x-ray absorption spectroscopic study of platinum supported on sulfate ion-treated zirconium oxide

Applied Catalysis A: General, 102 (1993) 79-92

Elsevier Science Publishers B.V., Amsterdam

79

APCAT A2560

X-ray absorption spectroscopic study of platinum supported on sulfate ion-treated zirconium oxide

Kohki Ebitanil, Tsunehiro Tanaka’ and Hideshi Hattori

Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060 (Japan)

(Received 3 July 1992, revised manuscript received 22 March 1993)

Abstract

Platinum particles supported on a sulfate ion-treated zirconium oxide (SO:- -ZrO*) were character- ized by means of X-ray absorption near edge structure (XANES) /extended X-ray absorption fine structure (EXAFS) and temperature-programmed reduction (TPR) in order to elucidate the nature of the sulfur-aided metal-support interactions. X-ray edge shift and area calculation, which give the number of unoccupied d states of the platinum particles, are suggestive of electron transfer from the platinum particle to the support and/or existence of the platinum unreduced cations. Fourier transforms (FTs) of /?-weighted EXAFS reveal that the Pt particles on the SO:- -ZrO, support after hydrogen treatment at 623 K are a mixture of Pt metal ( Pt-Pt bond length = 2.77 A) and Pt oxides, whereas the Pt particles on ZrO, exist as metal (Pt-Pt bond len@h = 2.73 A). TEM reveals that the presence of sulfate ion causes an increase of the Pt particle size (30 A-50 A). The low reducibility of the Pt particles on the SO,‘- - ZrO, support is evidenced by TPR results. The nature of the platinum-support interaction in the Pt/ SO:- -ZrQ catalyst is understood in terms of a Redox (reduction-oxidation) metal-support interaction.

Key words: EXAFS; platinum; TPR; XANES; zirconia; X-ray absorption

INTRODUCTION

In previous papers [ 1,2] it was reported that the addition of platinum to SOi--ZrO, enhances catalytic performance in the skeletal isomerization of alkanes. The characteristic feature is that the activity of the catalyst (Pt/ SOi--ZrO,) f or skeletal isomerization persists for a long period when the re- action is carried out in the presence of molecular hydrogen, which could be

Correspondence to: Dr. H. Hattori, Graduate School of Environmental Earth Science, Hokkaido

University, Sapporo 060, Japan. Fax. (+81-3)7575995. ‘Present address: Department of Chemical Engineering, Faculty of Engineering, Tokyo Institute

of Technology, 2-12-1 Ookayama, Meguro-ko, Tokyo, 152, Japan. ‘Present address: Department of Hydrocarbon Chemistry and Division of Molecular Engineering,

Kyoto University, Kyoto 606-01, Japan.

0926-860X/93/$06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.

80 K. Ebitani et al. / Appl. Catal. A 102 (1993) 79-92

explained both by the removal of the coke by hydrogenation [ 1 ] and the generation of protonic acid sites on the support [ 2,3].

Another important characteristic feature of the Pt/SOz--ZrOz catalyst is the absence of alkane hydrogenolysis and alkene hydrogenation abilities of the supported platinum particles, even though platinum particles on a ZrOz support exhibit higher activities for both reactions [ 2 1. IR study of adsorbed CO on the Pt/SOz- -ZrOz catalyst revealed no appreciable band representing adsorbed CO molecules [ 21, suggesting a low concentration of the exposed surface metallic platinum sites. Thus the state of the platinum in the Pt/SO;- - ZrO, catalyst should be different from that of usual supported platinum. The presence of sulfur might cause changes in the electronic state of the dispersed platinum particles [ 4-61.

The electronic state and valence of dispersed metal particles [ 7-101 on the oxide supports can be conventionally analyzed by the XPS technique. Both Pt metal and Pt cations were detected in the hydrogen-treated Pt/SOi--ZrO,, the binding energy of 4f electrons (7/2 and 5/2) of the platinum metal on the SOi- -ZrO, support being lower than that on Pt/ZrOz [ 111. However, sepa- ration of the final-state effects (screening of charge) from initial-state effects (electronic interaction) in the origin of the binding energy shift is impossible [ 7-101. Besides XPS, information about the core electron binding energy can in principle be evaluated from the X-ray absorption edge position [ 121.

