X-ray Photoelectron Spectroscopy Study of Pd and Pt ...

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X-ray Photoelectron Spectroscopy Study of Pd and Pt Ions in Type Y-Zeolite Published on 01 January 1978. Downloaded by Universite Pierre et Marie Curie on 28/08/2015 08:20:59.

Electron Transfer between Metal Aggregates and the Support as Evidenced by X-ray Photoelectron Spectroscopy and Electron Spin Resonance BY JACQUES c. VEDRINE,* MICHELDUFAUX, CLAUDE NACCACHE AND BORISIMELIK Institut de Recherches sur la Catalyse, C.N.R.S., 79, Boulevard du 11 Novembre 1918, 69626 Villeurbanne Cedex, France Received 9th March, 1977 Spectroscopic study by ESCA and e.s.r. techniques was performed for Pt and Pd ions in type Y-zeolites. Pt and Pd ions were found to be ionically bonded to lattice oxygen ions. Atomically dispersed Pto and Pd* were shown to give significant X.P.S. positive chemical shifts : 1.3 and 1.4 eV for Pt 4f and Pd 3d lines, respectively. These shifts were assigned to smaller electron relaxation energy. Metal aggregates of small diameter ( 4 < 20 A) and narrow size distribution were prepared in zeolite or on silica. The X.P.S. chemical shift was quite significant (-+0.7 eV) for Pt on zeolite support with respect to those (c+0.3 ev) for Pd on zeolite or Pt or Pd on silica support and did not depend on the particle size. Electron donor and electron acceptor properties of the materials were studied by following the e.s.r. formation of charge transfer complexes (radical ion) as a function of pt or Pd loading. Pt aggregates were shown to form charge transfer complexes with Lewis acid sites and to increase electron donor properties of the zeolite no matter what the particle size. This increase was shown to arise from electron donation from the metal to the zeolite lattice. However, Pd aggregates were shown by e.s.r. not to give rise to significant electron transfer.

+

+

~

Investigations on the nature of the effect of the carrier on the electronic and catalytic properties of supported metal catalysts have recently been carried out. The most intriguing results were obtained with zeolite carriers2 From studies of palladium supported zeolite it has been concluded that Pd clusters interacted either with Lewis acid sites by electron transfer 3* or with acidic OH groups with 6+ 6-

subsequent formation of charge transfer complex Pd , . . HO-zeol . . . The enhanced catalytic properties of zeolite supported platinum were also attributed to electron transfer from Pt clusters to the zeolite.2 Direct evidence of electron transfer from the metal should be obtained by study of the X.P.S. line chemical shifts which are directly dependent on the effective charge of the atom considered, i.e. on the ionic character of bonds, on the oxidation state and also on electronic transfer.6 For instance, a + 0.5 eV shift of the 4f2 level of Pt, when tungsten was added to Pt on silica catalyst, was detected.' It was thus postulated that interaction between Pt and W resulted in transfer of the electronic density from Pt to W atom. Zeolitic materials possess surface electron acceptor and electron donor centres 8 , which may interact with organic molecules, such as perylene (Pe) or anthracene (A) and tetracyanoethylene (TCNE) or trinitrobenzene (TNB), to form respectively paramagnetic positive and negative radical ions. The principal object of this work was to investigate the local charge density of small Pd or Pt clusters supported on zeolite using ESCA technique and to examine by e.s.r. the effect of Pd or Pt on the redox properties of zeolites. 440

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J . C . V E D R I N E , M . D U F A U X , C . N A C C A C H E A N D B . IMELIK

