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Interaction of oxygen and hydrogen with Pd-Au alloys: An AES and XPS study
Surface Science 103 (1981) 125-140 0 North-Holland Publishing Company
INTERACTION OF OXYGEN AND HYDROGEN WITH Pd-Au ALLOYS: AN AES AND XPS STUDY L. HILAIRE, P. LfiGARl$
Y. HOLL and G. MAIRE
Institut de Chimie, Universitk Louis Pasteur, CNRS, ERA 385, 4, rue Blaise Pascal, F-6 7000 Strasbourg, France
Received 19 February 1980; accepted for publication 16 September 1980
Pd-Au alloys of three different concentrations have been studied with Auger Electron Spectroscopy and Photoelectron Spectroscopy. A quantitative method shows that the decontaminated surfaces possess the same composition as the bulk. Interaction of oxygen up to 600°C with gold rich alloys (>85%) is very weak. It starts around 300°C on the other two alloys and results in a significant palladium surface enrichment. On the 60 at% Pd-Au alloy, at 500580°C, the surface may be completely covered with PdO. A hydrogen treatment at 350°C leads to a complete reduction of the oxide without reequilibration of the surface.
1. Introduction During the past twenty years a lot of investigations have been done on alloys. Particularly important is the knowledge of their surface composition which may be in many binary alloys quite different from the bulk concentration [ 11. The development of modern surface techniques, especially photoelectron and Auger electron spectroscopies, have made available unique tools for this purpose provided that electrons of suitable inelastic mean free path are chosen which enable one to really probe the surface. Quantitative measurements are reliable only if a proper calibration method is developed. It may rely upon comparison with the signal intensities of pure elements [2] and absolute measurements can be made, based on an exponential decrease of the Auger signal [3]. Corrections for back scattered [4] and forward scattered electrons [5], or more generally, to all matrix effects [6] may be included. Various approaches have been developed to predict surface segregation in alloys. If strain energy may be neglected, that is if the lattice constants of the constituents are nearly equal [7], the simplest model is the regular solution theory which is based on differences in surface energies on the components [8,9]. Some refinements may be brought to this model, including for example corrections for surface relaxation effects [IO] . In spite of the interest of palladium, for example in catalysis, relatively few 125
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oJ’oxygen
md hydrogen
with Pd -Au
studies have been devoted to Pd-Au. It is a simple system for it presents a continuous solid solution without miscibility gap in the whole range of concentrations and current theories predict no or very little surface segregation in this alloy. This prediction was confirmed by Auger electron spectroscopy on microspheres and supported catalysts of different concentrations [ 1 l] and on a 22 at% Pd-Au alloy [2] and by low energy ion scattering on a 4 at% Pd-Au surface [ 121. However Somorjai et al. [13] found a definite gold surface enrichment on alloys of different concentrations. A small but significant chemisorption induced segregation due to oxygen was detected on a 22 at% Pd-Au alloy [2] but Wood and Wise [ 1 l] found conflicting results on microspheres of different concentrations pretreated in an oxygen flow. We report here on the interaction of oxygen with three different Pd-Au alloys in a wide range of temperatures and oxygen pressures including such conditions that an oxide can be formed. A few experiments with hydrogen have also been done. The techniques used were Auger electron and photoelectron spectroscopies.
2. Experimental 2.1. Apparatus Auger experiments were performed in an ultra high vacuum chamber with 4-grid optics and a Varian cylindrical mirror analyser (CMA) as Auger detector. Incident electrons of 2 kV struck the sample at normal incidence and the beam current was 10 PA. Auger spectra were all recorded at room temperature in the dA@)/d@) = f(E) mode. The alloys samples were set up on a “carrousel” type device together with pure elements, namely palladium and gold. Temperatures were measured with a Ni-Cr thermocouple which was attached to a platinum reference. Consequently, the temperatures given below are maximum temperatures and the uncertainty may reach up to 30°C. The samples could be heated up to 1200°C with an Osram infra-red lamp inserted within the experimental chamber and thoroughly outgassed. XPS experiments were done in a Vacuum Generators ESCA III spectrometer. Residual gas pollution, especially by CO, was reduced to a very low level by means of a helium cryogenic pump and a vacuum better than lo-” Torr was easily reached in the analysis chamber. The experiments were performed in a separate chamber where UHV (in the low 10 -lo Torr range) was achieved with an oil diffusion pump. The Koc transition of Mg at 1253.6 eV was used as photon source. The analyser was of the hemispheric type with a channeltron multiplier and a pass energy of 10 eV. The samples could be heated by resistivity and the temperature measured with a good accuracy by means of a chromel-alumel thermocouple attached to the sample.
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127
2.2. Samples The alloys were prepared by melting at high temperature Johnson-Matthey specpure alloy wires. They were given a circular form approximately 1 cm in diameter. Bulk compositions were checked by X-ray diffraction and X-ray fluorescence. Alloys with three different compositions, namely 60, 22 and 13 at% Pd were studied. Sputtered 400 V A.r+ions (Ar pressure 2 X 10 -4 Torr) with a collected current of 10 PA were used in Auger for the cleaning procedure. In the XPS apparatus the conditions were less smooth: 3 kV Ar’ ions, collected current 60 PA. A few cycles of ion bombardment (5-10 min) followed by annealing at 600°C (15 min) were sufficient to clean the Pd-Au samples and to equilibrate them as already shown by Somorjai [ 131. 2.3. Transitions The main Auger transitions of Pd and Au are the M45N45N45 327 eV and N704s04s 69 eV peaks respectively. They are well suited for this study because they correspond to a reasonably low escape depth of the emitted electrons as can be seen from the well-known “universal curve” [ 141: approximately 2 layers for Au and 3-4 for Pd. Care must be taken to thoroughly clean the samples since the 69 eV transition of Au is of the XW type; it may be therefore very sensitive to surface contaminants. In this respect a difficulty may be expected since a small 276 eV Pd transition overlaps the 272 eV carbon peak. However, the resolution of the CMA is good enough to make the carbon peak appear as a definite shoulder. Furthermore small amounts of carbon do not affect the height of the 327 eV Pd and 69 eV Au peaks while large quantities of C cause a dramatic decrease of the latter. In XPS the strong transitions are the 3d doublet at 335-340 eV (in binding energy) and the 3p peak at 531 eV for palladium and the 4f doublet at 83-87 eV for gold. As the incident energy is 1253.6 eV it turns out that these transitions correspond to escape depths lying in the range 700-1100 eV (in kinetic energy), which is substantially higher than in Auger. The thickness of the sample probed in this way may be estimated to -5-7 atom layers, when the angle between the normal to the surface and the trajectory of the electrons is small (0 - 20”). To cope with this problem we have also analysed electrons of low take-off angles (0 - 75”). In these conditions the escape depth was comparable to what we had in Auger. 2.4. Calculations of the concentrations 2.4.1. Mean concentrations In first approximation, for a given alloy AB, if I, and In are the intensities of two chosen transitions of A and B respectively in the Auger or XPS spectrum of the
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alloy, the surface concentration CA =IA/(IA+QA/dB)
of oxygen
and hydrogen
of the constituent
with P&Au
A may be given by:
9
where &A/n is the ratio of the same transitions in the spectra of pure A and B recorded in identical conditions [2]. In Auger spectra “intensity” refers to peak-topeak heights. In XPS spectra the relative area of the peaks was measured by weighing. We understand that this method of calculation is a crude one. However it is worth noting that correction coefficients including matrix effects recently published [6] are quite close to unity for the present Pd-Au system. In Auger an averagk of over 30 spectra gave Qpd/Au = 1.76 + 0.10 for the Pd M45N45N45 transition at 327 eV and the Au N704s04s peak at 69 eV. From Palmberg’s Handbook we deduced a value (Y= 1.71, but the incident energy of the electrons was 3 kV [ 151. In XPS we found (Yp,J/A, = 1.08. The transitions chosen were Pd 3d3” and Au 4f5” to avoid overlapping with other peaks. The problem of attenuation of the peaks by oxygen must be considered. We have checked on pure gold and palladium that attenuation was very low, about lo%, and nearly the same on both metals, so that it can be neglected, when small amounts of oxygen were present on the surface. This was the case in most of the Auger experiments. When palladium oxide was formed the palladium peaks were reduced by a factor of up to 1.8, whilst Au peaks were not affected. One may think that in these conditions our calculations minimize the palladium concentration. However one must realize that the Pd atom density in PdO is 1.7 time less than that of metallic palladium. Therefore it does not make sense to introduce a coefficient of attenuation in our calculations. 2.4.2. Concentration profiles As the preceding method gives only estimations of a mean composition on several layers, we have tried to calculate concentration profiles, with, in particular, the purpose of getting a better value of the concentration on the very first layers. The method has been developed for R-Ni and is being published with more details in another paper [ 161. It is based on the wellknown exponential decrease of the Auger signal which gives for a pure element: I pure =1&l
- e--l?
