J. Phys. Chem. 1992,96,9927-9933 (9) Ahn, D. H.; Lee,J. S.; Nomura, M.; Sachtler, W. M.H.; Moretti, G.; Woo. S. I.; Ryoo, R. J . Carol. 1992, 133, 191. (10) Ryoo, R.; Pak, C.; Chmelka, B. F. Zeolites 1990, 10, 790. (11) Ryoo, R.; Pak, C.; Ahn, D. H.; de Menorval, L.-C.; Figueras, F. Catal. h t r . 1990, 7, 417. (12) Scharpf. E. W.; Crecely, R. W.; Gates, B. C.; Dybowski, C. J . P h p . Chem. 1986,90,9.
9927
(13) Fraieeard, J.; Ito, T. Zeolites 1988, 8, 350 and references therein. (14) Ito, T.; Fraissard, J. J. Chem. Soc., Faraday Trans. 1 1987,83,451. (IS) Yang, 0. B.; Woo, S.I.; Ryoo, R. J . Coral. 1990,123, 375. (16) Gustafson, B. L.;Lunsford, J. H.J . Carol. 1982, 74, 393. (17) Bart, J. C.; Vlaic, G. Ado. Caral. 1987, 35, 1. (18) Cox, A. D. In Characterization ofCaralysrs; Thomas J. M., Lambert, R. M., Eds.;Wiley: Chichater, 1980; p 255.
Catalytic Activity and Structural Analysis of Polymer-Protected Au-Pd Bimetallic Clusters Prepared by the Simultaneous Reduction of HAuCi, and PdCi2 Naoki Toshima,*gt Masafumi Harada; Yoshinao Yamazaki,t and Kiyotaka Asakurat 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: June 3, 1992; In Final Form: August 12, 1992)
The colloidal dispersions of the poly(N-vinyl-2-pyrrolidone)-protectedgold/palladium bimetallic clusters were prepared by simultaneous reduction of HAuC14and PdCl2 in the presence ofpoly(N-vinyl-2-pyrrolidone). The bimetallic clusters were relatively uniform in size (e.g., 1.6 nm in average diameter) and were used as a catalyst for the selective partial hydrogenation of 1,3cyclooctadiene at 30 OC under 1 atm of hydrogen. The initial rate of the hydrogenation depended on the metal composition of the cluster catalyst, and the maximum catalytic activitieswere achieved at Au/Pd = 1/4. The catalytic activities of the mixtures of the monometallic Au and Pd clusters were higher than those expected for the two kinds of the monometallic clusters without reciprocal actions. The structurcs of the bimetallic clusters were studied using extended X-ray absorption fine structure (EXAFS).The structural parameters were determined by fitting the experimental data for the two absorption edges simultaneously using improved computer program. As a consequence of this improvement, the coordination numbers of Au atoms around the Pd atom and of Pd atoms around the Au atom could be determined with greater confidence. The EXAFS data of the Au/Pd( 1/4 and 1/1) bimetallic clusters prepared by simultaneous reduction indicate that the surface of the particle entirely consists of palladium atoms and the core of gold atoms (an Au core structure).
Introduction Metal clusters composed of two kinds of metal elements are of interest in catalysis. Bimetallic clusters such as Cu-Ru,' CU+,~ and Cu-Rh3 dispersed on a carrier, commonly silica or alumina with high surface area, have been studied extensively for the industrial applications. The bimetallic clusters revealed different effects, such as the catalytic activity, selectivity, and stability. However, the colloidal dispersions of bimetallic clusters in a homogeneous solution have been little investigated. Recently, we have reported the preparation and the structural analysis by extended X-ray absorption fine structure (EXAFS) of the Pd/Pt bimetallic colloids$ Liu et al.5 reported the preparation and XPS characterization of the Au/Pd bimetallic colloids. Esumi et a1.6 prepared the Pd/R bimetallic colloids in organic solvent by solvent extraction-reduction. The analysis of EXAFS appears very promising for structural studies of noncrystalline as well as crystalline materials. The method is especially interesting for the colloidal dispersions of bimetallic clusters, since it is difficult or almat impossible to obtain structural information on such clusters by other methods. From an analysis of EXAFS data, one can get information on the number and type of neighboring atoms about a given absorber atom and on the interatomic distances. The colloidal dispersions of noble metals, 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-2pyrrolidone).' The colloidal dispersions thus obtained are stable and composed of fine metal particles from 1 to 3 nm in average
