7448
J . Phys. Chem. 1991, 95,1448-1453
Structural Analysis of Polymer-Protected Pd/Pt Blmetalllc Clusters as Dispersed Catalysts by Using Extended X-ray Absorption Fine Structure Spectroscopy Naoki Toshima,**+Masafumi Harada,+Tetsu Yonezawa; Kakuta Kushibashi; 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: March 6, 1991; In Final Form: April 30, 1991)
Extended X-ray absorption fine structure (EXAFS) was applied to the determination of the structure of colloidal dispersions of the poly(N-vinyl-2-pyrrolidone)-protectedpalladium/platinum bimetallic clusters, which work as the catalysts for selective partial hydrogenation of 1,3-cyclooctadieneto cyclooctene. The catalytic activity was found to depend on the structure of the bimetallic clusters. The EXAFS data on the Pd/Pt (4/1) bimetallic clusters, which are the most active catalysts, indicate a Pt core structure, in which the 42 Pd atoms are on the surface of the cluster particle and 13 Pt atoms are at the center of the particle, forming a core. In contrast, the Pd/Pt (1/1) bimetallic clusters are suggested to have a modified Pt core structure, in which 28 Pt atoms connect directly with each other, being located both in the core and on the surface, and 27 Pd atoms form three islands on the surface of the cluster particle.
Introduction Metal catalysis can be affected by adding the second metallic component. The additive components may form an alloy, improving the activity and selectivityof a metal catalyst. The effect of the additives could be asserted in terms of an ensemble and/or a ligand effect.' Supported metal catalysts are widely investigated in connection with their industrial applications. Such catalysts consist of small metal particles dispersed on oxide supports, such as A1203,Si02, TiOl, etc. However, little attention has been paid to the colloidal dispersions of small metal particles in a homogeneous solution, especially to the structure of the dispersed particles. EXAFS (extended X-ray absorption fine structure) is a powerful tool for investigation on local structure of the metal cluster particles in colloidal dispersions as well as small metal particles in supported catalysts. EXAFS measurements can give information on the environment about a particular atom in the metal clusters, Le., the number, the kind of neighboring atoms, and their distances from the X-ray absorbing atom, without requirements for long-range ordered structures.2 Development of an X-ray absorption spectroscopy technique during the 1970s facilitated the characterization of the exact structure of the bimetallic clusters in supported metal catalysts. Sinfelt et al.' used EXAFS to characterize Ir/Rh,4 A U / C U , ~ Ru/Cu? and Os/Cu7 bimetallic clusters supported on silica. For catalysts containing Ru/Cu bimetallic clusters? it has been concluded from EXAFS measurement that the surface can consist entirely of copper in the form of a monolayer and the core consists of ruthenium. The colloidal dispersions of noble metal clusters protected by polymers can be prepared by reducing the noble metal salts by refluxing the alcoholic solution containing water-soluble polymers such as poly(N-~inyl-2-pyrrolidone).~The colloidal dispersions thus obtained are stable for months at room temperature and are composed of ultrafine particles of an average diameter from 1 to 3 nm with a narrow size distribution. They work as active catalysts for selective hydrogenation of olefin^,^ selective partial hydrogenation of diene to monoene,'O*t'visible light-induced hydrogen generation from water,'*J3 and so on. Recently, we have succeeded in preparing the colloidal dispersions of bimetallic clusters by refluxing the alcoholic solution containing two kinds of noble metal salts in the presence of poly(N-~iny1-2-pyrrolidone).'~~'~ The catalytic activity of the bimetallic clusters protected by polymer depends on the composition of the clusters. For example, in the case of Pd/Pt bimetallic clusters thus prepared under air, the highest catalytic activity for 'Department of Industrial Chemistry, Faculty of Engineering. *Department of Chemistry, Faculty of Science.