,
where h is the inelastic mean free path of the electrons and lo the non-attenuated signal which would be given by a single layer. The signal recorded for the same metal in the alloy will be:
Zdlll0y= Ipure(1 - e-1/h) C Ci e-j/’
,
i=O
where Ci is the concentration on the ith layer. The calculations consist in a comparison by iteration
of the experimental
signal
L. Hilaire et al. /Interaction of oxygen and hydrogen with Pd-Au
129
recorded on the alloy with a spectrum calculated theoretically: varying Ci allows to find the best fit between the two methods. The solution obtained in this way is of course not unique and Ci was varied in such a way that a reasonably quick convergence gave a satisfactory variation of the concentration.
3. Results 3.1. Clean surfaces Palladium concentrations, calculated from Auger and photoelectron spectra for the clean surfaces of all three alloys are summarized in fig. 1. Experimental points are values averaged on a number of measurements varying from 12 (for 13 at% PdAu; XPS) to 38 (60 at% Pd-Au; AES) and the bars reflect the scatter of the results obtained in this way. XPS spectra taken in either position of the sample gave identical surface compositions. On the same figure are reported surface concentrations calculated from spectra taken immediately after an ion bombardment of the sample, that is before anneal-
(surface)
lOO._
60 .-
40 '-
20 .-
C
60
30
100
atom
“a Pd
(bulk) Fig. 1. Palladium surface composition as a function of bulk composition: (o) calculated from Auger spectra, (0) calculated from XPS spectra, (v) after ion bombardment with 400 eV A+, (a) after ion bombardment with 3.5 keV A*. The bars reflect the dispersion of the results.
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of ox)lgen and hydrogen
with Pd--Au
ing. With the ion gun used in the Auger apparatus, the palladium concentrations were 5 to 7% higher than before bombardment. In more drastic conditions the surface concentration can reach up to 23% with the 13 at% Pd-Au alloy. Concentration profiles for two of the alloys are given in fig. 2. Obviously there is no gradient of concentration in the 60 at% Pd-Au alloy. The first layer of the 22 at% Pd-Au alloy seems slightly richer in gold than the bulk but the small value of the enrichment (4%), although fairly reproducible, may not be very significant.
Fig. 2. Concentration profiles calculated from Auger spectra: (1) clean 22at% Pd-Au; (2) 1 Torr 02, 5OO”C, 60 min; (3) clean 60 at% Pd.-Au; (4) 2 Torr 02, 500°C, 15 min; (5) 2 Torr 02,5OO”C, 60 min; (6) 1 Torr 02,6OO”C, 30 min.
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131
3.2. Interaction with oxygen: Auger results The interaction of oxygen with all three alloys has been studied in a wide range of pressures (10e6 to 1 atm) and temperatures (25 to 600°C). At room temperature no adsorption was detected. Interaction of oxygen with the 60 at% Pd-Au alloy (fig. 3) became noticeable at 300°C provided that the oxygen pressure was not too low: there was no adsorption at 10e4 Torr but with 1 Torr at the same temperature an appreciable amount of oxygen was chemisorbed. Actually the adsorption was really important at 500°C with pressures at least equivalent to 1 Torr. At temperatures higher than 6OO”C, whatever the pressure might be, no adsorption occurred. As can be seen on the figure adsorption of oxygen always resulted in a surface segregation of palladium. Concentrations of up to 90% could be reached. This surface enrichment was important only at 500-600°C for oxygen pressures of 1 Torr or more. Concentration profiles from Auger spectra corresponding to experiments of the upper part of the curve (fig. 3) are given in fig. 2. Values of the palladium surface concentrations obtained in this way were substantially higher for the very first layer and could even reach up to 100% (curve 6). Surface enrichment is shown to be significant only on the 5 or 6 upper atomic layers of the sample. In a previous paper experiments on the 22 at% Pd-Au alloy [2] at one tempera-
%c: , 90 .
80
IO
.
I
.3
+
Fig. 3.60 at% Pd-Au alloy + 02. Palladium surface concentrations calculated from Auger spectra as a function of oxygen peak-to-peak heights in arbitrary units: (1) 10e6 Torr, WC, 60 min; (2) lo4 Torr, 300°C, 15 min; (3) 3 X low2 Torr, SOO”C, 15 min; (4) 2 Torr, 5OO”C, 15 min; (5) 1 Torr, 3OO”C, 45 min; (6) 2 Torr, .5OO”C, 60 min; (7) 1 Torr, 6OO”C, 30 min.
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of oxygen and hydrogen with Pd-Au
"0
Fig. 4. 22 at% Pd-Au alloy + 02. Palladium surface concentrations calculated from Auger spectra as a function of oxygen peak-to-peak heights in arbitrary units: (i)) and (0) this paper; (+) ref. [2]; (1) 2 Torr, 25”C, 60 min; (2) 1 atm, 3OO”C, 60 min; (3) 1 atm, 3OO”C, 7 h; (4) 1 atm, 35O”C, 8 h; (5) lo-’ Torr, 5OO”C, 60 min; (6) 1 Torr, SOO”C, 60 min; (7) 0.4 Torr. 7OO”C, 40 min; (8) 7 + 15 min at 800°C.
ture, 5OO”C, were shown to give the same features: there was a definite relationship between the amount of oxygen chemisorbed on the surface and the palladium surface segregation. Some further experiments were done on this alloy at lower temperatures (fig. 4) with the same conclusion. The concentration profile of fig. 2 (curve 2) shows that only one layer was involved in the segregation process. On fig. 4 results of experiments done at higher temperatures are far away from the curve. At 7OO”C, the amount of oxygen on the surface was less than at 300500°C and no surface enrichment occurred. Prolonged heating, up to 800°C after an experiment at 700°C (but not at lower temperatures) resulted in an increase of the amount of oxygen on the surface without modification of the surface composition. The oxygen disappeared after heating at 800-850°C. Chemisorption of oxygen on the 13 at% Pd-Au alloy was much more difficult. It was only possible in a narrow range of temperatures, 500-6OO”C, if the pressure was at least a few Torr. The amount of oxygen on the surface was small and the palladium surface enrichment, hardly significant, but still reproducible, was only 4%. 3.3. XPS results The interaction of oxygen has been studied in XPS, at least with the 60 at% PdAu alloy, in such experimental conditions, that drastic effects on the surface composition could be expected (500-55O”C, a few Torr or more), as deduced from Auger experiments (fig. 3, upper part of the curve). Furthermore, we knew that in these conditions palladium oxide is easily formed with pure palladium [ 17 1.