diameter with narrow size distributions. They work as catalysts with high activity and selectivity for the hydrogenation of olefins: selective partial hydrogenation of diene to monoene?JO the light-induced hydrogen generation from water,11J2and so on. We have recently reported in a series of papersl3-l5that the colloidal dispersions of the bimetallic clusters can be prepared by refluxing the alcoholic solution containing two kinds of noble metal salts in the presence of poly(N-vinyl-2-pyrrolidone). EXAFS analysis has clearly shown that the Pd/Pt(4/1) bimetallic clusters prepared under nitrogen, which is the most active catalyst for the selective partial hydrogenation of 1,3-cyclooctadieneto cyclooctene in a series of Pd/Pt bimetallic clusters, have a core composed of FY atoms and the surface layer entirely composed of Pd atoms.4 In the present paper, the colloidal dispersions of the polymer-protected gold/palladium bimetallic clusters are prepared by simultaneousreduction. The catalytic activity of the bimetallic clusters is examined for the selective partial hydrogenation of 1,3-~yclooctadieneto cyclooctene and compared with that of the mixtures of the monometallic clusters. The structure determination is performed by the experimental data of the EXAFS associated with the L3-edge of gold and the K-edge of palladium. In determining the structural parameters, the EXAFS data for the two edges are fitted separately and simultaneously, and then both results are compared. Here we describe the determination of the structure of the Au/Pd( 1/4) and Au/Pd( 1/ 1) bimetallic clusters. ExperimentalSection Repuatiaa of the Au/Pd Bimetallic Clusters by Si"s
Reduction. The colloidal dispersions of the Au/Pd bimetallic clusters protected by polymers were prepared by an alcohol-reduction method. Solutions of tetrachloroauric acid (0.033 "01 in 25 mL of water) were prepared by dissolving the corresponding
'Department of Industrial Chemistry, Faculty of Engineering. *Department of Chemistry, Faculty of Science.
0022-3654/92/2096-9927503.OOlO - _ _ ~ ., Q 1992 American Chemical Society I
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9928 The Journal of Physical Chemistry, Vol. 96, NO. 24, 1992
crystal in water. Ethanol solutions of palladium(I1) chloride (0.033 mmol in 25 mL of ethanol) were prepared by stirring dispersions of PdClz powder in ethanol. Both solutions were mixed at various ratias to produce a bimetallic ethanol/water (1/1 v/v) (50 mL) solution containing poly(N-vinyl-2-pyrrolidone) (PVP, K-30, 151 mg, mw 40000, 1.36 mmol of monomeric units) as a protecting polymer. The total amount of both metals was always kept as 3.3 x mol in SO mL of the mixed solution. The mixed solution was refluxed at about 100 OC for 2 h under nitrogen. For the preparation of the mixture of the Au clusters and the Pd clusters, both dispersions of the monometallic clusters, prepared separately, were mixed at room temperature. cheracterizatioa of the Au/Pd Bimecllllic chater& Electronic spectra of the colloidal dispersions of the bimetallic clusters were measured with a Hitachi Model 340 spectrophotometer. Transmission electron micrographs of the colloidal dispersions were taken with a HU-12A and H-7000 electron microscope. The diameter of each particle was determined from the enlarged photographs. The histogram of the particle size distribution and the average diameter were obtained on the basis of the measurements of about 300 particles. Hydrogenatioa of 1,3-Cy* C a t d y d by the colloidal Dispersions of the Bimetallic Clusters. The catalytic activities of the bimetallic clusters of various Au/Pd ratio were measured by the initial rate of hydrogen uptake in the hydrogenation of 1,3cyclooctadiene (25 mmol/L) in ethanol at 30 OC under 1 atm of hydrogen while keeping the total concentration of noble metal at 0.01 mmol/L. EXAFS Measurement. The samples for the EXAFS measurement were prepared by concentrating 1500 mL of the above obtained colloidal dispersions 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 and Au/Pd( 1/9) alloy foil, which serve as preferable references, were thankfully presented by Tanaka Kikinzoku Kogyo K.K. The EXAFS spectra were measured at a BL-1OB station of Photon Factory, The National Laboratory for High Energy Physics (KEK-PF) using synchrotron radiation at rmm temperature. Fourier transformationof k3x(k)was carried out over the region 30-160 nm-l. The peak in the Fourier transform was filtered and inversely Fourier transformed into k-space (the region 40-1 50 nm-l) again. The Fourier-filtered data were then analyzed with a curve-fitting There is a simple relation shown in eq 1,18 which must be satisfied concerning the coordination number of Au atoms around the Pd atom (MdAU) and the coordination number of Pd atoms around the Au atom (NAm). In eq 1, XA, and X , are the atomic fractions of Au and Pd in the colloidal dispersion, respectively. While the values of the parameters NWA" and NAm are dependent on the structure of the cluster, the relation shown in eq 1 is independent of structure. In this case, the modified method of analysis,Is in which all of both Au-L, and Pd-K edge EXAFS data are curve-fitted simultaneously, was also used instead of the previous method. The comparison of both methods, curve-fitted separately and simultaneously, will be discussed later.