X-ray absorption edge and extended X-ray absorption fine structure spectroscopy (EXAFS) [ 131 can provide us with information about the electronic and atomic structure in the immediate vicinity of the absorbing atom, without the stringent requirement of long-range structural order. Thus, these spectroscopic techniques provide us with structural information about the highly dispersed supported metals [ 14-171.

The sharp and narrow absorption bands at the L3 and L2 X-ray absorption edges, which are called white lines [ 181, correspond to the electronic transition from 2p core level states, 2p 3,2 and 2p,,,, respectively. The final vacant d states for each edge are ds12, d5,2 and d3,2 of the absorbing atom, respectively [ 18,191. Using the X-ray absorption L edges (both L3 and L2), Mansour et al. [ 201 have developed a quantitative evaluation of the number of holes in the d bands of the dispersed platinum particles on TiO, [ 141, A1203 and Si02 [21]. We have also used Mansour’s method for our catalyst system.

Additionally, temperature-programmed reduction (TPR) [22,23] was performed in order to obtain information about the ‘reducibility’ (reactivity with hydrogen molecules) of the supported platinum particles.

EXPERIMENTAL

Preparation

The method of preparation of the Pt/SOz- -ZrO, and Pt/ZrO, samples (5.0

K. Ebitani et al. / Appl. Catal. A 102 (1993) 79-92 81

wt.-% Pt) has been described elsewhere [3]. The amount of sulfur that re- mained in the resulting catalyst was 1.5 wt.-%, as determined by XRF. The mean particle sizes determined by the TEM histograms are 30 +- 5 A and 50 + 5 A for the Pt/ZrO, and the Pt/SOi- -ZrOa samples, respectively.

Ch4zracterization

XANES/EXAFS The platinum L edge (L3, L2) X-ray absorption spectra were obtained at

room temperature in a transmission mode at EXAFS facilities installed on BL- 6B line of the Photon Factory at the National Laboratory for High Energy Physics (KEK-PF), Tsukuba, Japan, with a ring energy of 2.5 GeV and a storage positron current of 340-280 mA, using a double-crystal Si (111) mon- ochrometer. The X-ray beam height is 1.0 mm at 25 m from the X-ray source. Energy calibration was performed using Cu K-edge absorption (8981.0 eV). The energy step of the measurement in the XANES region was 0.3 eV.

The samples were pretreated at 623 K for 2 h in flowing hydrogen (30 ml min-‘), cooled to room temperature in the presence of hydrogen, then evacu- ated for 15 min. The reduced samples were mixed with polyethylene and pressed into thin wafers in a nitrogen-filled glovebox without exposure to air. Sulfided Pt/ZrOz was obtained by heating the Pt/ZrO, in flowing H,S/H, (30 ml min-’ ) at 623 K for 2 h before subjecting to the X-ray absorption measurement using the same procedures described above.

Evaluation of number of unoccupied d state The fractional change in the number of d band vacancies (unoccupied d

states) from that of bulk Pt, fd, can be defined as [ 20 ]

f~=d(hT)/(hT)Pt=:(~As+l.ll~An)I(A~+1.11A2)Pt 0)

where: d(h) = (hTLmple- V&G dA3= (A3Lmple- (A&; dA2= (A2Lmple - (AZ)R; h,,. = total number of unoccupied d states; Aj= edge area for the jth edge; Pt = bulk platinum.

The data are normalized so that the EXAFS of both edges symmetrically overlap one another at energies greater than 40 eV above the X-ray absorption edge to compensate for the adsorption coefficient due to the s states [20], as shown in Fig. 1. The areas are determined by using the part of the X-ray absorption spectrum that extends from 10 eV below the edge to 15 eV above the X-ray absorption using Simpson’s method.

Analysis of EXAFS data The extraction and normalization of the EXAFS oscillation was performed

by a method reported previously [ 24-261. Fourier transformation (FT) of the Iz3-weighted normalized EXAFS oscillation was performed over the


I I I I I I

-20 0 20 40 60

Energy /eV

Fig. 1. Normalized L, X-ray absorption edge of the Pt foil (solid line) plotted over the normalized Lz X-ray absorption edge (dashed line).

3.5 <k-c 13.5 A-’ range in order to obtain the radial structure function [ 27,281. The N (coordination number of scatterers), R (distance between an absorbing atom and scatterers) and Debye-Waller factor were estimated by curve-fitting analysis with the inverse FT (2.3 <R < 3.1 A), assuming single scattering [ 131. Parameters were obtained from the EXAFS of the Pt foil.

