1 1 1) surfaces. The decomposition pathways are investigated using a combination of temperature programmed
desorption, Auger electron spectroscopy, high-resolution electron energy loss spectroscopy, and soft X-ray photo-
electron spectroscopy. All these surfaces exhibit high activity toward the decomposition of NO, and the only N-con-
taining products are N2 and N2O. Furthermore, all three surfaces preferentially produce N2 over N2O from the
decomposition of NO. Oxygen atoms, produced from the decomposition of NO, react with carbide surfaces to produce
gas-phase CO at high temperatures. In addition, our results demonstrate that cycles of alternate NO/hydrocarbon
treatments can regenerate the carbide overlayer on W(1 1 1), and the regenerated C/W(1 1 1) surface remains active in
the decomposition of NO.
� 2002 Published by Elsevier Science B.V.
Keywords: Nitrogen oxides; Carbides; Tungsten; Electron energy loss spectroscopy (EELS); Thermal desorption; X-ray photoelectron
spectroscopy
1. Introduction
In the past few decades, Pt-group metals, such
as Rh, Pd and Pt, have been extensively utilized as
the main catalytic components in catalytic con-
verters to abate automobile emissions [1–4]. NOx
emissions include mainly harmful NO and NO2,
which provoke environmental problems related to
the acid rain and greenhouse effect. The growing
concerns for the environment have resulted in in-
creasingly stringent NOx emission standards [5]. It
is therefore imperative that the Pt-group metals
should be more effectively utilized to meet the
regulations. The high demand for Pt-group metals
has motivated a large number of studies examiningthe reaction of NO on well-defined surfaces of Pt-
group metals [6–14]. The main objectives of these
studies were to obtain a better understanding of
the interaction of NO with Pt-group metals, which
*Corresponding author. Tel.: +1-302-831-0642; fax: +1-302-
831-4545.
E-mail address: [email protected] (J.G. Chen).
0039-6028/02/$ - see front matter � 2002 Published by Elsevier Science B.V.
PII: S0039 -6028 (02 )02335 -X
Surface Science 522 (2003) 112–124
www.elsevier.com/locate/susc
should help the more efficient utilization of Pt-
group metals in catalytic converters.
The primary motivation of our work is to
search for less expensive and more abundant al-
ternative catalysts to replace Pt-group metals for
the dissociation of NO. The carbides of Groups 4–6 early transition metals often show catalytic ac-
tivities that are similar to Pt-group metals [15,16].
Our research group has investigated the reactivi-
ties of carbide-modified surfaces by using a variety
of chemical probe reactions [17]. For example, in
the dehydrogenation of cyclohexene to benzene,
we found that the benzene-yield on carbide-mod-
ified tungsten [18] and molybdenum [19] are verysimilar to that on Pt(1 1 1). Furthermore, in a
recent letter we have reported temperature pro-
grammed desorption (TPD) results of the struc-
ture-dependent activity and product selectivity
from the decomposition of NO on C/W(1 1 1), C/
W(1 1 0) and C/Mo surfaces [20].
The reaction of NO on tungsten carbides has
been investigated in several previous catalytic [21]and surface science [22,23] studies. For example,
Leclercq�s group has reported catalytic studies oftungsten carbide in synthetic exhaust gases [21]. In
addition, Freund�s group has performed detailedangular resolved UPS studies of the adsorption and
dissociation of NO on a single crystal WC(0 0 0 1)
surface. These authors have also reported the TPD
spectrum of mass 28 amu, which was contributedby the reaction products of CO and N2 [22,23].
The detection of N2 clearly indicates that the
tungsten carbide surface is active toward the dis-
sociation of NO and the subsequent production
of N2.
In this paper we attempt to determine the
decomposition pathways and product selectivity
of NO on the C/W(1 1 1) surface by using TPD,high-resolution electron energy loss spectroscopy
(HREELS) and soft X-ray photoelectron spec-
troscopy (SXPS). The utilization of the isotope15NO in TPD allowed us to determine the product
yield of CO and 15N2, as well as that of another
reaction product, 15N2O. And the spectroscopic
characterization using HREELS and SXPS en-
abled us to identify the reaction intermediates anddecomposition pathways of NO on the C/W(1 1 1)
surface. In addition, because of the presence of
oxygen on carbide is unavoidable under realistic
De-NOx reaction conditions, we have investigated
the NO decomposition on oxycarbide-modified
W(1 1 1) surface. Finally, we have evaluated the
possibility of regenerating the C/W(1 1 1) surface
to explore the potential catalytic application oftungsten carbides as De-NOx catalysts in the
presence of NO and hydrocarbons.
2. Experimental
The experiments were conducted using three
separate ultra high vacuum (UHV) systems. Thetypical base pressure was in the range of 2� 10�10–
8� 10�10 Torr. The first two systems are located at
the University of Delaware and both contain fa-
cilities of Auger electron spectroscopy (AES),
TPD, low energy electron diffraction (LEED), and
ion sputtering. One of these systems is also
equipped with an LK ELS3000 high resolution
electron energy loss spectrometer. The third sys-tem is located at the U12 Beamline at the National
Synchrotron Light Source at Brookhaven Na-
tional Laboratory, which is equipped with capa-
bilities for soft X-ray photoemission spectroscopy
(SXPS) measurements.
The tungsten (1 1 1) single crystal was mounted
on sample manipulators by spot welding to two Ta
wires. The surface can be cooled by contact with aliquid nitrogen reservoir and heated resistively
through the Ta mounting wires. Sample prepara-
tion first entailed cleaning by several cycles of
sputtering with 2 keV Neþ bombardment at 300 K
followed by annealing in vacuum to 1200 K.