Results Pad Discussion Reprvrtion and Chuacterizntioa of Au/Pd Bimetallic Cluster. The colloidal dispersions of Au/Pd bimetallic clusters prepared by simultaneous reduction have brown color and are stable for months at room temperature. The formation of the colloidal dispersions of Au/Pd bimetallic clusters by simultaneousreduction was examined by electronic spectra and transmission electron microscopies. Figure 1 shows a series of the electronic spectra of the colloidal dispersions of the Au/Pd bimetallic clusters and the mixture of monometallic Au and Pd colloidal dispersions. As for the bimetallic colloidal dispersions prepared by simultaneous
Toshima et al.
A u / P d = l / O (-1 AU / Pd = 4 / 1 (----) Au / Pd = 3 / 2 (-.-.-) AU / Pd
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AU / Pd
300
400
500
600
700
800
Wavelength / nm Figure 1. A series of UV spectra of (a) Au/Pd bimetallic colloidal dispersions and (b) mixtures of monometallic Au and Pd colloidal dispersions at Au/Pd ratios = 110, 411, 312, 213, 114, and 011.
reduction (Figure la), thc spectra of the colloidal dispersions composed of more than 60% palladium are monotonous and nearly the same as the colloidal dispersions of the monometallic palladium clusters, while, at less than 60% palladium, the absorption peaks near 560 nm are increasing with the increase of the Au/Pd ratio. As for the mixture of the colloidal dispersions (Figure lb), the peak height is increasing in order with increase of Au/Pd ratio even at small Au/Pd ratio. Figure 2 shows the transmission electron micrographs of the colloidal dispersions of Au/Pd bimetallic clusters, monometallic palladium and gold clusters, and the mixtures of both monometallic clusters. The gold clusters prepared by photoreduction are red in color and very large in size, 10-50 nm in diameter, while the palladium clusters are dark brown and 1-2 nm in diameter. The Au/Pd bimetallic clusters, prepared by the simultaneousreduction of both Au and Pd ions, are more homogeneous in size distribution and smaller in average diameter than each monometallic cluster, especially when the mole ratio of gold is less than 80%. The average diameters of the bimetallic clusters are increasing with increase of the Au/Pd ratio. The size distributions are shown as histograms in Figure 3. Figure 3a indicates an example of narrow size distribution for the Au/Pd( 1/4) bimetallic clusters, which are composed of ultrafine particles of almost uniform size of 1.6-nm average diameter. In the case of the mixture of the monometallic clusters, the small palladium particles can be de-
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9929
Polymer-Protected Au-Pd Bimetallic Clusters
U 0
500A
Figure 2. Electron micrographs of (a) Au/Pd(l/4) bimetallic, (b) Pd monometallic, (c) Au monometallic, and (d) mixtures of Au and Pd monometallic clusters.
is 2.5 nm (Figure 3d). The results on the electron micrographs as well as the electronic spectra suggest that the dispersions prepared by the simultaneous reduction is not the mixture of the monometallic particles but is composed of the Au/Pd alloy particles. Furthermore, the disappearance of the peak near 560 nm in UV-vis spectra of the bimetallic cluster with less than 40% Au suggests that not gold but palladium atoms are located on the surface of the bimetallic cluster particles. Hydrogenation of 1,3-Cyclooctadiene Catalyzed by the Au/Pd Bimetallic Clusters. 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-cyclooctadiene to cyclooctene in ethanol at 30 OC under 1 atm of hydrogen. Dependence of the catalytic activity upon the metal composition of the Au/Pd bimetallic clusters (0.01 mM) prepared by different methods was investigated. Figure 4 shows composition dependence of catalytic activities of the colloidal dispersions of the Au/Pd bimetallic clusters prepared by simultaneous reduction and the mixture of the monometallic clusters at various Au/Pd ratio, both separately prepared by alcohol-reduction under nitrogen. The colloidal dis-
reference Pd foil Au foil Pd cluster Au cluster Au/Pd(9/1) foil Au/Pd(l/9) foil
edge Pd-K Au-L~ Pd-K Au-L~ Pd-K Au-L3
bond Pd-Pd Au-AU Pd-Pd Au-AU Au-Pd Au-Pd
N 12 12 6.2 12 12 12
r, A AE,eV 2.73 -10.28 2.85 -3.21 2.76 -12.73 2.86 2.78 2.83 -8.11 2.75 -1.21
u, A
0.066 0.073 0.074 0.072 0.072 0.060
persions of the monometallic Au clusters, prepared by the same method, have almost no activity. However, the Coexistence of Au and Pd in the clusters increases the catalytic activity, and the maximum activity is achieved by the bimetallic clusters prepared by the simultaneous reduction at the composition of Au/Pd = 1/4. The significance of the 1/4 ratio in the bimetallic clusters will be discussed later. The corresponding mixtures of both Au and Pd monometallic dispersions did not produce such high activities as the dispersions prepared by the simultaneous reduction but still higher activities than those estimated as the sum of the activities of the two kinds of the monometallic clusters without reciprocal actions. EXAFS Analysis of the Au/Pd Bimetallic Clusters Prepared by SimultaneousReduction. In the EXAFS analysis of colloidal
9930 The Journal of Physical Chemistry. Vol. 96, No. 24, 1992
Toshima et al. 60
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Figure 3. Particle size distribution histograms of (a) Au/Pd( 1/4) bimetallic, (b) Pd monometallic, (c) Au monometallic, and (d) mixtures of Au and Pd monometallic clusters. The dashed lines indicate the average diameters of all the particles and the dashed-and-dotted lines indicate the average diameters of more than and less than 5 nm particles, respectively.
