rhodium

J. Phys. Chem. 1994,98, 2653-2662

2653

Structural Analysis of Polymer-Protected Platinum/Rhodium Bimetallic Clusters Using Extended X-ray Absorption Fine Structure Spectroscopy. Importance of Microclusters for the Formation of Bimetallic Clusters Masafumi Harada,? Kiyotaka Asakura,$ and Naoki Toshima'J Department of Industrial Chemistry, Faculty of Engineering, and Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received: October 6, 1993; In Final Form: January 2, 1994'

The colloidal dispersions of polymer-protected platinum/rhodium bimetallic cluster particles are easily prepared by refluxing solutions of hexachloroplatinic(1V) acid and rhodium(II1) chloride in ethanol/water (1/1 v/v) in the presence of poly(N-vinyl-2-pyrrolidone). These colloidal dispersions of the R / R h bimetallic cluster particles are very stable and have a size distribution from 2 to 7 lim in diameter. The electronic spectra and the transmission electron micrographs suggest that the colloidal dispersionsare not a mixture of monometallic Rh and monometallic Pt particles but are mostly composed of Pt/Rh alloy particles. Electron microprobe analysis indicates that the metal composition of each alloy particle with nearly average diameter is kept as the charged mole ratio of Pt/Rh. Extended X-ray absorption fine structure analyses, high-resolution electron microprobe analysis, and scanning tunneling microscopyindicate that colloidal alloy particles consist of an assembly of Pt/Rh bimetallic microclusters, each of which has a Pt core and a modified Pt core structure of colloidal dispersions of R / R h ( l / 4 ) and Pt/Rh( 1/1 or 4/1) bimetallic clusters, respectively. The importance of the microcluster is discussed in detail for the formation of Pt/Rh bimetallic cluster particles.

Introduction A primary goal of recent research on the colloidal dispersions of bimetallic clusters has been to determine the structures necessary for both the high catalytic properties (activity and selectivity) and for the high stability as colloidal dispersions. An understanding of the additive effect of the second metallic component in the bimetallic systems is requisite, from a fundamental and practical point of view, for improving and controlling the catalytic activity and selectivity of colloidal dispersions and supportedmetal catalysts (the latter which are composed of small metal particles dispersed on oxide supports such as A1203,SiOz, and TiOz). In recent years, considerableresearch effort has been directed toward studying colloidal dispersions of small metal particles in homogeneous solutions. Bradley et a1.l investigated the synthesis of elemental metal clusters in fluid media by stabilizing colloidal transition metal particles in nonpolar hydrocarbon solutionsusing polymers to prevent aggregation of the metals. Harriman et aLz reported the rate constants for interfacial electron-transfer reactions between colloidal Pt particles and reducing radicals in an aqueous solution. Liu et ale3studied a novel method for transferring colloidal noble metal particles from an organic phase to an aqueous phase, and vice versa, by using soluble organic ligands. Despite many investigationsdealing with the preparation and application of these metal particles to physics, chemistry, and biology, especially as they apply to catalysis, no detailed information on the structure of colloidal dispersions of the metal clusters has been obtained. We have, therefore, characterized the colloidal dispersions of noble metal clusters using EXAFS (extended X-ray absorption fine structure) spectroscopy. One of the advantages of EXAFS is its ability to elucidate the relationship between the structure of metal clusters and their catalytic activity (particularly with colloidal dispersions and supported metal clusters). Sinfelt et al.4 have characterized IrRh,5 Au-Cu,6Cu-Ru,7 and Cu-Os8 bimetallicclusters supported on silica. Furthermore, Koningsberger et al. have extensively t Department of Industrial Chemistry, Faculty of Engineering.

