J. Phys. Chem. 1993,97, 5103-51 14
5103
Catalytic Activity and Structural Analysis of Polymer-Protected Au/Pd Bimetallic Clusters Prepared by the Successive Reduction of HAuC14 and PdClz Masafumi Harada,? Kiyotaka Asakura,* and Naoki Toshima'vt 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: November 2, 1992; In Final Form: January 22, 1993
Colloidal dispersions of the poly(N-vinyl-2-pyrrolidone)-protected gold/palladium bimetallic clusters were prepared by successive reduction in order to compare the catalytic activity and the structure of the clusters. The catalytic activity of the colloidal dispersions of the Au/Pd( 1/4) clusters for the selective partial hydrogenation depends on the reduction order in the preparation of the clusters. The clusters prepared by successive reduction starting from the reduction of gold ions (Au Pd) have higher activity than those prepared by the reverse order (Pd Au). The EXAFS measurements indicate that the Au/Pd(l/4 and 1/1) bimetallic clusters prepared by the successive (Pd Au) reduction have the cluster-in-cluster structure. In the case of the successive (Au Pd) reduction, however, the dispersions contain both large monometallic Au clusters and monometallic Pd clusters, not Au/Pd bimetallic clusters. Mixing the dispersions of the Au and Pd monometallic clusters changes their structures at room temperature in solution, resulting in bimetallic alloy structures in the course of time, which is suggested by both the TEM observation and the electronic spectra.
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Introduction The metal cluster catalysts composed of two different metallic elements are of interest from both scientific and technological points of view. Bimetallic catalysts have long been valuable for investigating the relationship between the catalytic activity and the structure of catalysts. Bimetallic clusters such as Pt/Re,l Ir/Pt,2 and Pt/Sn,3 dispersed on a high surface area carrier of metal oxide such as A1203,SO2, Ti02, and MgO, have been studied extensively in the petroleum industry for hydrocarbon reforming. The Pt/Re bimetallic clusters4 are superior to the monometallic platinum clusters as the catalyst. The nature of the interaction between the platinum and the rhenium metals was already discussed. However, the colloidal dispersions of bimetallicclusters in a homogeneous solution have received much less attention. Thecolloidaldispersionsof noblemetals,protected by polymers, can be prepared by reducing the noble metal ions in the refluxing solution of alcohol or alcohol/water containing water-soluble polymers such as poly(viny1 alcohol) and poly(N-vinyl-2-pyrr~lidone).~ The colloidal dispersions thus obtained are stable and composed of fine metal particles from 1 to 3 nm in an average diameter with narrow size distribution. They work as catalysts with high activity and selectivityfor the hydrogenation of olefins,6 selective partial hydrogenation of diene to m~noene,~J the lightinduced hydrogen generation from ~ a t e r , ~ Jand O so on. We have recently reported in a series of papers11J2that the colloidal dispersions of the bimetallic clusters can be prepared by refluxing the alcoholic solution containing two kinds of noblemetal salts in the presence of poly(N-vinyl-2-pyrrolidone). EXAFS (extended X-ray absorption fine structure) analysis has clearly shown that thePd/Pt(4/1) and theAu/Pd( 1/4) bimetallic clusters, which are the most activecatalysts for the selective partial hydrogenation of 1,3-cyclooctadieneto cyclooctene in a series of Pd/Pt and Au/Pd bimetallic clusters, respectively, have the surface layer entirely composed of Pd atoms and a core composed of R atoms and Au atoms, respectively. The Pd/Pt bimetallic catalysts supportedon alumina washcoat were reported by Skoglundh et a1.13 to be used for the oxidation of xylene. The Pd/Pt(4/1) bimetallic catalysts supported on Department of Industrial Chemistry, Faculty of Engineering.
* Department of Chemistry, Faculty of Science.
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hydrothermally treated alumina are effective and economically attractive catalysts for the oxidation. Liu et al.I4 characterized the Au/Pd bimetallic clusters by XPS, which indicated that the constituent elements are metallic and, especially, that the palladium atoms are concentrated on the surface of the alloy cluster. EXAFS studies have been especially useful in providing structural informations about the noncrystalline, as well as the crystalline, materials. The analysis of EXAFS allows determination of local structural parameters, such as interatomic distance and coordination number, which are difficult to measure by any other method. Since EXAFS has been found to be a valuable probe for establishing the presence of bimetallic clusters and for obtaining information on their structure, we have conducted an EXAFS investigation associated with characteristic absorption edges of the colloidal dispersions of metal clusters. In the present paper, the colloidal dispersions of the polymerprotected gold/palladium bimetallic clusters are obtained by successive reduction in order to investigate on the relationship between the preparation method and the catalytic activity. The clusters prepared by the successive reduction can be used as catalysts for the selective partial hydrogenation of 1,3-cyclooctadiene to cyclooctene and the catalytic activity is compared with that of the mixtures of the monometallicclustersand the bimetallic clusters prepared by simultaneous reduction. The structure determination is performed by the EXAFS associated with the K edge of palladium and the L3 edge of gold. The structural change with time of the mixtures of the Au and Pd monometallic clusters and of the dispersed clusters prepared by the successive reduction is determined on the basis of the TEM observation and the electronic spectra. Comparison of the structures by EXAFS measurementsof the Au/Pd clustersprepared by the simultaneous and successivereductions has indicated that both Au microclusters and Pd microclusters play an important role in the formation process of the Au/Pd bimetallic clustersprepared by simultaneous reduction. The details will be discussed.
