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A photoemission study of the oxidation of platinum in Pt-based alloys: PtPd, PtRu, PtIr
Surface Science 146 (1984) 5699582 North-Holland, Amsterdam
569
A PHOTOEMISSION STUDY OF THE OXIDATION P&BASED ALLOYS: Pt-Pd, Pt-Ru, Pt-Ir L. HILAIRE *, G. DIAZ and G. KRILL ** Institut
Received
Le Bel, Unioersit~
GUERRERO
Louis Pasteur,
2 May 1984; accepted
*.***, P. LtiGARk
4 Rue Blake
for publication
OF PLATINUM
Pascal,
F-67000
*, G. MAIRE
Strasbourg,
IN
*
France
28 June 1984
Photoemission technique was used to study the surface composition of Pt-Pd. Pt-Ru and Pt-lr alloys in a wide range of bulk compositions. The samples were submitted to in-situ oxygen exposures at atmospheric pressure and various temperatures in the 200 to 6OO’C range. Oxygen induced changes of surface composition were followed. Special attention was paid to the various oxidation states of the metals characterized by high-binding energies shifts of the core-levels. Oxidation properties of platinum but also of Pd. Ru and Ir drastically changed by alloying. The Pt-based alloys could be oxidized provided the impurity (i.e. Pd, Ru or Ir) surface concentration was higher than a limit situated between 13% and 208 for all three systems. Above this limit, the impurity and the platinum atoms were both oxidized. Three different Pt oxidation states were detected and identified with PtO, a-PtO, and P-PtO,, the occurrence of which depended on the composition and the temperature of oxidation treatment. In the low impurity content range, neither the impurity nor platinum could be oxidized. Several hypotheses (kinetics of oxidation, oxygen spill-over, percolation effects, changes in local electronic states) are discussed to elucidate this phenomenon.
1. Introduction Numerous studies have been devoted to the problem of the interaction of oxygen with platinum surfaces in a wide range of temperature and pressure. In the past few years, several oxygen-adsorbed states were proposed by various authors [l-3] in spite of the difficulties arising from the presence at the surface of minute amounts of impurities (calcium, silicon, aluminum) more avid for oxygen than platinum itself [4-61. In photoemission, apart from Hecq et al. [7], who prepared bulk platinum oxides by sputtering, no chemical shifts, characteristic of oxidation, were * Laboratoire de Catalyse et Chimie des Surfaces, UA 423, CNRS. ** Laboratore de Magnetisme et de Structure Electronique des Solides. *** Permanent address: Departamento de Quimica, Universidad Autonoma palapa, 09340 Mexico, DF, Mexico.
0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Metropolitana
Izta-
reported on the platinum 4f transitions when gas phase oxygen was interacting with pure platinum surfaces even at atmospheric pressure and high temperature. However, such shifts were found by Sedlacek et al. [8] on a 20 at% Ni-Pt alloy treated at 500°C by 1 atm of oxygen. As part of a general study on the oxidation of alloys, we have investigated the interaction of oxygen with different Pt-based alloys: Pt-Pd. Pt-Ru, Pt-Ir. Special attention was paid to surface segregation in these systems, pure and oxygen-covered, and to the modifications of the platinum core levels as observed in photoemission.
2, Ex~ri~ental Pt-Pd alloys were prepared by melting at high temperature in an induction oven Johnson-Matthey specpure mixtures of the elements. The higher melting points of iridium and ruthenium made necessary the use of a plasma discharge apparatus for Pt-Ir and Pt-Ru alloys, which were prepared by progressively enriching a pure platinum sample with iridium and ruthenium. The bulk compositions and homogeneity were checked by X-ray analysis and, in a few cases (Pt-Ru), by electron microprobe. The samples were polycrystalline discs of 8 mm diameter and 0.5 mm thickness. The specimens were inserted within a VG ESCA III photoemission spectrometer and decontaminated by argon ion bombardments and annealings at high temperature (700°C) to reach surface equilibrium. The spectrometer itself has already been described [9]. Let us only mention that it possesses two chambers where UHV down to the lo-” Torr range could be achieved. The analyser is of the hemispheric type and the resolution 0.75 eV. All the specimens studied were submitted, in the so-called preparation chamber, to a standard treatment, viz. 1 atm oxygen, 3 h at different temperatures. The sample was then cooled down to room temperature under gaseous atmosphere, the oxygen was pumped off, and the photoelectron spectra were recorded in the analysis chamber using the unmonochromatized Ka radiation of an aluminium source (1486.6 eV). In another paper [IO] we have thoroughly developed a layer-by-layer calculation of the concentrations from Auger spectra. We have limited ourselves here to an estimation based on a simple relationship which has already been discussed [9]. For an alloy AB, if Z, and I, are the intensity of two chosen transitions on the XPS spectrum of the alloy and (Y+,~ the intensities ratio of the same transitions for the pure elements, the “surface” concentration of the constituent A may be given by:
We have measured
(Yby recording
spectra of the pure metals under identical
L. Hiloire et al. / Oxidution
of Pt rn Pt - bused alloys
571
conditions and comparing with cross sections given in the literature (11). The agreement was fairly good. The XPS core-level spectra have been reproduced using the Doniach-Sunjic line shape [12] convoluted by a gaussian contribution taking into account the experimental resolution of the spectrometer (FWHM = 0.75 eV). The energy position of the core-level, the value of the asymmetry parameter (Y(correlated with the local density of states at the Fermi level) and the lifetime core hole created by the photoemission process can be deduced from such a line shape analysis.
