J. Phys. Chem. B 1997, 101, 5285-5293
5285
EXAFS Characterization of Rh-Pt Metal Clusters Supported on NaY Zeolite F. Cimini and R. Prins* Laboratory of Technical Chemistry, Swiss Federal Institute of Technology (ETH), UniVersita¨ tstrasse 6, 8092 Zu¨ rich, Switzerland ReceiVed: December 11, 1996; In Final Form: March 26, 1997X
The size and shape of monometallic Rh and Pt and bimetallic Rh-Pt clusters on zeolite NaY were determined by EXAFS. In the monometallic as well as in the bimetallic samples, the first coordination numbers were independent of the metal loading (1.5-4.8 wt % for Pt and 0.8-2.5 wt % for Rh in monometallic samples, and 0.8-2.4 wt % for Pt and 0.4-1.2 wt % for Rh in the bimetallic samples). All first coordination numbers were around 7, demonstrating that all monometallic and bimetallic particles have similar dimensions. The second- and third-shell coordination numbers of the monometallic clusters are in accordance with a spherical shape of the metal clusters which just fit into the supercages of the Y zeolite structure. Both Rh and Pt have the tendency to bind preferentially to their own kind. The observed coordination numbers, as well as the EXAFS spectra obtained after CO absorption, showed that no substantial surface enrichment is present in the bimetallic RhPt/NaY samples. Since EXAFS data are averaged over all particles, no conclusion could be drawn about the compositional homogeneity of the particles.
Introduction
TABLE 1: Metal Loadings of the Three Series of Samples
The structure of small metallic clusters has been of great interest in catalysis for many years. Such clusters are usually dispersed on a support, such as silica or alumina, and the catalytic activity of the metal clusters depends on their size and on the physical characteristics of the support. Only the metal atoms in the surface are active in the catalytic process, and a general aim of heterogeneous catalysis is, therefore, to reduce the size of metal particles and thus to use the metal as efficiently as possible. The activity and selectivity of metal clusters can be drastically affected by the presence of a second metal component.1-5 Many bimetallic clusters have been synthesized and characterized so far, i.e., Pt-Ir, Ru-Cu, Os-Cu, Pt-Mo.1,2,6-11 To control and improve the catalytic activity of supported metal clusters, it is of prime importance to know and understand their structure. In the case of bimetallic clusters, complete knowledge of clusters involves not only the size and shape of the clusters but also the distribution of the metals inside the cluster. Several distributions may occur. The two metals may have very limited miscibility in the bulk, i.e., Cu-Ni,12 or they may be completely miscible, as in Au-Cu.12 When the clusters are small, the number of surface atoms is a significant fraction of the total number of atoms. Even if the metal components are completely miscible in the bulk, it is still possible that the component with the lowest surface energy concentrates in the surface. The total composition can hence be different from that at the surface as well as from that in the interior, and this can have a strong influence on the catalytic activity of the bimetallic clusters. Extended X-ray absorption fine structure (EXAFS) has been proven to be a very efficient tool for studying such materials,13-19 giving information on the environment of a particular atom in the metal clusters, i.e., the number and kind of neighboring atoms and their interatomic distances. On the other hand, the structural parameters obtained by the EXAFS analysis are aVerage data. This can lead, in case of nonhomogeneity of the clusters, to an incomplete understanding of their structure.20 In this paper we report the structure of monometallic (Pt, Rh) and bimetallic (Pt-Rh) clusters supported on zeolite NaY, X
Abstract published in AdVance ACS Abstracts, June 15, 1997.
