An amorphous approach to the structure of a catalystPt–Fe/c-Al2O

3characterized by XAFS

Li-wu Lin,¤ Yuan Kou,* Ming Zou and Zhen Yan

College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

Received 18th January 2001, Accepted 6th March 2001First published as an Advance Article on the web 29th March 2001

A catalyst has been investigated by means of Pt Fe K-edge X-ray absorption ÐnePtÈFe/c-Al2O3 LIII-andstructure (XAFS). X-Ray absorption near-edge structure (XANES) clearly indicates the formation of highlydispersed zerovalent Pt atoms on the surface after reduction. The iron precursor is reduced toFe2O3well-dispersed FeO. Six Pt neighbors at 2.72, 3.14 and 3.83 respectively, are found for each Pt atom onA� ,

However, only one type of Pt neighbor, at 2.75 with a low coordination number of 0.8, isPt/c-Al2O3 . A�detected for the Pt sites on An amorphous approach to rationalizing a possible structure ofPtÈFe/Al2O3 .

has been discussed at the atomic level by consideration of the surface coordinate geometry. ThePtÈFe/Al2O3XAFS-derived parameters are highly commensurate with a sandwich model. An iron oxide phase with thestoichiometry is present as an interface between Pt and while zerovalent Pt atoms areFe17O19 c-Al2O3randomly distributed on the iron oxide surface as a mixture of monomers, dimers and trimers, well separatedfrom each other.

1. IntroductionThe addition of a second metallic component to supportedplatinum catalysts results in a series of bimetallic platinumcatalysts. Investigation of such catalysts has been a topic offundamental interest for more than 30 years but detailedinvestigation has only been possible more recently followingthe development of X-ray absorption spectroscopy.1h13 Basedon structural studies of the Pt active center, it has beendemonstrated that addition of the second component signiÐ-cantly improves the dispersion of the Pt atoms. The averagesize of the Pt particles, for example, is reduced from 110 inA�diameter on 1.0 wt.% to about 35 onPt/Al2O3 A�

The electronic properties of the second com-PtÈRe/Al2O3 .9ponent, for instance the reducibility of the Re species8 or theinterface function of the Sn and Fe species10h12 make a sig-niÐcant contribution to the catalytic behavior of the platinumin such bimetallic phases. Most recently, EXAFS (extendedX-ray absorption Ðne structure) studies of a cluster-derivedmodel catalyst suggest a simpliÐed model for PtÈRe/c-Al2O3at the atomic level.13,14 units are proposed to be theRe2Ptbuilding blocks of the supported bimetallic cluster, which canbe represented to a good approximation as moietiesRe4Pt2residing on the (111) face of It is of general interestc-Al2O3 .to note that this simple model provides a good conceptualbasis and/or starting point for the analysis of metalÈsupportinteractions in more complex bimetallic working catalysts.

There still remain many difficulties however in using con-cepts derived from the cluster-based system to rationalize thestructure of a working catalyst. This stems from the highlyamorphous features of the surfaces in the latter systems.Unlike the cluster-derived system, few of the many workingcatalysts are believed to have uniform ensembles on thesurface. Small clusters consisting of several well-ordered atomsare extremely difficult to form, especially under reaction con-ditions. The XAFS technique is a very sensitive probe of such

¤ Guest Professor of Peking University. Affiliated with the State KeyLaboratory of Catalysis, Dalian Institute of Chemical Physics,Chinese Academy of Sciences, Dalian 116023, China.

short-range-ordered systems. In contrast to crystallography,however, XAFS provides only limited information aboutradial structure. The site-symmetries, coordination numbersand interatomic distances obtained are averaged over all thedi†erent types of coordinations. Thus, when a structuralmodel is considered, these XAFS-derived parameters are oftenfound to be consistent with more than one possible structure.This phenomenon is usually considered to be a technical limi-tation of XAFS but is seldom recognized as a limitationcaused by the amorphous system itself. Highly dispersed sup-ported catalysts are just such a case. It is noteworthy thatXAFS observes catalysts in the same manner as that describedin the physics of amorphous solids,15 and thus enables chem-ists to gain an insight into the structure of these extremelynon-uniform phases in detail. However, the lack of a funda-mental approach rather than the limitation of the technique,usually makes understanding difficult and conclusions areoften thought to be speculative.