3. Results 3.1. Pt-Pd
alloys
Before studying these alloys, we felt it necessary to get a good knowledge of the behaviour of the pure metals towards oxygen. The results have already been published [13,14], but it is worthwhile recalling their general trends. An XPS study of the interaction of oxygen with platinum in a large domain of temperature ranging from 200 to 800°C did not reveal any change in the platinum 4f transitions. The binding energy of the 4f,,, peak was stable at 71.1 f 0.1 eV and the shape of the peak was not modified; the full width at half maximum was constant within experimental errors and equal to 1.55 + 0.05 eV. 100 c”pd
B CPd
100
Fig. 1. Pd surface concentration as a function of bulk concentrations for Pt-Pd alloys: (0) surface; (A) after interaction with 0, at 500°C; (I) after interaction with 0, at 600°C.
clean
The situation with palladium was quite transitions of palladium were not modified.
different. up When 1 atm
to 400°C. of oxygen
the 3d reacted
with the same surface between 450 and 6OO”C, a chemical shift of 1.9 f 0.1 eV towards higher binding energies revealed the formation of the oxide PdO. At higher
temperatures,
no shift
occurred,
the oxide
being
unstable.
A series of five Pt-Pd alloys, ranging from 3 to 75 at% Pd, were then studied after decontamination and after interaction with oxygen at temperatures varying from 200 to 7OO’C.
N(E A.U.
334
Fig. 2. Pd 3d,,, 300°C;
(d) 0,.
338
transitions 500°C.
of the 50 at% Pd-Pt
BE CeV)
alloy:
(a)
clean
surface;
(b) 02,
2OO’C; (c) 0,;
In fig. 1 we have reported the palladium surface concentrations as a function of the bulk concentrations for the five alloys after decontamination and equilibration at 700°C. It can be seen that a significant palladium surface segregation occurred in these alloys except for the sample already rich in palladium. In the following the alloys will be named after their bulk concentrations, unless otherwise specified, but one will have to keep in mind this surface enrichment. The palladium and platinum core levels were then recorded after interaction with oxygen. The results are given in figs. 2 and 3 for the Pt50--Pd50 alloy, at the various temperatures studied. Obviously the behaviour of both metals in the alloy is quite different from what was observed with the pure elements.
NCE A.&
Fig. 3. Pt 4f transition (d) 0,. 6OOT.
of the 50 at’% Pd-Pt
alloy: (a) clean surface;
(b) 0,.
3OOT;
(c) 0,.
4oO°C;
574 100
100
i , ,a. !
I
rf
50
50
;
,
.
I ;
I’
\ ‘0
/I
;,’ I’I
:I
(’
.J 300
600
--m--_-.._..i 300
TOC
.-t.
._-L.
_
600
TOc
Fig. 4. Percentages Pd.
of oxides formed on the 10 at% Pd-Pt
alloy at different
temperatures:
(0)
Pt:
Fig. 5. Percentages Pd.
of oxides formed
alloy at different
temperatures:
(0)
Pt;
(0)
Pt;
(A)
on the 25 at% Pd-Pt
(A)
200 Fig. 6. Percentages of oxides formed (A)
Pd.
500
1%
on the 50 at% Pd-Pt alloy at different temperatures:
L. Hitaire et al. / Oxrdation of PI in Pt - based alloys
515
Even at temperatures as low as 300°C and up to 650°C broadenings of the peaks and chemical shifts of their maxima, characteristic of oxide formation, were definitely observed. The different transitions were then deconvoluted in order to estimate the percentages of oxide formed with respect to the remaining metal, both for palladium and platinum. In figs. 4, 5, 6 and 7 we give the percentages of oxide formed as a function of the temperature of reaction for the 10, 25, 50 and 75 at% Pd-Pt alloys respectively. Although the whole range of temperatures was covered with the 50Pt-50Pd alloy only, it can be seen that the formation of oxides went through a maximum around 4OO’C. The percentage of platinum oxide at this temperature was always higher than 50% while the oxidation of palladium was total or nearly total. A puzzling result is that the situation was quite different with the 3 at% Pd-Pt alloy. Whatever the reaction temperature could be, no modifications in the position and the shape of the Pt 4f and Pd 3d transitions were observed. In all cases where oxides were formed, a further palladium surface enrichment was observed. this was particularly true for the palladium rich alloys as can be seen in fig. 1. For example, after interaction with oxygen at 500 and 600°C the surface of the 75 at% Pd-Pt alloy was almost completely covered with PdO. 3.2. Pt-Ru
alloys
Six Pt-Ru
alloys
with 4, 11.5, 15, 20, 30 and
55 at% ruthenium
were
100 A--
______&
I
I
I
I/ I/
I/
: 4 300
I
\ \
1 600
Tot
Fig. 7. Percentages of oxides formed on the 75 at% Pd-Pt
(A) Pd.
alloy at different temperatures:
(0)
Pt;
L. Htlaire et ul. / Oxidation of Pt 111PI-bused
516
aNo_ps
100 Cs,”
1uu
LI
‘Ru Fig. 8. Ru surface concentrations surface;
(A) 0,.
as a function
of bulk concentrations
75 Fig. 9. Pt 4f transitions (c) o,,
5oooc.
for Pt-Ru
alloys:
(0)
clean
400°C.
of the 55 at% Ru-Pt
alloy;
BE
raw spectra:
(d!)
(a) clean surface;
(b) 0,.
400°C;
L. Hlluire et al. / Oxidurion of Pt WI Pt - bused alloys
studied. In fig. 8 we give the ruthenium XPS
spectra,
as a function
surface concentrations,
of the bulk concentrations
571
calculated
for all six alloys.
from It can
be seen that in all cases the surface was enriched with platinum. The surfaces were then submitted to oxygen treatments at atmospheric pressure
and temperatures
low ruthenium
contents
varying
from 200 to 600°C.