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dispersions of Au/Pd bimetallic clusters, tweshell fitting has been carried out using theoretical phase-shift and amplitude. The reference compounds used in the present work were Au foil, Pd foil, Au/Pd(9/1) alloy foil, and Au/Pd(l/9) alloy foil.l9 Table I shows the curve fitting analyses of EXAFS data for these reference compounds and the colloidal dispersions of mo-
nometallic clusters of Au and Pd. In the previous paper: we have reported the EXAFS analysis of the Pd/Pt bimetallic clusters. There are large correlations among the ten fitting parameters, and they might converge to physically unreasonable values. To avoid this, the Au/Pd bimetallic clusters were analyzed by using the value of AE and S of the Au foil, Pd foil, Au/Pd(9/1) alloy foil, and Au/Pd( 1/9) alloy foil as well as the value of u of the monometallic clusters. To estimate the errors in Niand r,, in the fmt sta e, AE and u's were varied within the range of i5 eV and fO.O1 from that of Au-L3 and Pd-K edge EXAFS of Au and Pd foil, respectively. Second, the estimation of errors for Ni and r, has been carried out by changing the R factor (reliability factor) from the minimum value (Rm), which can be obtained when the best fitting of the curve is performed, to 21/2(Rd,,).zo Figure Sa shows the Fourier transforms of Pd K-edge EXAFS of the colloidal dispersions of the Au/Pd bimetallic clusters prepared by the simultaneous reduction at the Au/Pd ratio = O/ 1, 1/4, and 1/ 1. Increasing the ratio, the main peak splits into two peaks. This spiitting arises from the interference between the EXAFS oscillations of the Pd-Pd and those of the Au-Pd because the phase shift of Pd-Pd and Au-Pd are w a d different from each other, though the cormponding distances are expected to be the same. To obtain detailed information about the Pd-Pd and Au-Pd bonding, the data were Fourier-filtered Over 0.154.30 nm and analyzed by curve fitting technique. Figure 5b shows the Fourier transforms of Au-L3 edge EXAFS of the colloidal dispersions of the Au/Pd bimetallic clusters of Au/Pd ratio = 4/1,1/1, and 114. Decreasing the Au/Pd ratio,
1
The Journal of Physical Chemistry, Vol. 96. No. 24, 1992 9931
Polymer-Protected Au-Pd Bimetallic Clusters
q
TABLE JIk Coordinrb Numbem Around the Pd md Au Atom8 of the Au/Pd = 1/1 Bimcbllic Cluster Dctcrmined by the Scpurte
E 201 L
Q,
U
3
.-
c1
C
Fitting
A u / P d = O / l (-)
0 C
AU / Pd e 1 / 4 (-.-.-)
absorbing metal Pd Pd Au Au
Au / Pd I 1 / 1 (-----)
'*I
x3 Distance
R (i) 1 A ~ / P d n 4 / 1(-) AU / Pd 1 / 1 (-----)
t
h
2
scattering metal Pd Au Au Pd
interatomic distance r, A 2.78 f 0.03 2.78 f 0.03 2.82 f 0.03 2.77 f 0.03
Coordination
no. N obad 1.8 f 1.1 2.9 f 1.6 6.1 f 2.1 1.9 f 1.3
TABLE YY: coordinrtloa Numbera Around the Pd md Au Atomr of the Au/Pd = 1/4 MmatalUc Cluster Deterdmd by the Simplclwopr Fitting rad T h e c.lcpI.ted for the Models interatomic absorbing scattering distance r, coordination no. N metal metal A obsd Aucore' randomb Pd Pd 2.77 f 0.03 4.2 f 0.8 6.4 6.9 1.4 1.7 Pd Au 2.78 f 0.03 1.2 f 0.3 Au Au 2.82 f 0.03 6.2 f 1.9 6.3 1.7 Au Pd 2.78 0.03 4.5 f 1.1 5.7 6.9 'See Figure 6a. bSee Figure 6b.
