Reaction induced surface segregation in amorphous CuZr, NiZr and PdZr alloys—an XPS and SIMS depth profiling study

Journal of ALLOYS

A~D COMPOUI~D$

E L S E V I E R Journal of Alloys and Compounds 236 (1996) 137-150

Reaction induced surface segregation in amorphous CuZr, NiZr and PdZr alloys an XPS and SIMS depth profiling study

M . K i l o a, M . H u n d a, G . S a u e r a, A . B a i k e r b, A . W o k a u n b 'c '*

"Physical Chemistry H, University of Bayreuth, D-95440 Bayreuth, Germany b Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zurich,

Switzerland CPaul Scherrer lnstitut, General Energy Technology Department, CH-5232 Villigen, Switzerland

Received 30 July 1995

Abstract

Amorpaous alloys based on zirconium are used as precursors for the preparation of highly active CO 2 and CO hydrogenation catalysts. The activation process of glassy CuZr, NiZr, and PdZr alloys is characterized by XPS and SIMS depth profiling. Upon exposure to CO2/H 2 or C O / H 2 reactant gas mixtures at 523 K, the formation of zirconium oxide occurs in spite of a large excess of hydrogen, indicating that the zirconium component is highly reactive towards the carbon oxides.

In the Ni64Zr36, Cu30ZrTo, and Pd25Zr75 systems, a distinct overlay of almost pure zirconium oxide develops, which contains traces of Cu and Pd respectively for the latter two alloys. The thickness of this layer increases with treatment time in the range 10 to 90 min, and varies between 5 and 30 nm (Cu3oZr7o : up to 80 nm). With NiqlZr 9 the zirconium component is oxidized, but no compact surface zirconia layer is formed, while with CUToZr30 the formation of alternating Zr-rich and Cu-rich bilayers is observed. Upon prolonged exposure to reaction conditions, the depth over which segregation phenomena occur in this alloy increases to values in excess of 1 tzm.

From an extensive set of sputter depth profiles, changes in the oxidation state of zirconium have been traced as a function of depth for Cu3oZr7o, Ni64Zr36, and PdzsZr75. At the interface to the unreacted alloy, the oxidation state of the zirconium is smaller than 4+, and the presence of zirconium suboxides is detected. With the zirconium-rich alloys Cu30ZrT0 and Pd2~Zr75, the relative contribution of the formal oxidation state Zr ~3+) is maximum close to the outermost ZrO 2 layer, whereas Zr ~'+) is predominantly found close to the unreacted core of the alloy. In contrast, for Ni64Zr36, with a low content of zirconium, both Zr ~3+ ~ and Zr ~ +) reach their maximum concentrations at the interface between the oxide and the bulk. These observations are system-specific and, therefore, not due to sputter-induced processes.

After activation, the second metal is present in the metallic state for catalysts based on copper (CUToZr30 and Cu30Zr70 ) and on nickel (Ni64Zr36 and Ni9aZrg). In contrast, for the catalyst based on palladium (Pd2~ZrT~), there is strong evidence for the formation of PdO even in the subsurface region.

Keywords: Amorphous alloys; Catalyst activation; Zirconium; Oxidation; Segregation

1. Introduction

A m o n g various o ther applications [1,2[, glassy a m o r p h o u s alloys based on z i rconium have been successfully used as precursors for the p repara t ion of highly active catalysts for C O 2 and C O hydrogenation, as well as for o ther catalytic react ions (for a review 3n the applicat ion of metallic glasses in

* Corresponding author.

0925-8388/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0925-8388(95 )02143-4

catalysis, see Ref. [3]). The act ivated catalysts exhibit high CO 2 reduct ion rates at relatively low pressures, and excellent selectivities to methanol [4] or m e th a n e [5], depending on the alloy composi t ion. U p o n exposure of the alloy to C O 2 / H 2 or C O / H 2 reactant mixtures, these materials undergo a series of chemical and physical changes, including partial oxidation, segregation, agg lomera t ion and crystallisation, which finally result in active catalysts with unique structural proper t ies [3].

D e p e n d i n g on the composi t ion of the alloy, ei ther z i rconium segregates or an enr ichment of the second

138 M. Kilo et al. / Journal of Alloys and Compounds 236 (1996) 137-150

metal component is observed, with binary alloys containing zirconium and palladium [6-10], nickel [9,11- 16], or copper [17-20] respectively. This segregation phenomenon, its driving force and dependence on composition, has so far not been investigated in detail. In particular, different results have been reported in the literature on the state of the zirconium component in either the amorphous or the crys- tallized states of the alloys. The zirconium was stated to be either fully or partly oxidized (e.g. Refs. [15,21]), and was observed to form hydrides ZrH x [12,22,23], carbides ZrC x [6], or intermetallic phases [24], depending on treatment conditions. In the past, XPS has been used by Morant and coworkers [25,26] to study the initial stages of the oxidation of poly- crystalline Zr at room temperature and low oxygen pressures. Oxygen exposures of more than 0.6L at room temperature lead to the formation of a closed ZrO2 layer and different suboxides located in the Zr /ZrO 2 interface. The thickness of the ZrO 2 layer grows continuously as oxidation proceeds. The authors suggested [26] the presence of four oxidation states of Zr, shifted about 1.1 eV per metal-oxygen bond. The four proposed oxidation states at the metal/oxide interface correspond to the four possible coordinations of Zr with the oxygen atoms (i.e. ZrO2, Zr203, ZrO and Zr20 ) [25,26].

In this study, alloys of composition Cu30Zr70 , Cu70Zr30, Ni64Zr36 , Ni91Zr9, and Pd25Zr75 are investigated by means of XPS depth profiling in order to determine the dependence of the oxidation state on the parameters of exposure to CO 2 and CO hydrogenation conditions. As it is known that the surface state of the activated catalyst would change upon exposure to air, the samples were transferred directly to the UHV analysis chamber subsequent to treatment under reaction conditions. For this purpose, a high pressure chamber flanged to the preparation chamber of the surface analysis system has been used.

Special attention was paid to changes in the oxidation state of the metals. This information was obtained by deconvoluting the XPS signals, recorded as a function of depth, into contributions from the relevant oxidation states. In addition, SIMS depth profiling has been performed in order to achieve an increased depth resolution in the outermost surface layers.