0022-3654/9 1 /2095-1448302.50/0
the selective partial hydrogenation of 1,3-cyclooctadiene to cyclooctene was observed for the Pd/Pt bimetallic clusters at a molar ratio of Pd to Pt = 4/1 [abbreviated as W/A (4/1)].14 The Pd/A (1/1) clusters show activity a fraction of that of the most active Pd/Pt (4/1) clusters. The similar dependence of the catalytic activity upon the composition can also be observed in the colloidal Pd/Pt bimetallic clusters prepared under nitrogen instead of air.I6 Although the catalytic activity is related to the area and structure of the surface of the bimetallic clusters, a careful analysis of the EXAFS spectra of the colloidal dispersions of the bimetallic clusters has not been carried out. In order to determine the structure of the Pd/Pt bimetallic clusters at various ratios, we have structurally characterized these bimetallic clusters by EXAFS in the present paper. EXAFS measurements are performed on the Pd K-edge and Pt L3-edge, providing information about the coordination environment in these clusters. EXAFS analysis of the Pd/Pt bimetallic clusters has not been reported even for the system of supported metal catalysts. Since the Pd/Pt cluster has Pd-Pd, Pd-Pt, Pt-Pt, and Pt-Pd bond contributions and the contributions of these bonds have nearly the same value in EXAFS, EXAFS analysis has difficulties in separating these contributions. The analysis requires exact references taking account of the contributions of the Pd-Pd, Pd-Pt, Pt-Pt, and Pt-Pd bonds. In the present paper, for example, the Pd-Pt contribution from the Pd/Pt (1/9) alloy foil serves as a preferable reference because the values of the coordination number determined from EXAFS are approximately the same as those (1) Sinfelt, J. H. J . Coral. 1973, 29, 308. (2) Via, G.H.; Sinfelt, J. H.; Lytle, F. W. J . Chem. Phys. 1979, 71,690. ( 3 ) Sinfelt. J. H. Acc. Chem. Res. 1987, 20, 134. (4) Meitzner, G.;Via, G.H.; Lytle, F. W.; Sinfelt, J. H. J . Chem. fhys. 1983. 78. 2533. ( 5 ) Meitzner, G.;Via, G.H.; Lytle, F.W.; Sinfelt, J. H. J . Chem. fhys. 1985, 83, 4793. (6) Sinfelt, J. H.; Via, G . H.; Lytle, F. W. J . Chem. fhys. 1980, 72,4832. ( 7 ) Sinfelt, J. H.; Via, G . H.; Lytle, F. W.; Grccgor, R. B. J . Chem. Phys. 1981, 75, 5527. (8) Hirai, H.; Toshima, N. In Tailored Meral Catalysis; Iwasawa, Y.,Ed.; Reidel: Dordrecht, 1986; pp 87-140. (9) Hirai,.H.; Nakao, Y.; Toshima, N. Chem. feir. 1978, 545. (10) Hirai, H.; Chawanya, H.; Toshima, N. Bull. Chem. Soc. Jpn. 1985, 58, 682. **( I I ) Hirai, H.; Chawanya, H.; Toshima, N. Reacrloe Polymers 1985, 3, 3
ILI,
(12) Toshima, N.; Kuriyama, M.; Yamada, Y.; Hirai, H. Chem. feii. 1981, 793. (13) Toshima, N.; Takahashi, T.; Hirai, H. J . Mucromol. Sci.. Chem. 1988, A25 (5-7), 669. (14) Toshima, N.; Kushihashi, K.; Yonezawa, T.; Hirai, H. Chem. b i t . 1989, 1769. (15) Zhao, B.; Toshima, N. Chem. Express 1990, 5, 721. (16) Toshima, N.; Yonezawa, T.; Harada, M.; Asakura, K.; Iwasawa, Y. Chem. b i r . 1990, 815. Q 1991 American Chemical Society
-
. . . .
"
_ . , _ _ . ...
I.
structural Analysis or Y q P t Bimetallic clusters
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 1449
expected from the ratio of the metal composition. Here we describe the determination of the structure of the P d / R (4/1)and Pd/Pt (1/1) bimetallic clusters by use of the four preferable references for analysis of the EXAFS spectra.
Experimental Section Preparation of Pd/Pt Bimetallic Clusters. The colloidal dispersions of the Pd/Pt bimetallic clusters were prepared by an alcohol-reduction method. An ethanol/water (1/1 v/v) (50 mL) solution containing both Pd(I1) and R(IV) ions was prepared by mixing palladium(I1) chloride (0.033mmol in 25 mL of ethanol) and hexachloroplatinic acid (0.033mmol in 25 mL of water) at various ratios, followed by adding poly(N-vinyl-2-pyrrolidone) (PVP, K-30,MW 40OO0,151 mg, 1.36"01 of monomeric units) as a protecting polymer. The total amount of both metals was always kept as 3.3 X mol in 50 mL of the mixed solution composed of ethanol/water ( 1 / 1 v/v). Refluxing of the solution at about 100 O C for 1.5 h under nitrogen gave the stable and dark brown homogeneous solution of palladium/platinum bimetallic colloidal dispersions. l6 Cbaracterization of the Pd/Pt Bimetallic Clusters. Electronic spectra of the colloidal dispersions of the bimetallic clusters were measured with a Hitachi Model 340 spectrophotometer. The transmission electron micrographs of the colloidal dispersions were obtained with a Hitachi Model 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. EXAFS Measurement. The samples for the EXAFS measurement were prepared by concentrating 1500 mL of the above-obtained colloidal dispersions under reduced pressure of nitrogen to 30-50 mL. The concentrated dispersions were kept under nitrogen in the cells having polyimide film (KAPTON500H,125-pm thickness, kindly provided by Toray Co. Ltd.) windows. The cells with optical path lengths of 50 and 5-10 mm were used for Pd K-edge and Pt L,-edge measurements, respectively. The Pd/Pt (1 /9) alloy foil and Pd/Pt (9/1)alloy foil, which serve as preferable references, were thankfully presented by Tanaka Kikinzoku Kogyo K.K. For example, in the case of the Pd/R (1 /9)alloy foil, the Pd/Pt metallic composition is in the ratio 1.00:9.00.The thickness of the Pd/Pt (1 /9) alloy foil is about 10 pm. 