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L. Hilaire et al. f Interaction of oxygen and hydrogen with Pd-Au
Table 1 Variation of the calculated palladium surface concentration 60% Pd-40% Au ahoy (XPS results)
in two series of treatments on the
Surface treatment
CPd (%) Surface treatment
Decontaminated Pd-Au 2 Torr Oa,52O”C, 7.5 min, 0 = 20” e = 75’ 375°C 1 min, 0 = 20” e = 75” 400°C a few min, 0 = 20” 6OO”C, 8 min, 0 = 20” 0 = 75” IB, 6OO”C, 5 mm, 0 = 20”
61.6 80.2 89.3 72 83 69.9 67.8 68.5 64.1
Decontaminated Pd-Au 1 atm 02,54O”C, 11 h, 0 = 20” 0 = 75” 300°C 2 min, 0 = 20” e = 75” 1 atm Hz, 3OO”C, 11 h, 0 = 20” e = 75” 5OO”C, 20 min, e = 20” IB, 600°C, 15 min, 0 = 20”
CPd (%) 61.4 100 100 100 100 100 100 79.7 67
Two types of experiments on the 60 at% Pd-Au alloy are presented in this paper. Firstly this surface was treated with 2 Torr of oxygen at 520°C for 75 min. Modifications of the surface composition, 3d peaks and the conduction band of palladium are given in table 1, fig. 5 and fig. 6. The analysis of the 0 1s transition was difficult since it overlapped the Pd 3p peak. After exposure to oxygen the calculated surface composition of palladium was 80.2 and 98.3% for 0 = 20’ and 75” respectively. Each peak of the Pd 3d doublet (fig. 5) was splitted with shoulders at energies corresponding to the original Pd peaks (334.8 and 340.2 eV) and maxima shifted of nearly 2 eV towards higher energies (336.7 and 342.1 ev). It is worth noting that for 0 = 75” the shoulders A and B were definitely less intense than for 0 = 20”. The Au 4f doublet remained unaffected in shape and energy. In fig. 6, the conduction bands of this alloy, pure and after interaction with oxygen, are compared to those of Pd, Au and PdO. All the spectra given in this figure have been recorded without monochromator. Both Pd and Au exhibited broad bands with a main maximum at 1.5-2 eV below the Fermi level and, for Au, a supplementary hump at 6.5 eV. The band of 60 at% Pd-Au was very much alike, with a main maximum at 1.5-2 eV and a small hump at 6.5 eV. The band of PdO was much narrower and its maximum was 3 eV below the Fermi level. After interaction of the alloy with oxygen in the conditions mentioned above, the spectrum recorded with 0 = 20” revealed three accidents: a main maximum at 3.1 eV, a shoulder at 1.7 eV (not present with f3 = 75’) and a very small hump at 6.4 eV. The sample was then heated in vacuum at different temperatures from 375 to 6OO’C for very short times. Figs. 5 and 6 show that peaks identical in shape and positions to those of the surface before interaction with oxygen were restored very quickly at a temperature of about 400°C. Meanwhile the palladium surface concentration decreased but further heating at 600°C was necessary to approach the initial composition. In fact, the latter was restored by ion bombardment and annealing at 62O’C.
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I.. Hilaire et al. /A tcraction 0,f’oxygen and hydrogen with Pd- Au
Fig. 5.60 at% Pd-Au alloy + O2 photoelectron spectra of the Pd 3d transitions: (1) decontaminated 60 at% PdLAu and Pd; (2) PdO 1171;(3) 60 at % Pd--Au t 2 Torr 02, 52O’C. 15 min, 0 = 20”; (4) @ = 75”; (5) 1 min at 375”C, 0 = 20”; (6) B = 75”; (7) a few minutes at 400°C. H = 20” and 0 = 75”; (8) 60 at%, Pd-Au + 1 atm 02, 540°C. 11 11, 6 = 20” and 6 = 75”; (9) 2 min at 3OO”C, 0 = 20”; (10) I atm Hz, 3OO”C, 11 h. 0 = 20” and 0 = 75”.
I,. Hilaire et al. /Interaction
of oxygen and hydrogen with Pd-Au
135
Fig. 6.60 at% Pd-Au alloy + 02, Pd, Au, PdO. Photoelectron spectra of the conduction bands (the heights of the peaks are in arbitrary units): (1) Pd; (2) PdO [ 171; (3) Au; (4) decontaminated 60 at% Pd-Au; (5) 60 at% Pd-Au + 2 Torr 02, 52O”C, 75 min, 0 = 20”; (6) 0 = 75’; (7) 1 min at 375°C + a few minutes at 400°C; (8) 60 at% Pd-Au + 1 atm 02, 54O”C, 11 h, e = 20” and L?= 75’; (9) 2 min at 3OO”C, 0 = 20”; (10) 1 atm Hz, 3OO”C, 11 h, B = 20” and I9 = 7s”.
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L. Hilairc et al. /Interaction
ofoxygen
and hydrogen
with I’d- Au
Another type of experiment was done with 1 atm of oxygen at 540°C during 11 h. These pressure and temperature are the actual conditions of operation commonly used in catalytic processes. After exposure the gold peaks completely disappeared and each transition of the Pd 3d doublet was shifted of about 2 eV from 334.5 and 340 eV to 336.5 and 341.8 eV (fig. 5). As in the previous experiments the position of the peaks was not modified by changing 0 from 20 to 75”. The conduction band showed a narrow peak at 2.9 eV followed by two very small humps at 5.5 and 7 .O eV below the Fermi level (fig. 6). Heating the sample for 2 min at 300°C did not change the composition: no gold peak could be detected..But shoulders at 334.6 and 339.9 eV appeared in the Pd 3d doublet. In the conduction band a very small shoulder was detected at -1.2 eV below the Fermi level. The sample was then heated overnight at 300°C in the presence of hydrogen. Again no gold peak could be detected. But the Pd 3d appeared at 334.6 and 339.9 eV with no shoulders like in the clean surface. A broad conduction band appeared with a single maximum at -2.0 eV. Prolonged heating at 600°C was necessary to restore the initial composition. Experiments at lower (45O’C) or higher (600°C) temperatures did not result in any modification of the composition and of the shape and energy of the Au 4f and Pd 3d transitions. Several experiments were done with the 13 at% Pd-Au alloy at 1 atm of oxygen from 400 to 700°C during several hours. These experiments were all negative: no oxygen was detected on the surface and no modification occurred in the composition and in the main XPS peaks of p~ladium and gold. In a last experiment the same sample was ion bombarded with 5 keV Ar* ions during 6 min. The calculated palladium surface concentration increased from 9.9 to 22.7%. The surface was then exposed, before annealing, to 1 atm of oxygen at 350°C for 6 h. The calculated concentration was then 20.2%. The 3d doublet exhibited two strong maxima at 335 and 340.3 eV with two small shoulders at 336.3 and 341.7 eV.