Carbon that cannot be removed by sputtering was
cleaned by exposing to oxygen at 1000 K followed
by annealing at 1200 K. This process was contin-ued until there were no detectable amounts of
carbon and oxygen impurities by AES. The C/
W(1 1 1) thin film was made by several cycles of
exposing W(1 1 1) to 3 L (1 L ¼ 1� 10�6 Torr�s)cyclohexene at about 120 K followed by annealing
to 1200 K in vacuum. Cyclohexene decomposes to
produce gas-phase hydrogen and atomic carbon
on the surface [18]. The surface prepared in thisfashion has a C/W atomic ratio of approximately
0.55 and exhibits affiffiffi3
p�
ffiffiffi3
pR30� LEED pattern.
M.H. Zhang et al. / Surface Science 522 (2003) 112–124 113
Cyclohexene was used as the chemical agent for
making carbides due to prior knowledge and
characterization of C/W(1 1 1) prepared in a simi-
lar fashion [18]. The O/C/W(1 1 1) surface was
generated by exposing the C/W(1 1 1) to 0.5 L of
oxygen at 900 K, which generates an oxygen-modified C/W(1 1 1) with an O/W atomic ratio 0.12
and a C/W atomic ratio 0.44 [18].
The TPD experiments involved exposing the
surfaces to 1 or 10 L of 15NO (Cambridge Isotopes,
98% isotopically pure) at 100 K. The surfaces were
then heated to 1200 K at a constant rate of 3 K/s
and the desorption products were measured by a
mass spectrometer. Several desorption productswere measured by monitoring m=q ¼ 31 (15NO), 30
(15N2), 28 (CO), 46 (15N2O), 47 (
15NO2), 18 (H2O or15NH3), 32 (O2), and 2 (H2). The only species to
desorb appreciably from all surfaces were 15NO,15N2, CO and
15N2O. The15NO isotope was used in
the TPD measurements to differentiate the reaction
products of 15N2 and CO.
The HREEL spectra were all acquired afterexposing the surface to 14NO at 90 K and then
heating to the specified temperatures. All HREELS
spectra were recorded at 90 K. The intensity of the
elastic peak was typically 1� 105 counts per sec-
ond (cps) with a resolution of 40–50 cm�1. The time
to collect a spectrum was approximately 30 min.
All spectra were recorded in the specular scattering
geometry.
The SXPS spectra were recorded using incidentenergy of 450 eV for N1s and of 600 eV for O 1s.
The energy of the each spectrum was calibrated
using the Fermi energy of the valence state. The
end-station contained a VSW EA125 electrosta-
tic analyzer which was operated in constant pass
energy mode for photoemission measurements.
The instrument resolution was better than 0.5 eV.
3. Results and interpretation
3.1. TPD results
3.1.1. NO on clean W(111)
TPD spectra acquired following the exposure
to 1 and 10 L 15NO at 100 K on clean W(1 1 1) areshown in Fig. 1(a) and (b), respectively. The de-
composition of 15NO on W(1 1 1) is evident by the
observation of the 15N2 and 15N2O desorption
Fig. 1. TPD spectra after exposing W(1 1 1) to 1 L (a) and 10 L(b) 15NO at 100 K.
114 M.H. Zhang et al. / Surface Science 522 (2003) 112–124
peaks. After an exposure of 1 L, the majority of15N2 desorbs at around 1007 K from the clean
W(1 1 1) surface, as indicated in Fig. 1(a). In ad-
dition, a relatively weak 15N2 desorption peak also
occurs at 175 K, which coincides with the de-
sorption of 15N2O at the same temperature. Themolecular desorption of 15NO at �111 K is at-
tributed to the desorption from the heating wires
upon initial heating. In addition to the products
shown in Fig. 1, 15NO2 and15NH3 were also mea-
sured, but desorption of these compounds was
not detected in the TPD measurements. The TPD
results after 10 L of 15NO exposure (Fig. 1(b))
are very similar to the 1 L 15NO TPD, suggestingthat 1 L of 15NO corresponds to a near-saturation
coverage of the W(1 1 1) surface. More about the
effect of 15NO exposure will be discussed later.
3.1.2. NO on C/W(111) and O/C/W(111)
Fig. 2(a) presents the TPD spectra acquired
after exposing the C/W(1 1 1) surface to 1 L of15NO at 100 K. The decomposition of 15NO isclearly demonstrated by the formation of the gas-
phase 15N2, CO and 15N2O products. The decom-
position of 15NO occurs at relatively low temper-
ature, as evident from the desorption of the 15N2O
product around 175 K. The O atom produced
from the decomposition of 15NO combines with
the C atom of the C/W(1 1 1) surface to produce
gas-phase CO at 909 K. As a result, the formationof CO reduces the carbon concentration of C/
W(1 1 1), as confirmed by AES measurements after
TPD experiments. The 15N atom also recombines
and desorbs as 15N2 at the same temperature.
Similar to that discussed for clean W(1 1 1), the15NO desorption peak at 114 K is attributed to
initial desorption from heating wires.