AU / Pd = 1 / 4 (-.-.-)
i i
TABLE V Coordiartion N U " Arouod the Pd dAu Atom of the
c rc
0
Au/Pd = 1/1 B b W c m t e r Detemdd by the S i " Fitting dThow c.leuLtcd for tbe Modeb coordination no. N absorbing scattering interatomic Au 7 A u metal metal distancer,A obsd core! corea* randomC Pd Pd 2.78 i 0.03 1.5 i0.3 4.1 6.3 4.7 Au 2.78 i 0.03 2.5 i 0.4 3.2 Pd 2.6 4.7 Au 2.82 f 0.03 6.5 1.5 8.4 Au 7.3 4.7 Au Pd 2.78 i 0.03 2.3 f 0.4 3.1 2.6 4.7
*
0'
2 4 Distance R (i)
6
Figure 5. Fourier-transformed EXAFS spe-ctrum at (a) Pd K-cdge of the colloidal diapcrsiona of the Au/Pd bimetallic clusters at Au/Pd ratios = 0/1,1/4, and 1/1 and (b) Au-L, edge of the same dispersions at Au/W ratios = 4/1, 1/1, and 1/4. TABLE U C O O ~ ~Numbers O O Around the Pd md Au Atom of the Ao/Pd = 1/4 Bimetdlic Cluster Determined by the Separate
Fitting absorbing metal Pd Pd Au Au
scattering metal Pd Au Au Pd
interatomic distance r, A 2.76 f 0.03 2.75 f 0.03 2.81 f 0.03 2.77 f 0.03
coordination no. N obsd 4.1 f 1.2 3.1 f 1.7 6.2 f 2.7 4.4 f 1.7
the main peak of the Fourier transform at the position does not shift although the height of the main peak decreases. The data were Fourier-filtered over 0.20-0.30 nm in the same way as in the Pd-K edge EXAFS analysis. The structure parameters of the colloidal dispersions of the Au/Pd( 1/4) bimetallic clusters have been determined by fitting to each edge of the EXAFS data separately as shown in Table 11. The coordination numbers of Au and Pd atoms around the Pd atom are determined as 3.1 f 1.7 and 4.1 f 1.2, respectively. The coordination numbers of Au and Pd atoms around the Au atom are determined as 6.2 f 2.7 and 4.4 f 1.7, respectively. The errors for the coordination numbers were estimated in the same method! Then, the interatomic distances are also reasonable within the error bar.*' The structure parameters of the colloidal dispersions of the Au/Pd( 1/ 1) bimetallic clusters have been determined in the same way as those for the Au/Pd( 1/4) bimetallic clusters, as shown in Table 111. The coordination numbers of the Pd and Au atoms around the Pd atom are determined as 1.8 i 1.1 and 2.9 i 1.6, respectively, and those of Au and Pd atoms around the Au atom are as 6.1 2.1 and 1.9 i 1.3, respectively. As for the coordination numbers of the Au/Pd(l/4) and Au/Pd(l/l) bimetallic cluster, the conditions embodied in eq 1
*
"See Figure 7a. bSte Figure 7b. OSce Figure 7c.