In this paper, we present an amorphous approach to under-standing the structure of a working catalyst byPtÈFe/Al2O3using XAFS at both the Pt and the Fe K-edges. OurLIII-previous studies of by temperature-programmedPtÈFe/Al2O3reduction (TPR) with in situ 57Fe spectroscopyMo� ssbauerhave indicated complete reduction of the Fe3` to Fe2` intetrahedral (58% based on peak area) and octahedral (42%)vacancies corresponding to a TPR peak at 502 ¡C.11 COadsorption microcalorimetry studies show that, judged by thedistribution of the di†erential enthalpy of CO adsorption, themaximum appearing at 110È120 kJ mol~1 for Pt/c-Al2O3shifts to 100È110 kJ mol~1 for In this paper,PtÈFe/Al2O3 .10we report an XAFS investigation of the structural details, andthen depict the geometrically most probable model for the

by consideration of the surface coordinatePtÈFe/Al2O3geometry.

2. ExperimentalCatalyst samples were prepared by the Dalian Institute ofChemical Physics, Chinese Academy of Sciences.10h12 Threecatalysts, 0.375 wt.% and 0.375Pt/c-Al2O3 , PtÈSn/c-Al2O3

DOI: 10.1039/b100690h Phys. Chem. Chem. Phys., 2001, 3, 1789È1794 1789

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Table 1 EXAFS Ðts of some reference samples

Sample Edge na S02 Shell CN Distance p2 R-factor

Pt foil Pt LIII 2 0.75 PtÈPt 12.0 2.76 0.004 0.11a-PtO2 Pt LIII 2 0.75 PtÈO 6.0 2.02 0.002 0.11

PtÈPt 6.0 3.09 0.002PtÈO 12.0 3.76 0.012

Fe2O3 Fe K 3 0.75 FeÈO 6.0 2.00 0.004 0.08Fe3(CO)12 Fe K 3 0.55 FeÈCt 3.1 1.83 0.003 0.08

FeÈCb 1.6 2.07 0.006

a In kn.

wt.% PtÈ0.537 wt.% (Fe/Pt atomic ratio of 5 : 1)Fe/c-Al2O3were studied. The BET surface area of the was 156c-Al2O3m2 g~1. The catalyst was prepared by impregnat-Pt/c-Al2O3ing the with an aqueous solution ofc-Al2O3 H2PtCl6É6H2O,with 2% HCl added as a competing adsorbate.The and bimetallic catalysts werePtÈFe/Al2O3 PtÈSn/Al2O3prepared by the sequential impregnation method. Forexample, the precursor was Ðrst prepared byFe/c-Al2O3impregnation with an aqueous solution, followed byFe(NO3)3drying and calcination. Subsequently, the cata-PtÈFe/Al2O3lyst was prepared by impregnation of the with anFe/c-Al2O3aqueous solution of followed by drying and calcina-H2PtCl6 ,tion. All catalysts were dried at 60 ¡C for 4 h then at 120 ¡Cfor a further 4 h. After drying, the catalysts were calcined at250 ¡C and then at 480 ¡C in air for 4 h, followed by calcina-tion in steam for a further 4 h to remove the chloride ions.The catalysts were reduced in Ñowing at 480 ¡CH2(atmosphere pressure, Ñow rate 30 ml min~1) for 1 h. All oper-ations and characterizations were performed strictly withoutexposure to air or moisture. Samples maintained under highpurity (99.99%) were sealed in Ñasks. Specimens wereN2made by directly applying the Ðne powders to Scotch tapeinside a glove box kept under the high purity (99.99%),N2sealed by the tape and the XAFS data were then measuredimmediately.

