J. Phys. Chem. 1994, 98, 9865-9873
9865
Chemisorption of C1 on W S i 0 2 Determined by EXAFS A. P. Gloor and R. Prim* Luboratorium f i r Technische Chemie, Eidgenossische Technische Hochschule Zurich, 8092 Zurich, Switzerland Received: May 6, 1994; In Final Form: July 15, 1994@
A highly dispersed Rh/Si02 catalyst was prepared from [Rh(NH3)&1]C12 by the ion exchange method. Extended X-ray absorption fine structure (EXAFS) measurements showed that after calcination of this catalyst precursor at 573 as well as 673 K each Rh ion had on average slightly more than one C1 ligand. The Rh-0 coordination number was higher after calcination at 673 K, probably due to better ordering, and the total C1 0 coordination number was 6. Reduction at 673 K led to small Rh particles containing about 10 metal atoms, as evident from the small Rh-Rh EXAFS coordination numbers and the presence of a Rh-0 EXAFS contribution from Rh atoms at the metal-support interface. A Rh-C1 contribution at 2.25 A showed that during reduction at 673 K not all chlorine atoms had been removed and that the remaining C1 atoms were in contact with Rh atoms. These C1 atoms at the metal surface induced an outward relaxation of the surface Rh atoms, as shown by the Rh-Rh distance which was larger than in bulk Rh.
+
Introduction
The catalytic properties of supported metal catalysts are often influenced by the presence of additives, be it promoters or inhibitors, on the surface of the metal particles.' When the interaction between additive and support is strong, the additive will most likely be situated on the support, while in the opposite case when the interaction between additive and support is weak, the additive has a good chance of ending up on the metal. In this way, the catalyst properties can be influenced by the choice of the support even though the support does not play a direct role in the catalytic chemistry. Several additives have a strong interaction with A1203 and a weak one with Si02. Thus, impurities on Si02 affect the selectivity pattern of Rh/Si02 catalysts in the hydrogenation of CO to hydrocarbons and oxo product^,^-^ whereas they play no role on Rh/A1203 catalysts. Chlorine is known to strongly bind to A1203, and noble metals on A1203 catalysts prepared from metal chloride salts have similar properties as those prepared from other anion salts. On SiOz, on the other hand, the anion may influence the catalytic properties ~ubstantially.~ Recently, we observed that Rh/Si02 catalysts prepared from RhCl3 had a much higher activity in the CO hydrogenation than catalysts prepared from Rh(N03)3, while also the selectivity differed.6 These findings were ascribed to the presence of chlorine on the Rh surface in the catalysts prepared from RhC13, while the Rh particles in the catalysts prepared from Rh(N03)3 were free of chlorine. In the present work we describe an EXAFS study of the location of C1 on a Rh/Si02 catalyst prepared from [Rh(NH3)5ClIClp. This precursor salt was chosen for its chlorine content and because it allowed to attain a high dispersion of the eventual metallic Rh particles. EXAFS is a bulk technique, and if one wants to study C1 atoms on Rh particles by EXAFS, a substantial part of the Rh atoms should be at the metal particle surface. Only then will the Rh-Cl EXAFS contributions to the total EXAFS spectrum not be overwhelmed by the Rh-Rh contributions. Highly dispersed noble metal catalysts have been obtained on supports like Al2O3, MgO, and Ti02 by simple pore volume impregnation. In the EXAFS spectra of the resulting reduced catalysts, M-0 contributions from metal atoms in the metal@
Abstract published in Advance ACS Absrrucrs, September 1, 1994.