4. Discussion 4. I. Clean surfaces AES and XPS results (fig. 1) give for all three alloys surface concentrations which are in reasonable agreement with bulk compositions within experimental errors. Our values agree well with preceding calculations pub~shed by Wood and Wise [ll], Biloen et al. [12] and ourselves [2]. But Somorjai et al. 1131 found a considerable gold enrichment with clean Pd-Au alloys of different concentrations. This result is surprising since it is in contradiction with the regular solution model [9] which predicts no or very little surface enrichment with this system since
L. Hilaire et al. /Interaction
of oxygen and hydrogen
with Pd-Au
137
AH& = 88 kcal/mole and AH:& = 90 kcal/mole. Ion bombardment (fig. 1) leads to a small palladium surface enrichment: 5% with low energy ions, up to lo-12% with high energy ions. These results are consistent with values of sputtering yields given in the literature: 2.1 and 2.4 atoms/ ion for palladium and gold respectively with 500 eV Ar’ ions [ 181. 4.2. 60 at% Pd-Au alloy Depending on the range of temperature studied the interaction with oxygen may lead to different types of results. The most dramatic effects take place in a narrow range of temperature from 500 to 570°C approximately. If the oxygen pressure is at least 1 Torr, if time exposures are long enough, palladium surface concentrations up to 90% can be reached (fig. 3) and even 100% if one works at atmospheric pressure of oxygen. It is not surprising that palladium and not gold migrates to the surface since the Pd-0 bond is expected to be more easily formed than the Au-O bond in these experimental conditions. Examination of XPS spectra reveals interesting trends. The modifications of the conduction band are a clear indication of real oxidation of the surface (fig. 6, 1 Torr, 520°C): the band resembles very much, in position of the maximum and width, the one of PdO, as observed in a parallel study on pure Pd [17], with two humps due to residual gold (6.4 eV) and metallic palladium (1.7 eV); the latter is not detected when electrons issue from the surface at grazing angle (0 = 75’) which means that the oxide is preferentially formed at the surface. In the Pd 3d doublet (fig. 5), the peaks are shifted 2 eV towards higher energies but metallic palladium is still present and appears as shoulders. The height of the oxide peaks is, comparatively to the metallic humps, higher for 0 = 75” than for 0 = 20”. The same conclusions may be inferred from the modifications of the Pd 3d transition. Oxidation of the surface is connected with a gold depletion: this is clearly shown by the calculation of the palladium surface concentration since we find 82% for 19= 20” and 89% for 8 = 75’. Subsequent heatings of the surface (table 1, figs. 5 and 6) allows a reduction of the oxide which is completed after a few minutes at 400°C. However subsequent annealings at 600°C are necessary to restore the original atomic composition. Oxidation is total with 1 atm O2 at 540°C. The Pd 3d doublet is not splitted but shifted of -2 eV towards higher binding energies. The conduction band exhibits a single maximum at 2.0 eV and is characteristic of PdO [ 171. This result is hardly surprising but it is important to note that the temperature range or oxidation is very narrow (500-600°C) and is exactly the same with Pd and the alloy. This is different from what happens with Pd-Pt [ 191. In all these experiments no modifications of the Au 4f transitions were ever observed. This is not surprising since interaction of oxygen with gold occurs at much higher temperatures [20,21].
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of oxygen and hydrogen
with P&Au
Interaction of the oxidized surface with h$drogen at 300°C is most interesting. Examination of the Pd 3p and 3d peaks and the conduction band reveals that this treatment leads to a complete reduction of the oxide. However, the gold peaks lack completely in the spectrum which means that the surface is 100% metallic palladium. Further heatings at higher temperatures (500-600°C) are necessary to restore the initial surface composition. This result has important implications in catalysis. Supported catalysts are commonly regenerated or treated before use in an oxygen or air stream and then reduced with hydrogen. Such a treatment gave to O’Cinneide and Gault [22] surp~sin~y high values of activity with Pd-Au alloys. In view of our exceriments, the explanation is straightforward: the catalysts were enriched in palladium by the air treatment, completely reduced by hydrogen, but the temperature of reduction, 350°C, was too low for reequilibration of the surface. Interaction of oxygen with palladium at temperatures lower than 500°C did not get any modi~cation of the Pd 3p and 3d peaks or of the conduction band. Clearly no oxide was formed. However, as seen from Auger results (fig. 3), chemisorption took place and led to a substantial increase of the palladium surface concentration provided the pressure was at least 1 Torr and the temperature not lower than 3ooOc. No experiment was done in Auger at temperatures higher than 6OO*C but, at this temperature, no oxidation of Pd or Au could be observed in XPS spectra. 4.3. 22 at% Pd-Au
alloy
The results at 300 and 400°C (fig. 4) are quite consistent with experiments done at 500°C and already published [2] : interaction of this alloy with oxygen leads to a modest but significant palladium surface enrichment; it exists a nearly linear relationship of the palladium surface concentration as a function of oxygen present on the surface. Results at higher temperature are quite different. At 700°C oxygen chemisorbs on the surface but no surface segregation occurs, It must be recalled that at this temperature p~adlum itself does not interact with oxygen. However, chemisorption of oxygen with gold becomes important in this range of temperature (up to 800°C). Furthermore we have shown in a parallel study of pure gold [21] that this metal is able to dissolve appreciable amounts of oxygen which can later on migrate to the surface on annealing. We believe that these phenomena are able to explain our results on this gold-rich alloy at 700°C: Au and not Pd participates to the chemisorption bond, which explains that no p~adium segregation occurs; meanwhile some oxygen is dissolved in the matrix and further heating of the alloy leads to an increase of oxygen surface concentration.
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139
4.4. 13 at% Pd-Au alloy The interaction of this alloy with oxygen, studied in a wide range of temperature (25 to 600°C) is very weak. The only positive experiment led to the fixation of a small quantity of oxygen on the surface at 520°C with a very modest palladium enrichment. It seems that with Pd-Au alloys it exists a threshold of palladium concentration (around -15%) under which oxidation or even chemisorption is not possible. A partial oxidation, at 35O”C, was only possible with a previously enriched surface (up to 23%) by ion bombardment, as can be inferred from the small shoulders accompanying the Pd 3d XPS peaks. One may have expected some palladium surface enrichment but there is none. In fact this is easily understandable: as we have seen above, the segregation is connected to oxidation and the layers just under this surface have too low a concentration in palladium atoms to be oxidized. The temperature of this experiment is surprisingly low since, as we have already pointed out, no oxidation occurs with the 60 at% Pd-Au alloy under 5OO’C. The discrepancy may come from the special character, highly disorganized, of the surface created by ion bombardment. In conclusion, we will emphasize the importance of such studies for the knowledge of actual surfaces which take part to reaction processes such as catalytic ones. In particular common treatments like oxidation and reduction, depending on the conditions of temperature and pressure, may completely change the composition of alloy surfaces.
Acknowledgments The authors thank Dr. G. Krill for helpful discussions and Mr. P. Bernhardt for very skilful technical assistance.
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of’ oxygen and hydrogen
with Pd.-Au
1121 P. Biloen, R. Bouwman, R.A. van Santen and H.H. Urongersma, Appl. Surface Sci. 2 (1979) 532. 1131 A. Jablonski, S.H. Overbury and G.A. Somorjai, Surface Sci. 65 (1977) 578. [ 141 A. Joshi, LE. Davis and P.W. Palmberg, in: Methods of Surface Analysis. Ed. A.W. Czanderna (Elsevier, Amsterdam, 1975). 1151 P.W. Palmberg, Handbook of Auger Electron Spectroscopy (Physical Electronics, Edina. Minnesota, 1976). [ 161 J. Sedlacek, L. Hilaire, P. L&are and G. Maire, to be published. [ 171 Y. Hall, G. Krill, A. Amamou, 1,. Hilaire, P. Legare and G. Maire, Solid State Commun. 32 (1979) 1189. [ 181G.K. Wehnet, in: Methods of Surface Analysis, Ed. A.W. Czanderna (Elsevier, Amsterdal~t, 1975). [ 191 F. Weisang, L. Hilaire. P. LegarC and G. Maire. unpublished results. [20] M. Schrader, Surface Sci. 78 (1978) L227. [21] P. Legare, L. Hilaire, M. Sotto and G. Maire, Surface Sci. 91 (1980) 175. [ 221 A. OTinneide and F.C. Gault, J. Catalysis 37 (1975) 311.