As a destructive method, sputtering is known to cause chemical, compositional, and electronic changes. For example, it has been demonstrated that sputtering of ZrO2 results in loss of oxygen at the surface and the formation of Zr species in lower oxidation states [27]. Therefore, in the present study we focus on system specific differences between the three alloys investigated, rather than emphasizing absolute results.

2. Experimental

2.1. Apparatus

The system used (Leybold-Heraeus) comprises a high pressure chamber, a preparation chamber and an analysis chamber. The high pressure chamber has a volume of ca. 0.5 1, into which gases can be admitted up to a maximum pressure of 10 bar by means of a stainless steel gas dosing system. In order to suppress atmospheric contamination, the whole gas dosing system was evacuated and flushed with the reaction gas several times. Mass flow controllers (Brooks) were used to mix the gases in the desired concentrations, at flows up to 300 ml (STP) per minute. The preparation chamber has a base pressure of ca. 1 × 10 9 mbar, in which reactions at lower pressures may be carried out up to a maximum pressure of 1 bar. The analysis chamber houses the spatially separated XPS and SIMS analysis stations.

Sample manipulation and transfer between the chambers is achieved by means of a sample rod that may be cooled to 77 K, and heated up to 1073 K. In order to avoid contamination during the transfer between adjacent chambers, differential pumping in at least two stages was applied.

The XPS area of the analysis chamber is equipped with an energy analyser (Leybold, model EAl l ) , a twin-anode X-ray source (model 20/63) and an argon ion gun for sputtering (model 12/38, 5 kV excitation energy). In the present study, Mg K s radiation was used for excitation (h × v= 1253.6eV, high voltage 12 kV, emission current 10 mA).

The SIMS analysis system (Leybold, model SSM 200) comprises a quadrupole mass filter, and another argon ion gun of type 10/38 (energy 5 kV) for excitation. The ion beam was focused on 'a spot of ca. 300 Ixm diameter for both XPS and SIMS measurements. Mass spectrometric registration of the sputtered particles was restricted to 70% of the sputtered area of 1 x 1 mm 2 (edge size) by means of electronic gating. A flood gun (model FG 10/35) was used to compensate for charging effects during the SIMS measurements. Whereas the base pressure of the surface analysis system is less than 1 x 10 10 mbar, the working pressure during depth profiling was typically around 5 × 10 7 mbar as a consequence of the argon inlet.

Data acquisition was controlled by an HP 1000 computer system (model A400); the software package DS100 (Leybold) was also used for the first stage of spectral deconvolution. Further data processing, i.e. evaluation of intensities and binding energies as a function of sputtering time, was carried out on a personal computer.

M. Kilo et al. / Journal o f Alloys and Compounds 236 (1996) 137-150 139

2.2. Sample preparation and activation

Glassy alloys of the following compositions have been investigated, where the numbers indicate atomic fractions of the metals: Cu30Zr70 , Cu70Zr30 , Ni64Zr36 ,

Ni91Zrg, and Pd25Zr75. Ribbons of ca. 50 txm thickness of the alloys were prepared by melt spinning of the specified mixture of the metals under an argon atmosphere. It was confirmed by XRD that the prepared alloys were amorphous. The alloys were found to be stable against storage in air, except for the two copper containing alloys. The latter were therefore stored under argon at 273 K prior to the measurements.

Squares 4mm x 4mm in size were cut from the ribbons and placed onto the holder of the heatable sample rod. The samples were fixed by placing an aluminium sheet with a 3 mm radius hole and screwing down on top of the sample. This mask provided good electrical grounding of the sample, a defined area for XPS measurements in a reproducible manner, and guaranteed uniform sputtering of the entire area, from which signals were collected to obtain an optimum depth resolution of the time profiles. Measurements were performed on that side of the ribbons which had not been in contact with the cold wheel during the melt spinning preparation. It has been shown [6] that the latter side may carry contaminations from the wheel.

Gases used in the activation procedure, i.e. CO 2, CO, and H2, were obtained from Linde in a purity of 99.997%, and were used as received without further pretreatment. Their oxygen content was specified to be lower than 3 vpm. Prior to activation, each sample was cleaned by sputtering, as monitored by XPS. For activation, mixtures of CO2/H2 (1 : 9 by volume) and of CO/H 2 (1:6 by volume) at total pressures of 5-6 bars were used. Samples were heated to 523 K at a rate of 20 K min-1; the final temperature was main- tained for times between 15 and 90 min.

A low resolution wide scan (pass energy 200eV, check for impurities) and a high resolution spectrum (pass energy 50 eV) with an extensive accumulation time (ca. 60-120 min) were recorded before and after activation, as well as after completion of the XPS depth profile. In the depth profiling series, each single spectrum was measured with an energy step size of 0.1eV and a dwell time of 500ms. Signals were collected for the C ls, O ls, and Zr 3d signal regions, as well as one binding energy range characteristic of the second metal (i.e. the 2p3/2 region for the Ni and Cu containing samples). For the PdZr sample, the Pd 3d area was recorded together with the Zr 3p region, as the binding energies of these elements are quite similar. The total measurement time for one step of the profile thus amounted to 5-10min. The sputter

time between the recording of subsequent XP spectra was chosen to be half the measurement time. To avoid the crater effect observed in XPS depth profiling [28], the beam was scanned over an area much greater than the sample geometry, i.e. 6 × 6 mm 2 in size, whereas, because of the hole with diameter 3 mm in the aluminium mask, the sample area was only 7.1 mm 2 in size. Besides the reduced signal intensity, this also has the disadvantage that the A1 mask was sputtered during the measurements. A depth profile typically consisted of 100-150 cycles, thus requiring a full day of data acquisition.

In contrast, SIMS depth profiles were recorded in 1-2 h, owing to the smaller sputtered area. The sputter current measured at the sample was between 500 and 1000 nA, decreasing slightly with prolonged sputtering time. (Only for the Cu70Zr30 sample, where massive segregation was observed, was a sample current between 3 and 5 IxA chosen.) For converting sputter times into sputter charge densities, the sample current measured at the beginning of the depth profile was used consistently.

3. Results

3.1. Oxidation state of the metals

When evaluating changes in the oxidation state of the metals, any shifts due to a slight charging of the samples must be accounted for. As a consequence of sputtering, the intensity of the C ls peak was very low, such that the latter could not be used for re-calibration. Thus we decided to use the zirconium signal as an internal standard, because all samples contain this element.