S1089-5647(96)04053-9 CCC: $14.00
%Rh first set
2.5
second set
1.2 1.6
third set
0.8 0.8 0.4
%Pt 4.8 2.4 3.1 1.5 1.5 0.8
name Rh1 Pt1 RhPt1 Rh2 Pt2 RhPt2 Rh3 Pt3 RhPt3
as determined by an analysis of their extended X-ray absorption fine structure (EXAFS) spectra. The combined analysis of monometallic and bimetallic clusters allowed a better characterization of the bimetallic clusters. Further studies on our system were performed by means of the quick EXAFS (QEXAFS) technique, as described in the accompanying article. The results obtained by the QEXAFS analysis enabled us to obtain additional information on the structure of the particles. Experimental Section Preparation. Three sets of NaY-supported catalysts with different metal loadings were prepared. Each set included three catalysts, each containing the same total equivalents in metal atoms per fixed amount of zeolite support. The first sample was loaded with rhodium, the second with platinum, and the third with both metals in a 1:1 molar ratio. They were prepared by the ion-exchange method,21 adding zeolite NaY (Chemie Uetikon) to an aqueous solution of Pt(NH3)4(NO3)2 (Johnson Matthey) and/or Rh(NO3)3‚2H2O (Johnson Matthey) and stirring the mixture for 48 h. The mixture was filtered and washed, and the recovered solid was dried at 100 °C and further oxidized at 300 °C for 2 h under oxygen flow. Finally, the substrates were reduced at 300 °C for 2 h under hydrogen flow just before EXAFS measurements were carried out. Every sample was analyzed by X-ray diffraction (XRD) and EXAFS, and the metal content was measured by atomic absorption spectra; the metal loadings are reported in Table 1. EXAFS Analysis. EXAFS spectra were measured at the Synchrotron Radiation Source (SRS) in Daresbury, at the © 1997 American Chemical Society
5286 J. Phys. Chem. B, Vol. 101, No. 27, 1997 superconducting wiggler station 9.2. The Si 220 double-crystal monochromator was tilted to reduce the intensity to 50% and thus eliminate the radiation of higher harmonics. To obtain a wafer with an absorbance of about 2.5522 for measurements at the Pt as well as Rh absorption edge, different samples had to be prepared for each absorption edge by pressing the right amount of sample into a sample holder. Ionization chambers filled with a mixture of Ar and He were employed as detectors, and the Ar/He mixture was chosen to give 20% absorption in the chamber before the measuring cell and 80% in the chamber after the cell. The metal-containing samples were reduced in an in situ cell.23 Hydrogen was further evacuated, and the sample was stored in He atmosphere. All sample measurements were performed at the Pt LIII-edge and Rh K-edge, at liquid N2 temperature. To determine the coordination numbers and distances reliably, multiple-shell fitting in k space and in R space was done, using k1 and k3 weighting. Rh foil (for the Rh-Rh contribution), Pt foil (Pt-Pt), Rh2O3 (Rh-O), and Na2Pt(OH)6 (Pt-O) were used as reference compounds. For the Pt-Si and Rh-Si contributions, FEFF theoretical values24 were used for the backscattering amplitudes and phase shifts. The FEFF method has been reported to give reliable values for light elements but poorer values for heavy elements such as Pt.25 Therefore, the errors in the Pt-Si and Rh-Si coordination numbers, related to the use of the FEFF Si backscattering values, should be small, while the errors in the interatomic distances might be somewhat larger. For the RhPt (Rh is the absorber) and Pt-Rh contributions, it was decided to use different strategies because of the difficulty in finding a good reference compound. First, theoretical McKale26 and FEFF24 values were used, and second, semiexperimental references were calculated and used. Since the backscattering amplitude, Fj(k), depends basically on the backscatterer atom, the backscattering amplitude for the Rh-Pt contribution (for the Pt-Rh contribution the same procedure applies) can be reliably extracted from χ(k) of the Pt foil. The same was done with the overlap factor S2j (k). A value of 2.73 Å was used for Rj, representing the mean value between 2.68 Å (the Rh-Rh distance in pure Rh) and 2.77 Å (the Pt-Pt distance in pure Pt) as previously reported in the literature27 for analogous bimetallic alloys. The λj value was set at 5 Å. For the phase shift value, φj(k), the value of the FEFF codes was used. All other values were extracted from χ(k) of Pt foil. The reference signal χ(k), calculated as above, will be referred to as the SE reference (semiexperimental reference). A third set of references (E references) was obtained by using the 1/9 and 9/1 Pt/Rh foils prepared by Tanakara Kikinzoku Kogyo K. K.28 and kindly lent to us by Dr. M. Harada. Since Rh and Pt are two completely miscible metals, every Rh in the Rh/Pt 1/9 foil should have 1.2 Rh atoms and 10.8 Pt atoms in the first coordination shell. Therefore, before calculating the backscattering amplitude and phase shift of the Rh-Pt contribution, we subtracted the signal due to the Rh atoms, assuming an interatomic distance of 2.74 Å, as estimated for a Rh/Pt 1/9 alloy taking into account the deviation from the virtual crystal approximation29 and using the backscattering amplitude and phase shift extracted from the pure Rh foil. An identical procedure was worked out for the Rh/Pt 9/1 foil. The reliability of each bimetallic reference was examined as reported below. In analyzing the second and third coordination shells of Rh samples and the second coordination shell of Pt samples, references obtained from isolating the corresponding coordination shells of the pure metal foils were used.