are not fully satisfied. For example, in the case of the Au/Pd( 1/4) bimetallic cluster, the obtained cluster consists of XAU/Xw= 1/4, so the relation, that is, 4 X PAu = NAuw, must be satisfied. In fact, however, it is rather poor. The same tendency is found in the case of the Au/Pd(l/l) bimetallic cluster. Thus, there can be a high degree of uncertainty in the value of the parameter when the EXAFS data for the two absorption edges are fitted separately. In the EXAFS analysis of the bimetallic clusters, it is difficult to obtain a satisfactory set of absolute values of coordination numbers. Bemuse of a large number of correlation factors, the coordination numbers, which are physically reasonable, cannot be precisely determined. Recently, the modified method of EXAFS analysis has been found.l* The method is that in which both edges of the EXAFS data are fitted simultaneously. This has been made possible through extensive modification of the computer program. In this method, eq 1 is incorporated into the computer program, and there is a single value of the interatomic distance r for the pair of different atoms. For example, the relations, r m u = r A a and Fu = (XAu/Xw)NAyw, are satisfied. Consequently, the EXAFS data can be fitted with fewer adjustable parameters. However, one has greater confidence in the physical significance of the information obtained from the analysis. The results obtained by the new method of analysis for the Au/Pd( 1/4) and Au/Pd( 1/1) bimetallic cluster are shown in Tables IV and V, respectively. The EXAFS data used for obtaining these results, along with the associated Fourier transforms and filtered inverse transforms, were the same as those used in the previous method. With regard to the phaaeshift and amplitude functions, they have been obtained from Teo and Lee,'' as in the previous method of analysis. As a consequence of the incorporation of eq 1 into the analysis, the coordination numbers are physically reasonable. A comparison of values of interatomic distances shown in Tables I1 and IV reveals that the distance obtained by the new method agrees with that obtained by the previous method within 0.03 A. However, more signifhnt diffaencebetween the two methods are noted for results of the coordination number of Au atom around the Pd atom.
9932 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 It-- 1.7nm
ones. From the average size, the Au/Pd( 1/1) bimetallic cluster consists of the centered six atoms and the three layered atoms with fcc structure around the six atoms, totally containing 248 atoms. The coordination numbers suggest that one can take two types of models for the Au/Pd( 1/ 1) bimetallic cluster, as shown in Figure 7. One is the Au single core model, which occupies the Pd monolayer surface consisting of 122 atoms and the Au single core consisting of 126 atoms (Au core model). The other is the cluster-incluster model, which consists of 124 Au atoms forming seven cores and 124 Pd atoms locating around the seven Au cores and combining the Au cores. The consistency between the o b served values and those calculated for the core model or the random model, shown in Figure 7a or 7c, is not so good as that for Au/Pd( 1/4) clusters. However, the coordination numbers calculated for the cluster-in-cluster model, shown in Figure 7b, are nearly the same as those observed. Obviously, the clusterincluster model (Figure 7b) is preferred to other models (Figures 7a and 7c). R&tim between tbe Surface Structure and the Catalytic Activity. The Au/Pd( 1/4) bimetallic clusters were the more active catalysts for the selective partial hydrogenation than the Pd monometallic clusters or the mixtures of the Au and Pd clusters. The bimetallic clusters with the Au core structure, shown in Figure 6a, are more active than the Pd clusters. The high activity generally depends on the surface structure. In this case, both Au/Pd( 1/4) bimetallic and Pd monometallic clusters are composed of the Pd atoms on the surface of the particles in a similar manner. The difference is the atoms existing at the center of the particles. Thus, in the case of the Au/Pd( 1/4) bimetallic clusters, 74 Pd atoms are located on the surface of the cluster particle surrounding the Au core, while in the Pd monometallic clusters, about 42 Pd atoms are on the surface and the 13 Pd atoms are at the center of the particle. The difference is only in the central core. In the case of the Au/Pd( 1/ 1) bimetallic clusters, the sum of the coordination numbers around the Au atom becomes less than 12 because of the possible existence of the Au atoms on the surface of the cluster. This is considered to decrease the practical catalytic activity for the selective partial hydrogenation. In the case of the Au single core model, 16 Au atoms are located on the surface of the cluster particle, which has a total of 138 atoms on the surface, while 52 Au atoms are on the surface in the case of the cluster-in-cluster model. If the Au single core model is thought to be the case for the Au/Pd(l/l) bimetallic cluster, the catalytic activity expected from the structure amsisting of only 16 Au atoms on the surface might be higher than that experimentally obtained, since the catalytic activity is expected to be proportional to the number of surface Pd atoms. However, if the cluster-in-cluster model is appropriated, the catalytic activity expected from the structure consisting of 52 Au atoms on the surface might be as high as that experimentally obtained. Therefore, as a model for the Au/Pd(l/l) bimetallic cluster, it is also suggested from the viewpoint of the catalytic activity that the cluster-incluster model is better than the Au single core model.
IC-- 1.7nm 4
T
1.7nm
(a)
(b)
Figure 6. Crm section of models for the Au/Pd( 1/4) bimetallic cluster: (a) Au wre model and (b) random model.