Department of Chemistry, Faculty of Science. Abstract published in Advance ACS Abstracts, February 1, 1994.

investigated the structure of the interface between metal and metal oxides on supported metal clusters, Le., Rh/yAl2O3,9 [Os3(CO) 121 /y-A1203,l0 and [H3Re3(CO) 121/MgO. l However, there have been relatively few studies12J3 focusing on the structural and electronic properties of colloida1dispersions of noble metals, despite their use in many chemical systems. Colloidal dispersions of bimetallic clusters, which can be prepared by simultaneous reductions of alcoholic solutions containing two noble metal salts in the presence of poly(N-vinyl2-pyrrolidone), have been shown to exhibit a high degree of reproducibility and high catalytic activity with certain compositions of the clusters.1k18 We recently found19 that Au/Pd clusters prepared by successive reductionsshow catalytic activity that is a fraction of that of Au/Pd bimetallic clusters prepared by simultaneousreductions,suggesting that thecatalytic activity depends on the reduction method. The structure of colloidal dispersions of Pd/Pt>o Au/Pd,l6 and Pd/Rh21.22 bimetallic clusterscould be determined by EXAFS measurements. For the structure of Pd/Ptm and Au/Pd16 bimetallic clusters prepared under nitrogen, the core model and cluster-in-cluster model have been proposed (as described before). In addition, these models were examined in further studies by systematicallycomparing the structure of the bimetallic clusters prepared under air with those prepared under nitrogen and by determining their dependence on the preparation method. More recently, we observed a structural change in Pd/Pt bimetallic clusters during oxidation and re-reduction using both EXAFS measurementsand high-resolution electron microprobe analysis.23 This result suggests an important role for the microclusters(less than 10 A in diameter) in forming the cluster particle. Especially, an observed change in the average diameter of the bimetallic cluster particle during the re-reduction process stronglysuggeststhat the microclustersare stable in the colloidal dispersions. For further insight, we investigated the structure of Au/Pd bimetallic clusters prepared by the successive reduction method.19 In this case, EXAFS data suggest that microclusters form during the first stage of reduction while refluxing in alcoholic solution, and then the microclustersgather together, resulting in the formation of the bimetallic cluster particles. As a result, it is possible that the cluster particle might be composed of several 0 1994 American Chemical Societv

2654

Harada et al.

The Journal of Physical Chemistry, Vol. 98, No. 10, 1994

microclusters. Concerning this speculation, Wang et al.24have suggested that thearrangement ofsurfacemetalatomsof polymerprotected metal particles is irregular, since small metal clusters in the Rh particle are observed by scanning tunneling microscopy (STM) measurements. In the present paper, we determine the structure of the Pt/Rh bimetallic clusters by EXAFS analysis, high-resolution electron microprobe analysis, and scanning tunneling microscopy. The goal of the present study is twofold: (1) to determine the structure of Pt/Rh bimetallic cluster particles and (2) to establish the existence of the microclusters, which gather around one another and form larger cluster particles.