Experimental Section Preparation of the Au/Pd Bimetallic Clusters by Successive Reduction. The colloidal dispersions of the Au/Pd bimetallic clusters protected by polymers were prepared by an alcoholreduction method. Solutionsof tetrachloroauricacid (0.033 mmol
0022-3654/93/2091-5 103%04.00/0 0 1993 American Chemical Society
5104 The Journal of Physical Chemistry, Vol. 97,No. 19. 1993 in 25 mL of water) were prepared by dissolving the corresponding crystalline material in water. Ethanol solutionsof palladium(I1) chloride (0.033 mmol in 25 mL of ethanol) were prepared by stirring dispersions of PdCl2 powder in ethanol. An ethanol/ water (1/ 1 v/v) solution, containing one metal ion and poly(Nvinyl-2-pyrrolidone)(PVP,K-30,151 mg, MW 40 O00,l036mmol of monomeric units) as a protecting polymer, was refluxed at about 100 OC for 2 h under nitrogen. Subsequently the solution containing the other metal ion was added to the colloidaldispersion of the clusters prepared above, and then the mixture was refluxed again at about 100 OC for 2 h under nitrogen. The total amount of both metals was always kept as 3.3 X mol in 50 mL of the mixed solution. For preparing the Au/Pd( 1/4) clusters by the successive reduction, for example, the solution of HAuC14 was reduced first, and then the ethanol solution of PdClz was added and subsequently reduced. This successive reduction was abbreviated as Au Pd. In the other sample, the sequence was reversed (abbreviated as Pd Au). For the preparation of the mixture of the Au clusters and the Pd clusters, both dispersionsof the monometallicclusters, prepared separately,were mixed at room temperature. In order to compare the structure of the cluster prepared by the successive reduction with that of the cluster prepared by the simultaneous reduction, the preparation of the Au/Pd bimetallic clusters by simultaneous reduction was also performed.I2 Characterizationof the Au/Pd Bimetallic Clusters. Electronic spectra of the colloidaldispersions of the bimetallic clusters were measured with a Hitachi Model 340 spectrophotometer. Transmission electron micrographs of the colloidal dispersions were taken with an HU-12A and an H-7000 electron microscope. The diameter of the particles was determined from the enlarged photographs. The histogram of the particle size distribution and the average diameter was obtained on the basis of the measurements of about three hundred particles. In order to examine the change in the diameter of the cluster particle in the course of time, the TEM measurements of the three different samples, immediately after, kept for a day after, and kept for a month after the preparation, were performed. Hydrogenation of 1,3-CyclooctadieneCatalyzed by the Colloidal Dispersionsof the Bimetallic Clusters. The catalytic activities of the Au/Pd( 1/4) clusters prepared by the successive reduction were measured as the initial rate of hydrogen uptake in the hydrogenation of 1,3-~yclooctadiene(25 mmol/L) in ethanol at 30 OC under 1 atmof hydrogen by keeping the totalconcentration of noble metal at 0.01 mmol/L in order to compare the catalytic activities of the Au/Pd( 1/4) clusters prepared by the other reduction methods. EXAFS Measurement. The samples for the EXAFS measurement were prepared by concentrating 1500mL of the colloidal dispersions obtained above to 30-50 mL under reduced pressure of nitrogen. The cells with optical path lengths of 5-10 and 50 mm were used for Au L3 edge and Pd K edge measurements, respectively. The Au/Pd(9/ 1) alloy foil, Au/Pd( 1/ 1 ) alloy foil, and Au/ Pd( 1/9) alloy foil, which served as preferable references, were prepared by Tanaka Kikinzoku Kogyo K.K., Tokyo, on our request. The EXAFS spectra of the Au L3edge and the Pd Kedge were measured at room temperature at the BL-10B station of Photon Factory, the National Laboratory for High Energy Physics (KEKPF), using synchrotron radiation with a ring enbrgy of 2.5 GeV and ring currents between 100 and 300 mA. Both IOand I ion chambers were filled with Nz gases for the Au L3 edge. For the Pd K edge, the ZOand Zion chambers were filled with Ar and Kr gases, respectively. Fourier transformation of k3 x(k) of the EXAFS data, obtained from the colloidal dispersions of the Au/Pd bimetallic clusters, was carried out over the region 30-160 nm-1. The peak in the
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Harada et al. Fourier transform was filtered (the region 0.154.30 nm) and inversely Fourier transformed into k space (the k region 40-1 50 nm-1) again. The Fourier-filtered data were then analyzed with a modified curve-fitting technique, in which all of both Au L3 and Pd K edge EXAFS data are curve-fitted simultane~usly.~~ There is a simple relation shown in eq 1,which must be satisfied
concerning the coordination number of Au atoms around the Pd atom (WdAu)and the coordination number of Pd atoms around theAuatom (NAuPd).Ineq 1,X~~andXpdare theatomic fractions of Au and Pd in the colloidal dispersions,respectively. Although the parameters WdAu and NAuPd depend on the structure of the cluster, eq 1 is independent of the structure.
Results and Discussion Preparation and Characterization of the Au/Pd Bimetallic Cluster. The colloidal dispersionsof the Au/Pd bimetallic clusters prepared by simultaneous reduction have a brown color and are stable for months at room temperature. This is not always the case for the colloidal dispersions prepared by successive reduction. The dispersions prepared by first reducing the Pd ions, followed by reducing the Au ions (Pd Au), seem to consist of Au/Pd bimetallic clusters. However, first reducing the Au ions, followed by reducing the Pd ions (Au Pd), does not seem to produce bimetallic clusters, because the gold particles immediately aggregate and precipitate after the first reduction. Figure 1 shows the transmission electron micrographs of the colloidal dispersions of the Au/Pd( 1/4) bimetallic clusters immediately after the preparation by the successive and the simultaneous reduction, respectively, with their particle size distribution histograms. The average diameters of the bimetallic clusters prepared by the successive (Au Pd and Pd Au) and the simultaneous reduction are 2.7,2.7, and 1.6 nm, respectively, when the particles less than 5 nm in diameter are counted. Figure 2 shows the transmission electron micrographs of the colloidal dispersions of the Au/Pd( 1/ 1) bimetallic clusters immediately after the preparation by the successive and simultaneous reductions and their particle sizedistribution histograms. The average diameters of the bimetallic clusters prepared by the successive (Au Pd and Pd Au) and the simultaneous reduction are 2.9,2.6, and 2.4 nm, respectively, when the particles less than 6 nm in diameter are counted. In the case of the Au/Pd( 1/4 and 1/ 1) clusters prepared by the successive (Au Pd) reduction, large gold particles immediately aggregated and precipitated, so that the large gold particles, more than 100 nm in diameter, cannot be observed by the transmission electron micrographs in the supernatant of the colloidal dispersions of the clusters. On the other hand, the Au/ Pd(1/4 and 1/1) (Pd 4 u ) clusters might become bimetallic in the colloidal dispersions because the particle size distribution histogram of the clusters in the course of time has the same tendency as that of the Au/Pd( 1/4 and 2/3) bimetallic clusters prepared by the simultaneous reduction. The details will be discussed in the later paragraph. Hydrogenation of 1,3-CyclooctadieneCatalyzedby the Au/Pd Bimetallic Clusters Prepared by Various Reduction Methods. The colloidal dispersions of the Au/Pd bimetallic clusters protected by polymers were used as the active catalysts for the selective partial hydrogenation of 1,3-~yclooctadieneto cyclooctene in ethanol at 30 OC pqder 1 atm of hydrogen. The dependence of the catalytic activity of the Au/Pd bimetallic clusters (0.01 mM) upon the preparation method was investigated. Table I shows the catalytic activity of the colloidal dispersions of the Au/Pd( 1/4) clusters prepared by the different reduction methods. The colloidal dispersions of Au/Pd( 1 /4) bimetallic
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Polymer-Protected Au/Pd Bimetallic Clusters
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 5105
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TABLE I: Catalytic Activity for Hydr enation of 1,3-Cyclooctadiene at 30 OC over Au/Pql/4) Bimetallic Clusters Prepared under Nitrogen initial rate, normalized mmol/(s mmol surf. Pd activity, mmol/(s of metal) ratie mmol of surf. Pd) Pd) 3.16 0.61 1 5.17 Au) 2.66 0.42 1 6.32
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clusters prepared by the simultaneous reduction have the highest initial rate of hydrogenation among those investigated here. In the previous paper,'* the correspondingmixtures of both Au and Pd monometallic dispersions were shown not to have such a high activity as the dispersions prepared by the simultaneous reduction, with respect to the initial rate of hydrogenation, but still higher activitiesthan thoseexpected as the sum of the activities of the two kinds of the monometallic clusters without reciprocal actions. Moreover, in the case of the colloidaldispersionsof the Au/Pd clusters prepared by the successive reduction, the initial rate of
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catalytic hydrogenation of the Au/Pd clusters prepared by the (Au Pd) reduction is higher than that by the (Pd Au) reduction. The relationship between the catalytic activity for the hydrogenation and the surfacestructureof the Au/Pd clusters prepared by the different methods will be discussed in a later paragraph. Change of the Particle Size in tlie Course of Time. The PVPprotected noble metal cluster particle is usually very stable. In the present case, however, a change of particle size in the course of time was observed by transmission electron micrographs. Parts a and b of Figure 3 show the transmission electron micrographs and their particle size distribution histograms obtained from the correspondingphotographs of the colloidaldispersionsof the Au/ Pd( 1/4) bimetallic clusters prepared by the simultaneous reduction and the mixture of the monometallic Au and Pd clusters, respectively. In the case of the bimetallic clusters prepared by the simultaneous reduction, the average diameters, about 1.6 nm at first, are gradually increasing from 1.6 to 2.1 and then to 2.3 nm in the course of time, probably because of the formation of the Au/Pd alloy particles. In contrast, the mixture of the monometallic Au and Pd clusters has two sizes of the particles, Le., the larger particles, more than 5 nm in diameter, probably gold clusters, and the smaller ones, less than 5 nm, probably palladium clusters. In fact, the original dispersions of the gold and the
Harada et al.