In the case of the Au/Pd(l/4) and Au/Pd(l/l) bimetallic clusters, the coordination numbers of Au atom around the Pd atom obtained by the new method are 1.2 f 0.3 and 2.5 0.4, respectively. This indicates a degree of segregation of each component in the bimetallic cluster particle is significantly higher than that indicated by the previous analysis, which resembles the Pd/Pt bimetallic clusters with regard to the segregation of each component.22 From the results of the coordination numbers of the Au/Pd bimetallic cluster, especially, the coordination number of the Au atoms around the Au atom, it is expected that Au atoms are located at the center of the cluster and Pd atoms are enriched on the surface of the cluster. Modd of the Au/Pd Bimebllic chrsters Dete"d by EXAES. In the case of the Au/Pd( 1/4) bimetallic clusters, the average size has been determined to be 1.6 nm by the transmission electron microscope (TEM).Therefore, the Au/Pd( 1/4) bimetallic cluster particle consists of the centered six atoms and the two layered atoms with fcc structure around the six atoms, totally containing 110 atoms as shown in Figure 6. The coordination number of Au atoms around the Au atom, as shown in Table IV,suggests that the Au atoms are located at the center of the cluster (Au core structure as shown in Figure 6a). The observed coordination numbers are Werent from those calculated for the random model, where 88 Pd atoms and 22 Au atoms are located completely at random. As shown in Figure 6a, the 110 atoms form the fcc structure, in which 22 Au atoms of the 110 atoms are located at the center of the cluster particle and the other 88 Pd atoms are on the surface of the cluster. The coordination numbers calculated on the basis of the Au core model in Figure 6a are consistent with the values observed from the EXAFS data. On the basis of these results the Au core structure can be taken as a model for the Au/Pd( 1/4) bimetallic cluster. In the case of the Au/Pd( 1/ 1) bimetallic clusters, the average size has been determined to be 2.2 nm from TEM observation. The coordination number of Au atoms around the Au atom in the Au/Pd(l/l) clusters is as large as that in the Au/Pd(l/4)
*
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Toshima et al.
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F i i 7. Crass section of models for the Au/Pd(l/l) bimetallic cluster: (a) Au single wre model, (b) cluster-in-clustermodel, and (c) random model.
J. Phys. Chem. 1992, 96,9933-9939 The ionization potential of the Pd atom (8.33 eV) is known to be lower than that of the Au atom (9.223 eV).23 Thus, the reduction of the Au ions proceeds more easily than that of the Pd ions, so the Au core strWure is formed sooner than the Pd core structure is. Moreover, the electronic interaction between the Au awe and the Pd layer could provide an unevendistribution of electrons. Then, the Pd in the surface layer becomes poorer in electron density than in the Au core, which might make the Au/W( 1/4) bimetallic clusters more active than Pd clusters since the substrate having a double bond favors the electron-deficient surface.
Cdwioll (1) The polymer-protected gold/palladium bimetallic clusters
can be prepared by the simultaneous reduction of the corresponding ions in the presence of the poly(N-vinyl-2-pyrrolidone). The clusters were used as the catalysts for the selective partial hydrogenation of 1,3-cyclooctadiene with higher activities than those of the monometallic clusters. (2) 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, is prrsented as a model for the Au/Pd( 1/4) bimetallic clusters prepared by the simultaneous reduction. (3) For the Au/Pd(l/l) bimetallic cluster, it is suggested to use the cluster-in-cluster model, in which seven Au cores are located in the cluster and Pd atoms play a role to combine the Au cores.
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 in 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 'Macromolecular Complexes" (01612002) from the Ministry of Education, Science and Culture, Japan.
R-m
9933
NO. PVP, 9003-39-8; HAuC14, 16903-35-8;PdClz, 7647-
10-1; Au, 7440-57-5; Pd, 7440-05-3.