Data reductions were performed with the FACOM M-780 computer system of the Data Processing Center of Kyoto University.

TPR The TPR apparatus and procedures used in this study have been described

previously [28]. Prior to the TPR measurement the sample was calcined at 573 K for 2 h, and heated in flowing Ar in situ at 573 K for 2 h. The heating rate was 10 K min-l.

RESULTS

L edges and XANES

Fig. 2 shows the normalized L, and L2 absorption structure of the Pt/ZrO, sample after hydrogen treatment at 623 K in comparison with those for Pt foil (dotted line). The energy offset was taken to be the position of each edge. The areas of both L3 and L, absorption edges are significantly larger for the Pt/ ZrO, than for the Pt foil.


o. 1 I I 1 I 1 I 1 I -20 0 20 40 60

Energy /eV

Fig. 2. Normalized L, (a) and L, (b) X-ray absorption edges of the hydrogen-treated Pt/ZrOz (solid lines) plotted over the L, and L, absorption edges of the Pt foil (dotted lines).

Fig. 3 shows the normalized L3 and L2 absorption structure of the Pt/ SOi- -ZrO, sample after hydrogen treatment at 623 K in comparison with the Pt foil absorption edges (dotted line ). The areas of both edges are larger than those of the Pt foil. The differences between the white lines of the Pt/SOz- - ZrOz and the Pt foil are larger than those obtained by comparing the Pt/ZrO, and the Pt foil, indicating that there are more unoccupied d states in the Pt/ SO;- -ZrOz sample than in the Pt/ZrO, sample.

For both the Pt/ZrO, and the Pt/SO:- -ZrO, samples, the absorption structure in the range of O-40 eV above both L edges is not so clear as that obtained for the bulk Pt.

Table 1 summarizes the results of near edge X-ray absorption (L edge shifts, L edge areas, numbers of unoccupied d states). The L edge (L3 and L2 edges)


a

1 7% . . . . . . : . . . . . . . . . .

2 .$ E

z 1 i”--‘ Energy /eV

Fig. 3. Normalized L, (a) and Lz (b) X-ray absorption edges of the hydrogen-treated Pt/SOi-- ZrO, (solid lines) plotted over the L, and L, absorption edges of the Pt foil (dotted lines).

positions are referred to those of the Pt foil. The edge areas, fd (fractional change in the number of unoccupied d states) and bs (number of unoccupied d states per atom) are calculated by Mansour’s method, assuming that the number of unoccupied d states per platinum atom for the bulk Pt (br) is 0.3 [ 201 using the following equation (rearrangement of Eqn. 1):

hTs = (l.o+fd)h (2)

The L, and L, edge positions of the platinum particles supported on both the ZrOz and the SO:- -ZrOz shift to higher energies. The extent of the positive edge shift is larger for the Pt/SOz--ZrO, than for the Pt/ZrO, sample. For the sulfided Pt/ZrO, sample, the shifts in L3 and Lz edges are +0.7 eV and

K. Ebitani et al. / Appl. Catal. A 102 (1993) 79-92

TABLE 1

Near-edge X-ray absorption parameters of the samples after H, treatment at 623 K

85

Sample L:, edge L, edge AA, A& fd h Tan

(shift/eV ) (shift/eV)

Pt/ZrO* +0.2 +0.3 0.234 0.299 0.106 0.332 Pt/SO:- -ZrOp +0.6 +0.7 0.547 1.121 0.334 0.400

Pt foil 0.0 0.0 0.00 0.30

Estimated error +0.15 +0.15 +0.06 +0.06 +0.01 +0.01

“Number of unoccupied d states per atom.

+ 1.0 eV, respectively. The positive shifts in the L, edge are always larger than those of the L3 edges for all samples.

It can be seen that the hT, for the platinum particles supported on both ZrOa and SO:- -ZrOz are always larger than that for the Pt foil. In addition, the Pt particles on the SOi--ZrO, support have larger bs than the Pt particles on the ZrOa support. The charge transfer from the Pt particles to the SOi- -ZrOz support could be expected from the edge studies.