(up to 20%,
that is about
On the four alloys of
11% on the surface)
no
chemical shifts nor broadening of the Pt 4f and Ru 3d peaks were observed. The ruthenium rich alloys behave in a quite different way. In fig. 9 we give the raw spectra
of the platinum
and after oxygen treatments spectra
4f transitions
at 400 and 500°C.
for the Pt455Ru55
alloy, pure
The trends observed
on the raw
by deconvolutions. At 200°C the maxima of the did not reveal any chemical shift. The platinum core levels 4f,,, and 4f,,, same situation was observed after reaction at 300°C but a small hump appeared on the higher binding energy side of the spectra, corresponding to a shift of 2.2 eV. At 400”~ each transition due to metallic platinum was followed by
two
were confirmed
supplementary
peaks
characteristic
chemical shifts of 2.2 and 2.8 eV. metallic platinum almost disappeared chemical
of oxide
formation
indicating
At 500°C the peaks corresponding to and three transitions were detected with
shifts of 1.0, 2.2 and 2.8 eV. The same shifts were observed
at 600°C
but the major transition was then due to metallic platinum. Depending on the temperature, one, two or three contributions must be introduced in addition to the metallic state, in order to get a correct fit for the whole spectra. Two of these contributions, corresponding to chemical shifts of - 1 eV and - 2.2 eV, were actually observed in the raw spectra. The third one, with a chemical shift of - 2.8 eV, was not clearly distinguished and must therefore be considered with some caution. At all temperatures a chemical shift of l-l.6 eV was detected on the ruthenium 3d transitions which were significantly broadened. The 30 at% Ru-Pt alloy (which contained - 20% of ruthenium atoms on the surface) was intermediate between the preceding alloy and the platinum-rich ones. Up to 400°C no modifications of the peak could be observed. At 500°C only, a small hump was detected on the higher binding energies side of the platinum 3.3. Pt-ir Seven prepared.
transitions,
with a chemical
shift of 2.2 eV.
alloys Pt-Ir
alloys
The surface
fig. 10 as a function
with
2, 5, 10,
concentrations
15, 25, derived
40 and from XPS
of the bulk concentrations.
50 at% iridium spectra
were
are given in
It can be seen that a significant
surface enrichment of platinum occurred on all samples: for example, with the 50 at% Ir-Pt alloy, the iridium surface concentration was not higher than 24%. The same experiments as in the other two series of alloys were performed on the Pt-Ir samples. On all alloys of low iridium with oxygen did not result in any modifications
concentration the interaction of the platinum 4f and Ir 4f
L. Hilarre et (11./ Oxrdufion of Pt rn Pt hosed ullo~..~
578
100 c:r
cYr 100 Fig.
10. Ir surface
concentrations
as a function
of bulk
concentration:
(0)
clean
surface;
(A) 0,.
500°C.
N(E: A.U
70
60 Fig. Ir-Pt
11. Ir 4f and alloy+<),,
Pt 4f transition 500°C;
of Pt-lr
(c) 50 at% Ir-Pt
alloys: alloy,
80 (a) 40 at% Ir-PI
decontaminated.
alloy+O,,
BE 500°C;
eb’ (b) 50 at%
transitions. Only the two iridium-rich samples revealed new trends in the spectra as can be seen in fig. 11. Only one experiment, at 500’~ was performed on these samples. The iridium 4f peaks exhibited two maxima, characteristic of a partial oxidation. The platinum 4f transitions were largely modified in the same way as on the ruthenium-rich Pt-Ru alloys, i.e. they exhibited a - 2.2 eV chemical shift towards higher binding energies.
4. Discussion Numerous studies have been devoted to the prediction of surface segregation in binary alloys. From the quantitative point of view, it can hardly be said that the various current theories are really satisfactory. I-Iowever, it is possible now, in most cases, to foresee which element of an alloy AB will segregate to the surface, using relatively simple thermodynamical considerations. It has been shown that arguments based on the differences on the heats of vaporization of A and B [15] combined with the differences in their atomic sizes [16] could provide a satisfactory fit between qualitative predictions and experimental results in a series of more than 30 binary systems [17] although there are a few exceptions ]S]. In the present study we have observed a surface segregation of palladium in Pt-Pd alloys and of platinum in Pt-Ru and Pt-Ir alloys. The heats of vaporization of the four elements, platinum, palladium, ruthenium and iridium are 135, 90, 155.7 and 160 kcal/mole, respectively while their atomic radii are 1.37, 1.37, 1.32 and 1.35 A. Obviously, the differences in the sizes of all these elements are small and we can expect that the surface segregation due to the strain energy will be negligible. On the other hand the heats of vaporization vary significantly and, as we know that the element which possesses the lower heat of vaporization will segregate to the surface, we can predict that palladium in Pt-Pd alloys and platinum in Pt-Ru and Pt-Ir alloys will enrich the surface. These conclusions agree quite well with our experimental observations. The only puzzling result is the absence of segregation with the 75 at% Pd-Pt alloy (fig. I). A sample even more concentrated in palladium would be necessary to check whether this result is spurious or not. The main result of our studies is the possibility of oxidizing platinum in alloys while under the same experimental conditions a pure platinum sample does not react with oxygen. The reason for such a phenomenon is not straightforward. One may think of a kinetic effect. In the three alloys that we have studied, the second metal is easier to oxidize than platinum itself. A possible mechanism might involve the oxidation of this second metal as a first step. The new environment thus created around platinum atoms (oxide instead of metal) might enhance the possibility of oxidation of platinum, for example by a transfer of oxygen atoms from the second metal to platinum, by a
mechanism analogous to the so-called heterogeneous catalysis. The second interesting the concentration threshold.
For example
“spill-over”,
point of our studies is the absence
of the second
with the PdlO--Pt90
hydrogen
metal on the surface
the oxidation
of palladium
well known
of oxidation
when
is lower than a critical
and platinum
alloy but none of these elements
in
is possible
could be oxidized
in the
Pd3-Pt97 alloy. Let us recall that, after fig. 1, the corresponding palladium surface concentrations were 30 and 10% respectively. In order to get a more accurate and Pt-Ir
value of this critical concentration alloys by enriching
we have prepared
progressively
a platinum
a series of Pt-Ru
sample with ruthenium
and iridium. We can derive from the experimental results, keeping in mind the phenomenon of surface enrichment as shown in figs. 8 and 10, a critical surface concentration threshold between 13 and 22% in Pt-Ir and between 11 and 21% in Pt-Ru alloys. It seems therefore that, for the three systems, this critical concentration had a nearly constant value, in the 1520% range as a first approximation. Our method of calculation for the surface concentrations, which is a crude one, does not allow us to be more accurate.