Experimental Section Preparation of Pt/Rh BimetallicClusters. Colloidaldispersions of Pt/Rh bimetallicclusterswere prepared by an alcohol-reduction method.25.26 Rhodium(II1) chloride (0.033 mmol) was dissolved in25 mL of water, and hexachloroplatinic(1V) acid (0.033 mmol) was alsodissolved in 25 mL of water. Poly(N-vinyl-2-pyrrolidone) (PVP, K-30, MW 40 000, 151 mg, 1.36 mmol of monomeric units) was added as a protecting polymer to an ethanol/water (1/1 v/v) (50 mL) solution containing both Rh(II1) and R(1V) ions at a given concentration. The total amount of both metals was always 3.3 X 10-5 mol in 50 mL of the mixed solution. The mixed solution was stirred and refluxed at about 100 OC for 2 h under nitrogen, which resulted in a stable, dark brown, homogeneous solution of colloidal dispersionsof Pt/Rh bimetallic clusters. Characterizationof the Pt/Rh Bimetallic Clusters. Electronic spectra of colloidal dispersions of the bimetallic clusters were measured with a Hitachi Model 340 spectrophotometer. Transmission electron micrographs (TEM) of the colloidal dispersions were taken using a Hitachi Model HU-12A and H-7000 electron microscope operated at 100-kV acceleration voltage. Electron microprobe analysis was performed using a Hitachi Model HF2000 electron microscope operated at 200 kV. The high-resolution carbon-supportedcopper mesh, kindly supplied by Dr. K. Adachi, was used to support the samples. The diameter of each particle of the bimetallic clusters was determined from enlarged photographs. The histogram of the particle size distribution and the average diameter were obtained by measuring about 300 particles in an arbitrarily chosen area. Scanning tunneling microscopy was performed under air at room temperature with a Digital Instruments Co. nanoscope I unit modified with an original feedback system. The Pt-Ir tip supplied from MAS Co. was used in the measurements of the metal clusters adsorbed on a newly cleaved HOPG surface under the following conditions: bias voltage, 20 mV; set current, 0.2 nA; scan area, 10 X 10 nm2. EXAFS Measurement. For EXAFS measurements of the Rh-K edge and Pt-Ls edge of colloidal dispersions of Pt/Rh bimetallic clusters, the samples were prepared by concentrating 1500 mL of the above colloidal dispersions to 30-50 mL under a reduced pressure of nitrogen. The concentrated dispersions were kept under nitrogen in cells with optical path lengths of 50 and 5-10 mm for Rh-K edge and Pt-Ls edge measurements, respectively. These cells had polyimide film windows (KAPTON-SOOH, 125 pm of thickness, kindly provided by Toray Co. Ltd.). The Pt/Rh(9/1) alloy foil, Pt/Rh(l/l) alloy foil, and Pt/ Rh( 1/9) alloy foil were produced as preferable reference c0mpounds2~by Tanaka Kikinzoku Kogyo K.K. in accordance with our request. For example, in the case of the Pt/Rh(9/1) alloy foil, the Pt/Rh metallic composition was 9.00:l.OO. The thickness of the Pt/Rh(9/1) alloy foil was about 10 pm. The EXAFS measurements were performed at the Photon Factory, the National Laboratory for High Energy Physics (KEKPF), using the BL-1OB station. Monochromatic synchrotron radiation was obtained using a channel-cut Si(3 11) crystal. The

storage ring was operated at 2.5 GeV with approximately 100300 mA of ring current. The experiments were carried out in the transmission mode using concentrated dispersions at room temperature. The monochromaticX-ray beam was first passed through an ionization chamber (which measures the incident beam intensity l o ) , then through the sample, and finally through another ionization chamber (which measures the transmitted intensity Z). N2 was the detecting gas used in both ionization chambers for the Pt-Ls edge, while Ar and Kr gases were used in the ZOand I ionization chambers, respectively, for the Rh-K edge. the raw EXAFS data in energy space In the EXAFS (ln(Zo/l) vs E ) were reduced to the photoelectron wave vector ( k ) space as described elsewhere,29with the threshold energy EO, where k = [2m(E - Eo)/h2]1/2. The EXAFS spectra were extracted using a cubic spline method and normalized to the edge height. The k3x(k)functions vs k data and the corresponding Fourier transforms were obtained using a Hanning window function with 1/ 10of the FT ranges in the same way as b e f ~ r e ? ~ ? ~ ~ The typical range of Fourier transformation from the k space to the r space was 30-160 nm-1 both for the Rh-K and Pt-L3 edges. For the purpose of the curve fitting (using eqs 1 and 2), the high-frequencynoise was removed by a Fourier filteringtechnique, and the inverse Fourier transformation to the k space (the range 40-150 nm-l) was employed. k3x(k) =

where Nj denotes the coordination number, rj the bond distance, A&, the difference between the theoretical and experimental threshold energies, and uj the Debye-Waller factor of the j-th coordination shell. SI is an amplitude reduction factor, which can be obtained from the reference compounds as a constant function of k.30 The resulting filtered data were fitted, with theoretical phase shift ( 4 ( k ) )and amplitude functions (Fj(k)) tabulated by Teo and Lee,3IJ2 on the basis of the single-scattering planewave theory, to obtain detailed structural i n f ~ r m a t i o n . ~ ~ Results and Discussion