References used were Pt foil (Pta-PtO2 , LIII-edge), Fe3O4and (Fe K-edge). Correction of the coordinationFe3(CO)12number in highly dispersed systems was made by comparisonof bulk with cluster-derived as describedFe2O3 Fe3/cAl2O3previously.16

The Pt and the Fe K-edge XAFS data wereLIII-edgeobtained at the Photon Factory (Tsukuba, Japan) on beamline BL-7C. An Si(111) double-crystal monochromator with apositron beam energy of 2.5 GeV and an average storedcurrent of 250 mA was employed. Data were collected intransmission mode at room temperature. Spectra in the

Fig. 1 Best Ðts (dashed line) to the Pt EXAFS-derivedLIII-edgeFourier transforms (upper) and EXAFS oscillations Ðltered of Pt foiland a-PtO2 .

XANES regions ([20 to 50 eV about the edges) were scannedin steps of 1.0 eV for the and 0.5 eV for the K-edge.LIII-edge,The signal was integrated for 4 s at each point. The energyscale of the Pt L-edge (11 559.0 eV) and the Fe K-edge (7111.0eV) was calibrated relative to Pt foil and respectively,Fe3O4using the Ðrst derivative of the spectrum. The energyresolution was about 2.0 eV for the L-edge and 1.5 eV for theK-edge.

The data were processed on a PIII 450 microcomputerusing an improved version (XPKU) of EDAPN developed bythe Institute of Physics, Chinese Academy of Sciences.16h18The XANES spectra were normalized by Ðtting a cubic spline

to the EXAFS region of the data and by Ðtting a lineark0function to the pre-edge data. The spectra were then furthernormalized to unit step height. The EXAFS oscillation wasnormalized by the curve then Fourier transformed in thek0range k \ 1.35 to 13.5 for the K-edge and k \ 3.5 to 14.1A� ~1

for the The oscillation was Ðrst Fourier trans-A� ~1 LIII-edge.formed into r-space weighted by k1 and then the nearestoxygen peak was Ðltered into k-space. A least-squares curveÐtting was performed to determine the parameters of thenearest oxygens. After this pre-processing procedure, the knEXAFS (n \ 2 for the Pt edge and n \ 3 for the Fe edge) wasFourier transformed and the peaks of interest were entirelyÐltered. A Ðnal least-squares curve Ðtting was performed. Thetheoretically calculated components were determined by theFEFF6.0 code.19 The best Ðt was initially chosen by R-factor,and was then transformed into r-space weighted by k0 withthe same range to give a Ðt of the Fourier transform. Theparameter error estimates were calculated by the recommend-ed method.20 The correlation between the variables was esti-mated by the standard method.21

The details of the EXAFS data analysis have been describedelsewhere.17,18,22 A portion of the best Ðts of the referencesamples is summarized in Table 1. The best Ðts to the Pt LIII-EXAFS of and Pt foil are shown in Fig. 1.edge a-PtO223

Fig. 2 The Pt XANES spectra (left) and the Ðrst derivativeLIII-edgespectra of A: BÈD: reduced andPtO2 ; Pt/c-Al2O3 , PtÈSn/c-Al2O3respectively, E : Pt foil.PtÈFe/c-Al2O3 ,

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Fig. 3 Comparision of the Fourier transforms of (a), (b) and (c) before (thin line) and after reduction.Pt/Al2O3 PtÈSn/Al2O3 PtÈFe/Al2O3