0022-365419412098-9865$04.5010
support interface have been observed. Long M - 0 distances of about 2.6-2.7 8, were observed when supported metal catalysts were measured under H2,7s8whereas short distances around 2.1 8, were observed when the catalysts were evacuated.gJO In some systems even the contribution from noble metal to metal cation in the support was observed. Thus, a Rh-Ti contribution in R l ~ T i 0 2and ~ ~a Pd-Si contribution in Pdzeolite XI2 have been reported. EXAFS investigations of noble metals on Si02 catalysts have been p u b l i ~ h e d , ~but ~J~ metal-support M-0 EXAFS contributions were not observed. We will show that small Rh particles on Si02 can be obtained by preparation of a catalyst via the ion exchange method and that in this catalyst Rh-0 distances at the metal-support interface, as well as Rh-Cl distances due to C1 atoms adsorbed on the Rh particles, can be observed. Experimental Section Catalysts. To avoid contamination by impurities, we did not use commercial silica as a support, but instead prepared our own support by hydrolysis of tetraethyl orthosilicate (Fluka). To prevent the take-up of alkali and earth alkali metal cations from glass, the preparation and further experiments were carried out in polyethylene equipment, which had been cleaned with a hot 5 wt % boric acid solution. The hydrolysis of the tetraethyl orthosilicate in ethanol solution was carried out with a nitric acid solution, while cooling the beaker in an ice bath. After 1.5 h stirring a surplus of ammonia was added. The resulting gel was dried under vacuum at room temperature and subsequently by heating for 16 h at 393 K. Finally, the silica was calcined for 3 h at 723 K. The resulting material had a surface area of 504 m2/g, and the major part of the pore volume came from pores with diameters in the 4-7 nm range. A 0.73 wt % Rh/Si02 catalyst was prepared by stirring 5 g of this silica with 0.13 g of [Rh(NH3)5Cl]C12 in 25 mL of 3 N ammonia for 2 h at 353 K. After washing, the catalyst precursor was dried for 16 h at 393 K and subsequently calcined by heating from room temperature at 5 Wmin and holding for 1 h at either 573 or 673 K. These catalysts will be denoted C573 and C673, respectively. After cooling to room temperature, these materials were reduced in pure H2 by heating at 5 Wmin and holding for 1 h at 673 K. The resulting reduced catalysts are denoted C573 R673 and C673 R673, respectively. 0 1994 American Chemical Society
9866 .I Phys. . Chem., Vol. 98, No. 39, 1994
Gloor and Prins
TABLE 1: Crystallographic Parameters and Fourier Transform Ranges of the Reference Compounds ref shell compd N R, 8, krange, 8,-l Rrange, 8, Rh-Rh 1st shell 2nd shell 3rd shell Rh-0 Rh-c1 Rh3+-Rh3+
Rh foil Rh foil Rh foil a 2 0 3
RhC13 RhC13
12 6 24 6
6 3
2.687 3.800 4.654 2.048 2.310 3.434
2.59-24.87 2.54-24.87 2.53-24.87 2.67-24.66 3.03-20.34 3.15-20.20
1.82-2.70 2.82-3.78 3.77-4.62 0.73-2.08 0.96-2.34 2.60-3.62
Chemisorption was carried out, after reducing at 723 K for 1 h and evacuation for 1 h at the same temperature, by measuring the H2 adsorption at room temperature between 10 and 40 kPa and extrapolating to 0 E a . H/Rh values of 1.49 and 1.65 were obtained for the C573 R673 and C673 R673 samples, respectively. Using the EXAFS-chemisorption calibration of Kip et al.,15 this amounted to fractions exposed Rh of 0.90 and 0.93, respectively. EXAFS. Transmission EXAFS spectra of the calcined and of the calcined and reduced Rh/SiO2 samples were measured at the Wiggler station 9.2 at the Synchrotron Radiation Source in Daresbury, England, with a ring energy of 2 GeV and ring currents between 120 and 250 mA. The Si(220) double-crystal monochromator was detuned to 50% intensity to diminish the influence of higher harmonics. Catalyst samples were pressed in wafers, which were mounted in an in situ cell and heated during pretreatment and cooled during the EXAFS measurement.16 Measurements were always performed under helium at liquid N2 temperature. The EXAFS analysis followed standard procedures8 with Fourier transformations,l 7 the difference EXAFS technique, and phase- and amplitude-corrected Fourier transformations.l8 Since a single mathematical fit is not always unambiguous, we have fitted kl- as well as k3- weighted spectra in k space and took care that good fits were obtained for the kl- and k3-weighted Fourier transforms as well. Estimates of the coordination numbers N and distances R, and of the Debye-Waller factors A d and the zero energy EO (both relative to their respective reference values), were obtained by single-shell fitting, while keeping the maximum number of free parameters below the theoretical limit allowed by the Brillioun theorem. Final values were obtained by a fit of the complete k range. The trustworthiness of the parameters was checked by looking at the phases of the separate contributions in the difference EXAFS files and by verifying that the values obtained made sense chemically. The coordination numbers N determined in the EXAFS analysis were corrected for the difference in coordination distances r and r,f between sample and reference compound, respectively, with the formula Nc = N exp(2(r - rref)/A),in which the mean free electron path 1 was taken equal to 5 8,. Results Reference Compounds. Rh-Rh, Rh-C1, and Rh-0 amplitude and phase functions needed in the analysis of the EXAFS spectra of the catalysts were determined from Rh foil, waterfree RhCl3, and Rh2O3. The crystallographic data of the reference compounds, the shells analyzed, and the k and R Fourier transform ranges used are given in Table 1. The first-, second-, and third-shell Rh-Rh contributions in the EXAFS spectrum of Rh foil are well separated from each other and from other contributions and can therefore easily be analyzed. The same holds for the Rh-0 contribution in the spectrum of Rh203 and for the Rh-C1 and Rh-Rh contributions in the spectrum of RhCl3 (Figure 1). The second- and third-shell Rh-Rh contributions were used as references in the analysis of the
TABLE 2: Results of the Fit of the EXAFS Spectrum of the C673 R673 Catalyst (0.73 wt 70 Rh/SiOz, Calcined and Reduced at 673 KY backscatter N R, 8, Au2,AZ AEo, eV Ncb
c1
0.5 1.3 4.5 1.8 3.1
0 Rhl Rh2 Rh3
2.22 2.73 2.71 3.81 4.66
0.0007
-0.0040 0.0023 0.0038 0.0030
2.1 2.2 -4.5 -6.0 2.4
0.5 1.7 4.5 1.8 3.1
Fourier transformation range 2.72-19.88 kl, back transformation range 1.38-4.80 A, fit range 3.22-19.38 A-', number of allowed fitting parameters 35.2, variance of fit 0.0048. Corrected for the difference in distance with the reference shell.