Preparationand Characterizationof Pt/Rh Bimetallic Cluster. Colloidal dispersions of Pt/Rh bimetallic clusters protected by solublepolymers such as poly(N-vinyl-2-pyrrolidone)(PVP) were dark brown and were stable for months at room temperature. The formation of colloidaldispersionsof Pt/Rh bimetallicclusters protected by PVP was confirmed both by electronic spectra and by transmission electron microscopies. Parts a and b of Figure 1 show the electronic spectra of colloidal dispersions of R / R h bimetallic clusters and a mixture of monometallic colloidal dispersions of Rh and Pt clusters, respectively. The spectra of the bimetallic colloidal dispersions have a characteristic pattern which differs from that of thecolloidaldispersionsof monometallic Rh and Pt clusters and cluster mixtures. Figure 2 shows transmission electron micrographs of colloidal dispersionsof monometallic Rh and Pt clusters, Pt/Rh bimetallic clusters, and a mixture of both monometallic Rh and Pt clusters with a Pt/Rh ratio = 1/ 1. The size of each monometalliccluster of Rh and Pt in the colloidal dispersions is very small, but each cluster tends to aggregate and/or grow together, resulting in the formation of larger cluster particles in solutions. In contrast, the Pt/Rh bimetallic clusters neither aggregate nor grow over time, though each cluster seems to be the same in size distribution as each monometallic cluster of Rh and Pt. The mixture of the

The Journal of Physical Chemistry, Vol. 98, No. 10, 1994 2655

Polymer-Protected Pt/Rh Bimetallic Clusters P t / R h = O / l (-1 Pt / Rh = 1 / 4 (.........) Pt / Rh = 1 / 1 Pt / Rh = 4 / 1 (----) P t / R h = l / O (-)

(-.e.-)

I

300

400

I

500

I

600

I

I

700

800

Wavelength I nm 0.2

P t / R h = O / I (-) P t / R h = l / O (----)

300

400

500

600

700

800

Wavelength / nm Figure 1. Series of electronic spectra of (a) Pt/Rh bimetallic colloidal dispersions at Pt/Rh ratios = 0/1, 1/4, 1/1, 4/1, and 1/0 and of (b) Pt/Rh( 1/1) bimetallic clusters and mixtures of Rh and Pt monometallic clusters at Pt/Rh ratio = 1/1, besides Rh and Pt monometallicclusters.

monometallic Rh and Pt clusters is apparently different from the Pt/Rh bimetallic clusters. The particle sizedistributionsare shown as histograms in Figure 3. These histograms indicate that the colloidal dispersions of Pt/Rh( 1/1) bimetallic clusters have an average diameter of 40 A and are widely distributed from approximately 20-70 A. In the case of the mixture of monometallic clusters, the average diameter is 31 A, which is the intermediate value of the average diameter of the monometallic Rh clusters (33 A) and the monometallic Pt clusters (27 A). The same aggregation is observed in the mixture of the monometallic clusters as was observed with colloidal dispersions of monometallic Rh clusters. However, from TEM pictures shown in Figure 4a,b the average diameters of the Pt/Rh( 1/4) and the Pt/Rh(4/ 1) bimetallic clusters prepared under nitrogen are 44 and 36 A, respectively. The averagediameter (40 A) of the Pt/Rh( 1/ 1) bimetallicclusters is precisely the average of those of Pt/Rh(l/4) and Pt/Rh(4/1) bimetallic clusters. In order to measure the metal composition of each particle of Pt/Rh( 1/ 1) bimetallic clusters which has a diameter in the range 20-70 A, a high-resolution electron microprobe analysis was carried out. Figure 5 contains a table summarizing the results of the electron microprobe analysis of six particles chosen at random, as well as two electron micrographs showing the six particles. These particles have either the approximate minimum,