441

EXPERIMENTAL ESCA measurements were performed using a Vacuum Generator, ESCA 111, spectrometer equipped with an attached reaction chamber which allows heat treatment up to 650°C in a given atmosphere. The ESCA spectra were accumulated in a multichannel pulse height analyser to increase the signal over noise ratios, the scanning of the spectrometer having been monitored by computer. The data were then analysed, using a computer program, for the smoothing and the deconvolution of peaks, for the substraction of background and the determination of area, position, shape and width of the peaks. The powder was pressed by hand on a gilt grid and introduced into the reaction chamber for adequate treatment. A charging effect was observed and corrected for all samples taking known l o internal standard elements such as Si[Eb(2s) = 154.0 eV] or O[Eb(ls) = 532.4 eV]. Evaporation of very small amounts of gold Eb(4f3) = 84.0 eV proved that such a reference level was relevant. The accuracy in the determination of binding energy values was estimated to be +0.2 eV. E.s.r. spectra were recorded on an E-9 spectrometer at room temperature. Quantitative determinationswere estimated by comparison with standard samples (strong pitch or benzenic solution of DPPH). The charge transfer complexes (CTC) were formed by contacting the sample with a deoxygenated solution of the organic compound in benzene. The starting material was a Na-Y zeolite. The NHZ form was obtained by conventional ion exchange technique. Palladium or platinum exchanged samples were prepared by stirring the zeolite in Pd(NH,)Zf or Pt(NH,)$+ aqueous solutions. The degree of exchange was estimated from chemical analysis. Some experiments were performed on Pd or Pt over S O 2catalysts prepared by exchanging OH with Pt(NH,):+ complex at 80°C in ammonia solution for 24 h and then washing by distilled water. The Pd-Y zeolite samples were heated in an oxygen flow at 500°C to decompose the Pd(NH,);+ complex, resulting into Pd2f in cationic sites.ll Further contact with hydrogen at room temperature resulted in -60 % reduction of PdZ+with the formation of atomically dispersed PdO, which is known not to chemisorb hydrogen, and of a few percent of Pdf (911 = 2.33 and 91 = 2.10). Reduction at 160°C by hydrogen gave mainly atomically dispersed Pdo (280 %), while H2 treatment at 350°C gave metallic Pd crystallites (20A in diamefer).l2*l 3 The Pt-Y zeolite sampleswere heated according to one of the following three procedures :l4 (i) evacuation (p < Torr) at 500°C and further reduction by hydrogen at 300°C. This procedure gave 80 % atomically dispersed PtO and 20 % Pt crystallites of -20 A size. (ii) Evacuation at 350°C and reduction at the same temperature by hydrogen resulting in Pt crystallites 10 A in diameter (8-12 A size distribution). (iii) Evacuation at 500°C followed by reduction at 400°C giving Pt crystallites 20 A in diameter (15-25 A size distribution).

-

N

RESULTS A N D I N T E R P R E T A T I O N

ESCA SECTION ESCA measurements were performed on samples thermally treated under various conditions as described above (fig. 1). Table 1 summarizes the binding energy values of Pd 3 4 and Pd 3 4 lines. For comparison, table 1 also includes the binding energy values of the 3d levels of various palladium compounds where Pd ions are in different oxidation states. Table 1 shows that the palladium 3d binding energy values increase with the oxidation state of the metal ions. Pd (NH&+ exchanged zeolite outgassed at room temperature, in order not to decompose the complex, displayed X.P.S. lines at 337.6 and 342.9 eV attributed to 3d levels of Pd2+ ions. The positive line shifts with respect to PdO are due to the nature of the Me-ligand bonds which then are more covalent for PdO than for Pd (NH,)i+. This conclusion was further supported by the results obtained for Pd-Y samples thermally treated at 500°Cwhich gave larger binding energy values. Previous work l4 has shown that this thermal treatment resulted in the decomposition of the palladium

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442

X . P . S . OF Pd I N Y-ZEOLITE 337.6


I

(4 FIG.1.-Pd 3d X.P.S. peaks as a function of heat treatment conditions : (a) Pd(NH&+ complex in zeolite (starting material), (6)Pd2+in cationic sites (complex decomposed under O2flow at 50O0C), (c) Pd2+ and Pd+ (H2 introduced at room temperature), (c’) Pd+ as (c) but accumulated for 24 h (X-rays slowly reduced Pd2+ into Pd+ as evidenced by e.s.r.), (d) metallic Pd particles in zeolite formed by reduction by hydrogen at 350°C. Note that Pd ions (I or 11) gave satellite peaks at about 9.7eV, presumably of the shake-up type.