EXAFS

Fig. 4 shows the Fourier transforms of Iz3-weighted EXAFS for the Pt foil and the samples after oxidization at 773 K and hydrogen treatment at 623 K (phase shift was not corrected for). The FT magnitudes of EXAFS are smaller for the supported Pt samples (after oxidation and hydrogen treatment) than for the Pt foil, suggesting that the Pt particles supported on both Zr02 and SOi- -ZrOz are smaller than the Pt foil. The support effect is clearly observed on the oxidized samples. Two peaks, at about 1.5 and 2.7 A, are observed for the oxidized Pt/SOz- -ZrOz (c), whereas a single peak is detected at 1.5 A on the oxidized Pt/ZrO, sample (d).

After hydrogen treatment, the peak around 2.5 A, which is close to the main peak observed for the Pt foil, is observed for the Pt/ZrO, and the Pt/SOi-- ZrO, samples. This peak corresponds to the Pt-Pt shell in the metallic Pt phase. This peak intensity is larger for Pt/SOz- -ZrO, than for the Pt/ZrOz sample, indicating the larger number of neighboring Pt atoms (in the Pt metal phase) on Pt/SOz--ZrO,.

In addition to the peak at 2.5 A, a peak at about 2 A can be detected for the hydrogen-treated Pt/SOz--ZrOe sample. This peak position is close to those of the first peak for the oxidized Pt/SOz--ZrO, (c) and for the sulfided Pt/ ZrO, (b).


30 a

20

10

8

6

4

2

0

4

2

0

m e

0 2 4 6

Distance /A

8-d

0 2 4 6

Distance /_.&

Fig. 4. FT magnitudes of k3-weighted EXAFS functions. (a) Pt foil, (b) sulfided Pt/Zr02,(c) oxidized Pt/ZrO,, (d) oxidized Pt/SOi--Zr02, (e) hydrogen-treated Pt/Zr02, (f) hydrogen- treated Pt/SO:- -Zr02.

In order to obtain more detailed information on the structural parameters, we performed a curve-fitting analysis. Table 2 shows the EXAFS parameters (N= coordination number; R = distance; Debye-Waller factor) of the Pt-Pt shell of the hydrogen-treated Pt/ZrO, and the Pt/SOz--ZrOz samples. The interatomic distance of the Pt-Pt shell of the hydrogen-treated Pt/ZrOz (2.73 A) is smaller than that of the Pt foil (2.77 A), whereas the Pt-Pt bond distance of the hydrogen-treated Pt/SOz- -ZrO, is the same as that of the Pt foil.

The average coordination numbers (error + 10% ) of the first sphere in the platinum metal particles of the Pt/ZrOz and the Pt/SOi--ZrO, samples are smaller than that of the Pt foil. This confirms the smaller platinum particle sizes in both the Pt/ZrOz and the Pt/SOz- -ZrOz than that of the bulk Pt foil. The platinum particle sizes are evaluated from the coordination number (N) obtained for the Pt-Pt shell according to the method proposed by Greegor and


TABLE 2

EXAFS parameters for Pt-Pt shell of the samples after H2 treatment at 623 K

87

Sample R,‘A N D.W./A” Pt/ZrOz 2.73 2.2 0.0035 Pt/SO:- -ZrOa 2.77 4.9 0.00199

Pt foil (2.77) (12) (0)

Lytle [30], assuming spherical particles in face-centered cubic packing. For the Pt/ZrO, sample, NT (total number of atoms in the particle) was lo-30 atoms. The diameter of the platinum particle on the ZrOp support is 6-10 A. For the platinum particle on the SOi- -ZrOz support the diameter is estimated to be lo-20 A.

TPR

Fig. 5 shows the temperature-programmed reduction (TPR) profiles of the Pt/ZrO, (a) and the Pt/SOi- -ZrO, (b) samples. The features of the profiles are entirely different if the ZrOs support contains sulfate ion. The magnifica- tions are different for the two samples. The H, consumption was about ten times larger for the Pt/SOz- -ZrO, sample because the TPR peak includes reduction of both platinum particle and sulfate ions. The peak at about 800 K, observed for the Pt/SOi--ZrOa sample, is ascribed mostly to the reduction of sulfate ions. It is likely that reduction of platinum species starts at about 673 K. For Pt/ZrOz, reduction of platinum species starts at about 400 K and shows a maximum at about 473 K. The platinum particles on SOi- -ZrOz are much more difficult to reduce than those on ZrO,, the difference in the reduction temperature being about 200 K. The number of the hydrogen molecules con-

b

II 400 600 800

Temperature /K

Fig. 5. Temperature-programmed reduction (TPR) profiles of (a) Pt/ZrOz and (b) Pt/SOi-- ZrO,. Heating rate= 10 K min-‘.


sumed per total amount of platinum for the Pt/ZrO, sample is ca. 2, indicating complete reduction of the platinum particles on the ZrO, support after hydrogen treatment at 623 K [ 11,291.