corresponding shells of the reduced Rh/SiOz catalysts. The RhRh contribution in the spectrum of RhC13 was used to analyse the Rh3+-Rh3+contribution in the spectra of the calcined Rh/ Si02 catal sts, because the two Rh3+-Rh3+ contributions at 2.71 and 2.99 in the spectrum of RhzO3 overlapped too much with each other and with neighboring contributions to enable their use as reliable references. C673 R673. The measured data allowed a Fourier transformation of the EXAFS spectrum between 2.72 and 19.88 A-1 (Figure ZA), while an inverse Fourier transformation was performed in the 1.38-4.80 8, range. Rh-Rh contributions up to the fourth shell were clearly visible in the kl- and k3weighted Fourier transforms (Figure 2B,C). In addition, the kl-weighted Fourier transform showed a contribution between 1 and 2 8, (Figure 2B). Its lower intensity in the k3-weighted Fourier transform (Figure 2C) points to a contribution from a light backscattering element. In cases where small metal particles with small Rh-Rh coordination numbers were studied, often a Rh-02- metal-support contribution has been observed around 2.7 8,.8.19 The contribution between 1 and 2 A in our EXAFS spectrum (Figure 2B) could, however, not satisfactorily be fitted with such a Rh-02- contribution only. When performing a single shell fit between 1 and 3 A, addition of a Rh-02- contribution at about 2.05 A (as observed in rhodium oxide) did not lead to a good fit either. Addition of a Rh-Cl contribution, on the other hand, quickly led to convergence of the 1-3 8, shell. The 3-4.8 A range was first separately fitted for the second- and third-shell Rh-Rh contributions. Finally, a total fit was made in the whole 1.38-4.8 8, range. This led to a good fit of the EXAFS spectrum and to good fits of both the kl- and k3-weighted Fourier transforms (Figure 3). The resulting parameters (Table 2) are chemically and physically acceptable. The assignment of the difference EXAFS, obtained after subtraction of Rh-Rh and Rh-0 contributions from the measured EXAFS spectrum, to a Rh-Cl contribution was checked by a phase correction of its Fourier transform. When the difference spectrum was phase corrected for a C1 backscatterer, the maximum in the resulting imaginary Fourier transform indeed coincided with that of the absolute part of the Fourier transform.18 The absolute parts of the Fourier transforms of the difference spectra of the separate Rh (first shell), 0, and C1 contributions are presented in Figure 4. 0 and Rh contributions overlap strongly, while the C1 contribution is shifted to lower coordination distance. The analysis of the second and third Rh-Rh contributions was done with reference parameters from the respective RhRh contributions of Rh foil. The resulting fits were less good than those of the Rh first shell, 0, and C1 contributions, and therefore the coordination numbers for the second- and thirdshell Rh-Rh contributions are less accurate than the other coordination numbers (Table 2). C673. The EXAFS spectrum of the C673 sample could be Fourier transformed in the 2.78-19.63 A-1 k range. Back
K
J. Phys. Chem., Vol. 98, No. 39, 1994 9861
Chemisorption of C1 on Rh/SiOz Determined by EXAFS
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R [AI k [A-'] Figure 1. EXAFS spectra and &weighted Fourier transforms of the refe:rence compounds: (A) Rh foil, (B) Rhc13, and (C) RhzO3. Fourier transformation was done in the 0.69-3.19 A range (Figure 5). Three contributions could rather easily be analyzed by single-shell analysis of the low and high R ranges, one Rh3+02-contribution at 2.01 A and two Rh3+-Rh3+ contributions at 2.78 and 3.07 A, respectively. The Rh-Rh contributions at 2.78 and 3.07 A had only small coordination numbers of 0.2, but because of the excellent signal-to-noise quality of the original data, they could be easily distinguished in the higher R range in the total fit. A fourth contribution (around 2.0 A in Figure 5C) was separated by a sharp indentation from the Rh3+-