If we use, as a reference, the literature value of 178.7 eV [28-30] for the binding energy of pure metallic zirconium, the signals of the other metals are detected at binding energies which are apparently too low even for the metallic state of these elements. Consistent results were obtained only when assuming a binding energy of 179.5 eV for metallic zirconium, in agreement with a recent study on the alloys under investigation [6]. With this reference value one finds that copper and nickel are in the metallic state, whereas palladium is present in the oxidized form (Pd 2+). This result is consistent with earlier studies on this and similar systems [6-10]. Only on the unreacted alloys were nickel oxide and copper oxides found frequently. The measured binding energies and line widths of the metals are summarized in Table 1.

For Cu70Zr30 , the determined binding energy of 932.3 eV is a little lower than the literature value,for pure copper (932.67 eV), and would be closer to the

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Table 1 XPS binding energies" and line widths in sputter equil ibrium

System Element , spin Oxidation state Binding energy (eV) F W H M b (eV) orbit componen t

PdzsZr75 Pd 3d5~ 2 Pd 2+ 336.3 1.2 Ni64Zr3( ' Ni 2p3/2 Ni ° 852.6 1.55 Cu3oZr7o Cu 2P3/2 Cu ~j 932.9 1.6 Cu711Zr30 Cu 2P3/2 Cu ° 932.3 1.6 Nig~ Zr 9 Ni 2p3 ~2 Ni° 853.1 1.5

a Referred to a binding energy of 179.5 eV for e lemental zirconium. h Full width at half max imum, as defined in Eq. (1).

standard value for Cu20. (Reference measurements performed on Cu, Cu20, and CuO confirmed the literature values of these systems.) However, we note that in other alloys, such a s N i 6 6 C u 3 4 , the binding energy of Cu 2p312 was also found at 932.4 eV [31], i.e. a little lower than for pure copper. The existence of CuO can be excluded, because no shake-up satellite is observed in the Cu 2p region for our systems; the Auger parameter was repeatedly found to be greater than 1851 eV, excluding the presence of Cu20 (Auger parameter 1849.5 eV, [28,29,32]). In addition, the line width of the Cu 2p3/2 signal determined in the alloys is much larger than the value known for pure copper, and the signals exhibit a pronounced asymmetry (spectra not shown). These effects will be explained in Section 4.

3.2. Quantification of sputter time dependent intensities

In order to evaluate the concentration of elements as a function of sputter time, spectra were satellite subtracted and smoothed before further processing, according to standard procedures. Satellite subtraction is crucial for the PdzsZr75 sample in view of the overlap of the Pd 3d and Zr 3p signal regions, where a disregarding of satellites could lead to erroneous assignments of peak intensities. For the other spectral regions investigated, satellite subtraction was shown to give rise only to slight modifications of the obtained depth profiles. Smoothing of the spectra improved the quality of the fits and reduced the noise in the sputter time dependent intensity profiles. The spectra were fitted using a Gauss/Lorentz type function (Eq. (1), given, for example, in Refs. [33,34]):

/o fn(E) = [1 +c(FWHM/E~E° ~2]n]

; c = 4 ( 2 l," - 1 ) (1)

where E 0 represents the peak position on the energy scale, and I 0 is the associated peak height. The parameter n is a measure of the Gaussian contribution to the line shape: n = 1 corresponds to a pure Lorent-

zian line shape, while n ~ ~ means a pure Gaussian line shape. For a particular choice of n, the constant c is defined such that FWHM corresponds to the desired full width at half maximum. The second metal of the alloy may be well represented in terms of a single oxidation state. During a depth profile, FWHM and n were kept constant.

The procedure is illustrated for the example of a Pd25Zr75 alloy ribbon that had been activated for 90min in a CO2/H z mixture (1:9 by volume) at a pressure of 5 bar. The Pd 3d/Zr 3p range, and the corresponding Zr 3d region, are displayed for all spectra of this depth profile in Fig. 1 (raw data) The Zr 3d region (lower part of Fig. 1) clearly shows the presence of oxidized zirconium at the beginning of the profile, and the appearance of zirconium in lower oxidation states after prolonged sputtering.

After several sputtering steps, the Zr 3d spectrum shown in Fig. 2 is obtained. Vertically displaced, a trace is shown in which the 3d3/2 doublet component has been removed by numerical subtraction according to the procedure described, for example, in Refs. [25,26] (i.e. using the theoretical intensity ratio of 2:3 for the two components and a line splitting of 2.4 eV). From this spectrum one clearly recognizes a broad composite peak shape for the 3d5/2 doublet component, in which there is significant signal intensity between the bordering peaks corresponding to the fully reduced and the fully oxidized states. Thus, visual inspection shows that more than two oxidation states of zirconium are present.

To quantify the relative concentration of the various suboxides, extensive tests have been performed re- garding the best approach to fitting the complete set of sputter time dependent spectra. The intensity ratio of the 3d3/2 and 3d5/2 doublet components was consistently fixed at a value of 2:3. The obvious first attempt to fit the profile in terms of three oxidation states of zirconium, which is also reported in Ref. [27], was not satisfactory: we found that the Gauss/Lorentz mixing, line width, and binding energy of each component had to be individually adjusted for each spectrum. This results in lower binding energies and significantly broader line widths for the Z r 4+ c o r n -

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M. Kilo et al. / Journal of Alloys and Compounds 236 (1996) 137-150 141

Zr 5p/Pd C

° L , d / N N 3 4o 3.45 3_4 binding energy / eV A

c L88 ~84 ~.80 binding energy / eV

Fig. 1. Three-dimensional representation of the depth profiles for a PdzsZrv5 alloy activated by 90 min of exposure to CO2/H z at 523 K and a pressure of 6 bar. Upper part: Pd 3d/Zr 3p region of the XPS spectra; lower part: Zr 3d region (data not corrected for sample charging).

• / / W / / A ' z r / Y Vz ('

188 184 180 176

binding energy / eV Fig. 2. Zr 3d region of the XPS spectrum of the PdzsZr75 alloy after 90 min of exposure to CO2/H 2 and prolonged sputtering. Thin line: experimental spectrum; solid lines: synthetic spectrum from the fit and contribation of individual charge states. The vertically displaced trace corresponds to the experimental spectrum from which the contributions of the Zr3d3/2 doublet components have been subtracted, as described in the text.

ponent as compared with pure Z r O 2. The excessive line width hints at the existence of more than three different Zr species.