+

TABLE1.-ASSIGNMENT OF ESCA PEAKS ACCORDING TO TREATMENT Pdl 2 . Na19.s(NH4)1 5-Y z E o LITE

CONDITIONS OF

The binding energy values are referred to zeolite internal elements and are expressed in eV (+_0.2) Pd(NH3)Zf complex in zeolite

Pd 3dq Pd 3d+ reference

PdO Pd2+ Pd: in cationic in cationic atomically sites sites dispersed

PdO cristallites PdO (20A) metal

Pd Oads

PdO

337.6 339.2 337.1 336.4 335.2 335.0 335.6 336.3 342.9 344.3 342.5 341.6 340.6 this work this work this work this work this work (15) (15) (15)

Pd 0

2

337.9 (15)

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J. C. VEDRINE, M. D U F A U X , C . NACCACHE A N D B. IMELIK

443

tetramine complex, with subsequent migration of Pd2+ ions towards SI' sites where they ionically coordinated three lattice oxygen atoms. The X.P.S. line shifts to higher energy values show that PdIr ions are more ionically bonded to lattice 02than to NH, ligands and to a larger extent to oxygen in PdO. As Pd-Y sample initially heated at 500°C was contacted with hydrogen at room temperature the Pd 3d X.P.S. lines were modified with the appearance of an increasing shoulder, as shown in fig. 1 and summarized in table 1. From previous e.s.r. and X-ray diffraction studies * l 2 the formation of Pd' ions and atomically dispersed PdO atoms has been postulated. Hence, one can reasonably suggest that the X.P.S. lines at 337.1 and 336.4 eV are respectively due to Pd' ions and atomically dispersed PdO. Upon hydrogen reduction in the 300-400°C temperature range only the X.P.S. lines due to large (4 II: 20A) palladium crystallites were observed. In all cases the Pd line intensities are in good agreement with the chemically determined Pd content except when large Pd crystallites (4 2 20A) were formed resulting in a decrease in line intensity due to the aggregation.l 6 At variance, Minachev et al. l7 have observed an increase in line intensity upon reduction which they interpreted by the migration of metal particles at the surface of the grains. The binding energy values of A1 2s (119.6 eV), A1 2p (74.8), Si 2p (103.0) and 0 1s (532.4) remained constant in all cases within experimental error (k0.2 eV). , from the line area of the 2s lines and using the The Si/AI ratios ( ~ 2 . 5 )estimated theoretical cross sections given by Scofield * are in good agreement with the chemical composition of type Y-zeolite (Si/Al = 2.4). This shows that superficial enrichment in silicon l 9 did not occur in our case. The ESCA data concerning platinum exchanged Y-zeolites treated under different conditions are given in table 2 using the same internal standard reference as for Pd-Y 74.9

I

FIG.2.-Pt 4fand A12p peak in X.P.S. for Pt-Y zeolite as a function of treatment conditions following procedures described in the text : (a) (iii) 20A Pt aggregates, (b) (ii) 10 A Pt aggregates, (c) (i) monoatomically dispersed PtO.

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X.P.S.


444

OF

Pd

I N Y-ZEOLITE

zeolite samples. The Pt 4flines are difficult to analyse because of the overlapping with A1 2p lines except for Pt-SiO, samples, while Pt 4d lines which are broad, impede accurate determination of binding energy values. However a precision of &0.3 eV was estimated. Analysis of the data given in table 2 indicates that : (i) no detectable differences in binding energy values can be observed no matter what the size of metallic particles (10 or 20 A in diameter) ; (ii) for atomically dispersed P t O a significant positive chemical shift was observed as in the case of Pd-Y zeolite (1.3 eV) ; (iii) a positive chemical shift was detected for Pt-Y zeolite (21 +0.7 eV) (10 or 20 A particles) and for Pt-SiO, (2: +0.3 eV) (20 A particles) as well with respect to sputtered Pt films. TABLE 2.-ESCA

DATA OF BINDING ENERGY VALUES FOR Pt IN ZEOLITE WITH PURE Pt AND ITS OXIDES Pt 463

Pt2+ion in cationic sites Pto atomically dispersed Pt0-10 A Pt0-20 A Pt* metal foil sputtered by A ions ~ t 0 - 2 0A/Si02 Pt-sputtered film Pt 0 Pt 0 2 Pt-foil Pt a d s Pt 0 Pt 0 2

333.8 333.0 332.2 332.2 331.8 332.0

-

Pt 4ds

Pt 4x3.

