6926
J. Phys. Chem. 1995, 99, 6926-6936
Highly Dispersed MgO-Supported Model Pd-Mo Catalysts Prepared from Bimetallic Clusters S. Kawi,? 0. Alexeev: M. Shelef,' and B. C. Gates*9+ Department of Chemical Engineering and Materials Science, University of Califomia, Davis, Califomia 95616, and Scientific Research Stafi Ford Motor Co., Dearborn, Michigan 48121 Received: August 30, 1994; In Final Form: February 21, 1995@
In attempts to prepare highly dispersed supported palladium catalysts stabilized by molybdenum, an organometallic precursor with Pd-Mo bonds, [ P ~ ~ M O ~ ( C P ) Z ( C O > was ~(PP adsorbed ~ ~ ) ~ ~ on , MgO. The precursor Was adsorbed intact, as shown by infrared spectroscopy. For comparison, other samples were prepared from an organopalladium precursor, [PdClz(PhCN)2], and from a mixture of [PdC12(PhCN)2] [Mo(C0)6]. Each supported sample was treated in H2 at various temperatures to form metallic palladium. The palladium dispersions were characterized by chemisorption of H2, CO, and 0 2 ; transmission electron microscopy; temperature-programmed desorption of adsorbed CO; and extended X-ray absorption fine structure (EXAFS) spectroscopy, both at the Pd K edge and the Mo K edge. The data show that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form, with the supported clusters being smaller than about 10 A in average diameter. These clusters have a low capacity for chemisorption of hydrogen and of CO. They are stabilized by the oxophilic molybdenum, which exists preferentially at the interface between the palladium clusters and the metal oxide support. The sample prepared from the two monometallic precursors was characterized by larger palladium particles and by weaker PdMo interactions. The results suggest that the Pd-Mo interactions in the bimetallic precursor were the cause for the high dispersion of palladium in the reduced catalyst.
+
Introduction Selective catalytic reduction of NO, is one of the most important reactions in air pollution control, being applied on an enormous scale to emissions from motor vehicles and stationary fossil fuel power sources. Three-way catalysts, containing supported platinum, palladium, and rhodium, are widely used in automobile exhaust converters, allowing efficient simultaneous abatement of CO, hydrocarbons, and NO,.'-' Platinum and palladium catalyze mainly the oxidation of CO and hydrocarbons to give C02 and water, and rhodium is used for the selective reduction of NO, to give N:! and not NH3.6 Reports of searches for replacements for the expensive rhodium8-l0 include the observation",'2 that >20% loadings of molybdenum oxide incorporated with palladium give catalysts that are active and selective for NO, reduction to give N2 with minimal NH3 formation. The authors",I2 associated the activity and selectivity with Pd-Mo interactions. However, the potential advantages of the inexpensive Pd-Mo catalysts are offset by a lack of stability, as molybdenum oxide is volatile at temperatures >700 "C in the presence of steam. The authors postulated that losses of molybdenum oxide might be reduced by use of low loadings that would allow stable interactions with the support. However, in the catalysts that have been investigated, the selectivity for NO, reduction to give NZ decreased sharply as the molybdenum loading decreased, apparently because a large excess of molybdenum was necessary to allow significant Pd-Mo interactions.*-I2 Because oxophilic group 6 metals have been found to be preferentially located at the interface between metal oxide supports and dispersed group 8 metals in catalysts prepared from clusters having the two metals in close proximity to each other,I3 +
@
University of California. Ford Motor Co. Abstract published in Advance ACS Abstracts, April 15, 1995.
0022-365419512099-6926$09.00/0
we chose to use an organometallic Mo-Pd cluster to prepare supported catalysts. The premise is that, if the cluster frame is stable enough, the resultant supported catalysts might have the two metals in close contact with each other, with the oxophilic metal interacting strongly with the support. Thus our objective was to prepare such catalysts from [Pd2Moz(Cp)2(C0)6(PPh3)21 and to understand the nature of the Pd-Mo interactions, with the ultimate goal of being able to design supported catalysts with high activities, selectivities, and stabilities for NO, reduction. For comparison, catalysts were also prepared from a mixture of monometallic precursors, [PdC12(PhCN)z] and [Mo(co)6], and from [PdC12(PhCN)2] alone.