TPD spectra obtained following the adsorptionof 1 L 15NO at 100 K on O/C/W(1 1 1) are pre-
sented in Fig. 2(b). The O/C/W(1 1 1) is similarly
active to the decomposition of 15NO, as evident by
the formation of gas-phase 15N2 and15N2O. The
majority of 15N2 desorbs at 963 K and a weak 15N2
desorption peak appears at around 175 K. Again,
the low temperature 15N2 desorption peak coin-
cides with the desorption of 15N2O at 175 K.Carbon can be partially removed from O/C/
W(1 1 1) via the formation of CO. The desorption
Fig. 2. (a) TPD spectra after exposing C/W(1 1 1) to 1 L 15NO at 100 K and (b) TPD spectra after exposing O/C/W(1 1 1) to 1 L 15NO
at 100 K.
M.H. Zhang et al. / Surface Science 522 (2003) 112–124 115
temperature of CO occurs at 941 K on O/C/
W(1 1 1), which is slightly higher than the tem-
perature of 909 K on C/W(1 1 1). The desorp-
tion peak of 15NO at �115 K from O/C/W(1 1 1)is again contributed to desorption from heating
wires.
We have also performed TPD measurements of
H2 following the adsorption of NO on W(1 1 1)
and C/W(1 1 1) surfaces; the reasons for such TPD
measurements will become clear after the descrip-
tion of the HREELS results below. Fig. 3 shows
the H2 TPD measurements following the decom-position of 1 L NO on C/W(1 1 1) at 100 K. For
comparison, the H2 TPD from the decomposition
of 2.2 L cyclohexene on C/W(1 1 1) is also shown
in Fig. 3, which corresponds to the production of
0.22 H2 per W atom [18]. The comparison in Fig. 3
indicates that the amount of H2 production from
the NO/C/W(1 1 1) surface is negligible.
3.2. HREELS results
To facilitate the understanding of the NO de-
composition pathways, we obtained HREEL
spectra of NO on clean W(1 1 1), C/W(1 1 1), and
O/C/W(1 1 1) surfaces, which are shown in Figs. 4–
6, respectively. The exposures of NO were made
with the surface temperature at 90 K; the adsorbed
layers were then heated to the indicated tempera-
tures, and allowed to cool before the HREELspectra were recorded. The height of the elastic
peaks in all spectra has been normalized to unity,
and the expansion factor for each individual spec-
trum represents the multiplication factor relative
to the elastic peak. We have chosen 10 L NO ex-
posure instead of 1 L for the HREELS measure-
ments in order to assure saturation coverage, which
should help minimize the adsorption from theresidue impurity gases during the HREELS mea-
surements (about 30 min per spectrum).
Fig. 3. TPD spectra of H2 after exposing C/W(1 1 1) to 10 L15NO at 100 K and to 2.2 L c-C6H10 at 120 K.
Fig. 4. HREEL spectra after exposing W(1 1 1) to 10 L NO at
90 K and heating to indicated temperatures.
116 M.H. Zhang et al. / Surface Science 522 (2003) 112–124
3.2.1. NO on W(111)
Fig. 4 shows the HREEL spectra obtained afterexposing 10 L NO on the clean W(1 1 1) surface.
At 90 K, a relatively sharp peak is detected at 1772
cm�1, with unresolved shoulders at 1691 and 1847
cm�1. These features are attributed to the m(NO)modes of molecular NO [24]; the different fre-
quencies are likely due to the different adsorption
sites/environment of NO on the W(1 1 1) surface.
After heating to 300 K, two broad peaks centeredat �3328 and 3606 cm�1 begin to appear, which
are characteristics for the m(NHx) and m(OH)modes, respectively. The observation of the
m(NHx) mode suggests that the reaction between
H2 in the UHV background with the surface N
atoms that are produced from the decomposition
of NO; the detection of the m(OH) mode indicatesthe reaction of H2O in the UHV background dur-
ing the HREELS experiments. In addition, several
new features, at 1258, 1481, and 1610 cm�1, appear
at 300 K as the NO mode at 1772 cm�1 diminishes.
The new features, at 1258–1610 cm�1, are in the
frequency range for the m(NO) modes of stronglychemisorbed NO on metal surfaces [24]. The onset
of two additional peaks, at �846 and 994 cm�1,
is also observed at 300 K. After heating to 400 K,all vibrational modes remain in the spectrum.
Further heating to 600 K results in the disap-
pearance of all features at frequencies above 1100
cm�1. The 846 and 994 cm�1 peaks are tentatively
assigned to the m(W–N) modes of atomic N, re-sulting from the decomposition of NO. These two
features disappear after heating to 1000 K, which
coincide with the desorption of N2 in the TPDmeasurements. The two remaining features at 453
and 629 cm�1 are assigned to the m(W–O) modes,mainly because AES measurements indicate that
Fig. 5. HREEL spectra after exposing C/W(1 1 1) to 10 L NO
at 90 K and heating to indicated temperatures.
Fig. 6. HREEL spectra after exposing O/C/W(1 1 1) to 10 L
NO at 90 K and heating to indicated temperatures.
M.H. Zhang et al. / Surface Science 522 (2003) 112–124 117
oxygen is the only surface species after heating
NO/W(1 1 1) to 1000 K.
The inset in Fig. 4 provides a more detailed
comparison of the vibrational spectra after heating
the NO/W(1 1 1) layer to 150 and 200 K. As de-
scribed earlier, the N2O gas-phase product is de-tected at �175 K. The HREEL spectra in the insetindicate that N2O is not present as a surface in-
termediate, which should produce a m(N@NO)mode in the vicinity of the gas-phase value of 2224
cm�1 [25]. The combined HREELS and TPD re-
sults therefore suggest that the desorption of the
N2O product is a reaction-limited process.