INTERACTION OF OXYGEN AND HYDROGEN WITH Pd-Au ALLOYS: AN AES AND XPS STUDY L. HILAIRE, P. LfiGARl$
Y. HOLL and G. MAIRE
Institut de Chimie, Universitk Louis Pasteur, CNRS, ERA 385, 4, rue Blaise Pascal, F-6 7000 Strasbourg, France
Received 19 February 1980; accepted for publication 16 September 1980
Pd-Au alloys of three different concentrations have been studied with Auger Electron Spectroscopy and Photoelectron Spectroscopy. A quantitative method shows that the decontaminated surfaces possess the same composition as the bulk. Interaction of oxygen up to 600°C with gold rich alloys (>85%) is very weak. It starts around 300°C on the other two alloys and results in a significant palladium surface enrichment. On the 60 at% Pd-Au alloy, at 500580°C, the surface may be completely covered with PdO. A hydrogen treatment at 350°C leads to a complete reduction of the oxide without reequilibration of the surface.
1. Introduction During the past twenty years a lot of investigations have been done on alloys. Particularly important is the knowledge of their surface composition which may be in many binary alloys quite different from the bulk concentration [ 11. The development of modern surface techniques, especially photoelectron and Auger electron spectroscopies, have made available unique tools for this purpose provided that electrons of suitable inelastic mean free path are chosen which enable one to really probe the surface. Quantitative measurements are reliable only if a proper calibration method is developed. It may rely upon comparison with the signal intensities of pure elements [2] and absolute measurements can be made, based on an exponential decrease of the Auger signal [3]. Corrections for back scattered [4] and forward scattered electrons [5], or more generally, to all matrix effects [6] may be included. Various approaches have been developed to predict surface segregation in alloys. If strain energy may be neglected, that is if the lattice constants of the constituents are nearly equal [7], the simplest model is the regular solution theory which is based on differences in surface energies on the components [8,9]. Some refinements may be brought to this model, including for example corrections for surface relaxation effects [IO] . In spite of the interest of palladium, for example in catalysis, relatively few 125
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L. Hilairr et al. /Interaction
oJ’oxygen
md hydrogen
with Pd -Au
studies have been devoted to Pd-Au. It is a simple system for it presents a continuous solid solution without miscibility gap in the whole range of concentrations and current theories predict no or very little surface segregation in this alloy. This prediction was confirmed by Auger electron spectroscopy on microspheres and supported catalysts of different concentrations [ 1 l] and on a 22 at% Pd-Au alloy [2] and by low energy ion scattering on a 4 at% Pd-Au surface [ 121. However Somorjai et al. [13] found a definite gold surface enrichment on alloys of different concentrations. A small but significant chemisorption induced segregation due to oxygen was detected on a 22 at% Pd-Au alloy [2] but Wood and Wise [ 1 l] found conflicting results on microspheres of different concentrations pretreated in an oxygen flow. We report here on the interaction of oxygen with three different Pd-Au alloys in a wide range of temperatures and oxygen pressures including such conditions that an oxide can be formed. A few experiments with hydrogen have also been done. The techniques used were Auger electron and photoelectron spectroscopies.
2. Experimental 2.1. Apparatus Auger experiments were performed in an ultra high vacuum chamber with 4-grid optics and a Varian cylindrical mirror analyser (CMA) as Auger detector. Incident electrons of 2 kV struck the sample at normal incidence and the beam current was 10 PA. Auger spectra were all recorded at room temperature in the dA@)/d@) = f(E) mode. The alloys samples were set up on a “carrousel” type device together with pure elements, namely palladium and gold. Temperatures were measured with a Ni-Cr thermocouple which was attached to a platinum reference. Consequently, the temperatures given below are maximum temperatures and the uncertainty may reach up to 30°C. The samples could be heated up to 1200°C with an Osram infra-red lamp inserted within the experimental chamber and thoroughly outgassed. XPS experiments were done in a Vacuum Generators ESCA III spectrometer. Residual gas pollution, especially by CO, was reduced to a very low level by means of a helium cryogenic pump and a vacuum better than lo-” Torr was easily reached in the analysis chamber. The experiments were performed in a separate chamber where UHV (in the low 10 -lo Torr range) was achieved with an oil diffusion pump. The Koc transition of Mg at 1253.6 eV was used as photon source. The analyser was of the hemispheric type with a channeltron multiplier and a pass energy of 10 eV. The samples could be heated by resistivity and the temperature measured with a good accuracy by means of a chromel-alumel thermocouple attached to the sample.
L. Hilaire et al. /Interaction
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127
2.2. Samples The alloys were prepared by melting at high temperature Johnson-Matthey specpure alloy wires. They were given a circular form approximately 1 cm in diameter. Bulk compositions were checked by X-ray diffraction and X-ray fluorescence. Alloys with three different compositions, namely 60, 22 and 13 at% Pd were studied. Sputtered 400 V A.r+ions (Ar pressure 2 X 10 -4 Torr) with a collected current of 10 PA were used in Auger for the cleaning procedure. In the XPS apparatus the conditions were less smooth: 3 kV Ar’ ions, collected current 60 PA. A few cycles of ion bombardment (5-10 min) followed by annealing at 600°C (15 min) were sufficient to clean the Pd-Au samples and to equilibrate them as already shown by Somorjai [ 131. 2.3. Transitions The main Auger transitions of Pd and Au are the M45N45N45 327 eV and N704s04s 69 eV peaks respectively. They are well suited for this study because they correspond to a reasonably low escape depth of the emitted electrons as can be seen from the well-known “universal curve” [ 141: approximately 2 layers for Au and 3-4 for Pd. Care must be taken to thoroughly clean the samples since the 69 eV transition of Au is of the XW type; it may be therefore very sensitive to surface contaminants. In this respect a difficulty may be expected since a small 276 eV Pd transition overlaps the 272 eV carbon peak. However, the resolution of the CMA is good enough to make the carbon peak appear as a definite shoulder. Furthermore small amounts of carbon do not affect the height of the 327 eV Pd and 69 eV Au peaks while large quantities of C cause a dramatic decrease of the latter. In XPS the strong transitions are the 3d doublet at 335-340 eV (in binding energy) and the 3p peak at 531 eV for palladium and the 4f doublet at 83-87 eV for gold. As the incident energy is 1253.6 eV it turns out that these transitions correspond to escape depths lying in the range 700-1100 eV (in kinetic energy), which is substantially higher than in Auger. The thickness of the sample probed in this way may be estimated to -5-7 atom layers, when the angle between the normal to the surface and the trajectory of the electrons is small (0 - 20”). To cope with this problem we have also analysed electrons of low take-off angles (0 - 75”). In these conditions the escape depth was comparable to what we had in Auger. 2.4. Calculations of the concentrations 2.4.1. Mean concentrations In first approximation, for a given alloy AB, if I, and In are the intensities of two chosen transitions of A and B respectively in the Auger or XPS spectrum of the
128
L. Hilaire et al. /Interaction
alloy, the surface concentration CA =IA/(IA+QA/dB)
of oxygen
and hydrogen
of the constituent
with P&Au
A may be given by:
9
where &A/n is the ratio of the same transitions in the spectra of pure A and B recorded in identical conditions [2]. In Auger spectra “intensity” refers to peak-topeak heights. In XPS spectra the relative area of the peaks was measured by weighing. We understand that this method of calculation is a crude one. However it is worth noting that correction coefficients including matrix effects recently published [6] are quite close to unity for the present Pd-Au system. In Auger an averagk of over 30 spectra gave Qpd/Au = 1.76 + 0.10 for the Pd M45N45N45 transition at 327 eV and the Au N704s04s peak at 69 eV. From Palmberg’s Handbook we deduced a value (Y= 1.71, but the incident energy of the electrons was 3 kV [ 151. In XPS we found (Yp,J/A, = 1.08. The transitions chosen were Pd 3d3” and Au 4f5” to avoid overlapping with other peaks. The problem of attenuation of the peaks by oxygen must be considered. We have checked on pure gold and palladium that attenuation was very low, about lo%, and nearly the same on both metals, so that it can be neglected, when small amounts of oxygen were present on the surface. This was the case in most of the Auger experiments. When palladium oxide was formed the palladium peaks were reduced by a factor of up to 1.8, whilst Au peaks were not affected. One may think that in these conditions our calculations minimize the palladium concentration. However one must realize that the Pd atom density in PdO is 1.7 time less than that of metallic palladium. Therefore it does not make sense to introduce a coefficient of attenuation in our calculations. 2.4.2. Concentration profiles As the preceding method gives only estimations of a mean composition on several layers, we have tried to calculate concentration profiles, with, in particular, the purpose of getting a better value of the concentration on the very first layers. The method has been developed for R-Ni and is being published with more details in another paper [ 161. It is based on the wellknown exponential decrease of the Auger signal which gives for a pure element: I pure =1&l
- e--l?