Experimental Methods Materials and Catalyst Synthesis. Preparation of supported catalysts from organometallic precursors was carried out in the near absence of air with samples in a Braun MB-15OM drybox purged with N2 that recirculated through 0 2 - and moisturescavenging traps or on a Schlenk vacuum line that was purged with N2 (Matheson, 99.999%). The drybox was equipped with 0 2 and moisture detectors showing that the concentrations were -= 1 ppm. Reagent grade pentane was dried over sodium benzophenone ketyl. He and H2 (Matheson, 99.999%) were purified by passage through traps containing Cu20 and activated zeolite to remove traces of 0 2 and moisture, respectively. [PdClz(PhCN)2] and [Mo(C0)6] (Strem) were used without purification. [Pd2Mo2(Cp)2(C0)6(PPh3)2] was prepared by a literature methodI4 and the product identified by infrared spectroscopy. Partially dehydroxylated MgO powder was prepared by calcining MgO (MX-65-1, MCB Reagents) in flowing 0 2 (Matheson Extra Dry Grade) as the sample was heated to 550 "C at a rate of approximately 3 "Clmin and held for 2 h. The MgO was then evacuated at approximately Torr, held at 550 "C for 14 h, cooled under vacuum to room 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 18, 1995 6927
Highly Dispersed MgO-Supported Model Pd-Mo Catalysts temperature, and removed from the glassware in the drybox. The MgO surface area, measured by N2 adsorption, was 47 m2/ g. To prepare a supported sample, [PdzMoz(Cp)z(C0)6(PPh3)~1 (about 200 mg) was slurried with calcined MgO powder (2.0 g) in freshly distilled pentane (50 mL). The mixture was stirred under N2 for 24 h at room temperature, followed by drying under vacuum for 5 h at room temperature. Similar procedures were used to prepare supported samples from [Mo(C0)6], [PdClz(PhCN)2], and mixtures of [MO(C0)6] and [PdClz(PhCN)z].The samples contained 1.O wt % Pd; that made from [PdzMoz(Cp)2(C0)6(PPh3)2] contained 0.9 wt % Mo, and that made from the two monometallic precursors contained 1.0 wt % Mo. Infrared Spectroscopy. Transmission infrared spectra of the samples were recorded with a Brucker IFS-66V FTIR spectrometer with a resolution of 4 cm-'. Samples were pressed into semitransparent wafers in the drybox, mounted in the cell, and scanned 64 or more times. Transmission Electron Microscopy (TEM). TEM experiments were carried out with a Zeiss EM-109 instrument, typically operated at 100 keV. The samples were prepared by reduction in flowing HZat 400 or 500 "C and then loaded into vials capped with rubber septa. Particles of the powder in hexane were sonicated, and a drop of the slurry was placed on a holey carbon film on a copper grid, which was mounted on a sample holder. The hexane was removed by evacuation prior to recording of the micrographs. Samples came in contact with air for about 10 min. Hz, CO, and 02 Chemisorption Measurements. Chemisorption measurements were performed on a RMX-100 multifunctional catalyst testing and characterization instrument (Advanced Scientific Designs, Inc.). HZformed by electrolysis of water in a Balston 75-33 hydrogen generator was purified by passage through traps containing Cu20 and 4A zeolite particles. The samples (0.2 to 0.4 g) in the drybox were loaded into a quartz tube and connected to the system without exposure to air. The HZ flow rate was set at 99 mL (NTP)/min. The sample in flowing H2 was heated to the desired temperature at a rate of 10 "C/min, held for 2 h, and evacuated for 30 min at Torr and the desired temperature. After the sample had been cooled to room temperature, the first isotherm was measured immediately. After the first set of measurements, the sample was evacuated for 30 min at room temperature, and the procedure was repeated. The amount of chemisorbed hydrogen, CO, or oxygen was obtained by extrapolating the linear highpressure part of each isotherm to zero pressure and taking the difference between the two isotherms. To investigate the influence of 0 2 treatment, each sample was treated first in H2 at 500 "C and then in flowing 0 2 at 200, 300, 400, and 500 "C for 40 min. Following each of these treatments, the sample was again treated in HZat 500 "C for 1 h, and the hydrogen and CO chemisorption experiments were repeated. Accuracy in the determination of the atomic ratios H:Pd, CO:Pd, and 0:Pd values was fO.O1. Temperature-Programmed Desorption (TPD). Samples that had been characterized in chemisorption experiments were further characterized by TPD of CO in the RMX-100 instrument. CO was adsorbed on the samples at room temperature and 80 Torr, and then the samples were evacuated and heated at a rate of 10 "C/min to 900 "C. Desorbed gases were monitored by mass spectrometry. Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy. The EXAFS experiments were performed on X-ray beamline X-l1A at the National Synchrotron Light Source at Brookhaven National Laboratory, Upton, Long Island, NY. The ring energy was 2.5 GeV; the ring current was 80-220
TABLE 1: Crystallographic Data Characterizing the Reference Compounds and Fourier Transform Ranges Used in the EXAFS Analysie KFF sample calculation Pd foil PdO K2M004 Mofoil PdMo MoPd
crystallographic data shell N R, 8, Pd-Pd Pd-0 Mo-0 Mo-Mo Pd-Mo Mo-Pd
12 2.74 4 2.02 4 1.76 8 2.72 8 2.72 12 2.72
Fourier Ak, 4.22-17.9 3.86-13.2 3.51-16.9 3.56-18.9 3.45-18.7 2.88-18.8
transform Ar, 8, 1.90-2.90 0.80-2.07 0.53-1.50 1.88-2.86 1.99-2.86 1.88-2.88
n 3 1 1 3 3 3
Notation: N , coordination number for absorber-backscatterer pair; R, distance; Ak, limits used for forward Fourier transformation ( k is the wave vector); Ar, limits used for shell isolation ( r is distance); n, power of k used for Fourier transformation. (I
mA. The spectra were recorded with the sample in a cell that allowed treatment in flowing gases prior to the measurements. Two samples were characterized by EXAFS spectroscopy. One was prepared from [P~~MO~(CP)Z(CO)~(PP~~)~] and one from a mixture of [PdClz(PhCN)z] and [Mo(C0)6]. Each powder sample was pressed into a wafer with a C-clamp inside a glovebox. The amount of sample (approximately 200 mg) was calculated to give an absorbance of about 2.5 at the Pd K absorption edge. After the sample had been pressed, it was unloaded from the die, loaded into the EXAFS cell, sealed under a positive pressure of N2, and removed from the drybox. The sample was treated in flowing H2 (MG Industries, 99.999%) at 300 "C for 2 h and then at 400 "C for 2 h. The Hz (Matheson, 99.999%) had been purified by passage through traps containing particles of activated Cu and zeolites. The cell was cooled to room temperature in flowing H2, sealed under a positive pressure of H2, and aligned in the X-ray beam. The EXAFS data were recorded in the transmission mode after the cells had been cooled to nearly liquid nitrogen temperature. The data were collected with a Si( 111) double crystal monochromator that was detuned by 30% to minimize the effects of higher harmonics in the X-ray beam. The samples were scanned at energies near the Pd K edge (24 350 eV) and at energies near the Mo K edge (20 000 eV). EXAFS Reference Data. The EXAFS data were analyzed with experimentally and theoretically determined reference files, the former obtained from EXAFS data for materials of known structure. The Pd-Pd, Pd-Osuppon,Mo-Mo, and Mo-Osuppon interactions were analyzed with phase shifts and backscattering amplitudes obtained from EXAFS data for Pd foil, PdO, Mo foil, and K2M004, respectively. The Pd-Mo and Mo-Pd interactions were calculated by using the FEFF software of Rehr.Is The parameters used to extract these files from the EXAFS data are summarized in Table 1. EXAFS Data Analysis. The EXAFS data were extracted from the measured absorption spectra by the Koningsberger difference file t e ~ h n i q u e . ' ~ The , ' ~ normalized EXAFS function for each sample was obtained from the average of the X-ray absorption spectra from two scans by a cubic spline background subtraction. Normalization of the EXAFS function was done by dividing the absorption intensity by the height of the absorption edge. The main contributions to the spectra were isolated by inverse Fourier transformation of the final EXAFS function. The analysis was performed on these Fourier-filtered data. The parameters characterizing both low-Z (0)and high-Z (Pd, Mo) contributions were reliably determined by multiple-shell fitting in k space ( k is the wave vector) and in I space (I is the distance from the absorbing atom, Pd or Mo) with application of k1 and k3 weighting of the Fourier transform. The difference
Kawi et al.