Cimini and Prins
Figure 1. RhPt 1/1 foil. χ(k)k3 measured at the Pt (a) and Rh (b) edges.
Figure 2. RhPt 1/1 foil, Pt edge. Measured signal (s) and corresponding fit (- - -) obtained by using the SE reference for the PtRh contribution.
Results Rh-Pt References. To evaluate the reliability of the references, EXAFS spectra of a RhPt 1/1 foil were recorded and analyzed at both absorption edges. Figure 1a,b shows the χ(k)k3 measured at the Pt and Rh absorption edge, respectively. Figure 2 shows the Fourier transform (FT) of χ(k)k3 at the Pt edge after isolating the first shell region and the relative fit obtained by using the SE reference. The parameters obtained with the FEFF, SE, and E references are listed in Table 2. CNs, as determined by the fitting
Characterization of Rh-Pt Metal Clusters
J. Phys. Chem. B, Vol. 101, No. 27, 1997 5287
TABLE 2: Structural Parameters Obtained by Analyzing a Rh/Pt 1/1 Foil and by Making Use of the Different Sets of References FEFF
SE
E
abs-backsc
CN
R (Å)
CN
R (Å)
CN
R (Å)
Rh-Rh Rh-Pt Pt-Rh Pt-Pt
6.5 6.9 5.6 5.5
2.71 2.73 2.74 2.76
6.5 6.1 5.8 5.7
2.71 2.74 2.74 2.76
6.4 5.7 5.6 5.8
2.72 2.74 2.73 2.75
TABLE 3: Structural Parameters of the First Series of Samples sample Rh1 Pt1 RhPt1
absorber
backscatterer
CN
R (Å)
∆σ2
Rh Pt Pt Pt Rh Rh Rh Pt Pt
Rh Pt O Si Rh Pt Si Pt Rh
6.7 6.8 1.7 2.6 4.4 3.1 1.0 4.3 2.9
2.67 2.76 2.69 3.40 2.65 2.71 3.32 2.74 2.69
0.0053 0.0022 0.0010 0.0060 0.0048 0.0053 0.0013 0.0040 0.0045
procedure, are accurate to within 10-15%, as calculated according to the procedure described by Lytle et al.30 The McKale references were disregarded, because they gave quite unrealistic coordination numbers. Even in the analysis of the pure Rh foil, the coordination numbers were off by a factor of about 2, showing the unreliability of the McKale backscattering amplitude values for the monometallic and bimetallic contributions. The transferability of the backscattering amplitude from a pure metal foil to a system having a different absorber atom and the same backscatterer was already proven to be good.13,17,31-35 Therefore, the SE reference, obtained by using the phase shift given by the FEFF codes and the backscattering amplitude as obtained experimentally from pure metal foil, seemed to be reliable. The results obtained in the analysis of the Rh/Pt 1/1 metal foil confirmed this (Table 2). The expected coordination number (CN) for the Rh-Pt and Pt-Rh as well as for the Rh-Rh and Pt-Pt contributions is 6. Since the metals are completely miscible, the dimension of the FCC unit cell changes additively with composition in atomic percent.36 However, the deviation from the virtual crystal approximation29 suggests that interatomic distances should be as follows: Rh-Rh ≈ 2.70 Å, Rh-Pt ) 2.73 Å, Pt-Pt ≈ 2.76 Å. The FEFF references led to a slight overestimation of the Rh-Pt coordination number in the 1/1 metal foil (6.9 instead of 6) and were, therefore, not used for the catalyst samples. The parameters obtained by using the E references were as acceptable as those of the SE references (Table 2) and, in general, the use of SE or E references did not cause any significant change in the obtained parameters for the catalyst samples. Therefore, to simplify the discussion, only the parameters obtained by using the SE references will be considered. The latter were preferred over the E references, because the subtraction of a fixed contribution, as done with the E references method on the metal alloys to obtain the reference parameters, always includes a certain degree of approximation. To further check the reliability of the SE references, big RhPt clusters were synthesized in the ratio 1:12 by the so-called alcohol reduction method.37 In this method metal salts are dissolved in an ethanol/water solution, and the solution is heated under reflux for several hours. To determine whether big metallic particles were obtained, an X-ray diffraction spectrum was measured. The width of the peaks at 2θ ) 39.9° and 46°