intermediate, or maximum diameter in the particle size distribution and cover the whole region of the particle size distribution. As shown in the table, the larger the particle size becomes, the more the Rh content of each bimetallic cluster tends to increase. For example, the mole ratio of Rh/Pt (1 -35 and 1.78 instead of 1.OO) with 47.9- and 69.6-A diameters, respectively, differs considerably from the charged mole ratio. However, the particle (especially particle 3) with nearly average diameter, prepared under nitrogen, keeps the metal composition as the charged mole ratio of Pt/Rh = 1/1. The results of the electron microprobe analysis for colloidal dispersionsof the Pt/Rh( 1/4) and Pt/Rh(4/ 1) bimetallicclusters prepared under nitrogen are shown in parts a and b of Figure 6, respectively. Each inset table presents the compositions of five particles chosen at random from the corresponding electron micrograph. These particles also have either the approximate minimum, intermediate, or maximum particle size in the same way as the Pt/Rh( 1/ 1) bimetallic clusters shown in Figure 5. For the Pt/Rh( 1/4) bimetallic clusters, each cluster has the same tendency between the metal compositionof Rh/Pt and the particle size as for the Pt/Rh(l/l) ones, Le., the Rh/Pt ratio = 2.92 with diameter of 16.3 A or Rh/Pt ratio = 6.63 with diameter of 40.8 A. On the other hand, each Pt/Rh(4/1) bimetallic cluster tends to have much more Rh content than the expected value. These aspects might bedue to thedifferenceof the rateof cluster particle formation between the monometallic Rh and pt clusters.33 However, the particles (especially particle 5 in Figure 6a and particle 2 in Figure 6b) with nearly average diameter keep the metal composition as the charged mole ratio of Pt/Rh = 1/4 and 4/ 1, respectively, although the mole ratios of Pt/Rh in both cases are a little deviated from the expected ones. STM Measurements of the Pt/Rh Bimetallic Clusters. The STM of the bimetallic cluster particles was taken on the HOPG surface. The images were flowing in a direction of the surface, so the particle size estimated by the STM image is thought to be larger than the real partile.34 Even if the particle is flowing in one direction during the STM measurement, the STM image, which is perpendicular to the flowing direction, can be used to estimate the real particle size. Thus, the real particle size could be determined using the short length of the image. As a result, particles of the Pt/Rh( 1/1) bimetallic cluster are measured to be between 7 and 10 A in diameter. These observed particle sizes are due to metallic constitutions, which do not involve the protective polymer (PVP), since the polymer is an insulator and cannot be detected by STM. This result indicates that the real particle size obtained from the STM is quite less than the average diameter obtained from the TEM picture shown in Figure 2c. Therefore, we conclude that the cluster particle is composed of several small microclusters, less than 10 A in diameter (which has already been discussed in previous papers). 19.23 EXAFS Analysis of the Pt/Rb Bimetallic Clusters. In order to determine accurate structural parameters (r, PE, N, u, and S ) for colloidal dispersions of Pt/Rh bimetallic clusters at various Pt/Rh ratios using EXAFS, two-shell curve fitting has been carried out using theoretical phase shifts and amplitude functions. However, EXAFS analysis of the Pt/Rh bimetallic clusters in the Rh K-edge and Pt L3-edge, which provides information about the coordination numbers and the bond distances around an absorbing atom, has as much difficulty distinguishing the contributions of Rh-Rh, Rh-Pt, Pt-Pt, and Pt-Rh bonds as those of Pd-Pd, Pd-Pt, Pt-Pt, and Pt-Pd bonds in the Pd/Pt bimetallic clusters, because of the similarities between these bonds, as reported in previous p a p e r ~ . ~ o . ~ ~ In general, the correlation problems in the fitting parameters (r, AE, u, N, S ) for a pair of different metal atoms, such as the pair of Rh-Rh, Rh-Pt, Pt-Pt, and Pt-Rh bonds, are still difficult to solve because there are no good reference compounds for these

2656

The Journal of Physical Chemistry. Vol. 98, No. 10. 1994

Harada et al.