6928 J. Phys. Chem., Vol. 99, No. 18, 1995 50
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Chemisorption of [ P ~ ~ M o ~ ( C ~ ) ~ ( C O ) G on( PMgO. P~~)~] 10 When a slurry of [Pd2Mo2(Cp);!(C0)6(PPh3)2] in pentane under N2 was brought in contact with MgO, the initially white MgO 5 powder became gray. The infrared spectra in the vco region 0 characterizing [Pd2Mo2(Cp)2(C0)6(PPh3)2] mixed with KBr and 7 15 23 30 38 45 the species formed by adsorption of [Pd2Mo2(cp)2(co)6(PPh3)2] on MgO are shown in Figure 1, A and B, respectively. The vco spectrum of the MgO-supported sample is characterized Pd particle size, by bands at 1915w, 1859sh, 1838ssh, 1828s, and 1785s cm-'. A comparison with the spectrum of [Pd2Mo2(Cp)2(C0)6(PPh3)2] Figure 2. Distribution of metal particle sizes estimated by TEM for mixed with KBr shows that adsorption of [ P ~ ~ M o ~ ( C P ) ~ ( C O ) sample ~made from [PdC12(PhCN)2] on MgO: (A) following treatment in H2 at 400 "C; (B) following treatment in HZat 500 "C. (PPh3)2] on MgO led to a broadening of the vco bands, which is characteristic of metal carbonyl complexes that interact with However, no particles larger than 42 8, were observed. After several chemically distinct surface sites. treatment of this sample in H2 at 400 and at 500 OC, the d P d The fingerprint of the bridging carbonyl bands was almost values became 27 and 33 A, respectively. the same, before and after adsorption, consistent with the The micrographs characterizing the sample prepared from hypothesis that the bimetallic precursor did not decompose on the MgO surface and that the Pd-Mo bonds in the cluster were [Pd2Mo2(cp)2(CO)6(PPh3)2], following reduction in H2 at 300 and at 400 "C, are similar to those of MgO itself. Only after maintained on the support. The adsorbed species is suggested to interact with polar groups on the MgO surface via the CO treatment in H2 at 500 "C were dispersed clusters observed; ligands, as evidenced by the changes in the intensity of the they were smaller than 10 A. This result indicates very high metal dispersions in these samples and a strong resistance to strong vco band. sintering in H2 in the investigated temperature range. Transmission Electron Microscopy. As shown in Figure 2, the MgO-supported samples prepared from [PdC12(PhCN)2], H2 and CO Chemisorption. Chemisorption data characterfollowing reduction at 400 and at 500 "C, incorporated metal izing the samples that had been treated in H2 are summarized in Tables 2-4. A H:Pd atomic ratio of about 0.3 was observed particles, most of them in the range of about 15-40 8, in for the supported sample prepared from [PdC12(PhCN)2],which diameter. The electron micrographs of the MgO-supported sample prepared from [MO(C0)6] following treatment with H2 corresponds to a dispersion (fraction of Pd atoms exposed) of at 500 "C are indistinguishable from those of MgO itself. This 0.3 (assuming a 1:l H:Pd stoichiometry) and an average result indicates the absence of molybde!um-containing particles palladium particle size of about 35 A, which is in good agreement with that determined by TEM (Table 4). There was large enough to observe (roughly 10 A) on the MgO surface, less chemisorption of hydrogen and of CO per Pd atom on the consistent with the presence of either molecular species or twosample made from the combination of [PdC12(PhCN)2]-I- [Modimensional islands of Mo ions on the surface. Particle size distributions characterizing the samples made (co)6] than on the sample made from [PdC12(PhCN)2] (Table from the combination of [PdC12(PhCN)2] -I- [MO(C0)6] on MgO 2). Still less chemisorption per Pd atom was observed on the (Table 2). following treatment in H2 at 300, 400, and 500 "C are shown sample made from [P~~Mo~(CP)~(CO)~(PP~~)~] in Figure 3. After reduction at 300 "C, about 85% of the The influence of 0 2 treatment on the chemisorption properties observed particles were in the size range 7-21 A; the surface of the sample formed from [PdC12(PhCN)2] -k [MO(C0)6] is average size of the Pd particles (&d) is about 22 A. As the shown by the data of Table 3. With increasing temperature of temperature of treatment in H2 increased from 300 to 500 OC, treatment in 0 2 , beginning at 200 OC, the amounts of hydrogen the maximum in the particle size distribution shifted from 14 chemisorbed and CO chemisorbed per Pd atom increased. After to 28 A and the proportion of larger particles increased. treatment of this sample in 0 2 followed by treatment in H2 at
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6930 J. Phys. Chem., Vol. 99,No. 18, 1995
Kawi et al.