confirmed the presence of big metallic particles. The Rh-edge EXAFS spectrum of these metallic particles could be satisfactorily analyzed (with the SE references) with a single Rh-Pt contribution. The resulting coordination number of 12.3 and interatomic distance of 2.75 Å confirm the reliability of the SE references. First Set of Catalysts. The EXAFS signal of sample Rh1 (2.5% Rh) after data processing is shown in Figure 3a and the corresponding Fourier transformation in Figure 3b. The first coordination shell was isolated by backtransforming over the indicated R range. A fit was then carried out, leading to the parameters presented in Table 3. The fit was satisfactory for k1 as well as for k3 weighting (Figure 3c,d). As could be predicted from the Fourier transform of the raw data, the results confirmed the presence of Rh metallic clusters. The interatomic distance Rh-Rh (2.67 Å) is very similar to that of pure metal (2.68 Å). It is important to notice that for data of good quality (like the present ones) coordination numbers are accurate to within 10-20%, ∆σ2 within 20% and interatomic distances R within 0.002 Å. The accuracy of the data just mentioned was determined by following the procedure described by Lytle et al.30 The first coordination number had a value of 6.7. This indicated the presence of small metallic clusters, because in an infinite crystal with a FCC structure (like Rh or Pt), the first coordination number is 12. The χ(k)k3 of catalyst Pt1 (4.8% Pt) and its Fourier transform are shown in Figure 4a,b, while Figure 4c,d shows the firstshell fittings. The detected Pt-Pt distance has a value of 2.76 Å (Table 3), in good agreement with the Pt-Pt distance in pure Pt (2.78 Å). Besides the Pt-Pt contribution, two more contributions were identified due to the Si and O atoms contained in the framework of the zeolite, similar to results obtained with other metals in zeolite samples.38-41 The Pt-edge EXAFS spectrum of catalyst RhPt1 (2.4% Pt and 1.2% Rh) and its Fourier transform were quite different from the Pt1 spectrum and its Fourier transform respectively (Figure 5a,b). As a consequence, it was impossible to obtain an acceptable fit with only one Pt-Pt contribution. Undoubtedly this is due to overlapping of the Pt-Rh contribution with the Pt-Pt contribution, that is Pt atoms have Rh atoms in their first coordination shell as well, confirming that bimetallic particles were obtained. With the additional Pt-Rh contribution we obtained the fit shown in Figure 5c,d and the parameters reported in Table 3. On the other hand, at the Rh edge the FT profile did not seem to be strongly modified by the Pt atoms. Nevertheless, it was not possible to obtain an acceptable fit by using the RhRh contribution only. As could be predicted from the results at the Pt edge, a Rh-Pt coordination shell was also present. A complete analysis led to the results shown in Table 3 and Figure 6. When bimetallic clusters are analyzed at both metals edges, some checks on the reliability of the fits can be made. First, the interatomic Rh-Pt distance obtained by analyzing both edges has to be the same. Second, the Debye-Waller factors (∆σ2) of the Rh-Pt and Pt-Rh contributions must be of the same order of magnitude. Third, the coordination numbers of the Rh-Pt and Pt-Rh contributions must respect the condition: CNabXa ) CNbaXb, where a and b represent the two generic metals (Rh and Pt in our case), and CNab and CNba represent the number of nearest-neighbor atoms b about a and the number of neighbor atoms of a about b, respectively. Xa and Xb are the atomic fractions of a and b in the bimetallic cluster catalyst. For 1:1 bimetallic clusters, this means that CNab ) CNba. It is
5288 J. Phys. Chem. B, Vol. 101, No. 27, 1997
Cimini and Prins
Figure 3. Sample Rh1. Raw χ(k)k3 and related FT (a and b, respectively); measured signal (s) and corresponding fit (- - -) of χ(k)k3 and related FT of the first coordination shell (c and d, respectively).