316.8 315.9 315.2 315.2 314.8 314.9

76.4

-

74.8 75.2 74.6 75.5 77.4 74.0 75.1 76.6 77.5

(6.5 % Wt)

fs

Pt 4

-

72.8 72.2 72.2 71.5 71.8 71.3 72.2 74.1 70.7 71.8 73.4 74.2

COMPARED

references

this work this work this work this work this work this work (20) (20) (20) (21) (21) (21) (21)

Such a positive shift was also postulated for highly dispersed Pt on silica 2 2 but could not be determined because of residual charging effect. However a positive shift was observed for Ag clusters 23 deposited on carbon substrate and was found to decrease when the cluster size increases. Such a shift may characterize an electron donation of the Ag cluster to the substrate but may also arise from changes in electronic relaxation en erg^.^^'^' The magnitude of the latter effect could not be evaluated, since electronic relaxation accompanying photo-ionization of a core electron and leading to a compensatory net transfer of charge density depends on the ability of surrounding atoms to release electron density to the ionized atom and is unknown. However, in our case chemical shift depends on the support and on the metal but not on the particle size. It may then be tentatively suggested that this shift corresponds to a net electron transfer from platinum to the support due to the interaction between the cluster and the substrate. Such a transfer may thus modify the redox properties of the support, as will be discussed in the next section. In the case of Pd aggregates on zeolite and of Pd or Pt on SiO, the chemical shift is small compared with the estimated error of kO.2 eV, indicating that there must be little, if any, electron transfer. E.S.R. SECTION

The adsorption of tetracyanoethylene or trinitrobenzene on to Pd or Pt loaded zeolites produced anion radicals identified by their e.s.r. spectra. Perylene and anthracene formed radical cations by adsorption on to these catalysts. Such phenomena are indicative of electron donor and electron acceptor properties of

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J . C . V E D R I N E , M . D U F A U X , C . N A C C A C H E A N D B . IMELIK

zeolites, respectively. Quantitative measurements of radical ion formation were performed to study the effect of Pd or Pt upon the redox properties of zeolites. In tables 3 and 4, are collected the relative concentrations of anion and cation radicals formed on Na-Y zeolite with various Pd or Pt contents and metal particle sizes. For comparison the results obtained on SiOz samples are also given. The changes in redox properties of NH,-Y samples loaded with Pd or Pt were also examined, the samples being treated in oxygen and then in hydrogen at 350°C (table 5). TABLE ~.-E.s.R.

DATA FOR CTC YIELDS ON ZEOLITE AND SILICA SAMPLES AS A FUNCTION OF HEAT TREATMENT CONDITIONS AND OF Pt PARTICLE SIZE

The dashes denote no experimental data and 0 denotes < 10'' spins g-'. The percentages in weight refer to Pt loading and the symbols Tl0C/T2"Crefer to the treatment temperatures under successive O2 flow and Hz flow respectively treatment conditions under 0 2 fl0w/H2 flow

35OoC/35O0C 10 A Pt particles

~ O O ~ C20/ A~Pt~particles ~ C

of catalyst)

radical ion concentrations (lo17 spins g'

Pe+

samples

zeolite

silica (15A Pt particles) TABLE 4.-E.S.R.

TCNF

TNB'

Pe+

A+

TCNE-

TNB-

0

0

1.4

0.14

0

0

1.6

0.2

0.7

0

10.8

0.12

0.7

0

3.0

0.17

1.2

0

16.7

0.23

0.8

0

7.4

0.3

4.6

0

23.

2.0

1.9

0

+ co

-

-

-

-

2.2

-

SiO, Pt-SiO, 2 % Pt-SiO, 2.6 %

0

-

0 0

-

0.1 0.2 0.13

-

Na-Y NaPt-Y 3.74 % NaPt-Y 7.84 % NaPt-Y 12.5 % NaPt-Y 12.5 %

11

Pd loading

(in wt %)

zeolite Na-Y Na Pd-Y NaPd-Y NaPd-Y silica Si 0, Pd-SiOz Pd-SiOz

treatment conditions . ~ ~ ~ T1/T2I0Cunder successive 0 2 flow and H2 flow, respectively

0.2

0.08

~

350/350 500/160 500/160 500/350 350/350 5OO/130 500/130

0 3.55 10.6 10.5 0 0.7 1.3

1.1

1.3

DATA FOR CTC YIELD ON ZEOLITE AND SILICA LOADED WITH AS A FUNCTION OF HEAT TREATMENT CONDITIONS

samples

TABLE 5.-E.S.R.