TABLE 4: Oxygen Chemisorption and Hydrogen Titration Data Characterizing the MgO-Supported Samples Reduced at Various Temperatures precursor composition, wt % reduction temp, "C H:Pda O:Pdb hydrogen titration' OPdd O/Moef [MO(C0)61 2.0% Mo 400 0.0 0.50 0.0 0.47 500 [PdC12(PhCN)2] 1.O% Pd 400 0.27 0.35 1.05 500 0.32 0.30 0.87 [PdClz(PhCN)2]+ [MO(C0)6] 1.O% Pd, 1.O% MO 400 0.16 1.46 1.52 0.68 0.70 500 0.13 1.38 1.33 0.60 0.70 [ P ~ ~ M O Z ( C ~ ) ~ ( C O ) ~ ( P P1~.O% ~ ) Pd, ~ ] 0.9% MO 400 0.01 1.45 0.60 0.30 1.15 500 0.01 1.54 0.60 0.30 1.24 H:Pd values were determined after oxygen chemisorption followed by hydrogen titration and reduction of the catalyst at stated temperature. Total oxygen uptake, 0:Pd atomic ratio determined at 200 O C . Amount of hydrogen required for titration of chemisorbed oxygen, H:Pd atomic ratio; data measured at 200 "C, except for the sample containing no Mo, for which the measurements were made at room temperature. Calculated from hydrogen titration data at 200 "C taking into account the hydrogen chemisorption. e Amount of oxygen used for oxidation of molybdenum in bimetallic samples, determined as the difference between the amount of oxygen chemisorbed by the sample and the amount of oxygen chemisorbed by palladium (determined from hydrogen titration data). f Average molybdenum oxidation state determined from the equation: N M , = 6 - 2(0: Mo); see text for values. molybdenum ions in low oxidation states. The average oxidation state of the molybdenum in this sample, determined from the 0:Mo values, is about 3.5. Comparison of this value with that determined for the sample prepared from the mixture of [PdC12(PhCN)2] [Mo(C0)6] indicates that the fraction of molybdenum in low oxidation states is higher in the sample prepared from [Pd2Mo2(Cp)2(C0)6(PPh3)2]than in the sample prepared from the combination of the two monometallic precursors. Temperature-ProgrammedDesorption of Adsorbed CO. Temperature-programmeddesorption of CO from these catalysts was performed following reduction in H2 at 500 "C, evacuation during cooling to room temperature, and dosing with CO. TPD spectrum A (Figure 4)corresponds to CO desorption from the supported palladium catalyst. Broad desorption peaks observed at low temperatures are attributed to CO desorption from metallic palladium. Spectrum B, characterizing the catalyst made from the two monometallic precursors, is similar to the former spectrum in the low-temperature region; in addition, a small desorption peak was observed at higher temperature. Spectrum C, characterizing the sample made from the bimetallic precursor, includes no CO desorption peak at low temperatures. However, an intense CO desorption peak was observed in the high-temperature region. EXAFS Spectra. The normalized EXAFS function for each sample was obtained from the average of the X-ray absorption spectra from two scans by a cubic spline background subtraction. The EXAFS function was normalized by division by the height of the absorption edge. The raw EXAFS data characterizing the sample made from [PdC12(PhCN)2] [Mo(C0)6] at the palladium K edge (Figure 5A) show oscillations up to a value indicating the presence of k, the wave vector, of about 15 kl, of near-neighbor high-Z backscatterers around the absorbing Pd atoms, which are inferred to be Pd and/or Mo. The strong oscillations in the