Referema d Nom (1) Shfelt, J. H.; Via, G. H.; Lytle, F. W. J. Chem. Phys. 1980, 72,4832. (2) Sinfelt, J. H.; Via, G. H.; Lytle, F. W.; Gieegor, R. B. J. Chem. Phys. 1981, 75, 5527. (3) Meitmer, G.;Via, G. H.; Lytle, F. W.; Sinfelt, J. H. J. Chem. Phys. 1983, 78, 882. (4) Toshima, N.; Harada, M.; Yonezawa, T.; Kuehihashi, K.;Asakura, K. J. Phys. Chem. 1991,95, 7448. (5) Liu, H.; Mao, G.; Meng, S. 1. Mol. Catal. 1992, 74, 275. (6) Eaumi, K;Shiratori, M.; Ishimka, H.; Tano, T.; Torigoe, K.; Meguro, K. Lungmuir 1991, 7, 451. (7) Hirai, H.; Toshima, N. In Tailored Metal Catalysts; Iwasawa, Y., Ed.; D. Reidel Pub.: Dordrecht, 1986; pp 87-140. (8) Huai, H.; Nakao, Y.; Toshima, N. Chem. h i t . 1978,545. (9) Hirai, H.; Chawanya, H.; Toshima, N. Reactive Polym. 1985,3, 127. (IO) Hirai, H.; Chawanya, H.; Toahima, N. Bull. Chem. Soc. Jpn. 1985, 58, 682. (11) Toshima, N.; Kuriyama, M.; Yamada, Y.; Hirai, H. Chem. Lctt. 1981, 793. (12) Toshima, N.; Takahashi, T.; Hirai, H. J. Mucromol. Sci.-Chem.1988, A25(5-7), 669. (13) Toahima, N.; Kushihashi, K.;Yonezawa, T.; Hirai, H. Chem. h i t . 1989, 1769. (14) Zhao, B.; Toshima, N. Chem. Express 1990, 5(10), 721. (15) Toshima, N. J. Mucromol. Sci.-Chem. 1990, A27, 1225. (16) Teo, B. K.EXAFS Basic Principles and Data Analysis, Inorganic Chemistry Concepts; Springer-Verlag: Berlin, 1986; Vol. 9. (17) Teo, B. K.;Lce, P. A. J . Am. Chem. Soc. 1979,101, 2815. (18) Via, G. H.; Drake, K. F., Jr.; Meitzner, G.; Lytle, F. W.; Sinfelt, .I. H. Catal. Lett. 1990, 5, 25. (19) Renaud, G.; Motta, N.; Lancon, F.; Belakhovsky, M. Phys. Rev. B 1988,38, 5944. (20) Lytle, F. W.; Sayers, D. E.; Stem, E. A. In X-Ray Absorption Fine Structure-y;Leon, J. M., Stern, E. A., Sayers, D. E., Ma, Y., Rehr, J. J., Eds.; Elscvier/North-Holland New York, 1988; pp 701-722. (21) Wyckoff, R. W. G. Crystal Structures, 2nd 4.; Interscience Pub lishers: New York, 1963; Vol. 1. (22) Foger, K. In Catalysis;Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: New York, 1984; pp 227-305. (23) Lange, N. A. Handbmk ofChemistry; McGraw-HiU Book Company, Inc.: New York, 1961.
Epitaxial Formation of PbS Crystals under Arachidic Acid Monolayers X. K. Zhao,' J. Yang? L.D.McCormick,' and J. H.Fendler**2 Department of Chemistry, Syracuse University, Syracuse, New York 13244-41O0, and Jdt D Scientific, 1815 West 1st Avenue, Mesa. Arizona 85202 (Received: June 5, 1992; In Final Form: August 14, 1992) Lead sulfide (PbS) particulate films composed of highly oriented, equilateral-triangularcrystals have b n in situ generated by the exposure of arachidic acid (AA)-monolayer-coated aqueous lead nitrate [Pb(NO&] solutions to hydrogen sulfide (H2S). The AA-coated PbS particulate films, at different stages in their growth, were transferred to solid substrates and characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM),and electron diffraction measurements. Each individual crystal had its [ l l l ] axis perpendicular and its [TT2], [T2'f],and [21T] axes parallel (arranged in 3-fold symmetry at 120° angles) to the AA monolayer surface. The rate of HIS infusion influenced the size of crystals rown under the AA monolayer. The mean length of equilateral-triangular PbS crystals decreased from 607 A (U = 133 to 297 A (a = 91 A) upon diminishing the H2S exposure time from 30 to 5 min. The epitaxial growth of PbS crystals has been rationalized in terms of an almost perfect fit between the (11 1) plane of the cubic crystalline PbS and the (100) plane of the hexagonally closepacked AA monolayer. The presence of a monolayer and the slow encounter of the precursors have been found to be essential requirements for the oriented growth of PbS crystals. Exposure of Pb(N03)2solutions to HzS in the absence of monolayer surface coverage furnished only irregular PbS crystals at the air-water interface.
f)
Iatroductioa
Construction of advanced electronic devices with desired electric, optical, and electrooptical properties requires the availability of semiconductor nanostructures with controllable purities, sizes, shapes, and orientations. A variety of techniques, including molecular beam epitaxy (MBE),chemical vapor deposition (CVD), and sputter and vapor depositions, have been developed for semiconductor superlattice formation and band-gap engi0022-3654/92/2096-9933$03.00/0
neering. Indeed, systems with atomic dimensions and smoothness have been grown in ultrahigh v a c ~ u m . ~ ' ~ In situ generation of semiconductor particles and semiconductor particulate films at the interfaces of organized surfactant assemblies represents an alternative approach. Versatility, maneuverability, and relative simplicity are the advantages of this colloid chemical method. Particularly successful has bcen the formation of sizequantized metaliwlfide semiconductor particulate films 0 1992 American Chemical Society