A+

~

.

~

.

_

~

Pd OR UNLOADED

_

radical ion concentrations (1017 spins g - of catalyst) TCNEPe+

1.4 4.2 2.4 1.3 0.1 0.2 0.3

0 0.4 5.1 0.6 0

PdO particles size/A

-

atoms and PdI ions atoms and Pd'ions 20

-

-

37 23

DATA FOR CTC YIELDS ON NH4-Y ZEOLITE TREATED IN OXYGEN FLOW AT 350°C AND REDUCED UNDER HYDROGEN FLOW AT 350°C

The percentages refer to Pt loading and the dashes denote no experimental data. samples

NH4-Y NH4Pt-Y 0.95 % NH4Pt-Y 1.85 %

radical ion concentrations (1017 spins g-' of catalyst) A+ TCNEPe+

11

0.07

0.2

-

-

1.5

-

0.02

3.6

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X.P.S.

OF

Pd

IN

Y-ZEOLITE

The results given in table 3 indicate that, in the case of Na-Y zeolite, both radical cation and anion concentrations increased sharply with Pt content. Furthermore, while the calcination temperature did not modify the electron donating capacity of unloaded Na-Y zeolite, the change in radical anion concentration was strongly dependent on the calcination temperature for Pt loaded samples, the better dispersed platinum (10 A) producing the highest effect. However, it is worthwhile to note that if the number of Pt atoms at the surface of metal particles is considered (the dispersion is about twice as small for 20 8, than for 10 I$ particles) this effect is of the same magnitude. All these data provide convincing proof that, upon reduction, Pt exchanged Na-Y zeolites acquired oxidizing properties similar to those of decationated zeolites.** However a different effect was observed with NH, Pt-Y zeolite since a decrease in the electron acceptor capacity (table 5) occurred. Si02 and Pt-SiO, exhibited no electron acceptor capacity while the electron donor properties remained almost unchanged with the platinum content, the number of TCNE- radical ions being two orders of magnitude lower than for those formed on Na-Y zeolite with the same Pt content. The poisoning effect of CO adsorption on the electron acceptor and donor properties of PtNa-Y samples was also examined. The results, collected in table 3, show that the electron acceptor properties of CO poisoned PtNa-Y remained almost unchanged, while the electron donor centres decreased to about the same value as the unloaded Na-Y sample. The electron donor and electron acceptor properties of supported Pd samples do not change noticeably with respect to unloaded zeolite or silica in contrast with Pt zeolite. This result implies that Pd produces no significant change in the electronic structure of the zeolite. DISCUSSION

Formation of the perylene and anthracene cation radicals has often been used for characterizing the electron acceptor properties of solid oxides. As no e.s.r. signal was observed upon perylene adsorption onto Si02 and Pt or Pd-Si02 it can be concluded that these samples did not present electron acceptor centres. In contrast, electron acceptor sites developed on Na-Y zeolites as the Pt content increased. Since the results obtained on Pt loaded silica suggested that Pt could not be involved in the formation of cation radicals, it may reasonably be suggested that during metallization of Pt2+ exchanged zeolites electron acceptor sites identical to those formed on decationated type Y-zeolites appeared, i.e. tricoordinated aluminium ions following the scheme given below : Hz

+

To

Pt2+02--Ze +. Pto Ze-OH +. decationated zeolite. The suggestion that platinum is not directly involved in the perylene cation formation is further supported by the CO adsorption experiments. It is well known that CO is strongly adsorbed on Pt metal. Our experimental data did not reveal the existence of any effect of CO on the electron acceptor properties of PtNa-Y samples. The decrease observed for the anthracene radical cation concentration (table 5) when Pt loading of NH,-Y samples was increased strongly suggests that platinum aggregates poisoned the ionizing active sites of the zeolite. It is well established * * that decationated Y zeolites possess electron acceptor sites strong enough to ionize anthracene molecules. The poisoning effect of Pt on the oxidizing properties of decationated zeolite might result from the formation of a charge transfer complex between Pt aggregates and Lewis acid sites (tricoordinated aluminium ions). This