high-k region indicate relatively large metal particles in the sample. The raw EXAFS data characterizing the sample made from [Pd2Mo2(Cp)2(C0)6(PPh3)2] at the palladium K edge (Figure 5B) show oscillations up to a value of k of about 12 kl, indicating the presence of near-neighbor high-Z backscatterers. The relatively weak oscillations in the high-k region indicate the lack of large metal particles in this sample. Similarly, the raw EXAFS data characterizing the sample made from [Pd2Mo2(Cp)2(C0)6(PPh3)2]at the molybdenum K edge (Figure 5C) show oscillations up to a value of k of about indicating the presence of near-neighbor high-Z 12 k', backscatterers around Mo. The EXAFS reference data are summarized in Table 1. The raw EXAFS data at the palladium K edge for the sample
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+
prepared from the mixture of [PdC12(PhCN)21 and [Mo(C0)61 were Fourier transformed with a k' weighting over the range 3.95 < k < 15.8 A-1 with no phase correction. The Fouriertransformed data were then inverse transformed in the range 1.30 < r < 3.19 8, (where r is the distance from the absorber Pd atom) to isolate the major contributions from low-frequency noise and higher-shell contributions. The analysis was done similarly with the sample prepared from [Pd2Mo2(Cp)2(C0)6(PPh3)2]. The raw Pd edge data were Fourier transformed with a k1 weighting over the range 3.83 < k < 12.9 8,-l with no phase correction. The Fouriertransformed data were then inverse transformed in the range 1.17 < r < 3.37 8,. The analysis of the Mo edge data was
Highly Dispersed MgO-Supported Model Pd-Mo Catalysts 0.06
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0.02
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J. Phys. Chem., Vol. 99, No. 18, 1995 6931
TABLE 5: EXAFS Results at the Pd K Edge Characterizing the Moo-Supported Sample Prepared from and Treated in HZat 300 "C [ P ~ C ~ Z ( P ~ CfN[Mo(CO)6] )~] for 2 h and Then at 400 "C for 2 ha>
A
shell
N
R, A
A U ~ A2 ,
Pd-Pd Pd-Omppn Pd-0, Pd-01 Pd-Mo
10.7
2.76
0.0029
2.0
2.15 2.65 2.65
0.0010
A
'
1.0
0.6
0.0045 0.0009
AE,,, eV 3.99
EXAFS ref Pd-Pd
4.25 14.74
Pd-0 Pd-0 Pd-Mo
-3.0
Notation: N , coordination number; R , distance between absorber and backscatterer atom: Au2, Debye-Waller factor; AEo,inner potential correction. Estimated precision: N , +20% (Pd-Osuppon,*30%); R , & l % (Pd-O,,p,,,,n, f2%); Au2, 1 3 0 6 ; AEo, i l O % . (I
I
I
I
I
I
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4
6
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12
14
k,
16
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TABLE 6: EXAFS Results at the Pd K Edge Characterizing the MgO-Supported Sample Prepared from [P~ZMOZ(C~)~(CO)~(PP~~)Z] Following Treatment in Hz at 300 "C for 2 h and Then at 400 "C for 2 ha>
A U ~ , A ~A E ~eV ,
shell
N
R, A
Pd-Pd Pd-Osuppon Pd-Os Pd-01 Pd-Mo
4.5
2.85
0.0033
2.89
0.9 0.5
2.17 2.55 2.66
0.0008 -0.0111 0.0135
-13.4 12.84
3.0
8.01
EXAFS ref Pd-Pd Pd-0 Pd-0 Pd-Mo
"Notation as in Table 5 . bEstimated precision: N , &20% (PdOsuppon, f 3 0 % ) ; R , f l % (Pd-Osuppon,&2%);A d . 130%;A E o , *lo%.
TABLE 7: EXAFS Results at the Mo Edge Characterizing the MgO-Supported Sample Prepared from [P~~MO~(C~)Z(CO)~(PP~~)Z] Following Treatment in H2 at 300 "C for 2 h and Then at 400 "C for 2 ha> C
N
R,A
A U ~A2 ,
3.8
2.59
0.0094
Mo-Osuppon Mo-0.