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 room temperature. The channel-cut Si(3 1 1 ) monochromator was used. The storage ring was operated at 2.5 GeV, and the ring current was in the range of 10&300 mA. Io and I ion chambers were filled with N2gases for Pt L,-edges. For Pd K-edges Io and I ion chambers were filled with Ar and Kr gases, respectively. The EXAFS oscillation x(k) was extracted from the observed data p ( E ) by the subtraction of the smoothly varying part p s ( E ) , which was estimated by a cubic spline method and by the normalization of the oscillation by the absorption of the free atom p o ( E ) as shown in the equation:" x(k) = M E ) - ccs(E)l/ro(E) (1) where k is the wavenumber of the photoelectron and is related to the photon energy E and the threshold energy Eo by the equation: k = [2m(E - E o ) / h 2 ] ' / 2 (2) where m is the mass of the electron. Fourier transformation of k3x(k)was carried out over the region 30-160 nm-'. The peak in the Fourier transform was filtered and inversely Fourier transformed into k space (the region 40-1 50 nm-I) again. The Fourier-filtered data were then analyzed with (1 7) Teo, B. K. EXAFS Basic Principles and Data Analysis, Inorganic Chemisfry Concepts; Springer-Verlag: Berlin, 1986; Vol. 9.
oh 200
I
I
400
GOO Wavelcngth/nm
I
800
Figure 1. A series of UV-vis spectra of bimetallic colloidal dispersions of Pd/Pt ratios = 4/1,3/2,2/3, and 1/4, and of monometallic Pd (---) and Pt
(-a-).
a curve-fitting technique by using the theoretical EXAFS equations:" k3x(k) = 7SjNjFj(kj)kj2 exp(-2uj2kj2) sin [2kjrj kj = (k2 -
+ dJj(kj)]/rj2
(3) (4)
where Nj, r., AEoj,and crj represent the coordination number, the bond length, the difference between the theoretical and experimental threshold energies, and the Debye-Waller factor of the jth coordination shell, respectively. dJ,{k) and FLk) are phase shifts and amplitude functions, respectively, both of which can be obtained by theoretical calculation^.'^^'^ Sj is an amplitude reduction factor which arises from many-body effects and from inelastic losses in the scattering process. Since the many-body effects and the inelastic losses have the opposite k dependence each other, Sj can be regarded as a constant function of k.20
Results and Discussion Preparation and Characterization of Pd/Pt Bimetallic Chster. The colloidal dispersions of Pd/Pt bimetallic clusters protected by soluble polymers such as poly(N-vinyl-2-pyrrolidone) (PVP) have brown color and are stable for months at room temperature. The formation of the colloidal dispersions of Pd/Pt bimetallic clusters was suggested by electronic spectra and transmission electron microscopies.14 Figure 1 shows the electronic spectra of the obtained colloidal dispersions of the Pd/Pt bimetallic clusters protected by PVP. The spectra of the bimetallic colloidal dispersions were found not only to differ from that of the monometallic colloidal dispersion of Pd or Pt, or their mixtures, but also to have a characteristic pattern. The transmission electron micrographs are shown in Figure 2 for the colloidal dispersions of Pd/Pt bimetallic clusters at various ratios as well as for monometallic clusters. The monometallic clusters of Pd and Pt are small in size, but have a tendency to aggregate and/or grow, forming rather large particles during the storage. On the contrary, the bimetallic clusters are small in size, and neither aggregation nor growth is observed. Moreover, excellent uniformity is observed for each Pd/Pt bimetallic cluster with the different Pd/Pt ratios. From the results of the particle size distribution histogram shown in Figure 3, it is clear that the average diameter is in the range of 14.9-16.2A and that the monometallic clusters of Pd form the secondary aggregating particles, resulting in an average diameter of 24.5 A. EXAFS Analysis of Pd/Pt Bimetallic Clusters. In the EXAFS analysis of the colloidal dispersions of the Pd/Pt bimetallic clusters two-shell fitting must be carried out, where the correlation problems among the fitting parameters (r, AE) and (u, N,S) are serious. In order to solve the correlation problem and obtain more (18) Teo, B. K.; Lee, P. A. J . Am. Chem. Soc. 1979, IO!, 2815. (19) Teo, B. K. J . Am. Chem. SOC.1981, 103, 3990. (20) Tw,B. K.; Antonio, M. R.; Averill, B. A. J . Am. Chem. Soc. 1983, 105, 3751.
Toshima et a!.