3. Results and discussion3.1. Chemical state of the Pt on the surfaces

Fig. 2 illustrates the Pt XANES spectra of theLIII-edgesamples. The features in the region originate from theLIII-edgetransition of electrons to empty d orbitals.24 The lower2p3@2the d orbital occupation, the more intense the white line(absorption maximum) observed. It is known that the inten-sity of the white line gradually decreases as the absorbingmetal changes from Re, Os, Ir, Pt to Au because the d orbitalsare Ðlled in the same order. For example, there is a weakwhite line in the of bulk Pt but no white line in theLIII-edgeedge of bulk Au since the d orbitals of zerovalent Au are fullyoccupied. The same situation is illustrated by the Pt LIII-edgeXANES shown in Fig. 2. The platinum in is Pt4`, withPtO24 electrons less than the platinum in Pt foil and the white line

transition) observed for is therefore much(2p3@2] 5d PtO2more intense than that for Pt foil, as expected. It can be seenfrom Fig. 2BÈD that the Pt species after reduction are,without exception, in the zerovalent, metallic state.24

The general features of the EXAFS of and Pt foil arePtO2illustrated in Fig. 1. The EXAFS-derived Fourier transformsindicate the distribution of the neighboring atoms surround-ing the Pt centers. The Pt center in is octahedrally sur-PtO2rounded by 6 nearest oxygens and a strong peak (thestrongest one in the spectrum, Fig. 1) is observed withinPtO22 (phase shift is not corrected). The contributions of theA�nearest Pt neighbors and the second-nearest oxygens are thenobserved at about 3 Because each Pt center in crystallineA� .

is surrounded by 6 nearest Pt4` and 12 second-nearestPtO2O2~, the double peaks at about 3 are highly intense andA�well resolved. In fact, the peak is a Ðngerprint of long-range-order structure. On the other hand, each Pt atom in Pt foil issurrounded by 12 Pt atoms forming a close-packed structure,so a very strong single peak is observed in the range of 2 to

as expected.3 A� ,The Fourier transforms of the EXAFS for andPt/Al2O3before and after reduction are illustrated in Fig.PtÈFe/Al2O33. For comparison, the transform of is alsoPtÈSn/Al2O3included. Before reduction, the most intense peak in the

spectra of all the samples is at about the same position as thatobserved for but the double peak observed in the spec-PtO2 ,trum of at about 3 is of signiÐcantly lower intensity,PtO2 A�indicating that the oxide precursors are well dispersed on thesurface. In the Fourier transforms obtained after reduction ofthe samples, the intensities observed for all of the peaks areattenuated dramatically, implying a signiÐcant decrease incoordination number. It can be seen that a group of peakswith almost the same intensity is observed for in thePt/Al2O3range 2.2È3.7 (Fig. 3a), implying the presence of a smallA�amount of platinum crystallites with long-range order on thesurface of No similar peaks are found for thePt/Al2O3 .

(Fig. 3b) and (Fig. 3c) in the samePtÈSn/Al2O3 PtÈFe/Al2O3range, indicating the disappearance of long-range order, andinstead, the formation of highly dispersed Pt atoms on thesebimetallic catalyst surfaces.

3.2. Chemical state of Fe on surfacePt–Fe/Al2O

3

Iron oxides have three forms, FeO and Fe3`Fe2O3 , Fe3O4 .ions in and Fe2` in FeO are both in 6-coordinateFe2O3octahedral sites. Their 1s ] 3d transition is dipole-(Oh)forbidden. In all the Fe2` and one half of the Fe3`Fe3O4 ,ions are in sites while the other half of the Fe3` are inOh4-coordinate tetrahedral sites, of which the 1s ] 3d tran-(Td)sitions are dipole-allowed.

Fe K-edge XANES spectra are shown in Fig. 4. Fe K-edgeXANES features originate from the transitions of 1s electronsto empty 3d and 4p orbitals. At the pre-edge near the energyzero, only very weak features are observed for (Fig. 4A)Fe2O3and FeO (Fig. 4B) since the 1s] 3d transition is forbidden bythe symmetry. As stated above, one quarter of the Fe centersin are in sites, so a more intense 1s ] 3d peak isFe3O4 Tdobserved (Fig. 4C). the absorption maximum centered atabout 20 eV reÑects the 1s ] 4p transition. The half-Ðlledhighspin d5 Fe3` in has stabilized d orbitals and thusFe2O3gives the lowest (ionization threshold, 9.5 eV in Table 2)E0and the greatest energy di†erence (21.0 eV) between 3d and 4plevels. The features on the absorption edge ranging from 5 to20 eV result from the transition of 1s electrons to all of the