In accordance with Ref. [26], we obtained the best fit when we: (1) held the doublet splitting of all components fixed at the literature value of 2.4 eV; (2) used a constant line width and line shape for the entire profile (see below); (3) divided the constant 4eV interval between the highest and lowest binding energy into four equal parts, and determined relative concentration for formal oxidation states of 0, 1+, 2+, 3+, and 4+. In this manner, each spectrum was fitted using a total of six parameters, i.e. five intensities (concentrations) and the position of the entire multi- plet on the binding energy scale (in order to account for slight charging effects). The advantage of this procedure is that the deconvolution of the composite signal into contributions from different oxidation states is the same for all spectra of a sputter series, and hence changes in the relative concentrations can be consistently compared. With these assumptions, each component is represented by a doublet of Gauss/ Lorentz type bands in Eq. (1). The final fit is included in Fig. 2.

Of course one has to emphasize that the formal oxidation states introduced in this manner are a means of quantifying the substoichiometry of the zirconia,

142 M. Kilo et al. / Journal o f Alloys and Compounds 236 (1996) 137-150

and are not intended to imply the physical presence of species, such as Zr J+ or Zr 3+, which are known to be unstable, e.g. as ions in solutions. For this reason, the intermediate formal oxidation states are consistently written in brackets, i.e. Zr (1+), Zr (2+), and Zr (3+).

Integrated intensities of the individual components are obtained from each spectrum of a complete sputter series. To check the reproducibility of the deconvolution procedure, the Zr 3P3/2 region has been treated in a similar manner. In spite of the lower signal intensity, and the interference of the very intense Pd 3d5/2 signal, qualitatively the same depth profiles are obtained as from the Zr 3d region.

3.3. Pd25Zr75

Depth profiles of the Pd 3d5/2 and the Zr 3d5/2 signals (oxidation states 0 and 4+ only) are shown in Fig. 3 for different treatments with CO2/H z and CO/ H 2 respectively. On the untreated sample (Fig. 3A), a thin surface layer of zirconium oxide is present, which decreases exponentially immediately after the onset of

80004, pd 2+ ~ ' ~

,ooo t \ Zr o

> 8008-

m o_ o ~4ooo

" 8oo8 t-"

"O • ~ 4000 13 k .

13~ .4J

• - 0 15000-

10000 pd2+ - ~ Z p + _

5000 ~

0 o.oo o.~i o.~2 o.b3

charge density / As cm -2

Fig. 3. Depth profiles of the Pd25Zr75 alloy (A) untreated; (B) after 30min of exposure to CO/H2; (C) after 90min of exposure to CO/H2; (D) after 90 min of exposure to CO2/H2; at a temperature of 523 K and a pressure of 6 bar.

sputtering. After 30 min of treatment in C O / H 2 (B), a massive layer of ZrO z is seen to have formed. The thickness of this layer, which contains a small amount of Pd, increases with treatment time, as is evident from a comparison of profiles B and C (30 and 90 min of exposure respectively). The choice of reactant gas (CO or CO2) does not appear to change the qualitative appearance of the depth profile, as demonstrated by a comparison of Figs. 3C and 3D (90 min of treatment with CO/H z and C O 2 / H 2 respectively).

For the oxidation states of zirconium, the sputter charge dependence of the respective concentrations is shown in Fig. 4, both for the untreated alloy (Fig. 4A) and for the sample which had been exposed to CO 2 / H 2 for 90 min (Fig. 4B). Intensities have been normal- ized with respect to the sum of all species in Fig. 4; curves appear qualitatively similar when the absolute intensities are plotted instead. The species with a formal charge of (3+) exhibits a similar sputter time

A B 100

50"

Zr 3d5/2 Zr 4+

~2° I

~ 1 0 4 ~

~ o

01o4

oF

Zr4+ Z r °

Zr(2+)

~20| I

• ~ J ZrO+)l o lJ - " I o° l r° 0.00 0.()I 0 . ( )2 0.()3 0.00 0.()3 0.06

charge density / As cm -2 charge density / As cm -2

Fig. 4. Relative concentrations of the oxidation states of zirconium (see text) for the Pd25ZrT~ alloy shown as a function of sputter charge density, which is proportional to the sputter depth. (A) untreated sample; (B) after exposure to COz/H 2 for 90min. Included in the bottom graphs are the corresponding O ls depth profiles. Details of the fitting procedure are described in Sections 3.2 and 3.3.


dependence as the Z r 4+ component. In the activated sample (B), the Zr ~1+) concentration exhibits a maximum just below the interface between the oxide and the bulk metal. The concentration assigned to the intermediate oxidation state (2+) appears to be almost constant with time. The concentration of Zr ° exhibits a pronounced sigmoid increase, to asymp- totically reach the bulk concentration.

A corresponding behaviour (i.e. the sputter time dependence of Zr (3+) linked to that of Zr 4+, and the occurrence of a maximum in Zr (1+~ concentration in the depth zone where Zr ° approaches the bulk value), was found for all investigated treatments of Pd25Zr75.

For two representative samples, i.e. the untreated and 90 min treated alloy, the O ls signal was investigated in detail, and has been deconvoluted into four peaks. The highest energy component is not due to oxygen, but to the overlapping Pd 3P3/2 signal. The three oxygen peaks are assigned to Z r O 2 , A l 2 0 3

(arising from the AI mask), and to adsorbed oxygen or OH groups (see Section 4). The relative position and the width of the latter signals were fixed at the values given in the literature [31,35-37] for the relevant species. ]7he oxygen signal of A120 3 is shifted to higher binding energies by 1.35 eV relative to the O ls peak of ZrO 2, and the Pd 3p3/2 signal appears at a binding energy 3.5 eV higher than the oxygen of Zr 4+. Both the FWHM of 1.7eV and the Gauss-Lorentz mixing parameter were held constant for these four species. The remaining signal due to adsorbed oxygen was characterized by a shift of 2.25 eV relative to the oxygen of ZrO 2, and a smaller width of 1.2 eV. It was not possible to decompose the 'oxide' part of the O 1 s signal into contributions from PdO (binding energy 529.3eV [37]) and from ZrO2 (530.4eV [35]) (see Section ~).

The corresponding oxygen depth profiles are included in the lower part of Fig. 4, and may be correlated with the sputter time dependence of the Zr species. The three assigned O ls bands show the expected behaviour: oxygen from ZrO 2 decreases in parallel with the Zr 4+ species, whereas O ls from the thick A120 3 overlayer on the mask remains nearly constant. The depth profile of Pd 3p3/2 is similar to that of Pd 3d5/2 (Figs. 3A, 3D); both show a similar sputter charge dependence to the Zr ° signal, as will be discussed below.