7450 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991
. v
TABLE I: Curve-FittingAnalyses for EXAFS Jhh far the Modtl Reference Compounds reference edne bond N r I A AEIeV a/A 2.73 -10.28 0.066 Pd foil Pd K Pd-Pd 12 2.76 -2.09 0.059 Pt foil 12 Pt L3 Pt-Pt Pd K Pd-Pd 6.2 2.76 -12.73 0.074 Pd cluster Pt cluster Pt L3 Pt-Pt 8.0 2.75 0.80 0.065 2.76 -9.18 0.066 Pd/Pt (!/9) foil Pd K Pd-Pt 12 2.73 0.85 0.062 Pd/Pt (9/1)foil Pt L3 Pd-Pt 12
: :r *. .
.
'
. .. . .-,
.
. .*
E
.0 v1 c
*
O :
C
2
WlPt
I I
I 1 I O1((-.--) -)
WlPt
=
1 /1(----3
E
c
10
(c)
(d)
F i p 2. Electron micrographs of (a) Pd monometallic, (b) Pd/Pt (4/1) bimetallic, (c) Pd/Pt (1/1) bimetallic, and (d) Pt monometallicclusters protected by poly(N-vinyl-2-pyrrolidone). 60 I
I
60I S 50
i
! I
0
601
1'. I
5
IO 15 20 25 30 Diamelerl i
1
MomctUlA
Figure 3. Particle size distribution histograms of (a) Pd monometallic, (b) Pd/Pt (4/1)bimetallic, (c) Pd/Pt (1/1)bimetallic, and (d) Pt monometallic clusters protected by poly(N-vinyl-2-pyrrolidone).
reliable r and N , the curve fitting was carried out by using theoretical phase shift and amplitude. AE, S, and u of unknown compounds were assumed to be equal to those of the model compounds in the best fitting of their EXAFS using theoretical parameters. r and IV of unknown compounds can be determined by using the theoretical phase shift and amplitude together with value of AE, S, and u obtained empirically from model compoundsm The model compounds used in the present work were Pd foil, Pt foil, Pd/Pt (1/9) alloy foil, and Pd/Pt (9/1) alloy foil. Table 1 shows the curve-fitting analyses of EXAFS data for these model compounds and the colloidal dispersions of monometallic clusters of Pd and Pt. The bond distances of Pd-Pd and Pt-Pt in the Pd and Pt foils are equal to those obtained from crystallography, indicating the validity of theoretical phase shift.21 In order to check the transferability of AE, S, and u from the bulk metal foil to the cluster particle, one-shell fitting of the EXAFS data of the Pd and Pt clusters was carried out by using the theoretical parameters. The best-fitting results gave AE for the Pd-Pd and Pt-Pt bonds of the Pd and Pt clusters similar to those of Pd and Pt foil at similar Pd-Pd and Pt-Pt distances. The u's for Pd-Pd and Pt-Pt in the clusters were slightly larger than (21) Wyckoff, R. W.G. Crysrot Srrucrures, 2nd ad.; Interscience Publishers: New Yotk, 1963; Vo!. 1.
-0
2 4@ Distance R (A)
Figure 4. Fourier-transformed EXAFS spectrum at Pd K-edge of colloidal dispersions of the Pd/Pt bimetallic clusters at Pd/Pt ratios = 1 /O, l O / l , 4/1,and 1/1.
those of Pd and Pt foil. This is due to the contribution from the surface atoms, which are reported to have larger u values than the bulk atoms. When the spherical shapes are assumed for clusters, the expected Ns obtained from the average diameter shown in Figure 3 are 7.9 and 7.9 for Pd clusters and Pt clusters, respectively. In the case of the Pt clusters, the coordination number obtained from EXAFS is nearly the same value as the expected one, but for the Pd clusters, the coordination number obtained from EXAFS is slightly smaller than the expected. This is probably because the Pt clusters are more stable against the oxidation by air than the Pd clusters. The values of AE,S, and u for the Pd-Pt bond around a Pd atom and the Pt-Pd bond around a Pt atom were determined in the following way. First, a Pd atom is only surrounded by Pt atoms and a Pt atom is only surrounded by Pd atoms in the Pd/Pt (1 /9) alloy foil and Pd/Pt (9/1) alloy foil, respectively. It seems to be a good assumption in the first stage of analysis, because the distance between minor-major elements is almost equal to that of major-major elements; Le., the distance of Pd-Pt in the Pd/Pt (1/9) alloy foil is equal to that of Pt-Pt in Pt foil, and the distance of Pt-Pd in the Pd/Pt (9/1) alloy foil is equal to that of Pd-Pd in Pd foil. Moreover, AE and u*s are within the range of *3 eV and *0.004 A from that of Pd K- and Pt L3-edge EXAFS of Pd and Pt foil, respectively. Then the AE,S, and u were refined by including the minorminor element interaction, that is, the interaction of Pd-Pd in the Pd/Pt (1 /9) alloy foil and that of Pt-Pt in the Pd/Pt (9/ 1) alloy foil. As a result of it, the minor-minor element interaction could be neglected in this analysis. Finally, the Pd/Pt bimetallic clusters were analyzed by using the values of AE and S of the Pd foil, Pt foil, Pd/Pt (1 /9) alloy foil, and Pd/Pt (9/ 1) alloy foil and the values of u of the clusters. In this analysis, special attention was paid to the u, because u is different between surface and bulk. Before the analysis the distribution of Pd and Pt in the Pd/Pt bimetallic clusters was not known, so that the u*sof the Pd clusters and Pt clusters were used. Figure 4 shows the Fourier transforms of Pd K-edge EXAFS of the colloidal dispersions of the Pd/Pt bimetallic clusters at the Pd/Pt ratio = 1/O, 1O/ 1,4/ 1,and 1/ 1. In the case of the colloidal dispersions of Pd clusters, the main peak is assigned to a Pd-Pd bond, which was confirmed and also determined to be 0.276 nm in distance by the curve-fitting analysis. When the Pd/Pt ratio decreases, the above main peak splits into two peaks appearing