3.2.2. NO on C/W(111) and O/C/W(111)
Fig. 5 shows the HREEL spectra obtained after
exposing 10 L NO on the C/W(1 1 1) surface. At 90
K, the spectrum exhibits a relatively intense mo-
lecular feature of m(NO) at 1792 cm�1. After the
adsorbed layer is heated to 300 K, additional
modes appear at 846, 1001, 1705, and 3336 cm�1.
As described for the clean W(1 1 1) surface, the3336 cm�1 feature is characteristic of the m(N–H)mode of NHx species, which are likely produced
via the reaction of atomic nitrogen with H2 in
the UHV background during the acquisition of
HREEL spectra. As shown earlier the control
TPD experiments in Fig. 3, H2 is not produced on
NO/C/W(1 1 1) in the TPD experiment, which was
performed as soon as the C/W(1 1 1) surface wasexposed to NO. Therefore, the formation of the
NHx is most likely an experimental artifact that
involves the reaction of H2 with the atomic N on
C/W(1 1 1) during the HREELS measurements.
The 1705 cm�1 is in the frequency range that is
typical for molecular NO [24]. The lower fre-
quency modes at 846 and 1001 cm�1 are again
assigned to the m(W–N) modes. At 400 K thespectrum generally resembles that at 300 K, but
with diminished intensities for features between
1500–1800 cm�1 and increased intensities of fea-
tures at 846, 1001, and 3336 cm�1. These results
indicate that more NO is decomposed to produce
surface N, NHx and surface O species. The 600 and
750 K spectra are nearly identical, which reveal
that most of the NHx species have dissociated bythe disappearance of the m(N–H) mode at �3336cm�1. After heating to 1000 K, the m(W–N) modes
at 846 and 1001 cm�1 disappear, which coincide
with the detection of N2 and CO in the TPD
measurements (Fig. 2(a)). The 622 cm�1 at the
1000 K spectrum is assigned to the m(W–O) modeas described after the decomposition of NO on
clean W(1 1 1) in Fig. 4. The 331 cm�1 feature isassigned to the m(W–C) mode based on our pre-vious study of C/W(1 1 1) [18]. The thermal be-
havior of NO on the O/C/W(1 1 1) surface (Fig. 6)
is nearly identical to that on the C/W(1 1 1) sur-
face. Similar to the 90 K spectrum on the C/
W(1 1 1) surface, the 90 K spectrum of NO on O/
C/W(1 1 1) also exhibits an intense molecular fea-
ture at 1779 cm�1. After heating to 300 K, addi-tional modes appear at 994, 1691, and 3315 cm�1.
Similar to the prior two surfaces, the decrease in
the intensity of 1779 cm�1 peak, coupled with the
appearance of the 839 and 994 cm�1 features, in-
dicate that NO starts to decompose to produce
atomic N on the surface. The HREEL spectrum
after heating to 400 K is essentially the same as
that of 300 K, with the exception of a decrease inthe intensity of m(NO) vibrations. Between 600 and750 K, most of the NHx species are decomposed,
leaving only surface C, N, and O. After heating to
1000 K, the HREEL spectrum is characteristic of
an oxygen-modified C/W(1 1 1) surface [18], con-
sistent with the TPD detection of N2 at 963 K (Fig.
2(b)).
3.3. SXPS results
The most important conclusion from the
HREELS study of NO on C/W(1 1 1) and O/C/
W(1 1 1) surfaces is that a significant fraction of
NO decomposes to atomic N and O, as revealed by
the appearance and then disappearance of the
m(W–N) modes at approximately 839–846 and at994–1001 cm�1. To further confirm these vibra-
tional modes are related to atomic N, instead of
any NxOy molecular intermediates, we have per-
formed SXPS measurements following the disso-
ciation of NO on the C/W(1 1 1) surface. Fig. 7
shows the N (1s) and O (1s) spectra after exposing
C/W(1 1 1) to 10 L NO at 90 K, followed by
heating to 300 and 600 K. As reported previouslyin the XPS study of NO on W and other surfaces
[26,27], molecularly adsorbed NO is typically
118 M.H. Zhang et al. / Surface Science 522 (2003) 112–124
characterized by the N (1s) peak at �399.8 eV andO (1s) peak at �531.4 eV. As shown in Fig. 7, afterheating to 300 K, new N (1s) and O (1s) peaks are
shifted to 397.1 and 530.3 eV, respectively. The
detection of lower-energy N (1s) and O (1s) peaks
is consistent with the formation of atomic N and Oon the surfaces [26,27]. Finally, after heating to
600 K, the molecular NO peaks are completely
converted to those of atomic N and O. The SXPS
results in Fig. 7, in particular the N (1s) peak,
further reveal that a small fraction of NO under-
goes dissociation even at 90 K, as indicated by the
presence of the relatively weak N (1s) feature at
397.1 eV. Furthermore, based on the relative peakareas of the N (1s) peaks at 300 K, we estimated
that the ratio of molecular and dissociated NO is
approximately 25% after the adsorbed layer is
heated to 300 K.
3.4. Regeneration of the C/W(111) surface
Attempts were made to regenerate C/W(1 1 1)surfaces after the adsorption and reaction of NO,
and the subsequent consumption of a fraction of
carbon atoms by the formation of gas-phase CO.
The decomposition of 15NO over regenerated C/
W(1 1 1) was examined to determine if the regen-
erated C/W(1 1 1) remains active to this reaction.