3.4. Ni64Zr36

When the Ni6nZr36 sample is exposed to either CO/H 2 or C O : / H 2 reactive atmospheres, segregation of ZrO 2 is observed, similar to the behaviour described above (see Fig. 5). However, in contrast to the behaviour found with the PdZr sample, where a

24000-

16000

8000 >

c~ o 30000

-~20000

C

.~ 10000 "1o

o L o

~ .. Z r o A

B (::7J30000

r- ° ~

Ni ° 20000. f

Zr4+ ~ v 1oooo. C

o o.oo o.~1 o.b2 o.b3

cha rge dens i ty / As cm -2

Fig. 5. Segregation of ZrO: in an Ni64Zr36 alloy as a function of the time of activation in CO2/H2: (A) untreated sample: (B) 30min; (C) 90 min of treatment.

residual amount of Pd is always found at the surface, the activated NiZr ribbon is fully covered by ZrO 2 (i.e. no Ni signal is detected at the surface).

In agreement with the findings reported above for Pd25Zr75, one observes an increase of thickness with time of exposure towards the reactive gases for the Ni64Zr36 alloy (Fig. 5). However, in contrast to the former system, the concentration of Zr ° exhibits a maximum near the interface between the bulk and the oxide layer, indicating that part of the segregated Zr is not fully oxidized.

The depth dependence for the concentration of various oxidation states of Zr is shown in Fig. 6 for the sample activated by 90 min of exposure to CO2/H 2. In contrast to the Pd25Zr75 sample, the signals due to all lower oxidation states start at a low level after a region consisting of pure ZrO 2. Similarly, the Zr (3+) fraction does not parallel the profile of Zr 4+. The dependence of the Zr (1+) and Zr (3+) species shows nearly the same behaviour: both increase with sputter time towards a plateau value, while the concentration of Zr ~2+) increases only slightly with time. The above-mentioned maximum in the Zr ° concentration close to the inter-

144 M. Kilo et al. I Journal of Alloys and Compounds 236 (1996) 137-150

15000-

> 010000

El. O

5000

0 ° _

"~ 15o8 o ._= "0 1000-

°:t .__.

0.00

~ ,~ , , . - , . , , ~ Z r ( 2 + )

o.bl o.~2 o.~3 c h a r g e dens i t y / As c m -2

Fig. 6. Relative concentrations of the lower oxidation states of zirconium in an Ni64Zr36 sample treated in CO2/H2 for 90min. Note the pronounced maximum in the Zr ° concentration.

face between surface oxide and bulk is clearly visible in Fig. 6 as well.

3.5. Ni91Zr 9

Results of 90min treatment of the Ni91Zr 9 alloy with the reactant gases CO/H 2 and CO2/H 2 are displayed in Fig. 7. In contrast to the Ni64Zr36 sample described above, signals due to metallic nickel are seen both at the surface of the untreated sample and after treatment. Zirconium is again enriched at the surface, but does not cover the sample completely, i.e. neither in the untreated sample stored under atmospheric conditions nor after exposure to the reactant gases.

Three points should be emphasized as they are particular to Ni91Zr 9. First, nickel is found in the form of NiO on and below the surface of the unreacted sample, but the concentration of the oxide decreases rapidly with sputtering time, in favour of metallic nickel. Second, the zirconia layer covering the sample stored under atmospheric conditions does not exclu- sively consist of Z r 4+ oxide: some 10% Zr ° is detected. Third, the binding energy of the most highly oxidized Zr species is higher by only ca. 3.4 eV compared with Zr °, which is significantly lower than for the other systems investigated (ca. 4 eV). A similar observation had been reported in a previous study on the oxidation of this sample [15].

In the latter investigation [15], NiO enrichment of the surface had been observed during oxidative treatment at a temperature of 623 K. To check whether this effect was due to the higher temperature employed in Ref. [15] or to the different reaction medium (pure

6000 - Ni °, :10

4+ Z rO

4000.

o

2000- > o

o El. °

~'~6000

.~ ] / \ 7.4+

.~ 2000 7

o L

•

"~6000- . - -

2000-

o C 0.00 0.02 0.04 0.06

c h a r g e dens i t y / As crn -2

Fig. 7. Segregation phenomena in Ni91Zr 9. (A) untreated sample; (B) after 30min of exposure to C O J H z ; (C) after 30min of exposure to CO/H 2.

oxygen exposure), the C O 2 / H 2 activation experiment was repeated at 623 K. The observed time profile (not shown) did not exhibit an NiO segregation. The oxidation state of nickel did not change as compared with the 523 K treatment, and a zirconia enrichment was found at the sample surface as above.

3.6. Cu3oZr7o

The Cu30ZrT0 sample is found to behave in a manner similar to that of the Pd25Zr75 system. Upon activation an oxide layer is formed, the thickness of which increases with exposure time. Similarly, there is only a weak influence of the reactant gases (Fig. 8).

This is further supported by the depth dependence of the lower zirconium oxidation states shown in Fig. 9 for a sample that had been activated for 30 min in a CO/H 2 atmosphere. There is a significant fraction of Z r (3+) close to the surface. Z r (3÷) and Z r 4+ show almost identical profile shapes (in pronounced difference with respect to the behaviour observed for Ni64Zr36 ). The concentrations of Z r (2+) and Zr (1+)


10000-

> (D

m CL 0

5000

\ Z r 4+

A ~ 1 0 0 0 0 -

C 5000

C o - -

o -~'15000- 0 L C~

-~IO000- E

5000.

O- O.O0

Z r 4 + C u °

Z r 4+ . / f

0.01 0.02

charge densi ty / A s cm -2

C o.b3

Fig. 8. Segregation phenomena in the Cu3oZr70 sample: (A) as received; (13) after 30min of exposure to CO/H2; (C) after 30min of exposurz to CO~/H 2.

Cu30Zr70 system (compare, for example, Figs. 8A and as).