TABLE 4: Structural Parameters of the Second and Third Coordination Shells of Sample Rh1 and of the Second Coordination Shell of Sample Pt1 sample
absorber
backscatterer
CN
R (Å)
∆σ2
Rh1
Rh Rh Pt
Rh Rh Pt
2.3 7.1 2.4
3.80 4.65 3.93
0.0018 0.0026 0.0034
Pt1
important to stress that the condition CNabXa ) CNbaXb is independent of the structure of the clusters. In the present case all these conditions were met. In fact CNRh-Pt = CNPt-Rh, RRh-Pt = RPt-Rh, and ∆σ2Rh-Pt = ∆σ2Pt-Rh. In addition, all observed interatomic distances are in good agreement with expectations; the Rh-Rh distance is 2.65 Å (2.68 Å in pure Rh), the Pt-Pt distance is 2.74 Å (2.78 Å in pure Pt) and the Rh-Pt distance around 2.70 Å. An analysis of the second and third coordination shells of samples Rh1 and Pt1 was tried. At the beginning an attempt was made to separate the second and third shells of sample Rh1, but this did not lead to satisfactory results because of the overlap between these shells (Figure 7: the fitting around 3.9 Å is not very good, probably because of the presence of a side lobe of the third shell). It was, therefore, decided to perform the analysis of both shells together; the results are shown in Figure 8 and in Table 4. The quality of the fit can be considered to be satisfactory, although it is not as good as that obtained in the analysis of the first shell. This is due to several factors. First, the signal of higher coordination shells is much weaker than that of the first coordination shell (cf. Figure 3b) and, at constant
noise, this leads to a lower signal/noise ratio. Second, small ripples belonging to the first shell can be present in the R range of the second and third shells. This could be avoided by analyzing the spectrum after subtracting the signal due to the first coordination shell, but this could cause even bigger errors if the first coordination shell is not perfectly fitted. In the present work, analysis after isolation (without subtraction) was performed first, and the results were verified by an analysis after subtraction; there was no substantial difference. In sample Pt1 only an analysis of the second coordination shell could be performed (Table 4) because of the overlap of the third and fourth coordination shells (Figure 4b). An analysis of the higher coordination shells of the bimetallic clusters was impossible because of the interference between the shells of the two different metals, making an identification of the higher shells in the Fourier transformation problematic, as can be seen in Figure 5b. Second Set of Catalysts. In this set of catalysts the successful synthesis procedure was not altered, but the number of metal atoms was reduced by one-third. Again, the main contribution in the EXAFS signal of the Rh2 sample was due to the Rh-Rh first coordination shell of Rh metallic clusters, with a first coordination number of 7.4 at a distance of 2.69 Å. Small Rh2-O2- (2.11 Å) and Rh0-O2- (2.73 Å) contributions due to oxygen ions in the support42,43 were detected as well (Table 5). The results of analyzing sample Pt2 (3.1% Pt) are given in Table 5. Two small contributions of oxygen and silicon were
Characterization of Rh-Pt Metal Clusters
J. Phys. Chem. B, Vol. 101, No. 27, 1997 5289
Figure 4. Sample Pt1. Raw χ(k)k3 and related FT (a and b, respectively); measured signal (s) and corresponding fit (- - -) of χ(k)k3 and related FT of the first coordination shell (c and d, respectively).
TABLE 5: Structural Parameters of the Second Series of Samples sample Rh2 Pt2 RhPt2
absorber
backscatterer
CN
R (Å)
∆σ2
Rh Rh Rh Pt Pt Pt Rh Rh Rh Pt Pt
Rh O O Pt O Si Rh Pt O Pt Rh
7.4 0.9 1.9 7.3 1.9 1.7 4.4 3.0 0.5 4.7 2.6
2.69 2.11 2.73 2.76 2.62 3.43 2.70 2.74 2.14 2.73 2.72
0.0017 0.0010 0.0010 0.0024 0.0012 0.0063 0.0010 0.0039 0.0013 0.0042 0.0032
detected in addition to the main contribution of platinum; these were attributed to interactions between the clusters and the support. The first coordination number had a value of 7.3, showing a degree of dispersion very similar to that found in sample Pt1. Catalyst RhPt2 (0.8% Rh and 1.5% Pt) was analyzed at the Pt edge and at the Rh edge, giving the results shown in Table 5. The interatomic distances obtained in the analysis of samples Rh2, Pt2, and RhPt2 are in agreement with crystallographic values (pure metal foils). All “rules” confirming the reliability of the analysis of bimetallic clusters performed at both metal edges are obeyed, within the uncertainty error. The interatomic Pt-Rh and Rh-Pt distances are 2.72 and 2.74 Å, respectively,