Moreover
our
samples were polycrystalline, with possibly many defects, grain boundaries for example which might influence this value. Such a threshold for palladium oxidation was already reported with Pd-Au alloys [9]. The existence of this critical value may be related to the so-called tion transition.
In an alloy Pt, _ .M,
will react with oxygen on condition
percola-
(M = Pd, Ru, Ir) we can assume that M
that a continuous
path of M-M
neighbouts
can be found on the surface. If the M concentration decreases too much, the chain of neighbours will be broken and the reaction will no longer be possible. It is worthwhile
noting that this “percolation
transition”
for a square superfi-
cial lattice has been calculated and found close to 15%, which is not far from our experimental value. Furthermore, if the second metal can no longer be oxidized, it is easy to understand that platinum will not react either if we admit the “spill-over” mechanism proposed above. However, this latter mechanism is questionable for several reasons. First it implies more or less explicitly that the role of the second metal is the dissociation of oxygen to provide oxygen atoms to platinum so that the latter can be oxidized. However, it is well known that platinum itself is able to adsorb oxygen in a dissociated form [I]. It is therefore likely that a more complicated effect than a simple kinetic one must be involved to explain why pure platinum is not oxidized while oxidation takes place when platinum is alloyed. A second point is that this mechanism does not explain why palladium in Pd-Pt alloys can be oxidized, like platinum, under experimental conditions where pure palladium
does not react:
at 300 and 650°C
no oxidation
of pure
palladium takes place while at the same temperatures both palladium and platinum were oxidized in Pt-Pd alloys. We believe that our two main results (oxidation of platinum when alloyed,
L. Hiluire et (II. / Oxrdutron of Pt in Pt - bused alloys
581
existence of a critical concentration) may be explained in terms of electronic effects. It is obvious that the formation of an alloy and its interaction with oxygen will significantly change the local density of states around each of the metals. We have already studied the modification of the electronic structure of palladium due to its interaction with oxygen by recording the conduction bands using XPS and UPS [14]. However, the resolution of a photoelectron spectrometer is too poor to allow a detailed study of such modifications when we deal with such a complex system as an alloy interacting with a gas. Another approach may involve the study of the modifications of the core levels line shapes. It has been shown that a dissymmetry factor (Y, closely related to the density of states at the Fermi level, can be extracted from the core level spectra by deconvolution processes [12]. This needs a careful study but seems very promising. Work is in progress in our laboratory to try to correlate the modifications of the shape of the core levels peaks, especially Pt 4f. to the experimental results described in the present paper. A last point to discuss is the identification of the oxides formed. Bulk platinum oxides were prepared by sputtering by Hecq et al. [7] and their photoelectron spectra were recorded as long as a spectrum of Pt’. Three platinum oxides, namely PtO, cuPtO, and /?PtO,, were distinguished, which gave chemical shifts of 1.2, 2.2 and 2.8 eV, respectively, towards higher binding energies. These values were quite close to the shifts which we observed in this study. On Pt-Pd alloys a single chemical shift, around 1 eV, was observed. We may therefore conclude that the only oxide formed was PtO. On Pt-Ru alloys, depending on the temperature, chemical shifts of - 1.0, 2.2 and 2.8 eV were found, and these values correspond well to the shifts observed by Hecq et al. on PtO, aPt0, and /?PtO,, respectively. All three oxides could even coexist on the surface of the Pt,,-Ru,, alloy at 500°C. On Pt-Ir alloys, the observed shifts were around 2.6 eV, indicating probably the formation of pPt0,. The same conclusion could be drawn from a previous study on a 20 at% Ni-Pt alloys [8]. Acknowledgements
Many thanks are due to Mrs. M.F. Ravet for her help in preparing the samples. G.D.G. is grateful to the Consejo National de Ciencia y Tecnologia (Mexico) and to the Centre International pour les Etudiants et Stagiaires (Paris) for the organization of her stay in France and for a grant. References [l] J.L. Gland, [2] M. Salmeron,
B.A. Sexton and G.B. Fisher,
Surface Sci. 95 (1980)
L. Brewer and G.A. Somorjai,
[3] C. Smith, J.P. Biberian
and G.A. Somorjai,
Surface
587.
Sci. 112 (1981)
J. Catalysis
57 (1979)
207.
426.
[4] G. Maire, P. Legare and G. Lindauer, Surface Sci. 80 (1979) 238. [5] H.P. Bonzel, A.M. Franken and G. Pirug, Surface Sci. 104 (1981) 625. [6] J. Jupille, Surface Sci. 123 (1982) L674. [7] [X] [9] [lo] [II] [12] [13] !14] [15] 1161 :17]
M. Hecq, A. Hecq and J.P. Delrue. J. Less-Common Metals 64 (1979) 25. J. Sedlacek. L. Hilaire, P. L&are and G. Maire. Surface Sci. 115 (1982) 541. L. Hilaire, P. L~gar&. Y. Ho11 and G. Maire, Surface Sci. 103 (1981) 125. J. Sedlacek, L. Hilaire, P. L&are and G. Maire. Appl. Surface Sci. 10 (19X2) 75. R.C.G. Leckey. Phys. Rev. Al3 (1976) 1043. S. Doniach and M. Sunjic. J. Phys. C3 (1970) 285. P. Legart, L. Hilaire and G. Maire. Surface Sci. 141 (1984) 604. P. LCgarC, L. Hilaire, G. Maire. G. Krill and A. Amamou. Surface Sci. 107 (19X1 ) 533 F. Williams and D. Nason, Surface Sci. 45 (1974) 377. D. MC Lean. Grain Boundaries in Metals (Clarendon, Oxford, 1957) p. 147. F.F. Abraham, N.H. Tsai and G.M. Pound. Surface Sci. X3 (1979) 406.