5106 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993
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palladium clusters are 15.8 and 1.6 nm in average diameter, respectively. The averagediameter of the total number of particles in the mixture is 2.4 nm. In the course of time after mixing, the larger particles decrease from 15.8 to 9.2 and then to 9.9 nm in diameter, while the smaller particles increase from 1.6 to 2.3 and then 2.6 nm in diameter. This strange change suggests that Au and Pd clusters produce Au/Pd alloy particles by contact in solution in the course of time, since neither the dispersions of Au clusters nor those of Pd clusters themselves showed the same change in particle size under the same conditions. On the other hand, parts a and b of Figure 4 show the particle sizes in the course of time from the transmission electron micrographs of the colloidal dispersions of the Au/Pd( 1/4) clusters prepared by the successive (Au Pd) and (Pd Au) reduction, respectively. Their particle sizedistribution histograms obtained from the corresponding photographs are also shown, respectively.
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In the case of the cluster prepared by the successive (Au Pd) reduction, the average diameter, less than 5 nm does not change in the course of time. The gradual increaseof the average diameter of the particles, as shown in Figure 3b, was not observed in this case. This indicates that the structure of the clusters prepared by thesuccessive (Au- Pd) reduction might bedifferent from that prepared by mixing the corresponding monometallic clusters in terms of the change of the particle size. In order to clarify this difference, the precipitates obtained by the successive (Au Pd) reduction were observed by TEM. The TEM photographs of these precipitates are shown in Figure Sa. The average diameters of the particles are more than 100 nm, regardless of the course of time. In contrast, the clusters prepared by the successive (Pd +Au) reduction tend to keep the average diameter almost unchanged and clusters of more than 5 nm in diameter could not be detected there, as shown in Figure 4b.
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Polymer-Protected Au/Pd Bimetallic Clusters
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 5107
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immediatelyafter after a day after a month Figure 4. Electron micrographs and particle size distribution histograms showing the time dependence of the supematant of the colloidal dispersions of the Au/Pd(l/4) clusters prepared by (a) the successive (Au Pd) reduction and (b) the successive (Pd Au) reduction.
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The changes in the course of time of the average particle size of the Au/Pd( 1/4) clusters prepared by the various reduction methods are summarized in Table 11. It is obviousthat thechange of the particle size of the mixture of Au and Pd is strange. This change indicates that a partial formation of metallic alloy clusters occurs in solution. The change of the metal clusters was also observed by the electronic spectra. Since the dispersions of monometallic Au clusters are not stable, the Au clusters aggregate and form precipitates even a day after the preparation. As a result, the absorbance of the Au clusters at longer wavelength increases. In the case of the mixture of the monometallic Au and Pd clusters, on the contrary, the absorbances gradually decrease and become similar to that of the corresponding Au/Pd bimetallic clusters12 prepared by the simultaneous reduction in the course of time, as shown in Figure 6. This similarity of the electronic spectra suggests again that the gradual change of the mixtures of the monometallic clusters is attributed to the partial formation of bimetallic alloy clusters in solution. Parts a and b of Figure 7 show the transmission electron micrographs and the corresponding particle size distribution histograms of the colloidal dispersions of the Au/Pd(2/3)
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bimetallic clusters prepared by the simultaneous reduction and their mixture of the monometallic Au and Pd clusters at the molar ratio Au/Pd = 2/3, respectively. The same tendency was observed when they are kept in solution at room temperature as in the case of the Au/Pd( 1/4) bimetallic clusters shown in Figure 3. Also, the change of the electronic spectra of the mixtures of Au and Pd monometallic clusters at the ratio Au/Pd = 2/3 has the same tendency as that at the ratio Au/Pd = 1/4, as shown in Figure 6b,c. Parts a and b of Figure 8 show the transmission electron micrographs and their particle size distribution histograms obtained from the corresponding photographs of the colloidal dispersionsof the Au/Pd( 1/ 1) clusters prepared by the successive (Au Pd) and (Pd- Au) reduction, respectively, which indicate the change of particle size in the course of time. The same tendency is observed when they are kept in solution at room temperature as in the case of the Au/Pd( 1/4) clusters prepared by the successive reduction shown in Figure 4. Moreover, the precipitates are formed by the successive (Au Pd) reduction and the same change of the particle size is observed as in the case of the Au/Pd( 1/4) clusters prepared by the same reduction, as shown in Figure 5b.
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5108 The Journal of Physical Chemistry, Vol. 97, No. 19,1993
Harada et al.
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Figure 5. Electron micrographs showing the time dependence of the large gold particles in the precipitates of the colloidal dispersions of (a) the Au/Pd(l/4) clusters and (b) the Au/Pd(l/l) clusters prepared by the successive (Au Pd) reduction.