In these experiments, the C/W(1 1 1) surface was
first exposed to 10 L 15NO at 100 K and then he-
ated to 1200 K. The C/W atomic ratio was reducedfrom 0.56 to 0.23 based on the Auger measure-
ment; the loss of carbon is due to the fact that O
atoms react with C from the C/W(1 1 1) to produce
CO at �909 K (e.g., Fig. 2(a)). The C/W(1 1 1)
surface was then regenerated by exposing to 3 L
cyclohexene at 120 K and flashing to 1200 K in
vacuum. The C/W(1 1 1) atomic ratio increased to
0.53 after regeneration. The TPD spectrum fol-lowing the exposure of 15NO over the regenerated
C/W(1 1 1) surface is shown in Fig. 8. The detec-
tion of 15N2 over the regenerated C/W(1 1 1) sur-
face indicates that the regenerated surface remains
active toward the decomposition of 15NO. In fact,
the C/W(1 1 1) surface remains active after multi-
ple cycles of NO/cyclohexene treatments. On the
other hand, if the C/W(1 1 1) surface was not treatedwith cyclohexene after successive NO adsorption
and TPD measurements, all carbon atoms were
eventually consumed and oxygen atoms were left
Fig. 7. SXPS measurements following the adsorption of 10 L NO at 90 K.
M.H. Zhang et al. / Surface Science 522 (2003) 112–124 119
on the W(1 1 1) surface. For comparison, the TPD
spectrum following the adsorption of NO on
this O/W(1 1 1) surface is also included in Fig. 8.This surface is essentially inert toward the disso-
ciation of NO, as suggested by the absence of
the 15N2 peak and the relatively weak15N2O peak.
The comparison in Fig. 8 clearly demonstrates the
importance of regeneration, as well as the po-
tential feasibility of using tungsten carbides for
De-NOx via sequential injection of NO and hy-
drocarbons.
4. Discussion
4.1. Surface reactivity and product selectivity
4.1.1. Surface reactivity and product selectivity over
W(111)
The selectivity of the two 15N-containing
products, 15N2 and15N2O, can be estimated based
on combined AES and TPD measurements. AES
spectra were performed by collecting after expos-
ing the W(1 1 1) surface to 10 L 15NO at 100 K,
and after a newly prepared NO/W(1 1 1) surface
was heated to 300 K; these two surfaces corre-
sponded to the surfaces before and after the de-sorption of 15N2O, respectively. From the AES
measurements, we estimated that the atomic ratio
of N/O was 1.0 at 100 K and 0.96 at 300 K. In
addition, the O/W atomic ratio at 300 K was es-
timated from AES to be 0.76, using standard
sensitivity factors for the W(182 eV) and O(KLL)
transitions [28]. Eqs. (1) and (2) describe the mass
balance over the W(1 1 1) surface at 300 K:
a 15NO! a215N2OðgÞ þ
a2O ð1Þ
b 15NO! xb 15NOþ ð1� xÞb 15Nþ ð1� xÞbOð2Þ
Symbol a represents the number of 15NO mole-
cules leading to the production of 15N2O, and
symbol b represents the number of 15NO molecules
remaining on the W(1 1 1) surface, either in the
molecular or decomposed form, after heated to
300 K. Therefore, we obtain the following rela-
tionships at 300 K:
Fig. 8. Comparison of TPD spectra after the reaction of NO on regenerated C/W(1 1 1) and oxidized W(1 1 1) surfaces.
120 M.H. Zhang et al. / Surface Science 522 (2003) 112–124
N
O¼ b
a2
�.þ b
�¼ 0:96 ð3Þ
O
W¼ a
2
�þ b
�¼ 0:76 ð4Þ
The values for a and b can be solved as 0.06 and0.73, respectively, on the per W atom basis from
Eqs. (3) and (4). The surface reactivity of W(1 1 1)
in the decomposition of 15NO, A11 1, can then be
defined as the total number of 15NO molecules
ða=2þ bÞ that undergo decomposition on the perW atom basis, which corresponds to a value of0.76 15NO/W on clean W(1 1 1).
The selectivity of 15N2O from W(1 1 1) can be
estimated from the values of a and b as follows,with the symbol S15N2O Wð1 1 1Þ representing the
15N2O
selectivity over W(1 1 1):
S15N2O Wð1 1 1Þ ¼a
aþ b¼ 0:06
0:06þ 0:73
� �¼ 8%
Accordingly, the 15N2 selectivity is about 92% for
the 10 L 15NO/W(1 1 1) overlayer.
The activity and selectivity of W(1 1 1) surfaceafter 1 L 15NO exposure can be estimated by
comparing the 15N2 and15N2O TPD peak areas at
1 and 10 L of 15NO exposures. The mass balance
on the W(1 1 1) surface at 300 K can be described
by Eqs. (5) and (6) after exposing to 1 L 15NO and
heated to 300 K:
a0 15NO! a0
215N2OðgÞ þ
a0
2O ð5Þ
b0 15NO! xb0 15NOþ ð1� xÞb0 15Nþ ð1� xÞb0Oð6Þ
Symbol a0 represents the number of 15NO mole-cules that are involved in the production of 15N2O,
and symbol b0 represents the number of 15NO
molecules that are remaining on the W(1 1 1) sur-
face, either in the molecular or decomposed form,
after the surface was heated to 300 K. The overall
TPD peak area of 15N2 from C/W(1 1 1) is the sum
of the low-temperature (175 K) and high-temper-
ature (1007 K) peaks; the 175 K peak area is
normalized by subtracting the 11% contribution
from the cracking fragment [29] of 15N2O at thesame temperature. By comparing TPD peak areas
of 15N2O and 15N2 over the W(1 1 1) surface after 1
L 15NO exposure with those over W(1 1 1) after 10
L 15NO exposure, seen in Table 1, the values of a0
and b0 can be estimated on the per W atom basis as
follows:
a0 ¼ 5:08� 106
1:21� 107� a ¼ 0:025
b0 ¼ 1:06� 108
9:63� 107� b ¼ 0:80
The sum of a0=2 and b0 provides the surface reac-tivity in a value of 0.81 15NO/W. From the values
of a0 and b0, the selectivity to 15N2O and 15N2 is
determined to be 3% and 97%, respectively.