3. Z CUToZr30

In contrast to all other investigated systems, massive segregation of copper due to activation is observed with the CuToZr30 system. This effect may even be perceived visually by the appearance of a reddish taint on the sample surface after activation. Electron micro- scopy showed the formation of a layered system of metallic particles (micrographs not shown). XPS depth profiles for this system before and after CO2/H 2 treatment are presented in Figs. 10A and 10B. Most remarkable is the formation of a second layer with increased copper concentration, at a depth corresponding to a sputter charge density of around 0.15 As cm -2 (bottom diagram of Fig. 10), and even a third layer around the sputter charge density approximately 0.4 As cm 2. This copper enrichment, which is not as pronounced as in the first maximum, is accompanied by a corresponding decrease in the intensity of the zirconium component.

For this system, shifts in the binding energy of Cu 2p3/2 were followed as a function of depth, as shown in

80 .~

~ 4 0 .

m

?2 fn c I D o

Z r 4 +

'1~ 20 . Q)

.E

0 , 0.00 O.bl o.b2 o.b3 0.04

c h a r g e d e n s i t y / As c m -2

Fig. 9. Relative concentrations of low valent oxidation states of zirconium for the Cu30ZrTo sample exposed to CO/H 2 for 30 min, shown as ~L function of sputter depth.

reach their respective maxima at increasingly higher depths. The profiles of Zr (t+) and Zr ° are again running close to parallel.

There are two differences with respect to the Pd25Zr75 system. First, the interface between ZrO 2 and the bulk metallic alloy appears to be more gradual with C%oZrTo as compared with the Pd alloy. Second, identical treatments lead to a thicker ZrO 2 layer in the

A >

mn20000 0

° - -

10000

C ° - -

C ° - - 0

Zr 4+

> C u o ,

20000 cL o

° - -

c 1 oooo

._.g 0 , , , ,

0.0 0.1 0.2 0.3 0.4 charge densi ty / As cm -2

Fig. 10. Segregation phenomena in a Cu70Zr30 alloy: (A) untreated, and (B) exposed to CO~/H 2 for 30min. Changes in the binding energy (AEu) of Cu 2p3/2 are included as an inset in (B).


the inset of Fig. 10B. (In the fits, the line width and the Gauss -Loren tz mixing were held constant.) From the inset one recognizes that the binding energy shifts parallel to the intensity of Z r 4+. This suggests that the presence of Zr 4+ ions increases the binding energy of all peaks in the XP spectra slightly, as reported previously in Ref. [6]. Similar shifts in the binding energy of the metal as a function of the overall degree of oxidation, as measured by the Zr 4+ intensity, have also been found with the other samples: in almost all spectra the binding energy both of zirconium and of the second metal component decrease with decreasing degree of oxidation, as measured by the Zr 4+ concentration.

In order to investigate the outermost surface layers with higher depth resolution, depth profiles have been recorded using positive SIMS detection, which has a bet ter depth resolution than XPS depth profiling. The spectra (Fig. l l A ) show the formation of a very thin overlayer of copper close to the surface, even for the

~. 10000 ~" 0 O ~ ooooo ooooo

1ooooo ~ c 6oooo ~ . ~

3 0 0 0 0 1 - - -

"~ o - - ~ 0 .10 .

o.os - -~ o.oo I . , .

0 . 0 0 0 0.002

Cu* A

Zr +

ZrO +

_ _ . . . _

i 0.004 o.~o6 o.~os o.oto

charge density / As c m 2

200000

,ooooo ,,ooooo I

c 0 ~ ,,ooooo I - ooo o I

h 0.4

o .o 0 . 0 0

J J - _ _ B

Cu +

Zr +

J J ZrO +

' o . b l ' ' o .b2 ' ' o . b ~ ' ' o . b 4 charge density / As c m 2

Fig. 11. Depth profiles of Cu7oZr3o from positive SIMS measurements. Data were recorded (A) on the as received sample, and (B) after exposure to CO 2/H 2 at 473 K for 15 min. Note the different scales.

untreated sample. After activation (conditions C02/ H 2 = 1:12, T = 473 K), formation of a thick copper layer is found (Fig. l l B ) , in agreement with the XPS measurements. Even the range of sputter charge density (0.025 As cm-2) at which the first maximum in the Cu concentration is observed, is comparable with the results obtained by XPS analysis (Fig. 10B).

3.8. Comparison of the depth profiles

To compare the reactivity of all investigated amorphous alloys with regard to activation in either C O: / H 2 or C O / H : , we defined the layer thickness of the surface ZrO e by that point in the depth profile where the relative concentration of Z r 4+ was decreased to 50% of its initial value. An overview of all systems is given in Fig. 12, where the thickness of the layer (as represented by the sputter charge density) is plotted as a function of t reatment time. One recognizes that the thickness for comparable treatments decreases in the order Cu30Zr70 > Pd25Zr75 > Ni64Zr36. No significant difference appears between activation carried out in C O / H 2 or C O : / H : atmospheres. In the investigated range, the ZrO2 layer thickness increases roughly linearly with the time of treatment.

3.9. Calibration of the etch depth

The measured sputter charge densities can be con- verted into sputtered depths, and hence absolute layer thicknesses, using the sputter yield of ZrO 2 under the given conditions. Sputter yields of ZrO 2 for comparable conditions have not been reported in the literature to our knowledge. If we resort to using the yield

0,04 -

[] :C%oZrTo, CO / H 2 I • :Cu~Zr, o, CO 2 / Ha I . . ~ o :Pd25Zr75, CO / H=/

~ o.03 • :pd25Zr75 , CO 2 / H~

/

~. Z~ :Ni~Zr,,, CO / H=/

"~ 0.02

0 0.01

0,00 o ~b go 9'o

react ion t ime / min

Fig. 12. Thickness of ZrO 2 overlayers shown as a function of the reaction time with CO2/H 2 (filled symbols) and CO/H z (open symbols). Layer thickness is defined here by the sputter charge for which the Zr 4÷ signal has fallen to 50% of its surface values. For a correlation of charge densities and depths, see text. Circles, Pd2~Zr75; triangles, Ni64Zr36; squares, Cu30Zr70. The dashed lines serve as a guide for the eyes.

M. Kilo et al. / Journal o f Alloys and Compounds 236 (1996) 137-150 147

of 1.6 sputtered particles per primary ion of Ar, which is valid for pure metallic Zr [38], we obtain as a rough measurement a depth of 2 nm for a charge density of 1 mAs cm -2. This means that the thickness of our ZrO 2 layers (between 2.5 and 15 mAs cm -2 with the exception of the most extensively treated Cu30Zr70 sample, of. Fig. 10) ranges between ca. 5 and 30 nm. The intermediate area between the bulk and the oxide, where an accumulation of lower valent oxides is observed, has a similar thickness.