TABLE Ik Change in Course of Time of the Average Particle Size of the Au/Pd( 1/4) Clusters Prepared by the Various Reduction Methods
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preparation method successive (Au Pd)O successive (Pd Au) simultaneous mixture of Au and Pd
average particle size, nm immediately after after after adav amonth 2.7 2.6 2.5 2.7 2.1 .2.6 1.6 2.1 2.3 2.4b 4.1b 5.W 1 5.8' 9.2' 9.9' 1 .6d 2.3d 2.6d
Except the particles more than 5 nm in diameter. Average diameter of the total particles in the mixture solution. Average diameter of the particles more than 5 nm in diameter. Averagediameter of the particles less than 5 nm in diameter.
Finally, the change in the course of time of the average particle size of the Au/Pd(l/l) clusters prepared by the successive reduction methods is summarized in Table 111. By comparison of the change of the particle size shown in Table I1 with that shown in Table 111, it can be concluded that, in both cases of the Au/Pd clusters at the Au/Pd ratios of 1/4 and 1/ 1, the average particle sizes of the Au/Pd clusters prepared by the various reduction methods converge to nearly the same values after a month has passed, if the charged mole ratio is the
same. In other words, very likely, there is a stable particle size of about 2.5 and 3.0 nm in the case of the Au/Pd(l/4) and Au/Pd( 1/ 1) clusters, respectively. EXAFS Analysis of the Au/Pd Bimetallic Clusters Prepared by Successive Reduction. In the EXAFS analysis of colloidal dispersionsof the Au/Pd bimetallic clusters, two-shell fitting has been carried out by using theoretical phase shifts and amplitudes.'6J7 The reference compounds used in the present work were Pd foil, Au foil, Au/Pd(9/1) alloy foil, and Au/Pd(l/9) alloy foil.'* In the'previous papers, we have reported the EXAFS analysis of the Pd/Pt and Au/Pd bimetallic clusters.' 1 ~ When 2 the curvefitting of these bimetallic clusters was carried out, it was difficult for the structural parameters to be exactly obtained, owing to the large correlation problems among the five fitting parameters (r, AE,a, N, S),where r, AI?,u, N, and Srepresent the coordination distance, the differencebetween the theoretical and experimental threshold energies, the Debye-Waller factor, the coordination number, and the amplitude reduction factor, respectively. In order to determine the accurate structural parameters of the Au/ Pd bimetallic clusters, however, in the previous work the values of AE and S have been taken from the Pd foil, Au foil, Au/ Pd(9/ 1) alloy foil, and Au/Pd( 1/9) alloy foil and that of u from the monometallic Au and Pd clusters. To estimate the range of errors in Nand r, AE and u were varied within the range of 1 5
Polymer-Protected Au/Pd Bimetallic Clusters
The Journal of Physical Chemistry, Vol. 97,No. 19, 1993 5109
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the other hand, the (Pd Au) cluster has Pd-Au bonds the same as the Au/Pd bimetallic cluster prepared by the simultaneous reduction. In thecaseoftheAu/Pd( 1/4) cluster prepared by thesuccessive (Au Pd) reduction, the coordination number of Pd atoms around the Pd atom is determined as 6.3 f 1.9. So the (Au Pd) cluster has only Pd-Pd bonds similar to the monometallic Pd cluster, whose coordination number of Pd atoms around the Pd atom is determined as 6.2.12 This small coordination number suggests that the microcluster particles,20 less than 10 A in diameter, coexist in the colloidaldispersion and gather together, resulting in the formation of the monometallic Pd cluster, about 14 A in diameter, because the coordination numbers of the Pd cluster of 8 and 14 A in diameter are theoretically 5.5 and 7.9, respectively. These results indicate that the (Au Pd) sample is a mixture of monometallic Pd and Au clusters, so that the catalyticactivity (initial rate) of the sample for the hydrogenation reaction is not so high as that of the Au/Pd( 1/4) bimetallic cluster prepared by the simultaneous reduction, as shown in Table I. On the other hand, in the case of the Au/Pd(l/4) cluster prepared by the successive (Pd Au) reduction, the coordination numbers of Pd and Au atoms around the Pd atom are determined as5.0f 1.3and 1.1 f0.3,respectively. Similarly,thecoordination numbers of Au and Pd atoms around the Au atom are determined as 5.5 f 3.0 and 3.9 f 1.9, respectively. As shown in Table IV, the coordination numbers around the Pd atom and the Au atom in the cluster prepared by the simultaneous reduction have some similarities to those in the (Pd Au) cluster. This suggests that the (Pd Au) cluster has similarity in alloy structure to some extent to that prepared by the simultaneous reduction, even though they have different diameters. There is also observed the similarity in the pattern at a series of coordination numbers between the cluster prepared by the simultaneous reduction and that prepared by mixing the corresponding monometallic Au and Pd clusters, though the coordination numbers of Au atoms around the Pd atom and that of Pd atoms around the Au atom are different from each other. This is because the formation of alloy cluster is more difficult in the mixture than in the simultaneous reduction. Moreover, on the basis of the coordination numbers of the Au/Pd( 1/ 1) clusters prepared by the different reduction methods shown in Table V, the Au/Pd(l/l) (Au Pd) cluster has only the Pd-Pd bond similar to that of the monometallic Pd clusters, which is the same result as in the case of the Au/Pd( 1/4) (Au Pd) cluster. The coordination number of Pd atoms around the Pd atom is determined as 7.0 f 2.0. From the TEM observation, the average diameter of the Au/Pd( 1/1) (Au Pd) cluster is 2.9 nm, which is larger than that of the Au/Pd( 1/4) (Au Pd) cluster, 2.7 nm. Thus, this cluster size is in correspondence with the coordination number of Pd atoms around the Pd atom. On the other hand, in the case of the Au/Pd(l/l) cluster prepared by the successive (Pd Au) reduction, especially the coordination numbers of Pd atoms around the Pd atom (3.9 f 0.8) are larger than those of the bimetallic cluster prepared by the simultaneous reduction (1.5 f 0.3). However, the other coordination numbers are nearly the same as those of the bimetallic cluster prepared by the simultaneous reduction. These coordination numbers indicate that the (Pd Au) clusters mainly consist of bimetallicAu/Pd clusters,because the average diameter and the particle size distribution histogram of the (Pd Au) clusters are nearly the same as those of the clusters prepared by the simultaneous reduction as shown in Figure 2, parts b and c, respectively. There is, however, a remarkable difference between these clusters in the coordination number of Pd atoms around the Pd atom. The successive (Pd Au) reduction makes Pd ions reduced first, resulting in the formation of the monometallic Pd clusters at the first stage of the reduction. Thus, it might be concluded that Pd atoms aggregate by themselves more easily in
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300
400
500
600
700
800
-
0
300
WAVELENGTH / n m
400
500
600
700
800
WAVELENGTH / n m
-
-
01 300
' LCC
'
'
'
500
EO0
700
"
800
WAVELENGTH / n m
300
400
500
600
700
800
WAVELENGTH / n m
Figure 6. Electronic spectra showing the time dependence of (a) Pd monometallic, (b) mixtures of Au and Pd monometallic at ratio of Au/ Pd = 1/4, (c) mixtures of Au and Pd monometallic at ratio of Au/Pd = 2/3, and (d) Au monometallic clusters protected by poly(N-vinyl-2pyrrolidone).