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 7451
Structural Analysis of Pd/Pt Bimetallic Clusters
TABLE II: Coordination Numbers around tbe Pd a d Pt Atoms of the Pd/Pt = 4/1 Bimetallic Cluster Determined from EXAFS Data coordination number N absorbing scattering interatomic distance obsd Pt core’ random” metal metal rlA 4.4 f 1.0 4.6 6.0 2.74 f 0.03 Pd Pd Pt Pt “See Figure 7 5.50
I
2.3 f 1.3 5.5 f 1.7 3.5 f 1.5
2.73 f 0.03 2.73 f 0.03 2.72 f 0.03
Pt Pt Pd
Pd
1.9 1.9 6.0
for the structures.
P d / P t = 4 /1
3.30
-
2.0 5.5 6.5
P d l P t = O / 1 (-) w / Pt = 1 / 4 P d / P t = I / l (---+ W / P t : 4 / 1 (---I
C
E!
c 1.10
5
x
m
Y
-1.10
“0
-3.30
-5.50
4
5
6
7
8
9
10
11
12
13
14
15
k t A” Figure 5. Fourier-filtered EXAFS spectra (-) and best-fit curve (---) calculated by using both theoretical phase shift and amplitude function for the colloidal dispersions of the Pd/Pt (4/1) bimetallic clusters. at 0.21 and 0.28 nm. This can be attributed to a Pd-Pt bond as well as a Pd-Pd bond because the phase shift arises from the interference between the Pd and Pt atoms. The data were Fourier-filtered over 0.15-0.30 nm and analyzed by a curve-fitting technique to obtain the structure parameters. The EXAFS data of the colloidal dispersions of the Pd/Pt (4/1) bimetallic clusters are best fitted to a two-shell model consisting of Pd-Pd and Pd-Pt as shown in Figure 5 . The details will be discussed later. Figure 6 shows the Fourier transforms of Pt L3-edge EXAFS of the colloidal dispersions of the Pd/Pt bimetallic clusters of the Pd/Pt ratio = 0/1, 1/4,1/1, and 4/1. In thiscase the main peak around 0.275 nm is assigned to the Pt-Pt bond. When the Pd/Pt ratio increases, the main peak of 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 and were again best fitted to a two-shell model consisting of Pt-Pt and Pt-Pd in the same way as in the Pd K-edge EXAFS analysis. Structure of the Pd/Pt (4/1) Bimetallic Clusters Determined by EXAFS. The structure parameters of the colloidal dispersions of the Pd/Pt (4/1) bimetallic clusters have been determined on the basis of the two-shell fitting as shown in Table 11. The Pt-Pd distance (0.273 i 0.003 nm) obtained from Pd K-edge EXAFS is equal to the Pd-Pt distance (0.272 f 0.003 nm) obtained from Pt L,-edge EXAFS. The present EXAFS analyses using the theoretical parameters can estimate the Pd-Pt distance within an error of 0.003 nm. Hence the consistency of the distances indicates that the values of AE and u obtained from the reference compounds are correct. The coordination numbers of Pd and Pt atoms around the Pd atom are determined as 4.4 f 1 .O and 2.3 f 1.3, respectively. Similarly, the coordination numbers of Pt and Pd atoms around the Pt atom are determined as 5.5 f 1.7 and 3.5 f 1 S, respectively. In the first stage, the errors for the coordination numbers were estimated by varying the value of u in the range of d 0.01 ( d : the Debye-Waller factor of the reference compounds). For example, in the case of Pd/Pt (4/1) bimetallic clusters, the errors for the coordination numbers of Pd atoms around the Pd atom were estimated as 0.4 by varying the value of u in the range of 0.065-0.085 in regard to the Pd-Pd contribution.I6 Second, the estimation of errors for the coordination numbers has been carried
*
2 4 6 Distance R (A) Figure 6. Fourier-transformed EXAFS spectrum at Pt L3-edgeof the colloidal dispersions of the Pd/Pt bimetallic clusters at Pd/Pt ratios = 011, 1/4, 1/1, and 4/1. out by changing the R factor (reliabilityfactor) from the minimum value (Rmin),which can be obtained when the best fitting of the curve is performed, to 21/2Rmin.22The edge jump of Pt L3-edge in EXAFS measurement is so low in height that the coordination numbers cannot be determined precisely and the range of the errors becomes bigger, which results in the errors shown in Table 11. There is a simple relation shown in eq 5,23 which must be satisfied concerning the coordination number of Pt atoms around
Nw” = ( X ” / X P d ) P W (5) the Pd atom (PR) determined from Pd K-edge EXAFS and the
coordination number of Pd atoms around the Pt atom (Pw) determined from Pt L3-edge EXAFS. In eq 5 , XR and X , are the atomic fractions of Pt and Pd in the colloidal dispersions, respectively. Within the range of the error bars shown in Table 11, the present coordination numbers obtained can satisfy relation 5.