,
where h is the inelastic mean free path of the electrons and lo the non-attenuated signal which would be given by a single layer. The signal recorded for the same metal in the alloy will be:
Zdlll0y= Ipure(1 - e-1/h) C Ci e-j/’
,
i=O
where Ci is the concentration on the ith layer. The calculations consist in a comparison by iteration
of the experimental
signal
L. Hilaire et al. /Interaction of oxygen and hydrogen with Pd-Au
129
recorded on the alloy with a spectrum calculated theoretically: varying Ci allows to find the best fit between the two methods. The solution obtained in this way is of course not unique and Ci was varied in such a way that a reasonably quick convergence gave a satisfactory variation of the concentration.
3. Results 3.1. Clean surfaces Palladium concentrations, calculated from Auger and photoelectron spectra for the clean surfaces of all three alloys are summarized in fig. 1. Experimental points are values averaged on a number of measurements varying from 12 (for 13 at% PdAu; XPS) to 38 (60 at% Pd-Au; AES) and the bars reflect the scatter of the results obtained in this way. XPS spectra taken in either position of the sample gave identical surface compositions. On the same figure are reported surface concentrations calculated from spectra taken immediately after an ion bombardment of the sample, that is before anneal-
(surface)
lOO._
60 .-
40 '-
20 .-
C
60
30
100
atom
“a Pd
(bulk) Fig. 1. Palladium surface composition as a function of bulk composition: (o) calculated from Auger spectra, (0) calculated from XPS spectra, (v) after ion bombardment with 400 eV A+, (a) after ion bombardment with 3.5 keV A*. The bars reflect the dispersion of the results.
130
L. Hilaire et al. / Interactiorz
of ox)lgen and hydrogen
with Pd--Au
ing. With the ion gun used in the Auger apparatus, the palladium concentrations were 5 to 7% higher than before bombardment. In more drastic conditions the surface concentration can reach up to 23% with the 13 at% Pd-Au alloy. Concentration profiles for two of the alloys are given in fig. 2. Obviously there is no gradient of concentration in the 60 at% Pd-Au alloy. The first layer of the 22 at% Pd-Au alloy seems slightly richer in gold than the bulk but the small value of the enrichment (4%), although fairly reproducible, may not be very significant.
Fig. 2. Concentration profiles calculated from Auger spectra: (1) clean 22at% Pd-Au; (2) 1 Torr 02, 5OO”C, 60 min; (3) clean 60 at% Pd.-Au; (4) 2 Torr 02, 500°C, 15 min; (5) 2 Torr 02,5OO”C, 60 min; (6) 1 Torr 02,6OO”C, 30 min.
L. Hilaire et al. /Interaction
of oxygen and hydrogen with Pd-Au
131
3.2. Interaction with oxygen: Auger results The interaction of oxygen with all three alloys has been studied in a wide range of pressures (10e6 to 1 atm) and temperatures (25 to 600°C). At room temperature no adsorption was detected. Interaction of oxygen with the 60 at% Pd-Au alloy (fig. 3) became noticeable at 300°C provided that the oxygen pressure was not too low: there was no adsorption at 10e4 Torr but with 1 Torr at the same temperature an appreciable amount of oxygen was chemisorbed. Actually the adsorption was really important at 500°C with pressures at least equivalent to 1 Torr. At temperatures higher than 6OO”C, whatever the pressure might be, no adsorption occurred. As can be seen on the figure adsorption of oxygen always resulted in a surface segregation of palladium. Concentrations of up to 90% could be reached. This surface enrichment was important only at 500-600°C for oxygen pressures of 1 Torr or more. Concentration profiles from Auger spectra corresponding to experiments of the upper part of the curve (fig. 3) are given in fig. 2. Values of the palladium surface concentrations obtained in this way were substantially higher for the very first layer and could even reach up to 100% (curve 6). Surface enrichment is shown to be significant only on the 5 or 6 upper atomic layers of the sample. In a previous paper experiments on the 22 at% Pd-Au alloy [2] at one tempera-
%c: , 90 .
80
IO
.
I
.3
+
Fig. 3.60 at% Pd-Au alloy + 02. Palladium surface concentrations calculated from Auger spectra as a function of oxygen peak-to-peak heights in arbitrary units: (1) 10e6 Torr, WC, 60 min; (2) lo4 Torr, 300°C, 15 min; (3) 3 X low2 Torr, SOO”C, 15 min; (4) 2 Torr, 5OO”C, 15 min; (5) 1 Torr, 3OO”C, 45 min; (6) 2 Torr, .5OO”C, 60 min; (7) 1 Torr, 6OO”C, 30 min.