569
A PHOTOEMISSION STUDY OF THE OXIDATION P&BASED ALLOYS: Pt-Pd, Pt-Ru, Pt-Ir L. HILAIRE *, G. DIAZ and G. KRILL ** Institut
Received
Le Bel, Unioersit~
GUERRERO
Louis Pasteur,
2 May 1984; accepted
*.***, P. LtiGARk
4 Rue Blake
for publication
OF PLATINUM
Pascal,
F-67000
*, G. MAIRE
Strasbourg,
IN
*
France
28 June 1984
Photoemission technique was used to study the surface composition of Pt-Pd. Pt-Ru and Pt-lr alloys in a wide range of bulk compositions. The samples were submitted to in-situ oxygen exposures at atmospheric pressure and various temperatures in the 200 to 6OO’C range. Oxygen induced changes of surface composition were followed. Special attention was paid to the various oxidation states of the metals characterized by high-binding energies shifts of the core-levels. Oxidation properties of platinum but also of Pd. Ru and Ir drastically changed by alloying. The Pt-based alloys could be oxidized provided the impurity (i.e. Pd, Ru or Ir) surface concentration was higher than a limit situated between 13% and 208 for all three systems. Above this limit, the impurity and the platinum atoms were both oxidized. Three different Pt oxidation states were detected and identified with PtO, a-PtO, and P-PtO,, the occurrence of which depended on the composition and the temperature of oxidation treatment. In the low impurity content range, neither the impurity nor platinum could be oxidized. Several hypotheses (kinetics of oxidation, oxygen spill-over, percolation effects, changes in local electronic states) are discussed to elucidate this phenomenon.
1. Introduction Numerous studies have been devoted to the problem of the interaction of oxygen with platinum surfaces in a wide range of temperature and pressure. In the past few years, several oxygen-adsorbed states were proposed by various authors [l-3] in spite of the difficulties arising from the presence at the surface of minute amounts of impurities (calcium, silicon, aluminum) more avid for oxygen than platinum itself [4-61. In photoemission, apart from Hecq et al. [7], who prepared bulk platinum oxides by sputtering, no chemical shifts, characteristic of oxidation, were * Laboratoire de Catalyse et Chimie des Surfaces, UA 423, CNRS. ** Laboratore de Magnetisme et de Structure Electronique des Solides. *** Permanent address: Departamento de Quimica, Universidad Autonoma palapa, 09340 Mexico, DF, Mexico.
0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Metropolitana
Izta-
reported on the platinum 4f transitions when gas phase oxygen was interacting with pure platinum surfaces even at atmospheric pressure and high temperature. However, such shifts were found by Sedlacek et al. [8] on a 20 at% Ni-Pt alloy treated at 500°C by 1 atm of oxygen. As part of a general study on the oxidation of alloys, we have investigated the interaction of oxygen with different Pt-based alloys: Pt-Pd. Pt-Ru, Pt-Ir. Special attention was paid to surface segregation in these systems, pure and oxygen-covered, and to the modifications of the platinum core levels as observed in photoemission.
2, Ex~ri~ental Pt-Pd alloys were prepared by melting at high temperature in an induction oven Johnson-Matthey specpure mixtures of the elements. The higher melting points of iridium and ruthenium made necessary the use of a plasma discharge apparatus for Pt-Ir and Pt-Ru alloys, which were prepared by progressively enriching a pure platinum sample with iridium and ruthenium. The bulk compositions and homogeneity were checked by X-ray analysis and, in a few cases (Pt-Ru), by electron microprobe. The samples were polycrystalline discs of 8 mm diameter and 0.5 mm thickness. The specimens were inserted within a VG ESCA III photoemission spectrometer and decontaminated by argon ion bombardments and annealings at high temperature (700°C) to reach surface equilibrium. The spectrometer itself has already been described [9]. Let us only mention that it possesses two chambers where UHV down to the lo-” Torr range could be achieved. The analyser is of the hemispheric type and the resolution 0.75 eV. All the specimens studied were submitted, in the so-called preparation chamber, to a standard treatment, viz. 1 atm oxygen, 3 h at different temperatures. The sample was then cooled down to room temperature under gaseous atmosphere, the oxygen was pumped off, and the photoelectron spectra were recorded in the analysis chamber using the unmonochromatized Ka radiation of an aluminium source (1486.6 eV). In another paper [IO] we have thoroughly developed a layer-by-layer calculation of the concentrations from Auger spectra. We have limited ourselves here to an estimation based on a simple relationship which has already been discussed [9]. For an alloy AB, if Z, and I, are the intensity of two chosen transitions on the XPS spectrum of the alloy and (Y+,~ the intensities ratio of the same transitions for the pure elements, the “surface” concentration of the constituent A may be given by:
We have measured
(Yby recording
spectra of the pure metals under identical
L. Hiloire et al. / Oxidution
of Pt rn Pt - bused alloys
571
conditions and comparing with cross sections given in the literature (11). The agreement was fairly good. The XPS core-level spectra have been reproduced using the Doniach-Sunjic line shape [12] convoluted by a gaussian contribution taking into account the experimental resolution of the spectrometer (FWHM = 0.75 eV). The energy position of the core-level, the value of the asymmetry parameter (Y(correlated with the local density of states at the Fermi level) and the lifetime core hole created by the photoemission process can be deduced from such a line shape analysis.