and the coordination numbers of the unlike atom are 2.6 and 3.0, always in accordance with eq 3.2. The second and third coordination shells of catalyst Rh2 were analyzed as well, showing good agreement between experimental data and the fitted signal. The interatomic distances between the central Rh atom and the second (3.80 Å) and third (4.66 Å) Rh neighbors were consistent with the corresponding distances in the pure metal foil (3.80 and 4.65 Å, respectively) and in sample Rh1 (Table 4). The coordination numbers (2.5 and 7.6, respectively) were close to those in sample Rh1. Analysis of the second coordination shell in sample Pt2 (CN ) 2.3 and R ) 3.95 Å) led to values similar to those for sample Pt1 (Table 4). Third Set of Catalysts. In this set the metal amounts were reduced by 50% relative to those of set 1. The profile of the Fourier transform of the raw data of the bimetallic sample was similar to those of the previous sets, with two evident peaks at the Pt edge and one big peak at the Rh edge. The results were very similar to those obtained for the first two series, as shown in Table 6. Only the Rh-Pt and Pt-Rh coordination numbers were smaller. EXAFS after CO Absorption. To determine whether surface enrichment was present in the bimetallic samples, some EXAFS measurements were performed on sample RhPt2 after CO absorption. By comparing the EXAFS signals before and
5290 J. Phys. Chem. B, Vol. 101, No. 27, 1997
Cimini and Prins
Figure 5. Sample RhPt1, Pt edge. Raw χ(k)k3 and related FT (a and b, respectively); measured signal (s) and corresponding fit (- - -) of χ(k)k3 and related FT of the first coordination shell (c and d, respectively).
of the first shell
Figure 7. Sample Rh1, FT of χ(k)k3. Second shell after isolation (s) and corresponding fit (- - -).
after absorption at both metal edges, it should be possible to determine which metal is influenced most by CO and thus is present in excess on the surface. To prevent the disruption of our small metallic clusters by the action of CO molecules,44,45 the absorption was performed at liquid nitrogen temperature; in analogous systems at this temperature the IR spectra showed mainly signals from absorbed CO molecules.46 W(CO)6 as Reference. The backscattering amplitude depends basically on the backscatterer atom, and, therefore, one
can extract the metal-C and metal-(C)O backscattering amplitudes from a suitable carbonyl compound such as W(CO)6 and use them to calculate the Rh/Pt-C and Rh/Pt-(C)O references (SE references). As for the phase shift, the FEFF codes can be used. W(CO)6 has six CO molecules at equal distance from the metal atom and no further ligands, thus representing an ideal reference compound. To check the reliability of the obtained parameters, EXAFS measurements were performed on K2Pt(CN)4. The Pt-C contribution to the EXAFS signal was isolated through back-
Figure 6. Sample RhPt1, Rh edge. FT (s) of and corresponding fit (- - -).
χ(k)k3
Characterization of Rh-Pt Metal Clusters
J. Phys. Chem. B, Vol. 101, No. 27, 1997 5291
Figure 8. Sample Rh1: measured signals (s) and corresponding fits (- - -) of χ(k)k3 (a) and related FT (b) of the second and third coordination shells.
TABLE 6: Structural Parameters of the Third Series of Samples sample Rh3 Pt3 RhPt3
absorber
backscatterer
CN
R (Å)
∆σ2
Rh Rh Pt Pt Pt Rh Rh Rh Pt Pt
Rh O Pt O Si Rh Pt O Pt Rh
6.9 1.1 7.5 1.6 1.3 5.4 1.8 1.0 5.6 1.6
2.68 2.08 2.74 2.62 3.37 2.68 2.71 2.06 2.74 2.72
0.0014 0.0006 0.0029 0.0006 0.0010 0.0010 0.0010 0.0005 0.0028 0.0015
transformation, and the signal was analyzed via a fitting procedure, using the Pt-C reference obtained from the W(CO)6 spectrum. The fit was acceptable, and the Pt-C coordination distance of 1.97 Å and the coordination number of 3.6 were in good agreement with the crystallographic values of K2Pt(CN)447 and therefore confirmed the reliability of our Pt-C reference. A further check on the reliability of the W-C and W-(C)O backscattering amplitudes for the Rh-C and Rh-(C)O contributions was made by recording and analyzing the EXAFS signal of the [Rh(CO)2Cl]2 dimer with the SE references obtained from W(CO)6. The fit was quite good, and the contributions of Cl and C were in agreement with crystallographic data.48 RhPt Sample after CO Adsorption. Figure 9a compares the Fourier transform of sample RhPt2 at the Pt edge before and after CO absorption. The difference is small and mainly consists of a decrease in the peaks at 2.6 and 3.5 Å. To obtain
Figure 9. Sample RhPt2. FT of χ(k)k3 before (- - -) and after (s) CO absorption at the Pt (a) and Rh (b) absorption edge.