132
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of oxygen and hydrogen with Pd-Au
"0
Fig. 4. 22 at% Pd-Au alloy + 02. Palladium surface concentrations calculated from Auger spectra as a function of oxygen peak-to-peak heights in arbitrary units: (i)) and (0) this paper; (+) ref. [2]; (1) 2 Torr, 25”C, 60 min; (2) 1 atm, 3OO”C, 60 min; (3) 1 atm, 3OO”C, 7 h; (4) 1 atm, 35O”C, 8 h; (5) lo-’ Torr, 5OO”C, 60 min; (6) 1 Torr, SOO”C, 60 min; (7) 0.4 Torr. 7OO”C, 40 min; (8) 7 + 15 min at 800°C.
ture, 5OO”C, were shown to give the same features: there was a definite relationship between the amount of oxygen chemisorbed on the surface and the palladium surface segregation. Some further experiments were done on this alloy at lower temperatures (fig. 4) with the same conclusion. The concentration profile of fig. 2 (curve 2) shows that only one layer was involved in the segregation process. On fig. 4 results of experiments done at higher temperatures are far away from the curve. At 7OO”C, the amount of oxygen on the surface was less than at 300500°C and no surface enrichment occurred. Prolonged heating, up to 800°C after an experiment at 700°C (but not at lower temperatures) resulted in an increase of the amount of oxygen on the surface without modification of the surface composition. The oxygen disappeared after heating at 800-850°C. Chemisorption of oxygen on the 13 at% Pd-Au alloy was much more difficult. It was only possible in a narrow range of temperatures, 500-6OO”C, if the pressure was at least a few Torr. The amount of oxygen on the surface was small and the palladium surface enrichment, hardly significant, but still reproducible, was only 4%. 3.3. XPS results The interaction of oxygen has been studied in XPS, at least with the 60 at% PdAu alloy, in such experimental conditions, that drastic effects on the surface composition could be expected (500-55O”C, a few Torr or more), as deduced from Auger experiments (fig. 3, upper part of the curve). Furthermore, we knew that in these conditions palladium oxide is easily formed with pure palladium [ 17 1.
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L. Hilaire et al. f Interaction of oxygen and hydrogen with Pd-Au
Table 1 Variation of the calculated palladium surface concentration 60% Pd-40% Au ahoy (XPS results)
in two series of treatments on the
Surface treatment
CPd (%) Surface treatment
Decontaminated Pd-Au 2 Torr Oa,52O”C, 7.5 min, 0 = 20” e = 75’ 375°C 1 min, 0 = 20” e = 75” 400°C a few min, 0 = 20” 6OO”C, 8 min, 0 = 20” 0 = 75” IB, 6OO”C, 5 mm, 0 = 20”
61.6 80.2 89.3 72 83 69.9 67.8 68.5 64.1
Decontaminated Pd-Au 1 atm 02,54O”C, 11 h, 0 = 20” 0 = 75” 300°C 2 min, 0 = 20” e = 75” 1 atm Hz, 3OO”C, 11 h, 0 = 20” e = 75” 5OO”C, 20 min, e = 20” IB, 600°C, 15 min, 0 = 20”
CPd (%) 61.4 100 100 100 100 100 100 79.7 67
Two types of experiments on the 60 at% Pd-Au alloy are presented in this paper. Firstly this surface was treated with 2 Torr of oxygen at 520°C for 75 min. Modifications of the surface composition, 3d peaks and the conduction band of palladium are given in table 1, fig. 5 and fig. 6. The analysis of the 0 1s transition was difficult since it overlapped the Pd 3p peak. After exposure to oxygen the calculated surface composition of palladium was 80.2 and 98.3% for 0 = 20’ and 75” respectively. Each peak of the Pd 3d doublet (fig. 5) was splitted with shoulders at energies corresponding to the original Pd peaks (334.8 and 340.2 eV) and maxima shifted of nearly 2 eV towards higher energies (336.7 and 342.1 ev). It is worth noting that for 0 = 75” the shoulders A and B were definitely less intense than for 0 = 20”. The Au 4f doublet remained unaffected in shape and energy. In fig. 6, the conduction bands of this alloy, pure and after interaction with oxygen, are compared to those of Pd, Au and PdO. All the spectra given in this figure have been recorded without monochromator. Both Pd and Au exhibited broad bands with a main maximum at 1.5-2 eV below the Fermi level and, for Au, a supplementary hump at 6.5 eV. The band of 60 at% Pd-Au was very much alike, with a main maximum at 1.5-2 eV and a small hump at 6.5 eV. The band of PdO was much narrower and its maximum was 3 eV below the Fermi level. After interaction of the alloy with oxygen in the conditions mentioned above, the spectrum recorded with 0 = 20” revealed three accidents: a main maximum at 3.1 eV, a shoulder at 1.7 eV (not present with f3 = 75’) and a very small hump at 6.4 eV. The sample was then heated in vacuum at different temperatures from 375 to 6OO’C for very short times. Figs. 5 and 6 show that peaks identical in shape and positions to those of the surface before interaction with oxygen were restored very quickly at a temperature of about 400°C. Meanwhile the palladium surface concentration decreased but further heating at 600°C was necessary to approach the initial composition. In fact, the latter was restored by ion bombardment and annealing at 62O’C.
134
I.. Hilaire et al. /A tcraction 0,f’oxygen and hydrogen with Pd- Au
Fig. 5.60 at% Pd-Au alloy + O2 photoelectron spectra of the Pd 3d transitions: (1) decontaminated 60 at% PdLAu and Pd; (2) PdO 1171;(3) 60 at % Pd--Au t 2 Torr 02, 52O’C. 15 min, 0 = 20”; (4) @ = 75”; (5) 1 min at 375”C, 0 = 20”; (6) B = 75”; (7) a few minutes at 400°C. H = 20” and 0 = 75”; (8) 60 at%, Pd-Au + 1 atm 02, 540°C. 11 11, 6 = 20” and 6 = 75”; (9) 2 min at 3OO”C, 0 = 20”; (10) I atm Hz, 3OO”C, 11 h. 0 = 20” and 0 = 75”.
I,. Hilaire et al. /Interaction
of oxygen and hydrogen with Pd-Au
135
Fig. 6.60 at% Pd-Au alloy + 02, Pd, Au, PdO. Photoelectron spectra of the conduction bands (the heights of the peaks are in arbitrary units): (1) Pd; (2) PdO [ 171; (3) Au; (4) decontaminated 60 at% Pd-Au; (5) 60 at% Pd-Au + 2 Torr 02, 52O”C, 75 min, 0 = 20”; (6) 0 = 75’; (7) 1 min at 375°C + a few minutes at 400°C; (8) 60 at% Pd-Au + 1 atm 02, 54O”C, 11 h, e = 20” and L?= 75’; (9) 2 min at 3OO”C, 0 = 20”; (10) 1 atm Hz, 3OO”C, 11 h, B = 20” and I9 = 7s”.
136
L. Hilairc et al. /Interaction
ofoxygen
and hydrogen
with I’d- Au
Another type of experiment was done with 1 atm of oxygen at 540°C during 11 h. These pressure and temperature are the actual conditions of operation commonly used in catalytic processes. After exposure the gold peaks completely disappeared and each transition of the Pd 3d doublet was shifted of about 2 eV from 334.5 and 340 eV to 336.5 and 341.8 eV (fig. 5). As in the previous experiments the position of the peaks was not modified by changing 0 from 20 to 75”. The conduction band showed a narrow peak at 2.9 eV followed by two very small humps at 5.5 and 7 .O eV below the Fermi level (fig. 6). Heating the sample for 2 min at 300°C did not change the composition: no gold peak could be detected..But shoulders at 334.6 and 339.9 eV appeared in the Pd 3d doublet. In the conduction band a very small shoulder was detected at -1.2 eV below the Fermi level. The sample was then heated overnight at 300°C in the presence of hydrogen. Again no gold peak could be detected. But the Pd 3d appeared at 334.6 and 339.9 eV with no shoulders like in the clean surface. A broad conduction band appeared with a single maximum at -2.0 eV. Prolonged heating at 600°C was necessary to restore the initial composition. Experiments at lower (45O’C) or higher (600°C) temperatures did not result in any modification of the composition and of the shape and energy of the Au 4f and Pd 3d transitions. Several experiments were done with the 13 at% Pd-Au alloy at 1 atm of oxygen from 400 to 700°C during several hours. These experiments were all negative: no oxygen was detected on the surface and no modification occurred in the composition and in the main XPS peaks of p~ladium and gold. In a last experiment the same sample was ion bombarded with 5 keV Ar* ions during 6 min. The calculated palladium surface concentration increased from 9.9 to 22.7%. The surface was then exposed, before annealing, to 1 atm of oxygen at 350°C for 6 h. The calculated concentration was then 20.2%. The 3d doublet exhibited two strong maxima at 335 and 340.3 eV with two small shoulders at 336.3 and 341.7 eV.