The bulk metals of Pd and Pt are known to take a facecentered cubic (fcc) structure. In the Pd cluster the coordination distance is about 0.276 f 0.003 nm, which is the same as the coordination distance of the bulk metal in the fcc structure. Since the Pd cluster is 1.4 nm in original diameter, it can be estimated that the cluster particle consists of a three-layered fcc structure containing 55 atoms on the average. The coordination number of this structure agrees with the observed one obtained from the EXAFS analyses of the colloidal dispersions of Pd clusters. In the case of the Pd/Pt (4/1) bimetallic clusters, the average size has been determined to be 1.4 nm by the transmission electron microscope (TEM) observation as described before. Hence, the Pd/Pt (4/ 1) bimetallic cluster consists of a three-layered fcc structure containing 55 atoms. As shown in Table 11, the large coordination number of Pt atoms around the Pt atom suggests that the Pt atom coordinates predominantly to the other Pt atoms. Moreover, the coordination numbers are quite different from those calculated for the random model, where 42 Pd atoms and 13 Pt atoms are located completely at random. If the 55 atoms form the fcc structure, and 42 Pd atoms of the 55 atoms are on the surface of the cluster particle, and the other 13 Pt atoms are (22) Lytle, F. W.; Sayers, D. E.;Stern, E. A. In X-Ray Absorprion Fine S t r u c t u r c Y ; Leon, J. M., Stern, E. A,, Sayers, D. E.,Ma, Y.. Rehr. J. J..
Ed.; Elsevier/North-Holland: New York, 1988; pp 701-722. (23) Via, G. H.;Drake, K. F.,Jr.; Meitzner, G.; Lytle, F. W.; Sinfelt. J. H.Catal. Lett. 1990, 5, 25.
7452 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991
Toshima et al.
TABLE III: Coonlin8Hon Numbem around tbe Pd and Pt Atoms of the Pd/Pt = 1/1 Bimetallic Cluster Determined from EXAFS lhta coordination number N absorbing scattering interatomic distance metal metal r/A obsd Pt corea random" separated" Pd Pd 2.74 f 0.03 2.3 i 1.0 3.1 3.9 6.0 Pd Pt 2.74 f 0.03 3.0 f 1.1 3.0 3.9 1.7 4.8 f 1.7 6.6 3.9 6.4 Pt Pt 2.73 f 0.03 1.3 f 1.0 2.9 3.9 1.6 Pt Pd 2.73 f 0.03
"See Figure 8 for the structures.
(a)
(b)
Figure 7. Cross section of the Pd/Pt (4/1) bimetallic cluster models: (a) Pt core model and (b) random model.