For the CUT0Zr30 system, the sample was sputtered with an ion current one magnitude in order higher than the other systems. Therefore we succeeded in determining the depth of the sputtered hole with an alpha-stepper (Dektak) subsequent to the measurement. In this sample, the observed massive segregation and formation of alternating copper and zirconium rich layers gave rise to a visual difference between the sputtered crater and its surroundings, which made it possible to locate the crater ex situ in the profiling instrument. From these results, for each single layer a thickness between ca. 0.2 and 0.5 p~m has been esti- mated.

4. Discussion

Two sets of experimental results concerning the oxidation behaviour of the metals, and the substoichiometry of the zirconia produced, will be addressed. (1) Under the applied treatment conditions, Zr is oxidized in all investigated samples. The Ni component is not oxidized; copper is neither present in purely metallic nor in purely oxidic form; in PdZr, only Pd 2+ is found. (2) The thickness of the ZrO 2 layer increases approximately linearly with exposure time, in a way that is not strongly dependent on the composition of the reactant gases. Suboxides of Zr are formed; their depth-dependent distribution is different for the alloys investigated. These points shall be addressed in sequence.

4.1. Oxidation behaviour of the samples

It is remarkable that, in spite of a large excess of hydrogen, we consistently observe an oxidation of the surface, there are several sources of the oxygen: (i) during activation, the reactants CO, CO 2 and H 2 undergo chemical transformations on the catalyst surfaces. Over Cu/ZrO 2, methanol formation takes place [4], whereas methane is efficiently produced over Ni/ZrO 2 [5] regardless of whether CO or CO 2 is used as the carbon source. The activated Pd/ZrO 2 alloy is known I0 catalyze both reactions [8]. (ii) On all samples, the water gas shift/reverse water gas shift reaction (WGS/RWGS) is observed, which converts carbon dioxide into carbon monoxide and vice versa.

According to their stoichiometry, several of these reactions produce water as a by-product, which also acts as an oxidant. Thus, the oxidation of the surface may be effected indirectly via previously produced water, or (iii) directly by reaction of the carbon oxides with metallic zirconium.

Cooke and coworkers [39-41] have previously investigated the oxidation behaviour of amorphous Ni/ Zr alloys with different compositions. These authors also showed the presence of an oxide layer enriched in either ZrO z or NiO at the outer surface. The genera- tion of this layer was explained in terms of a general model of the oxidation [42], with the high free en- thalpy of formation representing the driving force for the ZrO 2 segregation. The oxidation temperature was shown to play a dominant role: ZrO2 segregates up to a temperature range of 473-523 K; above this temperature, and especially after long reaction times, NiO is formed [41]. In agreement with Cooke and coworkers, our results show the nearly exclusive formation of ZrO2. However, Ni is present only in the metallic state, even at a high reaction temperature of 523 K. This difference might be caused either by the different methods of sample preparation (Cooke and coworkers investigated splat-quenched foils), or by differences in the concentration (34at.% nickel in Refs. [39-41], 64 and 91 at.% in this work).

Concerning the oxidation state of the second metal component, the CuZr systems particularly show an unusual behaviour. The observations mentioned in Section 3.1 imply that the copper component is dis- turbed by its environment. This might be due either to the presence of oxygen, leading to a partly oxidized copper species, or to the presence of zirconium. Unfortunately, it was not possible to clarify this question with the aid of the O ls region, as there are too many components contributing to this area of the spectrum.

The fact that these effects were not observed with the NiZr systems, where the observed binding energies agree well with literature values, strongly suggests that the copper component of the CuZr alloys does not correspond to plain elemental metallic copper. How- ever, the type of perturbation cannot be quantified from the present data. The formation of mixed oxide species, e.g. CuxZryOz, would explain the asymmetry in the Cu 2p3/2 bands. In the SIMS depth profiles, no mixed clusters such as CuZr + were detected in significant amounts. However, as the formation of cluster ions in SIMS is strongly dependent on the nature of the system, the existence of micro-domains of mixed oxides cannot be excluded from the SIMS measurements.

A similar result holds for the PdZr system, where the analysis of the O ls signal was difficult due to the presence of several contributions to this band, in


addition to the contribution from the AI mask. With the Pd25Zr75 sample, palladium is present in the oxidation state 2+ according to the Pd 3d signal. We attempted to decompose the 'oxide' part of the O ls signal into contributions from PdO and ZrO 2, but the band was not sufficiently resolved to warrant a two- component fit. This observation may indicate either structural disorder or a clustering of PdO with ZrO2, which is equivalent to the formation of mixed compounds of the type PdxZryO z. In support of this suggestion, the width of the Pd 3d5/2 signal is very high (1.5 eV), i.e. much larger than the instrumental contribution limited by the width of the exciting radiation (0.7 eV for Mg Ka), and higher than the width for pure Pd compounds (1.0 eV).

Taking all these facts together, it can be stated that there is a strong interaction between copper and zirconium, as well as between palladium and zirconium, which is not found between nickel and zirconium.

For all samples investigated, it was not possible to fit the O ls region satisfactorily by considering just contributions due to O ls from ZrO 2 or AI203 (from the mask). In all cases a further species with a signal at higher binding energies, around 532-534 eV, has been found, which continued to be observable even after prolonged sputtering. Hydroxyl groups are known to give rise to signals at high binding energies, but hydroxides are usually assumed to be present only in the outermost surface layers of the sample, and should be rapidly removed by sputtering. At this point is should be mentioned that the activation can lead to the formation of microporous structures; this would allow the reactive gases to penetrate the sample deeply. Then, prolonged sputtering may lead to an equilibrium signal originating from adsorbed hydroxides which is significantly different from zero. This suggestion is confirmed by the time dependence of treatment: the linear time dependence hints at the growth of a porous layer controlled by the surface reactivity towards the oxygenated reactants and shows that an equilibrium of the surface reaction is not reached at any point on the investigated time scale. Shortly after beginning the activation, the catalytic reaction is high because of the high concentration of low-valent zirconium oxides.

4.2. Layered structure of the samples

A novel result arising from this investigation is the quantification of contributions from lower oxidation states of zirconium for the first time on alloyed systems. We have shown that, for the Ni64Zr36 system, their concentration is generally low; it is smallest at the outer surface, and significant only close to the transition towards the unreacted core of the metallic

alloy. In contrast, for the samples Cu30Zr70 and Pd25Zr75 there are significant amounts of lower valent oxides right at the onset of the transition between surface ZrO 2 and the bulk.