4. Discussion 4. I. Clean surfaces AES and XPS results (fig. 1) give for all three alloys surface concentrations which are in reasonable agreement with bulk compositions within experimental errors. Our values agree well with preceding calculations pub~shed by Wood and Wise [ll], Biloen et al. [12] and ourselves [2]. But Somorjai et al. 1131 found a considerable gold enrichment with clean Pd-Au alloys of different concentrations. This result is surprising since it is in contradiction with the regular solution model [9] which predicts no or very little surface enrichment with this system since
L. Hilaire et al. /Interaction
of oxygen and hydrogen
with Pd-Au
137
AH& = 88 kcal/mole and AH:& = 90 kcal/mole. Ion bombardment (fig. 1) leads to a small palladium surface enrichment: 5% with low energy ions, up to lo-12% with high energy ions. These results are consistent with values of sputtering yields given in the literature: 2.1 and 2.4 atoms/ ion for palladium and gold respectively with 500 eV Ar’ ions [ 181. 4.2. 60 at% Pd-Au alloy Depending on the range of temperature studied the interaction with oxygen may lead to different types of results. The most dramatic effects take place in a narrow range of temperature from 500 to 570°C approximately. If the oxygen pressure is at least 1 Torr, if time exposures are long enough, palladium surface concentrations up to 90% can be reached (fig. 3) and even 100% if one works at atmospheric pressure of oxygen. It is not surprising that palladium and not gold migrates to the surface since the Pd-0 bond is expected to be more easily formed than the Au-O bond in these experimental conditions. Examination of XPS spectra reveals interesting trends. The modifications of the conduction band are a clear indication of real oxidation of the surface (fig. 6, 1 Torr, 520°C): the band resembles very much, in position of the maximum and width, the one of PdO, as observed in a parallel study on pure Pd [17], with two humps due to residual gold (6.4 eV) and metallic palladium (1.7 eV); the latter is not detected when electrons issue from the surface at grazing angle (0 = 75’) which means that the oxide is preferentially formed at the surface. In the Pd 3d doublet (fig. 5), the peaks are shifted 2 eV towards higher energies but metallic palladium is still present and appears as shoulders. The height of the oxide peaks is, comparatively to the metallic humps, higher for 0 = 75” than for 0 = 20”. The same conclusions may be inferred from the modifications of the Pd 3d transition. Oxidation of the surface is connected with a gold depletion: this is clearly shown by the calculation of the palladium surface concentration since we find 82% for 19= 20” and 89% for 8 = 75’. Subsequent heatings of the surface (table 1, figs. 5 and 6) allows a reduction of the oxide which is completed after a few minutes at 400°C. However subsequent annealings at 600°C are necessary to restore the original atomic composition. Oxidation is total with 1 atm O2 at 540°C. The Pd 3d doublet is not splitted but shifted of -2 eV towards higher binding energies. The conduction band exhibits a single maximum at 2.0 eV and is characteristic of PdO [ 171. This result is hardly surprising but it is important to note that the temperature range or oxidation is very narrow (500-600°C) and is exactly the same with Pd and the alloy. This is different from what happens with Pd-Pt [ 191. In all these experiments no modifications of the Au 4f transitions were ever observed. This is not surprising since interaction of oxygen with gold occurs at much higher temperatures [20,21].
138
L. IIilaire et al. /Interaction
of oxygen and hydrogen
with P&Au
Interaction of the oxidized surface with h$drogen at 300°C is most interesting. Examination of the Pd 3p and 3d peaks and the conduction band reveals that this treatment leads to a complete reduction of the oxide. However, the gold peaks lack completely in the spectrum which means that the surface is 100% metallic palladium. Further heatings at higher temperatures (500-600°C) are necessary to restore the initial surface composition. This result has important implications in catalysis. Supported catalysts are commonly regenerated or treated before use in an oxygen or air stream and then reduced with hydrogen. Such a treatment gave to O’Cinneide and Gault [22] surp~sin~y high values of activity with Pd-Au alloys. In view of our exceriments, the explanation is straightforward: the catalysts were enriched in palladium by the air treatment, completely reduced by hydrogen, but the temperature of reduction, 350°C, was too low for reequilibration of the surface. Interaction of oxygen with palladium at temperatures lower than 500°C did not get any modi~cation of the Pd 3p and 3d peaks or of the conduction band. Clearly no oxide was formed. However, as seen from Auger results (fig. 3), chemisorption took place and led to a substantial increase of the palladium surface concentration provided the pressure was at least 1 Torr and the temperature not lower than 3ooOc. No experiment was done in Auger at temperatures higher than 6OO*C but, at this temperature, no oxidation of Pd or Au could be observed in XPS spectra. 4.3. 22 at% Pd-Au
alloy
The results at 300 and 400°C (fig. 4) are quite consistent with experiments done at 500°C and already published [2] : interaction of this alloy with oxygen leads to a modest but significant palladium surface enrichment; it exists a nearly linear relationship of the palladium surface concentration as a function of oxygen present on the surface. Results at higher temperature are quite different. At 700°C oxygen chemisorbs on the surface but no surface segregation occurs, It must be recalled that at this temperature p~adlum itself does not interact with oxygen. However, chemisorption of oxygen with gold becomes important in this range of temperature (up to 800°C). Furthermore we have shown in a parallel study of pure gold [21] that this metal is able to dissolve appreciable amounts of oxygen which can later on migrate to the surface on annealing. We believe that these phenomena are able to explain our results on this gold-rich alloy at 700°C: Au and not Pd participates to the chemisorption bond, which explains that no p~adium segregation occurs; meanwhile some oxygen is dissolved in the matrix and further heating of the alloy leads to an increase of oxygen surface concentration.
L. Hilaire et al. /Interaction
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139
4.4. 13 at% Pd-Au alloy The interaction of this alloy with oxygen, studied in a wide range of temperature (25 to 600°C) is very weak. The only positive experiment led to the fixation of a small quantity of oxygen on the surface at 520°C with a very modest palladium enrichment. It seems that with Pd-Au alloys it exists a threshold of palladium concentration (around -15%) under which oxidation or even chemisorption is not possible. A partial oxidation, at 35O”C, was only possible with a previously enriched surface (up to 23%) by ion bombardment, as can be inferred from the small shoulders accompanying the Pd 3d XPS peaks. One may have expected some palladium surface enrichment but there is none. In fact this is easily understandable: as we have seen above, the segregation is connected to oxidation and the layers just under this surface have too low a concentration in palladium atoms to be oxidized. The temperature of this experiment is surprisingly low since, as we have already pointed out, no oxidation occurs with the 60 at% Pd-Au alloy under 5OO’C. The discrepancy may come from the special character, highly disorganized, of the surface created by ion bombardment. In conclusion, we will emphasize the importance of such studies for the knowledge of actual surfaces which take part to reaction processes such as catalytic ones. In particular common treatments like oxidation and reduction, depending on the conditions of temperature and pressure, may completely change the composition of alloy surfaces.
Acknowledgments The authors thank Dr. G. Krill for helpful discussions and Mr. P. Bernhardt for very skilful technical assistance.
References [l] [2] [3] [4] [S] [6] [7] [8] [9]
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