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447

hypothesis implies that Pt aggregates are strongly interacting by electron transfer with the zeolite surface. Similar results and conclusions have previously been obtained for Pt supported on amorphous silica-alumina catalysts., * The authors have also suggested the existence of a charge transfer complex between the metal and the oxidizing site of the Si02-A1203 carrier. The concentration of TCNE radical anions formed on the zeolite increased with the Pt loading while silica gave almost identical radical concentration no matter what the Pt content. Although metal crystallites might serve as electron donor sites 2 9 the comparison of the results obtained for Pt-SiO, and Pt-zeolite samples militates against such an hypothesis, Indeed, for the samples characterized by approximately the same Pt content and particle size, the TCNE radical anion concentration was about ten fold less for Pt-SiO, than for Pt-zeolite samples. It is therefore possible to conclude that Pt does not act directly as an electron donor site for TCNE or TNB. It has already been established that the surface of an unloaded type Y-zeolite possesses reducing sites which may convert TCNE and TNB compounds into the corresponding anion radical. For Pt loaded zeolites the role of Pt in the electron transfer process may be to act as an electron donor compound. The transfer of a negative charge from the metal to the zeolite lattice would serve to increase the electron donating properties of the existing reducing sites. Thus, the sites, whose reducing power was not sufficient to convert adsorbed TCNE or TNB compounds into the corresponding anion radical in the absence of Pt, became active when Pt was present. Similarly it has been shown that a ten fold enhancement of the reducing power of zeolite occurred when electron donor molecules were adsorbed on the ~ u r f a c e . ~The ESCA results concerning Pt-zeolite samples showed a net decrease in the electronic density of Pt arising from a partial electron transfer from Pt to electron acceptor sites. This conclusion is in agreement with the suggestion that the interaction between Pt and tricoordinated aluminium ions is responsible for the decrease in oxidizing power of NH4Pt-Y zeolite and for the enhancement of its reducing properties. Furthermore when CO was adsorbed on Pt the strong bonding between CO and Pt depleted metal electrons resulting in a back donation of electrons from the zeolite to the metal. Thus, the charge density around the electron donor centres of the zeolite surface and the reducing power would decrease. It is worth noting that no difference in electron donor properties (reducing power) was observed for 10 and 20A Pt particles in zeolite. In the case of atomically dispersed PtO [treatment following procedure (i)] the TCNE radical anion concentration equalled 5.7 x lo1' per g for 12.15 % Pt content, i.e. was much smaller than for the other two samples (see table 3) of 10 and 20A particle sizes. It is then quite probable that this reducing power corresponded to the 20 % Pt aggregates of 20A in diameter contained in the sample, while atomically dispersed PtO has a negligible effect. The significant X.P.S. chemical shift observed for such Pto atoms should then mainly correspond to much smaller relaxation energy than for metallic clusters. The same explanation holds for PdO atoms. The weak donor properties of Pd-SiO, (table 4) determined from TCNEconcentration can be attributed to the Pd itself as it has been already proposed by Turkevich et aL1 However, in contrast with Pt loaded zeolite, the donor properties of Pd-zeolite do not noticeably change with respect to unloaded Na-Y zeolite. This result implies that the interaction of palladium with zeolite is small, in agreement with the small chemical shift observed by ESCA (table 1). The small increase in TCNE- concentration (table 4) observed for the 3.55 wt % Pd loaded sample reduced by H2 at 160°C may very well be due to the presence of Pd' ions which can act as electron donor centres. 1-1 5