Fig. 4 The Fe K-edge XANES spectra of A: B : FeO, C:Fe2O3 ,D : before reduction, and E: after reduction.Fe3O4 , PtÈFe/Al2O3

Table 2 Intensities and energy positions of Fe K-edge XANESfeatures

Pre-edge Edge MaximumFeature (intensity) (position/eV) (position/eV)

assignment 1s ] 3d E0 1s ] 4pFeO 0.07 12.0 18.0Fe2O3 0.07 9.5 21.0Fe3O4 0.10 12.9 19.8PtÈFe/Al2O3 , fresh 0.09 9.5 21.2PtÈFe/Al2O3 , reduced 0.07 10.4 19.6Fe2O3/Al2O318 0.07 9.5 21.0

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possible energy levels above the lowest empty d orbitals.Table 2 compares the intensities of the pre-edge peaks and theenergy positions of the features of the XANES of the catalystswith those of the three reference samples. It can be seen fromTable 2 that the iron species in before reductionPtÈFe/Al2O3(Fig. 4D) are energetically very similar to those in bulk Fe2O3while the iron species after reduction (Fig. 4E) seem to besimilar to that of shown in Fig. 4C. Table 2 also com-Fe3O4pares the intensity and the energy positions of the features of

with those of a highly dispersedPtÈFe/Al2O3 Fe2O3/c-It is noteworthy that the data for areAl2O3 .18 Fe2O3/Al2O3the same as those observed for bulk The iron speciesFe2O3 .

in reduced are clearly not Fe3` cations however.PtÈFe/Al2O3A larger (10.4 eV) and a signiÐcantly reduced energy di†er-E0ence between the 3d and 4p levels (19.6 eV) clearly suggeststhat the iron species in reduced are present in aPtÈFe/Al2O3lower oxidation state, by reference to the three bulk oxides. Asmentioned in the Introduction, results haveMo� ssbauerrevealed that the iron species in reduced arePtÈFe/Al2O3Fe2` cations. The reason why the XANES results for theirons in are not completely consistent with thosePtÈFe/Al2O3of bulk FeO is thought to be due to a geometric e†ect.XANES is very sensitive to the site-symmetry. Mixing of Tdand symmetries for either the Fe2` ions in orOh PtÈFe/Al2O3the Fe3`/Fe2` ions in bulk may give rise to veryFe3O4similar XANES features.

These conclusions are further supported by the EXAFSresults. Fig. 5 shows the Fe K-edge EXAFS-derived Fouriertransform (solid line) for after reduction. It canPtÈFe/Al2O3

Fig. 5 The Fe K-edge Fourier transform of (solid line)PtÈFe/Al2O3and (dashed line, ref. 18).Fe2O3/Al2O3

be seen that the transform, both the low-intensity triplet in therange 2.2 to 3.7 and the strongest nearest oxygen peak atA�about 1.5 (not corrected for phase shift), is very similar toA�that obtained for a highly dispersed catalyst Fe2O3/c-Al2O3(dashed line, ref. 18), indicating that the coordinate geometryof on surface can be applied to the reducedFe2O3 Al2O3system.PtÈFe/Al2O3