DISCUSSION

For both L3 and L2 absorptions of the platinum particles dispersed on both the ZrO, and the SOi--ZrOa supports, the absorption structures in the range of O-40 eV (XANES region) are not as clear as those for the Pt bulk. This is attributed to a reduction in coordination number of Pt in going from the foil to clusters [ 14,311. The decreases in the Pt particle sizes of the Pt/ZrOz and the Pt/SO:- -ZrOz relative to the Pt foil are supported by smaller coordination numbers (N) ( < 12)) determined by curve-fitting analysis of the inverse FTs (Table 2) and the TEM measurements.

In addition, the unclear part of the XANES, beyond the edge, may be caused by the superposition of a platinum oxide phase on the platinum metallic phase. Mixing of a non-negligible amount of the platinum oxide phase makes the spectrum less characteristic and plain. This also results in a decrease in the coordination number of the adjacent Pt atoms, as determined from curve-tit- ting analysis. The inconsistency of the particle sizes estimated from EXAFS and TEM may be due to such a mixing phenomenon.

In principle, the position of the absorption edge is related to the valence of the absorbing atom as the binding energy obtained from core-level electron spectroscopy (XPS) [ 13,211. The shift due to the final state effect (screening of charge by neighboring electrons) observed in XPS [g-10] should be ex- cluded in the X-ray absorption edge shift. Thus, the edge shifts to higher energies (Table 1) suggest that the states of the dispersed Pt particles on the Pt/ SO:- -ZrOz and the Pt/ZrO, samples are electron deficient in comparison to that of the Pt foil, which might be explained both by the electron transfer from the Pt particles to the supports and the existence of Pt cations.

The extent of the positive edge shift of X-ray absorption is larger for the Pt/ SOi- -ZrOz than for the Pt/ZrO, (Table 1) . This means that the core level electron binding energy of the Pt particles on the SOi- -ZrO, is larger than that of the Pt particles on the ZrOz support, suggesting the the Pt particles are more cationic on the SO:- -ZrO, support than on the ZrOz support.

For the sulfided Pt/ZrO, sample, the shifts in the L3 and L2 edges are + 0.7 eV and + 1.0 eV, respectively. These shifts agree with the previous report that the conversion from metal to sulfide shows up as an increase in the core-electron binding energies of the metal [ 321. The presence of a PtS phase in the Pt particles on the SOz--ZrOa after hydrogen treatment at 623 K might be one of the reasons for the positive edge shifts.

Consistent with the absorption edge shift, the numbers of unoccupied d states per atom ( hTs) of the dispersed Pt particles are always greater than that of the


bulk Pt (Table 1) . This also suggests an electron transfer from the Pt particles to the support oxides, as suspected from the results of the absorption edge shifts.

Since it is expected that the Pt particles exist in the form of Pt oxides after oxidation at 773 K, the peak at 2.5-3 A (Fig. 4d) is due to the Pt-Pt shell in the Pt oxides. This Pt-Pt peak (in Pt oxides) is not observed on the oxidized Pt/ZrOz sample. This suggests that the Pt particle size is larger on SO:- -ZrOz than on ZrOz.

The previous in-situ XPS investigation showed that most of the sulfur exists as S6+ after hydrogen treatment at 623 K, the conversion from S6+ to S2- occurs significantly on hydrogen treatment above 623 K [ 111. The valence of the sulfur in the PtS phase is considered to be 2-. Therefore, the possibility of PtS formation on the Pt particles for the Pt/SOz- -ZrOg after hydrogen treatment at 623 K is expected to be small. In addition, the formation of PtS on the Pt particles on the SOi- -ZrO, after hydrogen treatment at 623 K is expected to occur at the surface of the Pt particles; therefore, the contribution of the Pt-S shell to the EXAFS might be small since EXAFS gives the average structure. The peak at about 2 A might be due to the Pt-0 shell.