02contribution at 2.01 A (around 1.6 in the not yet phasecorrected k3-weighted Fourier transform in Figure 5C). This points to interference between the Rh3+-02- contribution and a contribution with a similar coordination distance, but with an opposite phase. A good fit could be obtained by adding a Rh3+-Cl- contribution at 2.32 8, and the difference signal indeed peaked positively when it was phase corrected for a RhC1 Contribution. After subtraction of the Rh-0, Rh-Rh, Rh-Rh, and RhC1 contributions from the EXAFS spectrum, a residual spectrum
9868 J. Phys. Chem., Vol. 98, No. 39, 1994
Gloor and Prins
2
6
10
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k [k']
Figure 2. (C) k3-weighted Fourier transform. TABLE 3: Results of the Fit of the EXAFS Spectrum of the C673 Catalyst (0.73 wt % Rh/SiOz, Calcined at 673 KP backscatter N R. A Aa2.A2 AEn.eV N,b 0 4.9 2.01 0.0012 2.9 4.8 0.0012 -1.9 1.2 c1 1.2 2.32 -7.2 0.3 0.3 2.78 -0.0012 Rh,3+ -0.0025 2.8 0.2 Rh*3+ 0.2 3.07 X' 0.8 2.54 0.0076 -6.4 0.9
Fourier transformation range 2.78- 19.63A-1, back transformation range 0.69-3.19 A, fit range 3.28-19.13 A-', number of allowed fitting parameters 25.2,variance of fit 0.0002. Corrected for the difference in distance with the reference shell. Unknown contribution. remained which showed still another EXAFS contribution. This residual spectrum could be fitted with a Rh3+-Cl- contribution at 2.54 A. The final fits are presented in Figure 6 and the final parameters in Table 3. The absolute parts of the Fourier transforms of the difference spectra of the separate Rh-0 and Rh-Cl contributions are presented in Figure 7. C573 R673. The data of the C573 R673 sample could be analyzed in the 2.73-18.41 A k range and back Fourier transformed in the 1.38-4.80 A R range. The k3-weighted spectrum and the k'- and k3-weighted Fourier transforms looked very much like those of the C673 R673 sample. Use of the results of the latter sample as starting values in the analysis of the C573 R673 data led to a fast convergence. The final fit
and TABLE 4: Results of the Fit of the EXAFS Spectrum of the C573 R673 Catalyst (0.73 wt % Rh/SiOz, Calcined at 573 K and Reduced at 673 K)" backscatter N R,A Aa2,A2 AEo,eV N: c1 0.9 2.29 0.0083 -4.3 0.9 0 1.7 2.79 -0.0020 0.8 2.3 Rhl 3.6 2.70 0.0030 -3.3 3.6 Rh2 0.8 3.81 0.0014 -4.8 0.8 Rh3 3.8 4.66 0.0031 -2.4 3.8 a Fourier transformation range 2.73- 18.41A-l, back transformation range 1.38-4.80 A, fit range 3.23-17.91A-1, number of allowed fitting parameters 32.2,variance of fit 0.0046. Corrected for the difference in distance with the reference shell.
values (Table 4) are close to those of the C673 R673 sample (Table 2). Differences are mainly in the Rh-C1, Rh-0, and Rh-Rh (first shell) distances and coordination numbers. The Rh-Rh coordination number was smaller, and the Rh-Cl and Rh-0 coordination numbers were higher than in the C673 R673 sample. Also in the C573 R673 sample, the Rh-C1 contribution could be confirmed by a phase-corrected Fourier transformation analysis. (2573. The EXAFS spectrum of the C573 sample could be analyzed in the 2.75-19.56 A range and back Fourier transformed in the 0.72-3.16 A range. As the similarity between the EXAFS spectrum and its Fourier transforms and those of
J. Phys. Chem., Vol. 98, No. 39, 1994 9869
Chemisorption of C1 on Rh/SiOz Determined by EXAFS 0.15
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k [A-'1 Figure 3. Fourier-filtered EXAFS spectra of the C673 R673 sample (0.73 wt % Rh/Si02, calcined and reduced at 673 K): (A) klX (B) kl-weighted , (D) k3-weighted Fourier transform; (-) Fourier-filtered experimental data, (-) fit. Fourier transform, (C) k 3 ~ and
0.15
the C673 sample already indicated, the C573 spectra could be fitted with parameters which are similar to those of the C673 sample. The main difference was the significantly lower coordination number of the Rh3+-02- contribution (Table 5 ) .