3.3. Theoretical Ðtting of the EXAFS data

It is known that the EXAFS signal of a highly dispersedsystem may have a large uncertainty, which often gives rise toa reduction in coordination number for the system. We solvethis problem by adjusting the (reduction factor) by refer-S02ence to the standard samples. The best Ðts to the Fe EXAFSfor andFe3(CO)12 , Fe3(CO)12/Al2O3 , Fe3/Al2O3(16) Fe(l3-demonstrated that correct coordinationCO)(CO)6/Al2O317numbers can be obtained for these highly isolated/dispersedsystems by simply adjusting the factor to 0.55, whereas aS02value of 0.75 is appropriate for condensed materials such asbulk metals and oxides including the standard samples used inthis work (Pt foil, andFe2O3 Fe3O4).The best Ðts to the Pt EXAFS for reducedLIII-edge

and the Pt and the Fe K-edge EXAFS forPt/Al2O3 LIII-edgereduced are given in Fig. 6 and Table 3. It isPtÈFe/Al2O3noteworthy that a possible PtÈFe coordination shell within 3

has not been found at either edge for thoughA� PtÈFe/Al2O3various attempts were made during the Ðtting process.The EXAFS-derived parameters reveal that (i) three PtÈPt

shells are observed on the surface of the clearlyPt/Al2O3 ,suggesting that the Pt species may remain in a three-dimensional structure. It is noteworthy that the Pt species isnot composed of Ðne crystalline particles. The coordinationnumber (CN) observed for the nearest PtÈPt shell at 2.72 A� ,indicative of a close-packed structure, is very low. Instead thePtÈPt shell at 3.14 seems to have the largest CN (CN\ 3.3)A�followed by the PtÈPt shell at 3.83 (CN\ 2.1). (ii) As men-A�tioned above, the dispersion of the Pt atoms on PtÈFe/Al2O3is signiÐcantly improved. The strong oscillation in the higherk-region in the Ðltered EXAFS of the (Fig. 6a) com-Pt/Al2O3pletely disappears in that of the (Fig. 6b). InPtÈFe/Al2O3contrast, only one PtÈPt shell at 2.75 is observed for theA�

The lower coordination number (0.8) suggestsPt-Fe/Al2O3 .that the Pt atoms are present in a two-dimensional arrange-ment. (iii) The iron oxides on are in a form veryPtÈFe/Al2O3similar to those on Three FeÈFe shells atFe2O3/Al2O3 .182.98, 3.41 and 4.02 are observed at almost the same dis-A�tances as those observed on (2.97, 3.42, 3.86Fe2O3/Al2O3 A� ,respectively). A particularly low coordination number (1.0) for

Fig. 6 Best Ðts (dashed lines) to the Pt EXAFS of (a) and the Pt (b) and the Fe K-edge (c) EXAFS ofLIII-edge Pt/Al2O3 LIII-edge PtÈFe/Al2O3 .Upper : Ðts to the Fourier transforms ; Lower : Ðts to the Ðltered EXAFS.

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Table 3 Best Ðts to the EXAFS of catalyst samples

Sample Edge na S02 Shell CN Distance/A� p2 R-factor

Pt/Al2O3 Pt LIII 2 0.55 PtÈO 2.4 2.11(1) 0.000 0.10PtÈPt 0.6 2.72(2) 0.001PtÈPt 3.3 3.14(2) 0.005PtÈPt 2.1 3.83(4) 0.014

PtÈFe/Al2O3 Pt LIII 2 0.55 PtÈO 3.5 2.11(1) 0.000 0.09PtÈPt 0.8 2.75(2) 0.004

Fe K 3 0.55 FeÈO 5.3 1.96(2) 0.004 0.12FeÈFe 1.0 2.98(4) 0.003FeÈFe 2.0 3.41(2) 0.001FeÈFe 0.7 4.02(3) 0.005

a In kn.

the Ðrst FeÈFe shell, compared with 1.8 on the Fe2O3/Al2O3 ,implies that most of the adjacent irons may appear at thepara-positions on the (111) surface.18 The possibleAl2O3changes in surface geometries will be discussed below.