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X . P . S . OF Pd

IN

Y-ZEOLITE


CONCLUSION

This work shows that ESCA and e.s.r. techniques are complementary. X.P.S. positive chemical shifts detected for metal Pt aggregates on zeolite compared with those for Pt on silica strongly suggest that electron donation occurs from the metal aggregates to the zeolite lattice,30 but eventual change in electron relaxation energy could preclude unambiguous assignment. E.s.r. data for charge transfer complexes show that the electron donor properties (reducing power) of the zeolite are sharply increased by Pt and that X.P.S. chemical shifts are, therefore, mainly due to electron donation from Pt aggregates rather than to relaxation energy changes. No differences in Pt donating properties could be detected for different particle sizes when considering number of Pt atoms at the surface of the aggregates. It was also suggested that charge transfer complexes are formed between the Pt aggregates and zeolite Lewis acid sites. The electron transfer from Pt to the support may thus modify the catalytic properties of the metal as has already been proposed. In contrast, such an electronic transfer was not detected by either techniques on Pd loaded zeolite. Atomically dispersed Pto and PdO are shown to give significant positive X.P.S. chemical shifts mainly due to smaller relaxation energies than for metal aggregates and not to modify the reducing properties of the zeolite surface. Pt and Pd ions in cationic sites on the zeolite are shown by X.P.S. to be bonded to lattice oxygen ions by ionic bonds, while more covalent bonds are involved in the case of Pt or Pd (NH3)3+complexes and to a larger extent in the case of PdO or PtO. Such differences in chemical bonding may sharply modify the catalytic properties of the ions. The authors acknowledge stimulating and interesting discussions with Dr. P. Gallezot, G. Hollinger, Y . Jugnet and Tran Minh Duc. M. Primet, J. M. Basset, E. Garbowski and M. V. Mathieu, J. Amer. Chem. SOC.,1975, 97, 3655. R. Dalla Betta and M. Boudart, Proc. Vth Znt. Congr. Catalysis (North-Holland, Amsterdam, 1973), vol. 2, p. 1329. C. Naccache, J. F. Dutel and M. Che, J. Catalysis, 1973,29, 179. F. Figueras, R. Gomez and M. Primet, Adv. Chem. Ser., 1973,121,480. G . D. Chukin, M. V. Landau, V. Ya. Kruglikov, D. A. Agievskii, B. V. Smirnov, A. L. Belozerov, V. D. Asrieva, N. V. Goncharova, E. E. Radchenko, 0. D. Konovalchikov and A. V. Agafonov, Proc. 6th. Znt. Congr. Catalysis, 1976 (The Chemical Society, London, 1977), p. 669. See for instance : K. Siegbahn, C. Nordling, A. Fahlman, R. Nordberg, K. Hamrin, J. Hedman, G. Johansson, T. Bergmark, S. E. Karisson, I. Lindgren and B. Lindberg, ESCA, Atomic Molecular and Solid State Structure Studied by means of Electron Spectroscopy (Almqvist and Weksells, Uppsala, 1967) ; C. K. Jargemen, Chimia, 1975, 29, 53 ; J. C. Vedrine, J. Microsc. Spectr. Electron., 1976, 1, 285. M. S. Ioffe, B. N. Kuznetsov, Yu. A. Ryndin and Yu. I. Yermakov, Proc. 6th Int. Congr. Catalysis, (The Chemical Society, London, 1977), p. 131 * F. R. Dollish and W. K. Hall, J. Phys. Chem., 1967,71,1005. B. D. Flockhart, M. C. Megarry and R. C. Pink, Adu. Chem. Ser., 1973, 121, 509. l o C. Defosse, R. M. Friedman and J. Fripiat, Bull. SOC.chim. France, 1975, 1513. l 1 M. Che, J. F. Dutel, P. Gallezot and M. Primet, J. Phys. Chem., 1976, 80, 2371. l 2 M. Primet and Y. Ben Taarit, J. Phys. Chem., 1977), 81, 1317. l 3 P. Gallezot and B. Imelik, Adu. Chem. Ser., 1973, 121, 66. l4 P. Gallezot, I. Mutin, G. Dalmai-Imelik and B. Imelik, J. Microsc. Spectr. Electron., 1976,1, 1. l 5 K. S. Kim, A. F. Gossmann and N. Winograd, Analyt. Chem., 1974,46,197. l6 D. Briggs, J. Electron. Spectr., 1976,9, 487 ; J. S. Brinen and J. L. Schmitt, J. Catalysis, 1976, 45, 274.

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J . C . V E D R I N E , M . DUFAUX, C . N A C C A C H E AND B . IMELIK

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