TABLE 7: Parameters of Sample RhPt2 after CO Absorption Pt edge
Rh edge
absorber
backscatterer
CN
R (Å)
∆σ2
Pt Pt Pt Pt Rh Rh Rh Rh
Pt Rh C O Rh Pt C O
5.1 2.3 0.4 0.5 4.6 2.7 0.5 0.4
2.73 2.71 2.08 3.18 2.70 2.72 2.14 3.20
0.0048 0.0026 0.0003 0.0000 0.0020 0.0021 -0.0001 -0.0001
a good fit, two additional Pt-C and Pt-O contributions had to be added to the Pt-Pt and Pt-Rh contributions; the results are shown in Table 7. The resulting interatomic distances are as expected. A very similar result was obtained in the analysis of sample RhPt2 at the Rh edge. The change in the spectrum due to CO absorption is clearly detectable (Figure 9b). The results of the fitting (Table 7) show two small Rh-C and Rh-O contributions in addition to the Rh-Rh and Rh-Pt contributions. In this case the Rh-C distance is slightly larger than expected, but still reliable.
5292 J. Phys. Chem. B, Vol. 101, No. 27, 1997 Discussion No significant differences were noticed between the first coordination numbers of the monometallic Rh clusters in the three sets of catalysts, despite the differences in metal loading. Samples Rh1 (2.5 wt % Rh), Rh2 (1.6 wt % Rh) and Rh3 (0.8 wt % Rh) have first coordination numbers of 6.7, 7.4, and 6.9, respectively; all around 7. The same holds true for the monometallic Pt samples, with first coordination numbers of 6.8, 7.3, and 7.5 for sample Pt1 (4.8 wt %), Pt2 (3.1 wt %), and Pt3 (1.5 wt %), respectively. The first coordination numbers give information about the size of the metal clusters; it is, therefore, concluded that the size of the monometallic Rh and Pt clusters is independent of metal loading under our synthesis procedure. The situation is similar for the bimetallic samples. It is noteworthy that bimetallic clusters were, indeed, obtained, as the EXAFS signals, and their Fourier transformations at both metal edges clearly show. The trend of the coordination numbers is similar to that of the monometallic clusters. The bimetallic samples RhPt1, RhPt2, and RhPt3 showed very similar first total coordination numbers. On average, every Rh atom of sample RhPt1 has 4.4 neighboring Rh atoms and 3.1 Pt atoms, a total of 7.5 atoms. Every Pt atom has 4.3 neighboring Pt atoms and 2.9 Rh atoms, a total of 7.2. As for sample RhPt2 every Rh atom has about 4.4 neighboring Rh atoms and 3.0 Pt atoms, a total of 7.4 atoms, and every Pt atom has around 4.7 neighboring Pt atoms and 2.6 Rh atoms, a total of 7.3. In sample RhPt3, every Rh atom has 5.4 neighboring Rh atoms and 1.8 Pt atoms, a total of 7.2, and every Pt has 5.6 Pt atoms and 1.6 Rh atom, a total of 7.2. Thus, the bimetallic cluster sizes are also unaffected by the metal loading, and these sizes are almost identical with those of the monometallic Rh or Pt clusters. In summary, all the metallic particle sizes obtained with our synthesis method were independent of metal loading in a range from 0.8 to 2.5 wt % for Rh (0.4-1.2% for the bimetallic clusters) and from 1.5 to 4.8 wt % (0.8-2.4% for the bimetallic clusters) for Pt. To check for the presence of big metallic particles, XRD spectra were measured as well. In the area around the (111) peak, where the overlap with the zeolite pattern was not so drastic, no measureable peak was found. Therefore, it is concluded that no big metallic clusters were present in our samples. While the first coordination number gives general information about the size of the particles, the higher coordination numbers provide information about the particle shape. The coordination numbers obtained from the analysis of the second and third shells of the Rh1 and Rh2 catalysts are 2.3 and 7.1 for Rh1 and 2.5 and 7.6 for Rh2; those of the second shells of catalysts Pt1 and Pt2 are 2.4 and 2.3, respectively. Considering the uncertainty with respect to the coordination numbers obtained by EXAFS, these coordination numbers are in good agreement with those predicted for spherically shaped particles containing 2834 atoms. Thus, even when changing the metal loading from Rh to Pt, metal clusters containing around 32 atoms were always formed. The zeolite NaY supercage can contain a maximum of about 32 atoms in a FCC structure,49 with a metal-metal distance of 2.75 Å. It thus seems reasonable to suppose that the independence of the particle size from metal loading is due to the support framework. Apparently, the metal particles grow during reduction until the supercages are filled. Further growth would require the removal of material from the surrounding zeolite lattice and would be quite difficult. Therefore, growth will continue, as far as possible, without disruption of the zeolite lattice, and eventually, spherically shaped particles, similar in