eV and fO.01 A from that of Pd K and Au L3 edge EXAFS of Pd and Au foil, respectively. The same curve-fitting technique12 as that in the previous work is applied to determine the structural parameters of the Au/Pd clusters prepared by the successive reduction in the present work. Figure 9a shows the Fourier transforms of the Pd K edge EXAFS of the colloidal dispersionsof the Au/Pd clustersprepared by the simultaneous and successive (Au Pd and Pd Au) reduction at the Au/Pd ratio = 1/4. To obtain detailed information about the Pd-Pd and Pd-Au bonding, the data were Fourier-filtered over 0.15-0.30 nm and analyzed by the curvefitting technique. In the present case, the peak height decreases in the order Au Pd > Pd Au > simultaneous reduction. This indicates that the samples prepared by the simultaneous reduction and the successive (Pd Au) reduction have Pd-Au bonds as well as Pd-Pd bonds. However, in the case of the sample prepared by the successive (Au Pd) reduction, the peak shape is similar to that of the monometallic Pd clusters. This suggests the sample have only Pd-Pd bonds. Figure 9b shows the Fourier transforms of Au L3 edge EXAFS of the colloidal dispersions of the same clusters shown in Figure 9a. The data were Fourier-filtered over 0.20-0.30 nm in the same way as in the Pd Kedge EXAFS analysis. The main peaks have nearly the same height but a different shape. The structure parameters of the colloidal dispersions of the Au/Pd( 1/4) clusters prepared by the different reduction method have been determined on the basis of the two-shell fitting as shown in Table IV. The errors for the coordination numbers are estimated in the same method.I9 Parts a and b of Figure 10 show the Fourier transforms of Pd K and Au L3 edge EXAFS of the colloidal dispersions of the Au/Pd( 1/ 1) clusters prepared by the simultaneous and successive (Au Pd and Pd Au) reduction, respectively. The structure parameters of the colloidal dispersions of the Au/Pd( 1/ 1) clusters have been determined in the same way as those for the Au/Pd(l/4) bimetallic clusters, as shown in Table V. As shown in Figure 5 , the colloidaldispersions prepared by the successive (Au Pd) reduction consist of large Au clusters and monometallic Pd clusters. So the large Au clusters immediately form precipitates. Hence, the Au edge EXAFSjump of the sample cannot be detected as shown by blanks in Tables IV and V. On
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Harada et al.
5110 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993
50
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:*
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Figure7. Electron micrographs and particle size distribution histogramsshowing the time dependenceof (a) the colloidal dispersionsof the Au/Pd(2/3)
bimetallic clusters and (b) the mixtures of the monometallic Au and Pd clusters at the same ratio.
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the (Pd Au) cluster than in the cluster prepared by the simultaneousreduction, which is different from that in the case of the Au/Pd( 1/4) cluster shown in Table IV. Structure of the Au/Pd Clusters Prepared by Successive Reduction. In the previous paper,I2 from the EXAFS analysis as well as TEM observation, the Au core structure, in which the Pd atoms are on the surface of the cluster particles, has been presented as a model for the Au/Pd(l/4) bimetallic cluster prepared by the simultaneous reduction. In the case of the Au/Pd( 1/4) bimetallic clusters prepared by the successive (Pd Au) reduction, the average size has been determined to be 2.7 nm by the TEM observation as described in the foregoing. This particle size is larger than that of the Au/Pd( 1/4) bimetallic clusters prepared by the simultaneous reduction. Hence, the present cluster is calculated to consist of a five-layeredfcc structurecontaining309 atoms from the particle size. The large coordination numbers of Pd atoms around the Pd atom and of Au atoms around the Au atom (Table VI) suggest
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that the Pd atoms and Au atoms tend to aggregate by themselves. This indicates that, in the case of the Au/Pd bimetallic clusters prepared by the successive (Pd Au) reduction, the metal combinationswith a wide miscibilitygapor complete immiscibility in the bulk can form biphasic particles, typical examples of which are Ru-Cu*' and 0 s - C dispersed ~ ~ ~ on SiOz, whereas singlephase alloy particles are only observed for systems completely miscible in the bulk, like Pd-Au,f3 Pt-Fe,24 and Fe-Pd.25 Moreover, the sum of the coordination numbers around the Au atom is less than 10, so that the exposure of the Au atoms on the cluster surface might be inferred from the coordination numbers. The observed coordination numbers are different from those calculated for the random model, in which 248 Pd atoms and 61 Au atoms are located completely at random. As shown in Figure 11, the 309 atoms form the fcc structure, in which 61 Au atoms form two cores and 248 Pd atoms surround the two Au cores (a cluster-in-cluster model). Then, the coordination numbers calculated for the cluster-in-cluster model in Figure 11 are
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The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 5111
Polymer-Protected Au/Pd Bimetallic Clusters
(a)
Prepared by Successive Reduction (Au + Pd) 28.66
ii "0
10
20 30 4 0 Diameter (A)
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b
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200w immediatelyafter
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200w after a month
after a day
-
-
Figure 8. Electron micrographs and particle size distribution histograms showing the time dependence of the colloidal dispersionsof the Au/Pd( 1/1) clusters prepared by (a) the successive (Au Pd) reduction and (b) the successive (Pd Au) reduction.
TABLE IIE Change in Course of Time of the Average
Particle Size of the Au/Pd( 1/1) Clusters Prepared by the Various Reduction Methods
--
preparation method successive (Au successive (Pd
Pd)o
Au)
average particle size, nm immediately after after after aday amonth 2.9 2.6
2.9 2.5
2.9 3.4
Except the particles more than 6 nm in diameter.
consistent with thoseobtained from the EXAFSdata. Therefore, it is possible to apply the cluster-in-cluster model for the Au/ Pd( 1/4) cluster prepared by the successive(Pd Au) reduction. The reason why the Au/Pd( 1/4) (Pd Au) cluster has such a cluster-in-cluster structure will be discussed later. As for the model of the Au/Pd( 1/ 1)bimetalliccluster prepared by the simultaneous reduction, it has been proposed to use the cluster-in-cluster model, in which seven Au cores are located in the cluster and Pd atoms fill the space between the seven Au cores, to play a role in combining ihe Au cores.12 In the case of the Au/Pd( 1/ 1) clusters prepared by the successive (Pd Au) reduction, the average size has been determined to be 2.6 nm from TEM observation as described
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before. On the basis of theTEM observation and themrdination numbers obtained by the EXAFS as shown in Table V, the Au/ Pd( 1/ 1) cluster prepared by the successive (Pd Au) reduction is considered to have a similar structure (cluster-in-cluster) as those prepared by the simultaneous reduction. The cluster-incluster model is better applicable for the Au/Pd( 1/ 1) (Pd Au) cluster than for that prepared by the simultaneous reduction, because the observed coordination numbers of the former are better fit to the calculated ones than those of the latter. Relation between the Surface Structure and the Catalytic Activity. The catalytic activity of the selective partial hydrogenation of 1,3-~yclooctadieneto cyclooctene is shown in Table I. The initial rate of catalytic hydrogenation decreasesdepending on the preparation method in the order simultaneous reduction > mixture of the monometallic clusters > successive (Au Pd) reduction > successive (Pd Au) reduction. This fact suggests that the surface structures of the clusters are different from each other, since it is well-known that the catalytic activity depends on the surface structure. The relationship between the surface structureand thecatalytic activity in the Au/Pd( 1/4 and 1/ 1) bimetallic cluster prepared by the simultaneous reduction has been discussed in the previous paper.12
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Harada et al.