Figure 2. Electron micrographs of (a) Rh monometallic, (b) R monometallic, (c) F’t/Rh(l/l) bimetallic, and (d) mixtures of Rh and Pt monometallic clusters at Pt/Rh ratio = I / I.

atomic pairs. In order tosolvethecorrelation problems andobtain more reliable r and N from the curve fitting, Rh foil, Pt foil, Pt/Rh(9/1) alloy foil, Pt/Rh(l/l) alloy foil, and Pt/Rh(l/9) alloy foil were used as the reference compounds in the present work. AE,S. and a of unknown compounds, i.e., the colloidal dispersions of the Pt/Rh bimetallic clusters, were assumed to be nearly equal to those of the reference compounds when the best curve fitting of their EXAFS using theoretical parameters could be carried out. An accurate value of r and N for unknown compounds can be determined by using the theoretical phase shifts and amplitude functions, together with fixed values of AE, S, and u obtained empirically from the referencecompounds.’6.zQ.lz Table I shows the curve-fitting analyses of EXAFS data for both the reference compounds and the colloidal dispersions of monometallic clusters of Rh and Pt. The bond distances of RhRh and Pt-Pt in the Rh and h foils are equal to those obtained from crystallography, indicating the validity of theoretical phase shifts. First, the values of AE,S, and u for the Rh-Rh, Rh-Pt, Pt-Pt, and Pt-Rh bonds were determined in the same way as those described in the previous paper.z0 In order to test the reliability of the values of AE, S, and u obtained from Rh foil, Pt foil, Pt/Rh(9/1) alloy foil, and Pt/Rh(l/9) alloy foil, the structural parameters, rand N,ofPt/Rh(l/l) alloy foil havebeenobtained from the curve fitting by using theoretical phase shifts together

with the fixed values obtained from these foils. The expected coordination numbersfor all atomic pairs are6,and bonddistances for Rh-Rh, Rh-Pt, Pt-Pt, and Pt-Rh bond expected from the Rh foil, Pt/Rh(9/1) alloy foil, Pt foil, and Pt/Rh( 1/9) alloy foil are 2.61, 2.15, 2.16, and 2.69 A, respectively. In the case of the Pt/Rh(l/l) alloy foil, the bond distances agree well with the expected ones within the systematic error of 0.03 A, and the coordination numbers also agree within a systematic error of 10%. Therefore, the reliability of the values of AE, S, and a, which were used as fixed constants in the calculation, was confirmed.” Thestatisticalerronin thecoordinationnumben a n d thebond distances were estimated by analyzing EXAFS of colloidal dispenionsofPt/Rh himetallicclusters. This analysiswascarried out by changing AE and u’s within the range of * 5 eV and f0.01 A from AE and u’s of Rh-K and Pt-L, edge EXAFS of the reference compounds. Finally, we conclude that Pt/Rh bimetallic clusters could be analyzed by using the values of AE,S, and u for the Rh clusters and the Ptclusters todeterminethecharacteristicsoftheRh-Rh and Pt-Pt bond, respectively, and using the Pt/Rh(9/1) alloy foiland the Pt/Rh( 1/9) alloyfoiltodetermine thecharacteristics of the Rh-Pt and Pt-Rh bond, respectively. In this analysis, the valuesof AEand a forthemonometallicclusters wereused instead of those for Rh and Pt foil. However, much attention was paid

Polymer-Protected Pt/Rh Bimetallic Clusters

The Journal of Physical Chemistry, Vol. 98, No. 10, 1994 2657

30 20 10

‘0

“0

10 20 30 4 0 5 0 6 0 70

Diameter

(c)

(i)

10 20 3 0 4 0 50 60 7 0

1 0 20 30 4 0 *50 60 70 Diameter (A)

(d)