Cimini and Prins size, are obtained. The dimension and shape of the clusters and the independence of the metal loading are all in good agreement with this model, since the zeolite supercages have a spherical shape. Although the metal particles have the same size as the supercages, the EXAFS results (Figure 9) prove that they are still accessible to CO. This is understandable, since the number of the metal particles is much smaller than that of the supercages, so that each metal particle is still accessible through the four 12-ring windows of the supercage. Bimetallic clusters of 32 atoms can have different structures. If one of the metal components were mainly present at the surface, it would have a much lower first total coordination number than the other component, because the atoms on the surface have many more vacant coordination sites than the atoms in the kernel. No difference in total coordination numbers was, however, found in our Pt-Rh clusters. In fact, as already pointed out, the Pt as well as Rh atoms have a first total coordination number around 7 in all samples, which excludes a large surface enrichment. Of course, a small enrichment cannot be excluded because of the uncertainty in the coordination numbers. The results obtained after CO adsorption confirm the absence of any substantial surface enrichment: since no relevant differences were detected at the two edges after CO addition, both metal atoms must be present at the surface of the clusters in a ratio of nearly 1:1. In the bimetallic clusters, the Rh-Rh and Rh-Pt, as well as Pt-Pt and Pt-Rh coordination numbers provide information about the structure of the clusters. A completely random arrangement would lead to CNRh-Rh ) CNRh-Pt ) CNPt-Pt ) CNPt-Rh because we are dealing with bimetallic particles having a metal ratio of 1:1. The observed coordination numbers are quite different, however, showing a clear tendency for each metal to bind preferentially to its own kind. In sample RhPt1, the coordination number for Rh-Rh is 4.4, for Rh-Pt 3.1, for Pt-Pt 4.3, and for Pt-Rh 2.9. In sample RhPt2, the coordination number for Rh-Rh is 4.4, for Rh-Pt 3.0, for Pt-Pt it is 4.7, and for Pt-Rh 2.6. This tendency is even clearer in sample RhPt3, because the Rh-Rh coordination number is 5.4 and that for Rh-Pt 1.8, while Pt-Pt CN is 5.6 and Pt-Rh CN is 1.6. Despite the detailed information obtained about the bimetallic clusters (spherical shape, no surface enrichment, tendency of each metal atom to bind to its own kind), more than one model can still fit the obtained results. A model that can explain the presence of bimetallic bonds and the tendency of each metal atom to bind to its own kind is the so-called cluster in cluster model,35 but it is not the only possible model. If the composition of the metal particles were nonhomogeneous, so that different particles had different compositions, the data could be equally well explained because EXAFS structural parameters are averages over the different particles. For instance, a sample containing monometallic Rh, monometallic Pt and bimetallic (randomly mixed) Rh-Pt particles could also fit our results. Results obtained in the QEXAFS analysis of the reduction step50 demonstrate that the composition of the metal particles is in fact inhomogeneous and that a distribution of monometallic Rh and bimetallic Rh-Pt clusters exists. References and Notes (1) Sinfelt, J. H. Chem. Eng. News 1972 (3), 18. (2) Sinfelt, J. H. J. Catal. 1973, 29, 308. (3) Biloen, P.; Helle, J. N.; Verbeek, H.; Dautzenberg, F. M.; Sachtler, W. M. H. J. Catal. 1980, 63, 112. (4) Burch, R.; Garla, L. C. J. Catal. 1981, 71, 360. (5) Shum, V. K.; Butt, J. B.; Sachtler, W. M. H. J. Catal. 1986, 99, 126.
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