located at the center of the cluster particle (Pt core model as shown in Figure 7a), then the Pd/Pt ratio is almost 4/1 and the coordination numbers calculated on the basis of the Pt core model are quite consistent with the values observed from the EXAFS, as shown in Table 11. However, the sum of the coordination numbers around the Pt atom might become less than 12 because of the possibility of the existence of the Pt atoms on the surface of the clusters. On the basis of these results the Pt core structure can be taken as a model for the Pd/Pt (4/1) bimetallic cluster.16 Structure of the Pd/Pt (I/]) Bimetallic Clusters Detepined by EXAFS. The structure parameters of the colloidal dispersions of the Pd/Pt (1 / 1 ) bimetallic clusters have been determined on the basis of the two-shell fitting in the same way as for the Pd/Pt (4/1) bimetallic clusters. The results are shown in Table 111. The Pt-Pd distance (0.274 f 0.003 nm) obtained from Pd K-edge EXAFS is equal to the Pd-Pt distance (0.273 f 0.003 nm) obtained from Pt L3-edgeEXAFS in the same sample. The coordination numbers of the Pd and Pt atoms around the Pd atom are determined as 2.3 f 1 .O and 3.0 f 1.1, respectively, and those of Pt and Pd atoms around the Pt atom are 4.8 f 1.7 and 1.3 f 1 .O, respectively. As shown in Table 111, the coordination numbers of both Pd atoms around the Pd atom and Pd atoms around the Pt atom decrease in the case of the Pd/Pt (1/1) bimetallic clusters on comparison with those of the Pd/Pt (4/1) bimetallic clusters. However, the coordination number of Pt atoms around the Pt atom in the Pd/Pt (1 /1) bimetallic clusters is not very different from that in the Pd/Pt (4/1) bimetallic ones, which suggests the existence of a Pd monolayer surrounding the Pt core. On the other hand, the coordination number of the Pt atoms around the Pd atom in the Pd/Pt (1/1) bimetallic clusters is larger than that in the Pd/Pt (4/1) bimetallic clusters. This increase in the coordination number indicates that, on the Pd monolayer surface of the Pd/Pt (1/1) bimetallic clusters, the Pd-Pt bond occupies a considerable fraction of the bonds in which the Pd atoms participate. From the TEM observation, the particle size of the Pd/R (1 / 1) bimetallic clusters is nearly the same as that of the Pd/Pt (4/1) bimetallic ones, and of course, the Pd/Pt ratio of the former is smaller than that of the latter. Therefore, Pt atoms must be substituted for the Pd atoms in a Pd monolayer of the Pd/Pt (4/1) bimetallic cluster to produce the Pd/Pt (1/1) cluster particle from the Pd/Pt (4/1) one. The presence of Pt atoms on the surface of the cluster particle is considered to decrease the catalytic activity of the Pd/Pt ( 1 / 1 ) bimetallic clusters for the selective partial hydrogenation of diene to monoene on comparison with that of the Pd/Pt (4/1) ones. Here, the model structure will be considered for the Pd/Pt (1 / 1) bimetallic clusters. The coordination numbers obtained are quite different both from those calculated for the random model, in which 27 Pd and 28 Pt atoms are located completely at random,
(c)
(d)
Figure 8. Cross section of the Pd/Pt (1 / 1) bimetalliccluster models: (a) modified Pt core model, (b) random model, (c) separated model, and (d) the three-dimensional picture of the modified Pt core model.
and from those calculated for the separated model, in which 27 Pd and 28 Pt atoms are located in both sides of a round particle, respectively, forming a biphasic ball. Thus, the modified Pt core structure is suggested as a model for the Pd/Pt (1 /1) bimetallic cluster. The 28 Pt atoms connect directly to each other, being located both in the core and on the surface, while the 27 Pd atoms form three islands on the surface of the clusters, as shown in Figure 8a,d. Relation between the Surface Structure and the Catalytic Activity. The Pd/Pt (4/1) bimetallic clusters were the more active catalysts for the selective partial hydrogenation than the Pd monometallic clusters or the mixtures of the Pd and Pt clusters. Thus, the Pt core structure, as shown in Figure 7a, is more active than the Pd clusters. The high activity generally depends on the surface structure of the catalyst. In the present case, however, both clusters have the same surface structure. Thus, in the case of the Pd/Pt (4/1) bimetallic clusters, 42 Pd atoms are located on the surface of the cluster particle surrounding the R core,while in the Pd monometallic clusters, about 42 Pd atoms are on the surface and 13 Pd atoms at the center of the particle. The difference is just in the central core, Le., 13 Pt atoms for the Pd/Pt (4/1) bimetallic cluster and 13 Pd atoms for the Pd cluster. The above consideration can provide an idea that the central atoms could affect the properties of the Pd atoms on the monolayer surface, which could influence the catalytic activity. Thus, in the case of the Pd/Pt (1 / 1) bimetallic clusters the presence of the Pt atoms on the surface of the clusters is considered to decrease the catalytic activity for the selective partial hydrogenation. The ionic potential of the Pd atom (8.34 eV) is known to be smaller than that of the Pt atom (9.0 eV). Thus, the electronic interaction between the Pt core and the Pd monolayer could provide an uneven distribution of electrons. Then, the Pd monolayer becomes poorer in electron density than in the Pt core. This is one kind of ligand effect of Pt upon the Pd of the surface, which might make the Pd/Pt (4/1) bimetallic clusters more active than Pd clusters since the substrate having a double bond favors the electron-deficient surface. There are many reports on metal carbonyl cluster complexes, the structures of which have been investigated energetically. Some of them are suggestive for the consideration on the present
J. Phys. Chem. 1991, 95,7453-7459
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using hE and u directly derived from suitable reference compounds in order to determine the accurate coordination numbers and the coordination distances. From the EXAFS analysis as well as TEM observation, the Pt core structure (Figure 7a), in which the 42 Pd atoms are on the surface of the cluster particle and 13 Pt atoms are in the core, is presented as a model for the Pd/Pt (4/1) bimetallic clusters. With decreasing in Pd/Pt ratio, the Pd atoms on the surface of the cluster are substituted for the Pt atoms. Then, the Pd/Pt (1/ 1) bimetallic clusters are suggested to have a modified core structure (Figure 8a,d), in which 28 Pt atoms are located both in the core and on the surface connecting directly each other and 27 Pd atoms form three islands on the surface of the cluster particle. 0 = Pt Figure 9. Metallic skeleton of the [Ni,8Pt,(Co)48H]J-cluster.24
structure of the bimetallic clusters. It can be empirically said for the bimetallic carbonyl clusters that the heavier metal atoms are located near the center of the cluster and the lighter metals are near the surface. For example, [Ni38Pt6(Co)aH]5- was reported to have the structure shown in Figure 9, where the heavier Pt atoms are located at the center and the lighter Ni atoms are near the surface.24
Conclusion The structure of the colloidal dispersions of the palladium/ platinum bimetallic clusters protected by poly(N-vinyl-2pyrrolidone) was studied by EXAFS measurement. Because of the similar contribution of Pd-Pd, Pd-Pt, and Pt-Pt coordination distance, the separation of each contribution is difficult. To analyze these contributions, the data analysis was carried out by (24) Heaton, B.T.; ingsllina, P.; Devenish, R.; Humphreys,C. J.; Ceriotti, A.; Longoni, G.; Marchionna, M. J . Chem. Soc., Chem. Commun. 1987,765.