,
0.12
Discussion C673 R673. The analysis of the EXAFS spectrum of the catalyst which had been calcined and reduced at 673 K resulted in an average Rh-Rh coordination number N u = 4.5. This number is consistent with the H/Rh = 1.65 chemisorption value, according to the model of Kip et al.15 If the rhodium particles are uniform and have a half-spherical shape, the value of N u = 4.5 means that the Rh particles consist of 8-10 metal atoms (Figure 8). The sum of the coordination numbers of the first three shells of the C673 R673 catalyst is N1 Nz N3 = 9.4 (Table 2). In view of the presence of a nonzero fourth-shell contribution (Figure 2A), the sum of the coordination numbers of all Rh-Rh shells is certainly equal to or larger than 10. This sum is equal to the number of metal atoms in the average Rh particle minus one, because in a metal particle containing N atoms, the total number of atoms other than the specific atom which undergoes photoionization is equal to N - 1. Thus, = N - 1. Taking into account the uncertainty in the determina-
+ +
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R [AI Figure 4. Absolute parts of the kl-weighted Fourier transforms of the difference spectra of the separate Rh-Rh (fist shell), Rh-0, and RhC1 contributions of the C673 R673 sample (0.73 wt % Rh/Si02, calcined and reduced at 673 K): (-) Fourier-transformed difference spectrum; (-) fit.
Gloor and Prins
9870 J. Phys. Chem., Vol. 98, No. 39, 1994
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R [AI Figure 5. EXAFS of the C673 catalyst (0.73 wt % Rh/SiOz, calcined at 673 K): (A) k3x, (B) kl-weighted Fourier transform, and (C) k3-weighted Fourier transform.
TABLE 5: Results of the Fit of the EXAFS Spectrum of the C573 Catalyst (0.73 wt % RWSiOz, Calcined at 573 K)" backscatter N R,A Aa2,W2 AEo,eV Ncb 0 c1 Rhl3f
3.2 1.4 0.3 0.1
2.01 2.31 2.71 3.08
0.0012 0.0023 0.0010 -0.0033
1.8 1.6 -8.4 -3.0
3.2 1.4 0.3 0.1
Rh2" Fourier transfFrmation range 2.75- 19.56 k', back transformation number of allowed fitting range 0.72-3.16 A, fit range 3.25-19.06 k', parameters 24.6, variance of fit 0.0042. Corrected for the difference in distance with the reference shell. tion of the higher shell coordination numbers (see below), we therefore conclude that the Rh particles contain 8- 12 metal atoms. The Rh-0 contribution at 2.73 A is in accordance with the sum of the rhodium metal and oxygen anion radii of 2.74 A. Following the analysis by van Zon et the Rh-Rh and Rh-0 coordination numbers (which are averaged over all Rh atoms in the Rh particle) can be used to calculate the true coordination of the rhodium atoms at the metal-support interface by neighboring oxygen ions in the support. With Nm = 4.5 and NO = 1.7 and again a half-spherical shape for the Rh particles, it is calculated that every average Rh atom in the interface is in direct contact with 2.5 oxygen anions. This is a realistic value, in between the values of 2, 3, or 4 for closed packed (1 1l), (1lo), or (100) oxygen surfaces, respectively.
The measured Rh-Rh distance of 2.71 A is 0.03 A longer than the value in bulk Rh and significantly longer than Rh-Rh values observed in Rh/A1203 catalyst^.^^'^ The Rh-Rh distance in Rh/Al2O3 catalysts was smaller than the 2.68 A bulk value because of inward relaxation of the surface Rh atoms. Since the Rh-Rh and Rh-0 coordination numbers of the C673 R673 Rh/Si02 catalyst were close to those of the Rh/Al2O3 catal y s t ~ ,the ~ , Rh ~ ~particle size of these Rh/Si02 and MA1203 catalysts must be similar. As a consequence, also an inward relaxation and a short Rh-Rh distance was to be expected for the Rh/Si02 catalyst. The explanation for the outward relaxation by 0.03 A must be the presence of C1 atoms in contact with Rh atoms (Table 2 ) . The most logic position for these C1 atoms is on top of the Rh particles, and the in- or outward relaxation of the Rh surface atoms is then determined by the balance between Rh-Rh and Rh-C1 forces. A cancellation of inward relaxation, or even an opposite outward relaxation, of metal surface atoms by adsorbed atoms has been observed before.20 That this outward relaxation was not observed in reduced Rh/A1203 catalysts prepared from RhCl3 is due to the strong interaction of C1 with the A1203 surface. Noble metals on A1203 catalysts prepared from metal chlorides contain substantial amounts of chlorine after reduction, but the chlorine is trapped on the A1203 surface. As a consequence, the Rh particles on A1203 are C1free and contract inwardly.