4.1.2. Surface reactivity and product selectivity over
C/W(111) and O/C/W(111)
Similarly, the mass balance on the C/W(1 1 1)
surface at 300 K can be described by Eqs. (7) and
(8) after exposing to 1 L 15NO and heated to 300 K,
c 15NO! c215N2OðgÞ þ
c2O ð7Þ
d 15NO! xd 15NOþ ð1� xÞd 15Nþ ð1� xÞdOð8Þ
Symbol c represents the number of 15NO mole-
cules decomposing to produce 15N2O. Sym-
bol d represents the number of 15NO molecules
Table 1
Comparison of surface reactivity and product selectivity
Surface Dosage of 15NO (L) 15N2 peak area15N2O peak area Surface reactivity
(# 15NO/W)
Selectivity of 15N2 (%)
W(1 1 1) 10 9:63� 107 1:21� 107 0.76 92
W(1 1 1) 1 1:06� 108 5:08� 106 0.81 97
C/W(1 1 1) 1 6:7� 107 4:39� 106 0.52 96
O/C/W(1 1 1) 1 6:49� 107 4:68� 106 0.50 96
M.H. Zhang et al. / Surface Science 522 (2003) 112–124 121
remaining on the W(1 1 1) surface, either in the
molecular or decomposed form, after the surface
was heated to 300 K.
By comparing TPD peak areas of 15N2O and15N2 from the C/W(1 1 1) surface after 1 L 15NOexposure with those over the W(1 1 1) surface after
10 L 15NO exposure, the values of c and d can beestimated in the unit of per W atom as follows:
c ¼ 4:39� 106
1:21� 107� a ¼ 0:02
d ¼ 6:7� 107
9:63� 107� b ¼ 0:51
The surface reactivity of the decomposition of15NO on C/W(1 1 1) can be derived from the sum
of c=2 and d, which corresponds to a value of 0.5215NO per W atom. From the values of c and d, theselectivity to 15N2O and 15N2 is determined to be
4% and 96%, respectively.
Repeating the same procedure of TPD peak
area analysis, the surface reactivity for the de-composition of NO on O/C/W(1 1 1) is estimated
to be 0.50 NO per W atom. The selectivity for15N2O and 15N2 on O/C/W(1 1 1) surface is deter-
mined to be 4% and 96%, respectively. The values
for the selectivity and surface reactivity over all
surfaces are summarized in Table 1.
It should be pointed out that the 1 L NO/
W(1 1 1) and 10 L NO/W(1 1 1) surfaces show aslight difference in the overall surface reactivity
(0.81 vs. 0.76) and N2 selectivity (97% vs. 92%).
Such differences are likely due to the uncertainty in
the TPD measurements and TPD peak area anal-
ysis. They could also be partially attributed to the
assumption that surface with higher NO coverages
should slightly favor the formation of N2O, which
should lead to a reduction in the overall surfacereactivity and in the selectivity to N2. More de-
tailed studies are necessary to determine the effect
of NO coverage on the product selectivity.
4.2. Decomposition pathways
The decomposition pathways over C/W(1 1 1)
and O/C/W(1 1 1), as investigated by the combi-
nation of HREELS, TPD, AES and SXPS, are
summarized in the equations below:
NO! NðaÞ þOðaÞ ð9Þ
NðaÞ þNOðaÞ ���!175 KN2O " or
2NOðaÞ ���!175 KN2O " þOðaÞ ð10Þ
NðaÞ þNðaÞ ����!>900 KN2 " ð11Þ
OðaÞ þ C=Wð111Þ����!>900 KCO " ð12Þ
Eqs. (9)–(11) are the only reaction pathways in-
volved in the decomposition of 15NO over W(1 1 1)
as CO is not produced from clean W(1 1 1) surface.
A fraction of NO molecules dissociate into ad-
sorbed N and O atoms at low temperatures over
all surfaces, and the decomposition is completed
after heating to 600 K, as confirmed by theHREELS results. Some of the N atoms combine
with adsorbed NO to desorb as N2O at 175 K; the
production of N2O could also result from the
disproportion reaction of two adjacent NO mole-
cules at 175 K. In addition, HREELS results (inset
of Fig. 4) suggest that the desorption of N2O is a
reaction-limited process. The remaining N atoms
stay on the surface and recombine to desorb as N2
at T > 900 K, with the O atoms staying on the
clean W(1 1 1) surface after heating to 1000 K.