Figure 3. Particle size distribution histograms of (a) Rh monometallic, (b) Pt monometallic, (c) Pt/Rh( 1/1) bimetallic, and (d) mixtures of Rh and Pt monometallic clusters at R/Rh ratio = 1/1. The dashed lines indicate the average diameters of all the particles.

to the values of u, because u is different between surface and bulk atoms. Before the analysis, the distribution of Rh and Pt atoms in the Pt/Rh bimetallic clusters was not known, so the u’s for a pair of metal atoms in the clusters were assumed to have the same values as those of the corresponding pairs of the reference compounds. Figure 7 shows the Fourier transforms of Rh K-edge EXAFS of the colloidal dispersions of the Pt/Rh bimetallic clusters at a Pt/Rh ratio = 0/1, 1/4, 1/1, and 4/1. In the case of colloidal dispersions of Rh clusters, the main peak is assigned to a Rh-Rh bond, which was determined to be 0.266 nm by the curve-fitting analysis. Decreasing the Rh content, the main peak splits into two. This can be attributed to a Rh-Pt bond as well as a Rh-Rh bond because of the phase shift arising from the interference between the Rh and Pt atoms, as observed in the case of the colloidal dispersions of the Pd/Pt bimetallicclusters.*O The data were Fourier-filtered over 0.1 5-0.30 nm and analyzed by curve fitting to obtain the structural parameters. The EXAFS data of colloidal dispersions of the Pt/Rh bimetallic clusters were best fit to a two-shell model consisting of Rh-Rh and Rh-Pt bonds. Figure 8 shows the Fourier transforms of the Pt L3-edgeEXAFS of colloidal dispersions of Pt/Rh bimetallic clusters with a Pt/ Rhratio = 1/0,4/l,l/l,and 1/4. Themainpeakaround0.275 nm for colloidal dispersions of the Pt monometallic clusters is assigned to the Pt-Pt bond. The height of the main peak of the Pt/Rh(4/1) bimetallic cluster is the same as that for Pt/Rh(1/4). However, the position of the main peak tends to shift to the left side. This suggests that the bond distances of the Pt-Pt and/or Pt-Rh change gradually with decreasingPt content. The data were Fourier-filtered over 0.20-0.30 nm and were again fit best to a two-shell model consisting of Pt-Pt and Pt-Rh in the same way as in the Rh K-edge EXAFS analysis. Structure of the Pt/Rh Bimetallic Clusters Determined by EXAFS. The structural parameters of colloidal dispersions of Pt/Rh( 1/ 1) bimetallic clusters have been determinedon the basis of the two-shell curve fitting as shown in Table 2. The Rh-Pt distance (0.272 i 0.003 nm) obtained from Rh K-edge EXAFS is nearly equal to the Pt-Rh distance (0.270 f 0.003 nm) obtained from Pt Ltedge EXAFS. The present EXAFS analyses using the theoretical phase shift can give the Rh-Pt distance within an error of 0.003 nm. Hence, the consistency of the distances indicates that the fixed values of hE obtained from the reference