J. Phys. Chem., Vol. 98, No. 39, 1994 9871
Chemisorption of C1 on WSiOz Determined by EXAFS
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The Rh-Cl coordination number was determined to be 0.5. Like all coordination numbers determined by EXAFS, this number is also an average, in this case over all Rh atoms. Furthermore, the C1 atoms can be located in different positions on a Rh surface. They can be singly coordinated on top of Rh atoms, coordinated to two Rh atoms in a bridge position, or coordinated to 3 or 4 Rh atoms in the 3- or 4-fold hollow positions, respectively. With N(Rh-Cl) = 0.5, this means that the atomic Cl/Rh ratio may vary between 0.5 and 0.125. The Rh-C1 distance of 2.22 8, can be used to estimate which position the C1 atoms occupy on the Rh surface. Mitchell derived a formula for a metal-adatom bond distance D as a function of the difference in electronegativities between metal and adatom and of the adatom valency v and coordination number n.21 For the couple Rh-Cl this formula gives the expression D(v,n) = R,,
+ R,
- 0.06 &, - x,)
- 0.8 log(v + 2a)/n
With single bond radii of RCI = 0.99 8, and R , = 1.25 A, electronegativities x, = 1.45 and xc1 = 2.83,22and a = 0.5, one obtains D(v,n) = 2.16 - 0.8 log(v
+ l)/n
Mitchell demonstrated that the D(v,n) formula leads to physically
meaningful distances when normal, or slightly higher than normal, valencies are used. In our case, the observed Rh-C1 distance of 2.22 8, would then together with the normal valency of v = 1 lead to n = 2.4. The average C1 atom would then be in direct contact with 2.4 Rh neighbors, which suggests that the bridge position or the 3-fold hollow site is more likely than the on-top position. The data obtained from the analysis of the second- and thirdshell Rh-Rh contributions should be considered semiquantitative, rather than quantitative, because no trustworthy uncoupling of the coordination numbers and the Debye-Waller factors could be obtained. It is well-known that lower or higher coordination numbers can be compensated by lower or higher Debye-Waller factors, respectively, and quite good fits of the EXAFS spectrum can in such cases be obtained over extended ranges of N and Aa2. The influence of the coordination number on the EXAFS spectrum is independent of the k range, but the Debye-Waller factor ha2influences the low and high k regions in a different way. This difference in k dependence can be used to uncouple Nand Aa2. By plotting the coordination numbers Nkl and Np (obtained from kl- and k3weighted Fourier transform analysis, respectively) as a function of Aa2, one obtains a crossing point which gives unique values for the N-Aa2 pair. With these values a good fit for both k' and k3 transforms is
Gloor and Prins
9872 J. Phys. Chem.. Vol. 98, No. 39, 1994 0.16
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t;: 0.08 0.04
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Figure 1. Absolute parts of the k’-weighted Fourier transforms of the difference spectra of the separate Rl-0. Rl-CI. and Rl-X contributions of the C673 catalyst (0.73 wt 9% RhISi02. calcined at 673 K): (-) Fourier-transformed difference SpeCtNm; (-) fit.
A
B
Figure 8. Ten-atom Rh particle on idealized Si02 surfaces: (A) ( I I I ) Si02 surface and (B)( I IO) Si02 surface.