5112 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993
m
Simul. (-) AuAPd(---) Pd +Au (---)
;
C
e
:I
..
I . I
c
TABLE V Coordination Numbers around the Pd and Au Atoms of the Au/Pd = 1/1 Cluster Prepared by the Various Reduction Methods and Those Calculated for the Model absorbing metal Pd Pd Au Au
Distance R
TABLE IV Coordination Numbers around the Pd and Au Atoms of the Au/Pd = 1/4 Cluster Prepared by the Various Reduction Methods
Pd Pd Au Au
Pd Au Au Pd
0
Pd
6.3 f 1.9 a
PPt PPt
coordination number N Pd- Au simul mixture 5.0 f 1.3 1.1 f 0.3 5.5 f 3.0 3.9 f 1.9
4.2 f 0.8 5.0 f 1.3 1.2 f 0.3 0.5 f 0.3 6.2 f 1.9 7.4 f 2.3 4.5 f 1.1 2.1 f 1.0
Not detected.
m
e C
I
c
"0
Simul. (-) AU +Pd (---) Pd -pAu (........)
2
40
PPt PPt
3.9 f 0.8 2.4 f 0.5 5 . 9 f 2.1 2.3 f 0.5
1.5 f 0.3 2.5 f 0.4 6.5 f 1.5 2.3 f 0.4
6
Distance R ( A )
metal
metal
observed
2 Au cores
random
Pd Pd Au Au
Pd Au Au Pd
5.0f 1.3
8.3 1 .o 6.1 4.1
1.7 1.9 1.9 7.1
1.1 f 0.3 5.5 f 3.0 3.9 f 1.9
particles, as shown in Figure 5 . This might suggest that the obtained dispersions have the same behavior as the mixture of the Au and the Pd monometallic clusters in terms of the initial rate of catalytic hydrogenation. As shown in Figures l a and 3b, however, the average diameters of the particles smaller than 5 nm in diameter are 2.7 and 1.6 nm in the case of the (Au Pd) cluster and themixture, respectively. Thus, theinitial ratedepends on the particle size, so that the initial rates of catalytic hydrogenation are different between the (Au Pd) clusters and the mixed clusters. In the case of the cluster-in-cluster model for the (Pd Au) clustersdescribed in the previous section,32 Au atoms are located on the surface of the cluster particle, which has in total 162 atoms on the surface as shown in Figure 11. The existence of Au atoms on the surface of the cluster-in-cluster model is considered to decrease the practical catalytic activity for the selective partial hydrogenation compared with the Pd monometallicclusters,since a Au atom has no activity for the catalysis. The relationship between the reduction method and the structure of the Au/Pd bimetallic clusters is summarized in Table VII. It is not clear why the smallest cluster with Au core structure, not the cluster-in-cluster structure, is produced in the case of the Au/Pd( 1/4) bimetallic clusters prepared by the simultaneous reduction. Finally, in order to clarify the relation between the surface structure prepared by the various reduction methods and the catalytic activity of the hydrogenation, the initial rates, as shown in Table I, have been reestimated. At first assumption, the Au/Pd( 1/4) clusters prepared both by the successive (Au Pd) reduction and by the mixing of the monometallic Au and Pd are composed of monometallic Au clusters a n d monometallic Pd clusters, not alloyed in the solution. The Au/Pd( 1/4) clusters prepared by the successive (Pd Au) reduction and the simultaneous reduction, however, have a twoAu-core structure (shown in Figure 1 1) and a Au core structure,12 respectively. In the case of the monometallic Pd clusters, the cluster particle contains 55 Pd atoms with fcc structure and 42 atoms of the 55 Pd atoms are on the surface of the particle.20 From these models, the surface Pd ratios have been calculated, as shown in Table I, and then the normalized activity, Le., the initial rate per millimole of surface Pd, has been calculated. As a result, in the case of the successive (Au Pd) reduction, the normalized activity of the obtained cluster is nearly the same as that of the monometallic Pd cluster. On the other hand, in the cases of the successive (Pd Au) reduction, the simultaneous
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Distance R
(i)
Figure 10. Fourier-transformed EXAFS spectrum at (a) Pd Kedge and (b) Au L, edge of the colloidal dispersions of the A u / P d ( l / l ) clusters prepared by the simultaneous and successive reductions.
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6.3 2.6 7.3 2.6
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(b) Au L3 edge of the colloidal dispersions of the Au/Pd(l/4) clusters prepared by the simultaneous and successive reductions.
Au-
a
TABLE VI: Coordination Numbers around the Pd and Au Atoms of the Au/Pd = 1/4 Cluster Prepared by the Successive (Pd Au) Reduction and Those Calculated for the Models coordination number N absorbing scattering
Figure 9. Fourier-transformed EXAFS spectrum at (a) Pd K edge and
scattering metal
7.0 f 2.0
Pd Au Au Pd
Not detected.
(i)
Distance R (i)
absorbing metal
coordination number N scattering metal Au- Pd Pd- Au simul 7 Au cores
In the case of the Au/Pd(l/4) clusters prepared by the successive (Au Pd) reduction, on the other hand, the colloidal dispersions consist of large gold particles and monometallic Pd
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Polymer-Protected Au/Pd Bimetallic Clusters
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 5113 SCHEME I Nuclear Formation Process 2Au3++ 3C2H,0H Pd2++ C2H,0H
@ : 248 atoms @ : 61 atoms
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2Au + 3CH3CH0+ 6H' Pd + CH,CHO + 2H'
Growing Process
Figure 11. Cross section of a model for Au/Pd(l/4) cluster prepared by the successive (Pd
-
Au) reduction.