Sputtering is known to potentially alter the surface state of oxidic samples. However, under comparable sputter conditions we are observing differences in the relative concentrations of suboxides and in their distribution for the various samples. Consequently, sputter-induced effects cannot be responsible for these observed alloy-specific differences. Being aware that sputter damage may change the quantitative aspects of the sputter time dependence, we are still in a position to interpret qualitative differences between the observed sputter depth profiles.

Next we address the identity of the five zirconium species that have been used in the fits presented in Section 3.2. From the literature, a linear relation between the atomic charge and a change in binding energy is known (see, for example, Ref. [43]). Assum- ing that the atomic charge and the oxidation state are proportional to each other, one might identify the five species with formal oxidation states of Zr. However, from the viewpoint of a chemist, substoichiometric Zr compounds are extremely unstable, and to our knowledge pure zirconium suboxides have not been syn- thesized. Thus we do not expect the existence of macroscopic domains of well-distinguished low-valent suboxides in our samples. However, the present results do suggest that between the outermost layer of stoi- chiometric ZrO 2 and the unreacted metallic zirconium alloy, there are intermediate zones in which Zr atoms are coordinated by varying numbers of oxygen atoms, as suggested by Morant and coworkers [25,26]; these are described in terms of the formal oxidation states used in the figures.

In related studies on the present systems it has been suggested [8,44] that clusters of palladium [6] or copper [45] surrounded by zirconia were generated upon activation, whereas no nickel clusters were observed [46]. From the present results one may assume that the formation of these clusters occurs as a result of a gradual oxidation of the zirconium component, as accompanied by a comparatively slow segregation of ZrO 2. For the Ni64Zr36 system, however, the segregation appears to be fast compared with the oxidation, such that a pure ZrO 2 layer on top of metallic Zr is produced. This suggestion may also account for the occurrence of metallic Zr in the untreated sample Ni91Zr9, in which the formation of a closed layer of ZrO 2 is not possible owing to the small concentration of zirconium.

Vanini et al. [47] explained the surface-enrichment of copper observed after exposing the system CUT0Zr30 to pure hydrogen at 473 K by the faster diffusion of copper, as compared with that of zirconium. In con-


trast, upon annealing the sample in vacuo at 473 K, no copper segregation was found to occur. This result was attributed to the formation of surface ZrO 2, representing a barrier to the segregation of the metastable alloy. From this comparison, Vanini and coworkers [45,47] stated that hydrogen was necessary for the disinte- gration of the alloy.

For the system Cu30Zr70, the concentration of low valent zirconium oxides appears to be somewhat higher than for Pd2sZr75. At the same time, the binding energies of copper are relatively low. These results can be correlated with the recently characterized catalytic properties of the activated alloys [5,48]. Catalyst,; derived from Ni64Zr36 and Pd25Zr75 are very active ir~ the formation of methane during CO 2 and CO hydrogenation [5,49], whereas methanol is predominantly formed o v e r Cu30Zr70 . From mechanistic studies, ,e.g. by FTIR [49], only a few intermediates on the route to methane have been identified, whereas several species on the reaction path leading to methanol were detected. Our results support the earlier suggestion [49] that these intermediates are bound to anion vacancies or substoichiometric areas (ZrOx) in the zirconia, which in the present study have been found in high concentrations on the activated Cu30ZrT0 catalyst. If methane synthesis takes place at other parts of the surface, the stabilisation and hence ac- cumulatJ:on of intermediates is less favourable than on the oxygen-deficient ZrO x, such that in the mechanistic studies no adsorbed compounds were observed [5].

In the system Cu70Zr30 several layers are formed, enrichect in copper and zirconium respectively. Treat- ing the samples in pure hydrogen for 16 h leads to the formation of one layer depleted in copper between the copper-enriched surface and the bulk, as is known from AES depth profiling, but no multilayer formation was reported [45,47]. It can be expected that prolonged ~:reatment with the system used may lead to macroscopic separated particles, some containing predominantly copper and others enriched in zirconium.

5. Conclusions

In the present study of amorphous binary alloys based on zirconium, the influence of activation treatments in CO2/H 2 or CO/H 2 atmospheres on the surface of amorphous alloys has been investigated by XPS and SIMS depth profile analysis. Upon activation the formation of a ZrO 2 overlayer was observed for most samples investigated. The thickness of this layer increases approximately linearly with exposure time. Its rate of growth is strongly influenced by the second metal present in the alloy (Ni, Pd, or Cu). In contrast, the nature of the reactant, CO or CO2, influences the segregalion behaviour to a lesser extent. This can be

explained by the fact that metallic zirconium is capable of dissociating CO according to the Boudouard equilibrium, leading to CO 2.

Formation of several lower valent oxides of zirconium is detected. The depth region in which these are enriched again depends on the nature of the second metal. With Cu and Pd containing systems, the concentration of Zr in the oxidation state (3+) follows that of Z r 4+, whereas the contribution of the low valent state Zr ~1+) increases with depth in parallel to the fraction of Zr °. In the NiZr alloys, the zirconium suboxides accumulate at the transition region between the activated surface layer and the unreacted metallic core of the alloy.

For one sample, CUT0Zr30 , extensive copper segregation is observed instead of the formation of a ZrO 2 overlayer. This segregation was found to give rise to layered structures made up of several domains, enriched in zirconium and copper respectively.

The state of the second metal is strongly influenced by the zirconium component. Nickel and copper were found to be present in the metallic state in the activated alloys, whereas in the Pd25Zr75 sample the palladium was in oxidized form (pd2+). The O l s signal indicates a superposition of several species, and the binding energy does not correspond to that of a pure oxide.

All these facts hint towards a strong interaction between the zirconium and the second metal. Earlier investigations have suggested that the existence of these interactions is a decisive factor influencing the catalytic behaviour of these alloys in the hydrogenation of carbon monoxide or carbon dioxide.

Acknowledgements

The authors thank R.A. K6ppel for performing the XRD measurements, and are indebted to Alusuisse- LONZA AG for providing the metal alloys. Financial support by the "Deutsche Forschungsgemeinschaft" (SFB 213) and by the "Verband der Chemischen Industrie" is gratefully acknowledged.

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Reaction induced surface segregation in amorphous CuZr, NiZr and PdZr alloys—an XPS and SIMS depth profiling study