obtained.[’ For the second and third Rh-Rh shells. however, this technique failed, because the N,t(Au2) line determined from the k’-weighted fit stayed very close to the Ni3(Ao2) line determined from the k)-weighted fit over a large range. Therefore. it is not possible to say which N-Ad combination is the best. A reason for this problem may be that second- and third-shell Rh-02- contributions (in which Rh is a metal atom and 02-an oxygen anion in the support, both near the metasupport interface, e.g. AC, AD, and EF in Figure 8) have about the same coordination distances as second- and third-shell RhRh contributions, since the radius of a rhodium atom is not much different from that of a 02-ion. Although the contributions of such Rh-02- contributions will not peak at exactly the same distance as equivalent Rh-Rh contributions (because of differences in phases), and although oxygen is a weaker backscatterer than Rh. the Rh-02- contributionswill hinder the analysis
of the Rh-Rh contributions. For that reason we refrain from a detailed analysis of the coordination numbers of the secondand third-shell Rh-Rh contributions. Suffice it to say that they are not in contradiction with the results of the first-shell RhRh, Rh-0. and Rh-CI contributions and confirm the presence of small Rh particles. C673. The analysis of the EXAFS spectrum of the C673 catalyst showed that the average Rh3+ ion has 4.8 oxygen and 1.2 chlorine neighbors after calcination of the [Rh(NH~)sCIICIz/ Si02 sample at 673 K. As to be expected for a Rh3+ ion. the coordination around the Rh3+ cation is octahedral. It is impossible to say if every Rh3+ ion is coordinated by at least one CI- ion or that 80% of the Rh3+ ions are octahedrally surrounded by oxygen anions and 20% of the Rh3+ ions by chloride anions, since the Rh3+-Oz- and Rh3+-CI- distances in both situations will hardly differ. Our results demonstrate that a normal coordination number of 6 is obtained when a careful EXAFS analysis is carried out for the first shell of a noble metal cation with oxygen and chlorine ligands. Studies of R,RRe, and RRh catalysts catalysts had indicated that 0 CI coordination numbers larger than 6 (between 7 and 9) could be obtained after calcination of HzPtC16 on A1201 cataly~ts.9.~~ These EXAFS analyses were carried out over a more limited k range than our analysis, however. It is possible that the combination of a strong interference between two partly overlapping CI and 0 contributions, with opposite phases, and a short k transformation range gives rise to a too high total coordination number. The Rh-Rh contributions at 2.78 and 3.07 8, point to the presence of R h 2 0 3 . The low coordination numbers suggest that either small RhzO3 particles are present (or an amorphous type of RhzO,) or that only a small part of the Rh is present as R h 2 0 3 and the major part as isolated Rh’+ ions. This would explain why the Rh-0 distance is shorter than in RhzO,. A fifth contribution in the EXAFS spectrum of the C673 catalyst at 2.54 8, was analyzed as a Rh-CI contribution. The maximum number of free parameters which can be fitted in the C673 spectrum is 2AkARln = 25.2. The analysis of five shells with four parameters each (N. R,A d , 4)is thus allowed. and there is no doubt that a contribution around 2.5 8, is present. The analysis with a CI reference led to convergence, while an attempt to analyse the 2.5 8, contribution with a 0 reference did not converge. This indicates that the phase of this 2.5 8, contribution is chlorine-like and that the corresponding backscattering atom could be CI or Si. A Rh-CI distance of 2.5 8, does not seem logic. First, such a distance is not present in RhCI3 and [Rh(NH,)5CI]Clz, compounds which could have been formed or be present on the Si02 surface. Second, with an octahedral coordination of five oxygen ions at 2.01 8, and one chlorine ion at 2.32 8, around the Rh’+ ions, there is no space for an additional chlorine ion at 2.5 8,. We therefore suggest that the contribution belongs to a Si atom at about 2.5 8, from the Rh ion. This is just the distance one would expect for a Rh3+ ion bonded to a Si4+ ion through two bridging 0 ions. Since. because of a lacking Rh-Si reference, we could not analyze this contribution with a Si reference. the coordination number and distance quoted in Table 3 should be used with caution. C573. The results of the EXAFS analysis for the C573 catalyst are hardly different from those of the C613 catalyst, as a comparison of the EXAFS spectra already suggested. Only the Rh-0 coordination number is significantly higher for the sample calcined at the higher temperature. while the Rh-CI coordination number might be somewhat smaller. The former might be due to the formation of a more ordered strucNre and
+