3. Results 3.1. Pt-Pd
alloys
Before studying these alloys, we felt it necessary to get a good knowledge of the behaviour of the pure metals towards oxygen. The results have already been published [13,14], but it is worthwhile recalling their general trends. An XPS study of the interaction of oxygen with platinum in a large domain of temperature ranging from 200 to 800°C did not reveal any change in the platinum 4f transitions. The binding energy of the 4f,,, peak was stable at 71.1 f 0.1 eV and the shape of the peak was not modified; the full width at half maximum was constant within experimental errors and equal to 1.55 + 0.05 eV. 100 c”pd
B CPd
100
Fig. 1. Pd surface concentration as a function of bulk concentrations for Pt-Pd alloys: (0) surface; (A) after interaction with 0, at 500°C; (I) after interaction with 0, at 600°C.
clean
The situation with palladium was quite transitions of palladium were not modified.
different. up When 1 atm
to 400°C. of oxygen
the 3d reacted
with the same surface between 450 and 6OO”C, a chemical shift of 1.9 f 0.1 eV towards higher binding energies revealed the formation of the oxide PdO. At higher
temperatures,
no shift
occurred,
the oxide
being
unstable.
A series of five Pt-Pd alloys, ranging from 3 to 75 at% Pd, were then studied after decontamination and after interaction with oxygen at temperatures varying from 200 to 7OO’C.
N(E A.U.
334
Fig. 2. Pd 3d,,, 300°C;
(d) 0,.
338
transitions 500°C.
of the 50 at% Pd-Pt
BE CeV)
alloy:
(a)
clean
surface;
(b) 02,
2OO’C; (c) 0,;
In fig. 1 we have reported the palladium surface concentrations as a function of the bulk concentrations for the five alloys after decontamination and equilibration at 700°C. It can be seen that a significant palladium surface segregation occurred in these alloys except for the sample already rich in palladium. In the following the alloys will be named after their bulk concentrations, unless otherwise specified, but one will have to keep in mind this surface enrichment. The palladium and platinum core levels were then recorded after interaction with oxygen. The results are given in figs. 2 and 3 for the Pt50--Pd50 alloy, at the various temperatures studied. Obviously the behaviour of both metals in the alloy is quite different from what was observed with the pure elements.
NCE A.&
Fig. 3. Pt 4f transition (d) 0,. 6OOT.
of the 50 at’% Pd-Pt
alloy: (a) clean surface;
(b) 0,.
3OOT;
(c) 0,.
4oO°C;
574 100
100
i , ,a. !
I
rf
50
50
;
,
.
I ;
I’
\ ‘0
/I
;,’ I’I
:I
(’
.J 300
600
--m--_-.._..i 300
TOC
.-t.
._-L.
_
600
TOc
Fig. 4. Percentages Pd.
of oxides formed on the 10 at% Pd-Pt
alloy at different
temperatures:
(0)
Pt:
Fig. 5. Percentages Pd.
of oxides formed
alloy at different
temperatures:
(0)
Pt;
(0)
Pt;
(A)
on the 25 at% Pd-Pt
(A)
200 Fig. 6. Percentages of oxides formed (A)
Pd.
500
1%
on the 50 at% Pd-Pt alloy at different temperatures:
L. Hitaire et al. / Oxrdation of PI in Pt - based alloys
515
Even at temperatures as low as 300°C and up to 650°C broadenings of the peaks and chemical shifts of their maxima, characteristic of oxide formation, were definitely observed. The different transitions were then deconvoluted in order to estimate the percentages of oxide formed with respect to the remaining metal, both for palladium and platinum. In figs. 4, 5, 6 and 7 we give the percentages of oxide formed as a function of the temperature of reaction for the 10, 25, 50 and 75 at% Pd-Pt alloys respectively. Although the whole range of temperatures was covered with the 50Pt-50Pd alloy only, it can be seen that the formation of oxides went through a maximum around 4OO’C. The percentage of platinum oxide at this temperature was always higher than 50% while the oxidation of palladium was total or nearly total. A puzzling result is that the situation was quite different with the 3 at% Pd-Pt alloy. Whatever the reaction temperature could be, no modifications in the position and the shape of the Pt 4f and Pd 3d transitions were observed. In all cases where oxides were formed, a further palladium surface enrichment was observed. this was particularly true for the palladium rich alloys as can be seen in fig. 1. For example, after interaction with oxygen at 500 and 600°C the surface of the 75 at% Pd-Pt alloy was almost completely covered with PdO. 3.2. Pt-Ru
alloys
Six Pt-Ru
alloys
with 4, 11.5, 15, 20, 30 and
55 at% ruthenium
were
100 A--
______&
I
I
I
I/ I/
I/
: 4 300
I
\ \
1 600
Tot
Fig. 7. Percentages of oxides formed on the 75 at% Pd-Pt
(A) Pd.
alloy at different temperatures:
(0)
Pt;
L. Htlaire et ul. / Oxidation of Pt 111PI-bused
516
aNo_ps
100 Cs,”
1uu
LI
‘Ru Fig. 8. Ru surface concentrations surface;
(A) 0,.
as a function
of bulk concentrations
75 Fig. 9. Pt 4f transitions (c) o,,
5oooc.
for Pt-Ru
alloys:
(0)
clean
400°C.
of the 55 at% Ru-Pt
alloy;
BE
raw spectra:
(d!)
(a) clean surface;
(b) 0,.
400°C;
L. Hlluire et al. / Oxidurion of Pt WI Pt - bused alloys
studied. In fig. 8 we give the ruthenium XPS
spectra,
as a function
surface concentrations,
of the bulk concentrations
571
calculated
for all six alloys.
from It can
be seen that in all cases the surface was enriched with platinum. The surfaces were then submitted to oxygen treatments at atmospheric pressure
and temperatures
low ruthenium
contents
varying
from 200 to 600°C.