Similar decomposition pathways also occur on
the C/W(1 1 1) and O/C/W(1 1 1) surfaces, except
that oxygen atoms from the decomposition of NO
react with the C atoms from the carbides and de-
sorb as CO at T > 900 K. Our results indicate that
the decomposition of NO over C/W(1 1 1) and O/C/W(1 1 1) is very facile, as evident by the pro-
duction of N2. As summarized in Table 1, the two
surfaces show similar activity for the decomposi-
tion of NO. The observation that CO desorbs at a
slightly higher temperature from the O/C/W(1 1 1)
surface (at 941 K), as compared with the C/
W(1 1 1) surface (at 909 K), suggests that the
presence of O in C/W(1 1 1) slightly enhances thestability of the carbide surface. A fraction of car-
bon atoms are stripped away from both C/W(1 1 1)
and O/C/W(1 1 1) due to the formation of CO.
However, the TPD results in Fig. 8 indicate that
NO decomposes over C/W(1 1 1) and regenerated
C/W(1 1 1) surfaces in a almost identical manner,
122 M.H. Zhang et al. / Surface Science 522 (2003) 112–124
which suggests that the C/W(1 1 1) surface can be
regenerated by reacting with hydrocarbons, such
as cyclohexene.
4.3. Brief comparison of C/W(111), O/C/W(111)
and Pt-group metal surfaces
In this section we will briefly compare the sur-
face reactivity of NO on tungsten carbides with
previous UHV studies on the surfaces of the cur-
rent commercial De-NOx catalysts, Pt and Rh.
Previous studies of NO decomposition over Pt-
group metals have indicated its structure sensitive
feature. Pt and Rh have the face-centered cubicstructure, and the (1 1 1) plane is the most closely
packed surface. In the previous study of NO de-
composition on Pt single crystal surfaces, negligi-
ble NO dissociation was reported on the (1 1 1)
plane [7,30], while a surface reactivity �0.25 NOper Pt atom was found on the more open-struc-
tured (1 0 0) plane [7,13,31,32]. Previous extensive
studies of NO adsorption and dissociation onsingle crystal Rh surfaces revealed similar general
trend that the more open and stepped Rh surfaces
show higher activities toward the decomposition
of NO [33]. Similar or lower surface reactivity was
found from Rh single crystal surfaces compared to
tungsten carbide surfaces, for instance, �0.37 [13]or �0.44 [34] NO per Rh atom for Rh(1 1 1) sur-
face, and 0.34 NO per Rh atom for Rh(1 0 0)surface [35]. However, the reconstruction of Rh
surfaces upon NO exposure results some compli-
cation in the comparison. Generally, NO to N2O is
not considered as an important reaction channel in
the decomposition of NO on the single crystal
surfaces of Pt and Rh. For instance, either unde-
tectable or negligible amount of N2O was reported
from Pt(1 1 1) [7,30], Pt(1 0 0) [7,8], Pt(1 1 0) [7],Rh(1 1 1) [13], Rh(1 1 0) [14] and Rh(1 0 0) [35]
surfaces under the UHV conditions that were
similar to the current study.
Overall, the decomposition of NO on C/W(1 1 1)
and O/C/W(1 1 1) show general similarities to those
observed on the Pt-group metal surfaces. Our re-
sults revealed that W(1 1 1), C/W(1 1 1), and O/C/
W(1 1 1) surfaces were highly active toward thedecomposition of NO. In addition, N2 and N2O
were the only N-containing products and no O2
was detected as the gas-phase products. It should
be pointed out that W has the body-centered cubic
structure. The relatively more open-structured
W(1 1 1), C/W(1 1 1) and O/C/W(1 1 1) surfaces all
exhibit relatively high activity in the decomposi-
tion of NO, and all surfaces preferentially producesignificant amount of N2 > 92%. The selectivity
of N-containing products is nearly the same over
all three surfaces, although clean W(1 1 1) has a
higher activity compared with the other two sur-
faces.
Our comparison of the reactivity of C/W(1 1 1)
with Pt-group metals are based on the number of
NO molecules undergoing decomposition on a permetal atom basis. However, another important
parameter regarding the decomposition of NO on
different surfaces is the desorption temperature of
the N2 product. As shown in our TPD results, only
a fraction of the N2 product desorb from the C/
W(1 1 1) surface at 175 K, while the majority de-
sorb at 909 K, Fig. 2(a). For comparison, the
recombinative desorption of N2 from the Pt-groupmetal surfaces typically occur in the temperature
range of 400–800 K [6–14]. Therefore, from the
perspective of N2 desorption, the Pt-group metals
should be considered as more active than the C/
W(1 1 1) surface toward the catalytic decomposi-
tion of NO.
5. Conclusions
Two competing reaction pathways are detected
in the decomposition of 15NO over W(1 1 1), C/
W(1 1 1) and O/C/W(1 1 1), which produce 15N2
and 15N2O, respectively. The activity of NO de-
composition on the per W atom basis is in the
order of Wð111Þ > C=Wð111Þ � O=C=Wð111Þ.All three surfaces produce almost entirely gas-
phase 15N2, and the selectivity of 15N2 is nearly
identical over the three surfaces (>92%). The Oatoms from the decomposition of 15NO strip away
a fraction of C atoms on C/W(1 1 1) and O/C/
W(1 1 1) and desorb in the form of gas-phase CO.
Finally, the TPD and AES results clearly indicate
that the C/W(1 1 1) surface can be regenerated viaa post-treatment of the surface by exposing to
cyclohexene at 120 K and flashing to 1200 K in
M.H. Zhang et al. / Surface Science 522 (2003) 112–124 123
vacuum, and that the regenerated C/W(1 1 1) sur-
face remains active toward the decomposition of
NO. Such finding suggests the potential feasibility
of using tungsten carbides for De-NOx via se-
quential injection of NO and hydrocarbons.