Chemisorption of C1 on Rh/SiOz Determined by EXAFS the latter to more chlorine removal during the calcination at higher temperature. C573 R673. A comparison of the EXAFS results for the C573 R673 and C673 R673 catalysts shows that the Rh-Rh coordination numbers are smaller and the Rh-0 and Rh-C1 coordination numbers are larger for the C573 R673 catalyst. This means that the Rh metal particles of the C573 R673 catalyst are smaller than those of the C673 R673 catalyst. The obvious conclusion would be that the higher calcination temperature induced a growth of the rhodium oxide particles. This is not directly clear from the EXAFS results of the calcined samples, however, since the Rh3+-Rh3+ coordination numbers are equal for both samples. Although the metal particles of the C573 R673 catalyst are smaller than those of the C673 R673 catalyst, the H2 chemisorption of the C573 R673 catalyst is smaller than that of the C673 R673 catalyst. The higher coverage with C1 of the Rh particles in the C573 R673 catalyst probably hinders the chemisorption of H2. As in the C673 catalyst, the presence of C1 at the Rh surface in the C573 R673 catalyst leads to an outward relaxation and to larger Rh-Rh distances than in bulk Rh. An analysis according to Mitchell,21 as done for the C673 R673 catalyst, leads to n = 2.9. This means that every C1 atom in the C573 R673 catalyst is in direct contact with 2.9 Rh neighbors and suggests that the C1 atoms occupy 3-fold hollow sites. EXAFS studies of adsorption of C1 and S on stepped Ni surfaces indicated that the C1 and S atoms occupied 3-fold hollow sites on the (1 11) terraces and 4-fold hollow sites at the Apparently C1 and S prefer the highest coordination sites. Conclusions A highly dispersed Rh/SiOz catalyst could be prepared via ion exchange of [Rh(NH3)5Cl]Clz on SiOz, calcination, and reduction at 673 K. After calcination each Rh ion had retained one C1 ion and had obtained five oxygen anions in its first coordination shell. After reduction still chlorine atoms in contact with rhodium were observed. They could be assigned to metallic Rh atoms, indicating that even after reduction at 673 K for 1 h, the Rh surface is not free of chlorine yet. These chlorine atoms caused an outward relaxation of the Rh surface atoms. The low Rh-Rh coordination numbers, and the
J. Phys. Chem., Vol. 98, No. 39, 1994 9873
substantial Rh-0 coordination numbers due to Rh atoms in contact with support oxygen ions, demonstrated that the Rh metal particles were very small; on average they contained 10 Rh atoms. References and Notes (1) Ponec, V. Stud. Surf. Sci. Catal. 1991,64, 117. 121 Nonnemann. L. E. Y.: Bastein. A. G.T. M.: Ponec. V.: Burch. R. Appl. Catal. 1990,62, L23. (3) Burch, R.; Petch, M. I. Auul. Catal. 1992,A88, 61. (4) Gloor, A. P.; Prins, R. R k k Trav. Chim., to be published. (5) Jackson, S. D.; Brandreth, B. J.; Winstanley, D. J . Chem. SOC., Faraday Trans. 1 1988,84, 1741. (6) Gloor, A. P.; Prins, R. Cafal.Left., to be published. (7) van Zon, J. B. A. D.; Koningsberger, D. C.; van’t Blik, H. F. J.; Prins, R.; Sayers, D. E. J . Chem. Phys. 1984,80, 3914. (8) van Zon, J. B. A. D.; Koningsberger, D. C.; van’t Blik, H. F. J.; Sayers, D. E. J. Chem. Phys. 1985,82, 5742. (9) Lagarde, P.; Murata, T.; Vlaic, G.; Freund, E.; Dexpert, H.; Bournonville. J. P. J. Catal. 1983. 84. 333. (10) Kampers, F. W. H.; Koningsberger, D. C. Faraday Discuss. SOC. 1990,89, 137. (11) Martens, J. H. A,; Prins, R.; Zandbergen, H.; Koningsberger, D. C. J . Phys. Chem. 1988,92, 1903. (12) Moller, K.; Koningsberger, D. C.; Bein, T. J . Phys. Chem. 1989, 93, 616. (13) Nandi, R. K.; Molinaro, F.; Tang, C.; Cohen, J. B.; Butt, J. B.; Burwell, Jr., R. L. J. Catal. 1982,78, 289. (14) Mizushima, T.; Tohji, K.; Udagawa, Y.; Ueno, A. J . Phys. Chem. 1990,94, 4980. (15) Kip, B. J.; Duivenvoorden, F. B. M.; Koningsberger, D. C.; Prins, R. J . Catal. 1987,105, 26. (16) Kampers, F. W. H.; Maas, T. M. J.; van Grondelle, J.; Brinkgreve, P.; Koningsberger, D. C. Rev. Sci. Insfrum. 1989,60, 2635. (17) Kampers, F. W. H.; Engelen, C. W. R.; van Hooff, J. H. C.; Koningsberger, D. C. J . Phys. Chem. 1990,94, 8574. (18) van Zon, J. B. A. D.; Koningsberger, D. C.; Prins, R.; Sayers, D. E. EXAFS and Near Edge Structure III; Hodgson, K. O., Hedman, B., Penner-Hahn, J. E., Eds.; Springer-Verlag: Berlin, 1984; p 89. (19) van’t Blik, H. F. J.; van Zon, J. B. A. D.; Koningsberger, D. C.; Prins, R. J . Mol. Catal. 1984,25, 379. (20)Somorjai, G.A,; van Hove, M. A. Prog. Surf. Sci. 1989,30,201. (21) Mitchell, K. A. R. Surf. Sci. 1985,149, 93. (22) Pauling, L. The Nature of the Chemical Bond; Come11 University Press: Ithaca, NY, 1960; p 224. (23) Bazin. D.: Dexoert. H.: Laearde. P.: Boumonville. J. P. J . Cafal. 1988,110,209. (241 Ohta, T. X-Rav Absomtion Fine Structure: Ellis Honvood: New York, ‘1991; p 248. (25) Ishii, H.; Asakura, K.; Namba, H.; Ohta, T.; Kitajima, Y.; Kosugi, N.; Kuroda, H. Jpn. J. Appl. Phys. 1993,Suppl. 32-2, 365, 368.
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