(up to 20%,
that is about
On the four alloys of
11% on the surface)
no
chemical shifts nor broadening of the Pt 4f and Ru 3d peaks were observed. The ruthenium rich alloys behave in a quite different way. In fig. 9 we give the raw spectra
of the platinum
and after oxygen treatments spectra
4f transitions
at 400 and 500°C.
for the Pt455Ru55
alloy, pure
The trends observed
on the raw
by deconvolutions. At 200°C the maxima of the did not reveal any chemical shift. The platinum core levels 4f,,, and 4f,,, same situation was observed after reaction at 300°C but a small hump appeared on the higher binding energy side of the spectra, corresponding to a shift of 2.2 eV. At 400”~ each transition due to metallic platinum was followed by
two
were confirmed
supplementary
peaks
characteristic
chemical shifts of 2.2 and 2.8 eV. metallic platinum almost disappeared chemical
of oxide
formation
indicating
At 500°C the peaks corresponding to and three transitions were detected with
shifts of 1.0, 2.2 and 2.8 eV. The same shifts were observed
at 600°C
but the major transition was then due to metallic platinum. Depending on the temperature, one, two or three contributions must be introduced in addition to the metallic state, in order to get a correct fit for the whole spectra. Two of these contributions, corresponding to chemical shifts of - 1 eV and - 2.2 eV, were actually observed in the raw spectra. The third one, with a chemical shift of - 2.8 eV, was not clearly distinguished and must therefore be considered with some caution. At all temperatures a chemical shift of l-l.6 eV was detected on the ruthenium 3d transitions which were significantly broadened. The 30 at% Ru-Pt alloy (which contained - 20% of ruthenium atoms on the surface) was intermediate between the preceding alloy and the platinum-rich ones. Up to 400°C no modifications of the peak could be observed. At 500°C only, a small hump was detected on the higher binding energies side of the platinum 3.3. Pt-ir Seven prepared.
transitions,
with a chemical
shift of 2.2 eV.
alloys Pt-Ir
alloys
The surface
fig. 10 as a function
with
2, 5, 10,
concentrations
15, 25, derived
40 and from XPS
of the bulk concentrations.
50 at% iridium spectra
were
are given in
It can be seen that a significant
surface enrichment of platinum occurred on all samples: for example, with the 50 at% Ir-Pt alloy, the iridium surface concentration was not higher than 24%. The same experiments as in the other two series of alloys were performed on the Pt-Ir samples. On all alloys of low iridium with oxygen did not result in any modifications
concentration the interaction of the platinum 4f and Ir 4f
L. Hilarre et (11./ Oxrdufion of Pt rn Pt hosed ullo~..~
578
100 c:r
cYr 100 Fig.
10. Ir surface
concentrations
as a function
of bulk
concentration:
(0)
clean
surface;
(A) 0,.
500°C.
N(E: A.U
70
60 Fig. Ir-Pt
11. Ir 4f and alloy+<),,
Pt 4f transition 500°C;
of Pt-lr
(c) 50 at% Ir-Pt
alloys: alloy,
80 (a) 40 at% Ir-PI
decontaminated.
alloy+O,,
BE 500°C;
eb’ (b) 50 at%
transitions. Only the two iridium-rich samples revealed new trends in the spectra as can be seen in fig. 11. Only one experiment, at 500’~ was performed on these samples. The iridium 4f peaks exhibited two maxima, characteristic of a partial oxidation. The platinum 4f transitions were largely modified in the same way as on the ruthenium-rich Pt-Ru alloys, i.e. they exhibited a - 2.2 eV chemical shift towards higher binding energies.
4. Discussion Numerous studies have been devoted to the prediction of surface segregation in binary alloys. From the quantitative point of view, it can hardly be said that the various current theories are really satisfactory. I-Iowever, it is possible now, in most cases, to foresee which element of an alloy AB will segregate to the surface, using relatively simple thermodynamical considerations. It has been shown that arguments based on the differences on the heats of vaporization of A and B [15] combined with the differences in their atomic sizes [16] could provide a satisfactory fit between qualitative predictions and experimental results in a series of more than 30 binary systems [17] although there are a few exceptions ]S]. In the present study we have observed a surface segregation of palladium in Pt-Pd alloys and of platinum in Pt-Ru and Pt-Ir alloys. The heats of vaporization of the four elements, platinum, palladium, ruthenium and iridium are 135, 90, 155.7 and 160 kcal/mole, respectively while their atomic radii are 1.37, 1.37, 1.32 and 1.35 A. Obviously, the differences in the sizes of all these elements are small and we can expect that the surface segregation due to the strain energy will be negligible. On the other hand the heats of vaporization vary significantly and, as we know that the element which possesses the lower heat of vaporization will segregate to the surface, we can predict that palladium in Pt-Pd alloys and platinum in Pt-Ru and Pt-Ir alloys will enrich the surface. These conclusions agree quite well with our experimental observations. The only puzzling result is the absence of segregation with the 75 at% Pd-Pt alloy (fig. I). A sample even more concentrated in palladium would be necessary to check whether this result is spurious or not. The main result of our studies is the possibility of oxidizing platinum in alloys while under the same experimental conditions a pure platinum sample does not react with oxygen. The reason for such a phenomenon is not straightforward. One may think of a kinetic effect. In the three alloys that we have studied, the second metal is easier to oxidize than platinum itself. A possible mechanism might involve the oxidation of this second metal as a first step. The new environment thus created around platinum atoms (oxide instead of metal) might enhance the possibility of oxidation of platinum, for example by a transfer of oxygen atoms from the second metal to platinum, by a
mechanism analogous to the so-called heterogeneous catalysis. The second interesting the concentration threshold.
For example
“spill-over”,
point of our studies is the absence
of the second
with the PdlO--Pt90
hydrogen
metal on the surface
the oxidation
of palladium
well known
of oxidation
when
is lower than a critical
and platinum
alloy but none of these elements
in
is possible
could be oxidized
in the
Pd3-Pt97 alloy. Let us recall that, after fig. 1, the corresponding palladium surface concentrations were 30 and 10% respectively. In order to get a more accurate and Pt-Ir
value of this critical concentration alloys by enriching
we have prepared
progressively
a platinum
a series of Pt-Ru
sample with ruthenium
and iridium. We can derive from the experimental results, keeping in mind the phenomenon of surface enrichment as shown in figs. 8 and 10, a critical surface concentration threshold between 13 and 22% in Pt-Ir and between 11 and 21% in Pt-Ru alloys. It seems therefore that, for the three systems, this critical concentration had a nearly constant value, in the 1520% range as a first approximation. Our method of calculation for the surface concentrations, which is a crude one, does not allow us to be more accurate.