TABLE MI: Relation between the Reduction Method and the Structure of the AdPd Bimetallic Clusters
- Pd,,, + CH3CH0+ 2 H - 2Au,+, + 3CH3CH0+ 6H' + 3CH,CHO + 6H' --Au,Pd2(Pd,Au) + CH,CHO + 2H'
Pd, + Pd" + C2H,0H 2Au, + 2Au" + 3C2H,0H 2Pd, + 2Au3++ 3C2H,0H Au, + Pd" + C2H,0H
~~
structure
--
preparation method successive (Pd successive (Au simultaneous
Au) Pd)
Au/Pd( 1/4)
Au/Pd( 1/ 1)
2 Au cores mixture Au core
cluster in cluster mixture cluster in cluster
reduction, and themixtureof themonometallic Au and Pdclusters, the normalized activities are higher than those of the monometallic Pd cluster. However, the above models have some speculations so that much attention has to be paid to the relation between the surface structure and the catalytic activity. For example, it has been observed that the alloy clusters form by only mixing the monometallic Au clusters and monometallic Pd clusters, as indicated by the change of particle size in the course of time as shown in Figure 3b, as well as by the EXAFS measurements as shown in Table IV. Thus, as a reasonable structure, the mixture of the monometallic Au and Pd clusters could have a strong interaction between large monometallic Au clusters and small monometallic Pd clusters, resulting in partial alloy clusters. Then, the initial rate per surface Pd has to be recalculated on this assumption. The surface Pd ratio of the mixture sample with partial alloy structure should be lower than that calculated for the pure mixtures (0.61 l), resulting in the larger normalized activity than 6.35. As for the structure of the Au/Pd(l/4) (Pd Au) cluster, two kinds of models can also be suspected from the TEM observation and the EXAFS data. One is a two-Au-core model (cluster-in-cluster model), as shown in Figure 11, and the other is an assembly in which several Au/Pd( 1/4) bimetallic clustershaving an Au core structure assembleto form a secondary particle. Now, if the assembly model can only be taken as the appropriate one, the surface atoms of the cluster are thought to consist of only Pd atoms and to achieve the same normalized activity as in the case of the Au/Pd(l/4) bimetallic clusters prepared by the simultaneous reduction. In fact, the normalized activity of the (Pd Au) cluster (6.32) is different from that of the cluster prepared by the simultaneous reduction (7.27). Therefore, the existence of Au atoms on the surface of the twoAu-core model is considered to decrease the practical catalytic activity compared with that of the Pd monometallic clusters. In order to compare the normalized activity of the bimetallic clusters prepared by the simultaneous reduction at different Au/ Pd ratios, the normalized activity of the Au/Pd( 1/1) bimetallic clusters has been calculated to be 10.59 by using its initial rate shown in the previous paper.12 This normalized activity of the Au/Pd( 1/ 1) bimetallic clusters is much higher than that of the Au/Pd(l/4) ones (7.27). This result suggests the interaction between the Au atoms and thepdatomsin thecluster prepared by thesimultaneous reduction. Moreover, in the case of the successive (Pd Au) reduction and the mixture, the higher normalized activity than that in the case of the monometallic Pd clusters could also be considered to be attributed to the interaction between the Au atoms and the Pd atoms in the cluster.
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Based on these data, it could be concluded that the interaction between the Au atoms and the Pd atoms has brought about the change in the electron density on the surface due to the difference in ionization potential between Au and Pd atoms. Thus, the electron-deficient Pd may achieve the higher activity for the hydrogenation of 1,3-cyclooctadiene, which has the double bond favoring the electron-deficient s ~ r f a c e . * ~ , ~ * Mechanism of the Formation of the Au/Pd Clusters. The ionization potential of the Pd atom (8.33 eV) is known to be lower than that of the Au atom (9.223 eV).29 Thus, the reduction of the Au ions proceeds more easily than that of the Pd ions. We have reported the in-situ UV-vis spectra during the formation of the Au/Pt bimetallic clusters in the presence of PVP.30 From these spectra, the tailing absorption due to the Pt clusters is observed at the whole wavelength, resulting in decreasing plasma absorption due to the Au clusters. This suggests that the Au core structure, in which the Pt atoms are on the surface of the cluster particle, has been presented as for the model of the Au/Pt bimetallic cluster prepared by the simultaneous reduction. In this case, because the ionization potential of the Pt atom (8.96 eV)29is lower than that of the Au atom, the Au ions are reduced more easily than the Pt ions. Then, after the reduction, the Au atoms aggregate to form Au clusters first, resulting in a plasma absorption peak, and then Pt atoms are deposited on the Au clusters. Here, in the formation of the Au/Pd bimetallic cluster, two typical processes could be considered. One is the nucleation process and theother is thegrowth process. Thereaction formulas of each process are shown in Scheme I. In the case of the nucleation process, one Au ion is reduced by ethanol, leading to the formation of one Au atom. Similarly, one Pd ion leads to one Pd atom. In the growth process, one Pd ion or one Au ion is reduced by ethanol, on the surface of the monometallic Au or Pd atom or cluster, leading to a larger cluster of Au or Pd, or a Au/Pd bimetallic cluster. Now, the cluster-in-cluster model is constituted by several microclusters. The microcluster, less than 10 A in diameter, can be thought to be a fundamental unit in the cluster particle.20 In other words, it can be considered that the Au/Pd( 1/ 1) bimetallic cluster presented by the cluster-in-cluster model is constituted by both 7 Au microclusters and some Pd mircoclusters. In the case of the Au/Pd( 1/1) bimetallic cluster prepared by the simultaneous reduction, the reduction of the Au ions proceeds more easily than that of the Pd ions from the viewpoint of the ionization potential. Thus, the Au microcluster is formed sooner than thepdmicrocluster. However,thesizeofthe Au microcluster might be kept small owing to the coexistence of the Pd ions and/ or Pd microclusters, as shown in TEM photographs in Figures ICand 2c, whereas the size of the Au microcluster becomes larger and larger during the reduction of Au ions, resulting in the formation of the large gold particles in the absence of the Pd ions and/or Pd microclusters in the case of the successive (Au Pd) reduction, as shown in TEM photographs in Figure 5 . Thus, the cluster-in-cluster model is reasonably appropriate for the model of the Au/Pd( 1/ 1) bimetallic cluster.