compounds are suitable for the two-shell curve fitting, because large correlationsamong the fitting parameters can be decreased. The coordinationnumbers of Rh and Pt atoms around the Rh atom are determined to be 2.6 f 0.5 and 3.1 f 0.8, respectively. Similarly, the coordinationnumbers of Pt and Rh atoms around the Pt atom are 6.3 i 1.0 and 1.7 f 0.8, respectively. The statistical errors for the coordinationnumbers were estimated by varying the value of u in the range of d f 0.01 (d: the DebyeWaller factor of the reference compounds). For example, in the case of Pt/Rh(l/l) bimetallic clusters, the errors for the coordination numbers of Rh atoms around the Rh atom were estimated to be 0.5 by varying the value of u for the Rh-Rh bond in the range 0.059-0.079. There is a simple relation shown in eq 3 which must be satisfied for the coordination number of Pt atoms around the Rh atom (IF”) and that of Rh atoms around the Pt atom (WRh).These coordination numbers are determined from Rh K-edge and Pt L3-edge EXAFS, respectively (3) where Xp,and XU are the atomic fractions of Pt and Rh in the colloidal dispersions, respectively. Within the range of the error bars shown in Table 2, the present coordination numbers can satisfy eq 3. Thus, there can be a high degree of certainty in the value of the coordinationnumbers when the EXAFS data for the two absorption edges are fitted separately. The structural parameters for colloidal dispersions of the Pt/ Rh( 1/4 and 4/1) bimetallic clusters have been determined on the basis of the two-shell curve fitting in the same way as for the Pt/Rh(l/l) bimetallic clusters. The results are shown in Table 3. In the case of Pt/Rh( 1/4) bimetallicclusters, the Rh-Pt bond distance (0.270 f 0.003 nm) obtained from Rh K-edge EXAFS is equal to the Pt-Rh bond distance (0.270 f 0.003 nm) obtained from Pt Lp-edge EXAFS in the same sample. The coordination numbers of the Rh and Pt atoms around the Rh atom are determined to be 6.0 f 1.2 and 1.4 f 0.6, respectively, and those of Pt and Rh atoms around the Pt atom are 4.6 f 0.8 and 4.5 f 1.0, respectively. In the case of Pt/Rh(4/1) bimetallicclusters, the Rh-Pt bond distance (0.274 f 0.003 nm) obtained from Rh K-edge EXAFS is also equal to the Pt-Rh bond distance obtained from Pt L3edge EXAFS. Coordinationnumbers of Rh and Pt atoms around the Rh atom are determined to be 0.6 f 0.2 and 2.9 f 0.9, respectively, and those of Pt and Rh atoms around the Pt atom are 7.1 f 1.2 and 0.5 f 0.1, respectively. AsshowninTables 2 aqd 3, theratioof thecoordinationnumber of Rh atoms around the Rh atom to that of Pt atoms around the Rh atom is equal to the charged mole ratio of Pt/Rh even though the colloidal dispersions of the Pt/Rh bimetallic clusters are prepared at various Pt/Rh ratios. For example, in the case of the Pt/Rh( 1/ 1) bimetallic clusters, the ratio of the coordination number (2.6 f OS) of Rh atoms around the Rh atom to that (3.1 i 0.8) of Pt atoms around the Rh atom is nearly 1/1, which is the charged mole ratio of Pt/Rh. However, the sum of the coordination numbers around the Pt atom is larger than that around the Rh atom in colloidal dispersions of Pt/Rh bimetallic clusters. Moreover, the large coordination number of Pt atoms around the Pt atom suggests that the Pt atom coordinates predominantly to the other Pt atoms. From the results of the coordination numbers of Pt/Rh bimetallic clusters, it is expected that Pt atoms are located at the center of the cluster and Rh atoms are enriched on the surface of the cluster. Thus, the Pt core structure or the modified Pt core structure can be taken as the model for the Pt/Rh bimetallic clusters, as in the case of the Pd/Pt bimetallic clusters.20.23 Model of the Pt/Rh Bimetallic ClustersDeterminedby EXAFS. In general, the particle size can be estimatedfrom the total average

2658

Harada e t ai.

The Journal of Physical Chemistry. Vol. 98, No. 10, 1994 50

-=

-E.-g 3020

0

‘i: n + 3 40

30

I

.-: + a 20 .-0 ~~~~~

.-ill

0

v)

lo

10

0

0

10 20 30 4 0

Diameter

50 60 70

(h)

0

1 0 20 30 4 0

0

-

Diameter

50 6 0 70

(A)

re* i

*

.-

. .

(a) FIplre4. Electron micrographs and particlesiredirtribution hi5togramsof (a) PtjRh( 114) bimetallic clusters and ( b j PtjRh(4j I ) bimctallicclusters. The dashed lines indicate the average diameters of a11 the particles

v -‘-+e - ,

-..._ .