Acknowledgements
We acknowledge partial support from the En-
vironmental Protection Agency (Grant No. EPA
STAR 82962401). The authors would like to thank
Johnson Matthey for partial financial support of
this work. We also like to thank H.-Y. Chen andP. Shady of Johnson Matthey for helpful discus-
sion. DRM is supported by the US Department of
Energy, under contract DE-AC05-00OR22725
with Oak Ridge National Laboratory, managed
and operated by UT-Battelle, LLC.
References
[1] K.C. Taylor, Catal. Rev.––Sci. Eng. 35 (1993) 457.
[2] G.A. Papapolymerou, L.D. Schmidt, Langmuir 1 (1985)
488.
[3] M. Shelef, G.W. Graham, Catal. Rev.––Sci. Eng. 36 (1994)
433.
[4] L.L. Hegedus, J.C. Summers, J.C. Schlatter, K. Baron,
J. Catal. 56 (1979) 321.
[5] G.C. Koltsakis, A.M. Stamatelos, Prog. Energy Combust.
Sci. 23 (1997) 1.
[6] J.M. Gohnrone, Y.O. Park, R.I. Masel, J. Catal. 95 (1985)
244.
[7] R.J. Gorte, L.D. Schmidt, J.L. Gland, Surf. Sci. 109 (1981)
367.
[8] J.M. Gohndrone, R.I. Masel, Surf. Sci. 209 (1989) 44.
[9] P. Jakob, M. Stichler, D. Menzel, Surf. Sci. 370 (1997)
L185.
[10] R.D. Ramsier, Q. Gao, H.N. Waltenburg, K.W. Lee, O.W.
Nooij, L. Lefferts, J.T. Yates, Surf. Sci. 320 (1994) 209.
[11] M. Hirsimaki, M. Valden, J. Chem. Phys. 114 (2001) 2345.
[12] M. Ishii, T. Hayashi, S. Matsumoto, Appl. Catal. A––
General 225 (2002) 207.
[13] T.W. Root, L.D. Schmidt, G.B. Fisher, Surf. Sci. 134
(1983) 30.
[14] V. Schmatloch, I. Jirka, N. Kruse, J. Chem. Phys. 100
(1994) 8471.
[15] S.T. Oyama, G.L. Haller, in: G.C. Bond, G. Webb (Eds.),
Catalysis Specialist Report, vol. 5, The Chemical Society,
London, 1981, p. 333.
[16] S.T. Oyama, The Chemistry of Transition Metal Carbides
and Nitrides, Blackie Academic and Professional, Glas-
gow, 1996.
[17] J.G. Chen, Chem. Rev. 96 (1996) 1477, and references
therein;
J.G. Chen, B. Fruhberger, J. Eng Jr., B.E. Bent, J. Mol.
Catal. A 131 (1998) 285.
[18] N. Liu, S.A. Rykov, H.H. Hwu, M.T. Buelow, J.G. Chen,
J. Phys. Chem. B 105 (2001) 3894;
N. Liu, S.A. Rykov, J.G. Chen, Surf. Sci. 487 (2001) 107.
[19] J.G. Chen, B. Fruhberger, Surf. Sci. 367 (1996) L102.
[20] M.H. Zhang, H.H. Hwu, M.T. Buelow, J.G. Chen, T.H.
Ballinger, P.J. Andersen, Catal. Lett. 77 (2001) 29.
[21] L. Leclercq, M. Prigent, F. Daubrege, L. Gengebre, G.
Leclercq, Catal. Automot. Pollut. Control (1987) 417.
[22] J. Brillo, R. Sur, H. Kuhlenbeck, H.-J. Freund, Surf. Sci.
397 (1998) 137.
[23] J. Brillo, R. Sur, H.-J. Freund, J. Electron Spectros. Relat.
Phenom. 88–91 (1998) 809.
[24] J.G. Chen, W. Erley, H. Ibach, Surf. Sci. 227 (1990) 79.
[25] G. Herzberg, Molecular Spectra and Molecular Structure,
Krieger Publishing Company, Malabar, 1989.
[26] R.I. Masel, E. Umbach, J.C. Fuggle, D. Menzel, Surf. Sci.
79 (1979) 26.
[27] S.H. Overbury, D.R. Mullins, Lj. Kundakovic, Surf. Sci.
470 (2001) 243.
[28] K.D. Childs, B.A. Carlson, L.A. LaVanier, J.F. Moulder,
D.F. Paul, W.F. Stickle, D.G. Watson, Handbook of
Auger Electron Spectroscopy, Physical Electronics, third
ed., 1995.
[29] http://webbook.nist.gov/chemistry/.
[30] C.T. Campbell, G. Ertl, J. Segner, Surf. Sci 115 (1982) 309.
[31] R.J. Gorte, L.D. Schmidt, Surf. Sci. 111 (1981) 260.
[32] H.P. Bonzel, G. Broden, G. Pirug, J. Catal. 53 (1978) 96.
[33] G. Comelli, V.R. Dhanak, M. Kiskinova, K.C. Prince, R.
Rosei, Surf. Sci. Reports 32 (1998) 167, and references
therein.
[34] H.J. Borg, J.F.C.-J. M. Reijerse, R.A. van Santen, J.W.
Niemantsverdriet, J. Chem. Phys. 101 (1994) 10052.
[35] M.J.P. Hopstaken, J.W. Niemantsverdriet, J. Phys. Chem.
B 104 (2000) 3058.
124 M.H. Zhang et al. / Surface Science 522 (2003) 112–124