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5114 The Journal of Physical Chemistry, Vol. 97, No. 19. 1993
In the case of the Au/Pd( 1/4) bimetallic cluster prepared by the simultaneousreduction, the surface structure is characteristic. In this case, the average particle size of the obtained cluster is 1.6 nm. This is the smallest among all the Au/Pd bimetallic clusters prepared at the various Au/Pd ratios. The same formation process might occur at thevarious Au/Pd ratios on the surface of the Au microcluster, which is formed sooner than the Pd microcluster, as describedbefore for the Au/Pd( 1/ 1) bimetallic cluster prepared by the simultaneous reduction. In thecaseoftheAu/Pd(l/l) cluster prepared bythesuccessive (Pd Au) reduction, on the other hand, Pd ions can be reduced first, resulting in the formation of the Pd nucleus and then in the formation of the Pd microclusters. Subsequently, Au ions are reduced more easily on the surface of the Pd microclusters,leading to the formation of the Au microclusters, less than 10 A in diameter, on the surface of the Pd microclusters. Both Au and Pd microclusters might be mobile, and the rearrangements of these microclusters might occur in the cluster particle, leading to the formation of the cluster-in-cluster structure. Considering the cluster-in-cluster structure of the Au/Pd( 1/ 1) cluster, the Au microcluster and the Pd microcluster play an important role in the reconstruction of the cluster. Therefore, the sameclusterin-cluster model of the Au/Pd( 1/ 1) cluster prepared by the successive (Pd Au) redpction can be taken as that prepared by the simultaneous reduction. In the case of the Au/Pd( 1/4) cluster prepared by the successive (Pd Au) reduction, however, the two-Au-core structure can be taken, as shown in Figure 11. This structure is different from the Au core model shown as the structure of the Au/Pd( 1/4) bimetallic cluster prepared by the simultaneous reduction. However, in the course of time, the structure of the (Pd Au) cluster might gradually change from the two-Au-core model to the Au core model, because the Au core structure is thermodynamically more stable. This change, of course, is derived from the rearrangements of the microcluster, which is a fundamental unit. If the rearrangements of the microcluster do not occur, the coordination number (5.0 f 1.3) of the Pd atoms around the Pd atom in the (Pd Au) cluster and the mixture, being different from that of the monometallic Pd cluster, cannot be observed, as shown in Table IV. Moreover, if the rearrangements of the atom, as the fundamental unit, take place, the drastic change of the coordinationnumber of the Au atoms around the Pd atom cannot occur for the (Pd Au) cluster and, especially, for the mixture. Here the example of the mobility of the microclusters has been shown. In the case of the Pd/Pt bimetaIlicclusters,2O the EXAFS measurements have indicated that Pd microclusters adhere on the surface, especially on the partly defected surface, of the Pd/ Pt bimetallic cluster, when the clusters are kept under hydrogen atmosphere. Thus, it could be generally concluded that the microclusters, not the atoms, are mobile and easily rearrange in the cluster particle, resulting in the reconstruction of the cluster.
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Acknowledgment. We gratefully acknowledge the assistance of Drs. Atsushi Oyama and Masaharu Nomura at the National Laboratory for High Energy Physics (KEK) for the EXAFS measurements and of Drs. Kouichi Adachi and Satoru Fukuda at the University of Tokyo for taking electron micrographs. This study was supported by a Special Grant by the Asahi Glass Foundation and a Grant-in-Aid for Scientific Research in the Priority Area of “MacromolecularComplexes” (016 12002) from the Ministry of Education, Science and Culture, Japan. References and Notes (1) Meitzner, G.; Via, G. H.; Lytle, F. W.; Sinfelt, J. H. J . Chem. Phys. 1987, 87. 6354. (2) Sinfelt, J. H.; Via, G. H.; Lytle, F. W.J. Chem. Phys. 1982,76,2779. (3) Meitzner, G.; Via, G. H.; Lytle, F.W.; Fung, S.C.; Sinfelt, J. H. J. Phys. Chem. 1988, 92, 2925. (4) Sinfelt, J. H. In Catalysis. Science and Technology; Anderson, J. R., Boudart, M., Us.Springer-Verlag: ; Berlin, Heidelberg, New York, 1981; Vol. 1, p 257. ( 5 ) Hirai, H.; Toshima, N. In Tailored Metal Catalysts; Iwasawa, Y., Ed.; D. Reidel Pub.: Dordrecht, 1986; pp 87-140. (6) Hirai, H.; Nakao, Y.; Toshima, N. Chem. Lett. 1978, 545. (7) Hirai, H.; Chawanya, H.; Toshima, N. React. Polym. 1985,3, 127. (8) Hirai, H.; Chawanya, H.; Toshima, N. Bull. Chem. Soc. Jpn. 1985, 58, 682. (9) Toshima, N.; Kuriyama, M.; Yamada, Y.; Hirai, H. Chem. Lett. 1981, 793. (10) Toshima, N.;Takahashi,T.; Hirai, H.J. Macromol.Sci.-Chem.1988, A25 (5-7), 669. (1 1) Toshima, N.; Harada, M.; Yonezawa, T.; Kushihashi, K.; Asakura, K. J. Phys. Chem. 1991.95.7448. (12) Toshima,N.; Harada, M.; Yamazaki, Y.; Asakura, K. J. Phys. Chem. 1992, 96, 9927. (13) Skoglundh, M.; Lowendahl, L. 0.;Ottersted, J. E. Appl. Catal. 1991, 77, 9. (14) Liu, H.; Mao, G.; Meng, S.J . Mol. Catal. 1992, 74, 275. (15) Via, G. H.; Drake, K. F., Jr.; Meitzner, G.; Lytle, F.W.; Sinfelt, J. H. Catal. Lett. 1990, 5, 25. (16) Teo, B. K.EXAFS Basic Principles and Data Analysis, Inorganic Chemistry Concepts; Springer-Verlag: Berlin, 1986; Vol. 9. (17) Teo, B. K.; Lee, P. A. J. Am. Chem. SOC.1979, 101, 2815. (18) Renaud, G.; Motta, N.; Lancon, F.; Belakhovsky, M. Phys. Rev. B 1988, 38, 5944. (19) Lytle, F. W.; Sayers, D. E.; Stern, E. A. In X-Ray Absorption Fine
Structure-V; Leon, J. M., Stern, E. A., Sayers, D. E., Ma, Y., Rehr, J. J., Eds.; Elsevier/North-Holland: New York, 1988; pp 701-722. (20) Harada, M.; Asakura, K.; Ueki, Y.; Toshima, N. J . Phys. Chem. 1992, 96, 9730. (21) Sinfelt, J. H.; Via,G. H.; Lytle, F. W.J. Chem. Phys. 1980,72,4832. (22) Sinfelt, J. H.; Via, G. H.; Lytle, F.W.;Greegor, R. B.J. Chem. Phys. 1981, 75, 5527. (23) Lam, Y.L.; Boudart, M. J . Catal. 1977,50, 530. (24) Bartholomew, C. H.; Boudart, M. J . Catal. 1973, 29, 278. (25) Garten, R. L. J . Catal. 1976, 43, 18. (26) Unpublished results. (27) Richard, D.; Couves, J. W.; Thomas, J. M. Faraday Discuss. Chem. SOC.1991, 92, 109. (28) Giroir-Fendler, A.; Richard, D.; Gallezot, P. Faraday Discuss. Chem. SOC.1991, 92, 69. (29) Lange, N. A. Handbook ofchemistry; McGraw-Hill Bookcompany, Inc.: New York, 1961. (30) Toshima, N.; Yonezawa, T. Makromol. Chem., Macromol. Symp. 1992, 59, 281.