Moreover
our
samples were polycrystalline, with possibly many defects, grain boundaries for example which might influence this value. Such a threshold for palladium oxidation was already reported with Pd-Au alloys [9]. The existence of this critical value may be related to the so-called tion transition.
In an alloy Pt, _ .M,
will react with oxygen on condition
percola-
(M = Pd, Ru, Ir) we can assume that M
that a continuous
path of M-M
neighbouts
can be found on the surface. If the M concentration decreases too much, the chain of neighbours will be broken and the reaction will no longer be possible. It is worthwhile
noting that this “percolation
transition”
for a square superfi-
cial lattice has been calculated and found close to 15%, which is not far from our experimental value. Furthermore, if the second metal can no longer be oxidized, it is easy to understand that platinum will not react either if we admit the “spill-over” mechanism proposed above. However, this latter mechanism is questionable for several reasons. First it implies more or less explicitly that the role of the second metal is the dissociation of oxygen to provide oxygen atoms to platinum so that the latter can be oxidized. However, it is well known that platinum itself is able to adsorb oxygen in a dissociated form [I]. It is therefore likely that a more complicated effect than a simple kinetic one must be involved to explain why pure platinum is not oxidized while oxidation takes place when platinum is alloyed. A second point is that this mechanism does not explain why palladium in Pd-Pt alloys can be oxidized, like platinum, under experimental conditions where pure palladium
does not react:
at 300 and 650°C
no oxidation
of pure
palladium takes place while at the same temperatures both palladium and platinum were oxidized in Pt-Pd alloys. We believe that our two main results (oxidation of platinum when alloyed,
L. Hiluire et (II. / Oxrdutron of Pt in Pt - bused alloys
581
existence of a critical concentration) may be explained in terms of electronic effects. It is obvious that the formation of an alloy and its interaction with oxygen will significantly change the local density of states around each of the metals. We have already studied the modification of the electronic structure of palladium due to its interaction with oxygen by recording the conduction bands using XPS and UPS [14]. However, the resolution of a photoelectron spectrometer is too poor to allow a detailed study of such modifications when we deal with such a complex system as an alloy interacting with a gas. Another approach may involve the study of the modifications of the core levels line shapes. It has been shown that a dissymmetry factor (Y, closely related to the density of states at the Fermi level, can be extracted from the core level spectra by deconvolution processes [12]. This needs a careful study but seems very promising. Work is in progress in our laboratory to try to correlate the modifications of the shape of the core levels peaks, especially Pt 4f. to the experimental results described in the present paper. A last point to discuss is the identification of the oxides formed. Bulk platinum oxides were prepared by sputtering by Hecq et al. [7] and their photoelectron spectra were recorded as long as a spectrum of Pt’. Three platinum oxides, namely PtO, cuPtO, and /?PtO,, were distinguished, which gave chemical shifts of 1.2, 2.2 and 2.8 eV, respectively, towards higher binding energies. These values were quite close to the shifts which we observed in this study. On Pt-Pd alloys a single chemical shift, around 1 eV, was observed. We may therefore conclude that the only oxide formed was PtO. On Pt-Ru alloys, depending on the temperature, chemical shifts of - 1.0, 2.2 and 2.8 eV were found, and these values correspond well to the shifts observed by Hecq et al. on PtO, aPt0, and /?PtO,, respectively. All three oxides could even coexist on the surface of the Pt,,-Ru,, alloy at 500°C. On Pt-Ir alloys, the observed shifts were around 2.6 eV, indicating probably the formation of pPt0,. The same conclusion could be drawn from a previous study on a 20 at% Ni-Pt alloys [8]. Acknowledgements
Many thanks are due to Mrs. M.F. Ravet for her help in preparing the samples. G.D.G. is grateful to the Consejo National de Ciencia y Tecnologia (Mexico) and to the Centre International pour les Etudiants et Stagiaires (Paris) for the organization of her stay in France and for a grant. References [l] J.L. Gland, [2] M. Salmeron,
B.A. Sexton and G.B. Fisher,
Surface Sci. 95 (1980)
L. Brewer and G.A. Somorjai,
[3] C. Smith, J.P. Biberian
and G.A. Somorjai,
Surface
587.
Sci. 112 (1981)
J. Catalysis
57 (1979)
207.
426.
[4] G. Maire, P. Legare and G. Lindauer, Surface Sci. 80 (1979) 238. [5] H.P. Bonzel, A.M. Franken and G. Pirug, Surface Sci. 104 (1981) 625. [6] J. Jupille, Surface Sci. 123 (1982) L674. [7] [X] [9] [lo] [II] [12] [13] !14] [15] 1161 :17]
M. Hecq, A. Hecq and J.P. Delrue. J. Less-Common Metals 64 (1979) 25. J. Sedlacek. L. Hilaire, P. L&are and G. Maire. Surface Sci. 115 (1982) 541. L. Hilaire, P. L~gar&. Y. Ho11 and G. Maire, Surface Sci. 103 (1981) 125. J. Sedlacek, L. Hilaire, P. L&are and G. Maire. Appl. Surface Sci. 10 (19X2) 75. R.C.G. Leckey. Phys. Rev. Al3 (1976) 1043. S. Doniach and M. Sunjic. J. Phys. C3 (1970) 285. P. Legart, L. Hilaire and G. Maire. Surface Sci. 141 (1984) 604. P. LCgarC, L. Hilaire, G. Maire. G. Krill and A. Amamou. Surface Sci. 107 (19X1 ) 533 F. Williams and D. Nason, Surface Sci. 45 (1974) 377. D. MC Lean. Grain Boundaries in Metals (Clarendon, Oxford, 1957) p. 147. F.F. Abraham, N.H. Tsai and G.M. Pound. Surface Sci. X3 (1979) 406.