3.4. Details of a sandwich model

It is clear from the XANES results that the Pt species on thesurfaces of the Pt catalyst and the PtÈFe, PtÈSn bimetalliccatalysts after reduction are in the zerovalent state, while theFe (and Sn) is present in oxide form. The XANES resultsreveal that the surface iron component may exist on the PtÈFecatalyst surface as a mixed site-symmetry system somewhatsimilar to The irons, as determined by areFe3O4 . Mo� ssbauer,Fe2` cations, of which 40% are in 6-coordinate octahedralsites while the other 60% are in 4-coordinate tetrahedralsites.11 However, neither nor units can exist onFeO6 FeO4the surface alone due to the low oxidation state of theAl2O3iron. It has been demonstrated for supported mixed oxidesthat low oxidation state, high coordination components, suchas Mn3` in tend to interact closely withMn/Na2WO4/SiO2 ,the oxide support in order to saturate their coordination.22On the other hand, the higher oxidation state, lower coordi-nation components tend to be enriched on the surface. Thissuggests that the iron, having an average coordinationnumber of 5.3, has to share bridging oxygens with the adja-cent iron(s) or aluminium center(s) to simultaneously achievestoichiometric balance (Fe/O\ 1) and electroneutrality.

Zerovalent Pt atoms are entirely di†erent however. Pt0 canbe ““ freely ÏÏ deposited on an inert surface because it does notneed to combine with any surface oxygen ions to attain agiven coordination number and stoichiometry.

In accordance with the surface coordinate geometry estab-lished for and based on the EXAFS-derivedFe2O3/Al2O318parameters, an ““average ÏÏ surface structural model for

after reduction is shown in Fig. 7. Fig. 7a showsPtÈFe/Al2O3how a group of Fe ions is randomly dispersed on theface. It can be seen that almost half of the FeÈFeAl2O3(111)

neighbors are in meta-positions and the others are in para-positions. For example, iron A has two neighbors (B and E) inmeta-positions. However, iron G has two para-position neigh-bors (F and N) and a further neighbor (M) at about 4 TheA� .distance between the meta-positions is consistent with theobserved shortest FeÈFe distance of 2.98 as reportedA� ,(2.97^ 0.09 previously.18 The distance between the para-A� )positions is times the distance between the meta-2/3 J3positions and calculated to be 3.44 when the nearest FeÈFeA�distance is 2.98 The observed value for our sample is 3.41A� .

also in good agreement with the estimation. After coverageA� ,by a layer of adventitious oxygens, it can be seen from Fig. 7bthat the iron ions on the positions labeled A, E, K, L, M andN are located in octahedrally 6-coordinate sites, while theother iron ions are in 4-coordinate tetrahedral sites. For eachiron center, the number of oxygens in the Ðrst coordination

shell varies, for example, it is 5 for iron A, and 4 for iron B.The average coordination number for each among a total of17 iron ions is 4.7. The corresponding value from the EXAFS-derived data is 5.3. The model-derived parameters indicatethat each iron may on average have 1.5 iron neighbors at2.98 2 irons at 3.4 and 0.8 at about 4 The corre-A� , A� A� .sponding parameters from the EXAFS-derived data are 1neighbor at 2.98 2 at 3.4 and 0.7 at about 4 It isA� , A� , A� .noteworthy that the surface iron oxide ensemble obtained hasan apparent stoichiometry of FeO It can therefore(Fe17O19).be concluded that the model-derived parameters are com-pletely consistent with the XAFS and the results,Mo� ssbauerand with the two-dimensional geometry predicted by thephysics of amorphous solids.15,25

The deposition of Pt atoms onto the iron oxide surface asshown in Fig. 7b is not so complex as might have beenexpected. Because no uniform PtÈFe shell is observed within 3

Pt atoms cannot be deposited on the triply bridged sitesA� ,opposite the irons labeled A, E, J, K, L, M, etc., since thiswould give particularly short PtÈFe distances (e.g. \2.5 A� ),but only on the other triply bridging positions. The model inFig. 7c is thus obtained. It can be seen that Pt atoms aredeposited on the surface of the iron oxide as a mixture ofmonomers, dimers and trimers, and on average have threeneighboring oxygens and nearly one neighboring Pt atom.This is consistent with the EXAFS results (three neighboringoxygens and 0.8 neighboring Pt) shown in Table 3. Theaverage diameter of the PtÈFe surface ensemble formed in this