Acknowledgment. We gratefully acknowledge the assistance of Dr.Kouichi Adachi in taking electron micrographs and of Drs. Atsushi Oyama and Masaharu Nomura a t the National Laboratory for High Energy Physics (KEK) for the EXAFS measurements. This study was supported by a Grant-in-Aid for Scientific Research in the Priority Area of “Macromolecular Complexes” (01612002) from the Ministry of Education, Science and Culture, Japan. Glossary x(k) EXAFS oscillation p(E) observed EXAFS data h Planck‘s constant/2r k wavenumber of the photoelectron E photon energy m mass of the electron coordination number of the j t h coordination shell Nj bond length of the j t h coordination shell rj difference between the theoretical and experimental threshA&, old energies Debye-Waller factor of the j t h coordination shell 3 phase shift of the j t h coordination shell 4j(k) amplitude function of the j t h coordination shell Fj(k) amplitude reduction factor reliability factor
2
I n Situ Infrared Spectroscopy of Carbon Monoxide Adsorbed at Iridium( 111)-Aqueous Interfaces: Double-Layer Effects on the Adlayer Structure Xudong Jiang, Si-Chung ChangJ and Michael J. Weaver* Department of Chemistry, Purdue University, West Lofayette, Indiana 47907 (Received: March 26, 1991; In Final Form: May 20, 1991)
Surface infrared spectra in the C-O stretching ( ~ ~frequency 0 ) region are reported for CO adsorbed on ordered Ir( 11 1) in aqueous media, primarily 0.1 M HClO,, as a function of CO coverage, Oca, and electrode potential, E. In contrast to other platinum-group (1 11) surfaces examined hitherto, the adsorption sites were found to be exclusively atop (or near-atop), even at potentials down to -0.8 V vs SCE (in 0.1 M NaCIOl + 1 mM KOH), as discerned from the appearance of a single uco feature at ca. 1960-2060 cm-I. Large (up to ca. 100 cm-I) frequency upshifts are observed at increasing dosed coverages, especially for adsorption in the “hydrogen” potential region (ca. -0.25 to 4 - 0 5 V vs SCE), where fractional CO coverages of up to 0.6 could be obtained. Significantly smaller saturated Bco values, ca. 0.45, were attained for adsorption in the “double-layer”region, 0 . 2 V. Adlayers formed in these two potential regions also exhibit different spectral and electrochemical properties. For example, CO adlayers prepared within the hydrogen region on I r ( l 1 1 ) exhibit extensive island formation during electrooxidative removal, as discerned from the relatively invariant vk0 values, in a similar fashion to adlayers on low-index Pt and Rh surfaces. For adlayers formed in the double-layer region on Ir( 11I), however, CO island formation is rather more limited under these conditions, as seen from a markedly larger dependence which matches more closely that obtained for partial coverages formed by direct dosing with dilute CO solutions.
Introduction A topic of continuing interest in our laboratory concerns the examination of carbon monoxide adsorption on ordered monocrystalline transition-metal surfaces in electrochemical environ‘Present address: Dow Chemical Co., Midland, MI 48667.
0022-3654/91/2095-7453$02.50/0
ments by means of in situ infrared reflection-absorption spectroscopy (IRAS).’+ These studies are motivated in part by the (1) For overviews, see: (a) Chang, s.-C.; Weaver, M. J. J . Phys. Chem. 1991, 95, 5391. (b) Chang, s.-C.; Roth, J. D.;Ho, Y.;Weaver, M. J. J . Electron Spectrosc. Relat. Phenom. 1990, 54155, 1185.
0 199 1 American Chemical Society