The shift of the hydrogen consumption peak to a higher temperature observed for the Pt/SOi- -ZrO, sample (Fig. 5) indicates a low reduction rate for the Pt/SOi--ZrO, sample as compared with the Pt/ZrOz sample. This indicates the incomplete reduction of the Pt particles on SOi- -ZrOz at 623 K; i.e., the presence of unreduced Pt oxides after hydrogen treatment at 623 K. It is thus proposed that the Pt-0 shell detected on the Pt/SOz- -Zr02 after hydrogen treatment originates from the unreduced Pt oxides. The presence of unreduced Pt cations (possibly PtO) was also suggested by the XPS result [ 171 on the hydrogen-treated Pt/SOz--ZrOz sample. The observed X-ray absorption edge shift and increase in the number of unoccupied d states per atom (Table 1) for the hydrogen-treated Pt/SOz- -ZrO, relative to the Pt/ZrOz must be due to the presence of unreduced Pt oxides rather than to electron transfer from the Pt particles.

The estimated diameter (from coordination number, N) of the Pt particles on the ZrO, (06-10 A) is smaller than that of the Pt particles on the SO:-- ZrO, (lo-20 A). This is confirmed by the TEM observation, although the particle sizes estimated from N are smaller than the mean particle sizes determined by TEM. The influence of the oxide supports on the Pt dispersion (particle size) was investigated by Adamiec et al. in terms of the acid-base properties of the support oxide [33]. According to these authors, the basic sites are re- sponsible for increasing dispersion during the 0, treatment because they can trap Pt species, thus retarding their migration, which results in an increase in the Pt dispersion. The addition of sulfate ion to ZrO,, results in the elimination of basic sites and the generation of strong acid sites [34-371, which makes the


migration of Pt species easier. This would lead to an increase of the Pt particle size on the SOi- -ZrOz support.

Metal-support interaction in Pt/SOz- -ZrO,

Because SMSI behaviors have not hitherto been reported for the metals supported on the ZrO,, the Pt/SOz- -ZrOz catalyst may possibly exhibit another type of metal-support interaction. With the Pt/Al,O, catalyst, the evi- dence for the sulfur-aided metal-support interaction has been presented after hydrogen treatment at 773 K [ 5-71. Kunimori et al. [ 51 have interpreted the decrease in H2 chemisorption in Pt/Al,O, as the result of sulfur-catalyzed alloy formation (deep reduction by hydrogen). In the present case, however, no fur- ther reduction of the catalyst occurred upon hydrogen treatment at 623 K, judging from the TPR and XPS results [ 111. Apestiguia et al. [6] have explained the change in the properties of the Pt particles upon the sulfur treatment in terms of surface poisoning (formation of PtS). In the case of Pt/ SO:- -ZrOg, most of the Pt cations formed on the hydrogen-treated Pt/SO2,- - ZrO, are considered to be those forming Pt oxides. The concentration of PtS on the Pt/SOi- -ZrOz is negligibly small, as discussed above. Thus, the sulfur- aided metal-support interaction in the Pt/SOi- -ZrOg catalyst seems to differ from that in the Pt/A1203 catalysts [ 5-71.

The observed metal-support interaction, resulting in a low reducibility of the platinum particles for the Pt/SO;--ZrO, can be interpreted in terms of the Redox (reduction-oxidation) effect. This type of metal-support interaction has been proposed by Turlier et al. [38] as a different type of metal- support interaction than SMSI. According to these authors, the reducibility of the dispersed metal particles depends on the acid-base properties of the support oxide; reduction of metal particles is facilitated on basic support oxides. This study also proposed that the strong acidity of the support (SO:- -ZrOa) retards the reduction of the dispersed metal particles. The changes in the X- ray absorption edge ( L3 and L2 edges) and edge areas of the hydrogen-treated Pt/SOz- -ZrO, are explained by the presence of a considerable amount of unreduced Pt cations.

ACKNOWLEDGMENTS

We thank Mr. Susumu Mochizuki for technical advice on the TEM measurements. We thank Professor Masaharu Nomura of KEK-PF for helpful technical advice. The X-ray absorption experiments were performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 90-154). This work is supportedby a Grant-in Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan.


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x-ray absorption spectroscopic study of platinum supported on sulfate ion-treated zirconium oxide

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