Fig. 7 Surface structural model of after reduction.PtÈFe/c-Al2O3

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way is estimated to be about 20 It is also possible thatA� .some Pt atoms may ““ randomlyÏÏ deposit onto the surface of

directly. Such a situation is also depicted in Fig. 7c.Al2O3

4. Conclusion1. In the freshly reduced 0.375 wt.% catalyst, zero-Pt/c-Al2O3valent Pt atoms are formed, as demonstrated by the PtXANES. The present results indicate only one type of PtÈOdistance at 2.11 However, the PtÈPt distances observed atA� .2.72, 3.14 and 3.83 having a PtÈPt coordination number ofA�6 in total do not support the hypothesis that there are pre-dominantly three-dimensional, extremely uniform clusters onthe surface of c-Al2O3 .

2. In the freshly reduced 0.375 wt.% PtÈ0.537 wt.% Fe/c-catalyst (atomic ratio Fe/Pt \ 5), a signiÐcantlyAl2O3improved dispersion for the Pt atoms is observed (Fig. 3).

Only one type of PtÈPt shell at 2.75 with a low coordi-A�nation number of 0.8 is detected. The measured PtÈO coordi-nation number of 3.5 is in good agreement with the proposedvalue for a two-dimensional Pt arrangement having threeinterface oxygen neighbors coordinating each Pt atom.

On the other hand, the iron oxides on the PtÈFe/Al2O3catalyst exhibit a Fourier transform very similar to that ofThe three EXAFS-derived FeÈFe shells atFe2O3/Al2O3 .18

2.98, 3.41 and 4.02 respectively, are almost the same asA� ,those observed for the Fe2O3/Al2O3 .

3. The XAFS-derived results for the freshly reducedcatalyst are consistent with a sandwich model.PtÈFe/Al2O3Iron oxide with a formula of is present as an inter-Fe17O19face between Pt and while Pt atoms are randomlyc-Al2O3distributed on the iron oxide surface as a mixture of mono-

mers, dimers and trimers well separated from each other. Inthis case, the e†ect of is transmitted to the O2~-coveredAl2O3surface through a layer of Fen` cations. Further, the PtÈMn`(Mn` \ Fen`, Snn`) interaction (through bridging oxygens,for example) may have an important inÑuence on maintainingthe zerovalence of the Pt sites. Since there are some sites (A, E,J, K, L and M in Fig. 7) that are unfavorable for Pt deposi-tion, the dispersion of the Pt atoms ““as a mixture of mono-mers, dimers, and trimers well separated from each other ÏÏ istherefore retained. It is likely that an interface with sufficientvacancies such as that shown in Fig. 7b can readily be formedunder the reaction conditions. The sites that are unfavorablefor Pt deposition may therefore play a key role in maintainingthe dispersion of the Pt atoms and thus the stability of thebimetallic catalysts during the reaction.

Following this idea, we may hope to improve the catalystmore rationally according to the practical requirements of dif-ferent reactions (such as reforming, dehydrogenation,hydrogenation), by selecting a suitable metal oxide interfaceand suitable preparation techniques. We should add here thatthe structural models derived from EXAFS are averagedresults, and the model shown in Fig. 7 is only one of the pos-sible ones matching the results. However, though the modelsderived are somewhat di†erent from each other, we have

found that changing the model would not change the conclu-sions of this paper. In fact, studies on amorphous solids haveclearly established that the varied forms of an amorphoussystem exhibit no di†erence in properties. This is perhaps alsothe case for a working catalyst under reaction conditions suchas the one discussed in this work.

AcknowledgementsWe are grateful to the Photon Factory for use of the BL-7Cfacilities and thank Professor T. Tanaka in Kyoto Universityfor assistance in XAFS measurements.

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