1.6
Mo-0; Mo-Pd
2.0 2.2
2.16 2.55 2.66
0.0006 -0.0072
shell Mo-MO
0
A
z 0
0.0070
A E ~eV ,
EXAFS ref
-0.21
Mo-MO
-6.54 12.33 12.96
Mo-0 Mo-0 Mo-Pd
Estimated precision: N , f20% (Moa Notation as in Table 5 . Oruppon. &30%);R , *l% ( M o - O ~*2%); ~ ~ ~ha2, ~ , &30%;A&, &lo%.
determined in the analysis of the Mo edge data matched the Pd-Mo distance determined in the analysis of the Pd edge data. The parameters determined in this fitting routine are sum-0.03 marized in Tables 5-7, and the comparisons of the data and 4 6 a 10 12 14 the fits, both in k space and in r space, are shown in Figures k, A'' 6-8. The residual spectra determined by subtracting the PdPd and Pd-Osupponcontributions from the EXAFS data (which Figure 5. Raw EXAFS (chi) data characterizing (A) Pd edge data for sample prepared from [PdC12(PhCN)2] [Mo(C0)6] on MgO; (B) Pd give evidence of the Pd-Mo interaction) are shown in Figures on edge data for sample prepared from [P~ZMOZ(C~)~(CO)~(PP~~)Z] 6E, 7E, and 8E. The Pd-Mo contribution characterizing the MgO; (C) Mo edge data for sample prepared from [Pd2Mo~(Cp)2(C0)6sample prepared from [P~~Moz(C~)~(CO)~(PP~~)Z] (Figure 8E) (PPh&] on MgO. is significant, whereas that characterizing the sample prepared [MO(C0)6] (Figure 6E) is indistinfrom [PdC12(PhCN)2] done similarly. The raw data were Fourier transformed with a guishable from the noise. The Pd-Pd contribution characteristic ko weighting over the range 4.04< k < 12.5 A-' with no phase is small, of the sample made from [P~zMo~(C~)~(CO)~(PP~~)~] correction. The Fourier-transformed data were then inverse whereas the Pd-Pd contribution characteristic of the sample transformed in the range 0.80 < r < 3.17 A. made from [PdC12(PhCN)2] [MO(C0)6] is large (Tables 5 With the difference file technique, the Pd-Pd contributions and 6). These results corroborate the result that the dispersion in each sample, the largest in the EXAFS spectra, were then of the palladium in the former sample was high, whereas the estimated. Additional contributions accounted for in the fitting dispersion of the palladium in the latter sample was low. were Pd-O,,,,, contributions; two Pd-0 contributions were The number of parameters used to fit the Pd edge data found, one with a distance of about 2.2 A and one with a representing each sample was 16; the statistically justified distance of about 2.6 A. The fit of the Pd edge data with the number for each of these samples was approximately 16, sum of these three contributions was not satisfactory, and it estimated from the Nyquist theorem,l8 n = (2AkArh) 1, was inferred that another contribution, Pd-Mo, had to be where Ak and Ar, respectively, are the k and r ranges used in accounted for. A good fit was obtained for each sample at the the forward and inverse Fourier transforms. Pd edge when this contribution was included. A similar analysis was done for the Mo edge data characterDiscussion A izing the sample made from [P~~MOZ(C~)~(C~)~(PP~)Z]. Adsorption of [Pd~Mo2(Cp)z(C0)6(PPh3)21on MgO. The good fit was obtained only when Mo-Mo, Mo-Pd, and two infrared spectra and the color of the solid show that the precursor Mo-Osupponcontributions were included. The Mo-Pd distance
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Kawi et al.
6932 J. Phys. Chem., Vol. 99, No. 18, 1995
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Figure 6. Results of EXAFS analysis near the Pd K edge obtained with the best calculated coordination parameters characterizing the MgOsupported sample prepared from [PdCl*(PhCN)*] [Mo(C0)6] following treatment in Hz as described in the text: (A) experimental EXAFS (solid Pd-0, Pd-01 Pd-Mo contributions (dashed line); (B) imaginary part and magnitude of Fourier line) and sum of the calculated Pd-Pd transform (kl-weighted, Ak = 4.50-12.30 k l ) of experimental EXAFS (solid line) and sum of the calculated Pd-Pd Pd-0, Pd-OI Pd-Mo contributions (dashed line); (C) imaginary part and magnitude of Fourier transform (k3-weighted, Ak = 4.50-12.30 A-l) of experimental Pd-0, Pd-01 Pd-Mo contributions (dashed line); (D) imaginary part and magnitude EXAFS (solid line) and sum of the calculated Pd-Pd of phase- and amplitude-corrected Fourier transform (k3-weighted, Ak = 4.50-12.00 A-]) of raw data minus the calculated Pd-0, Pd-01 Pd-Mo contributions (solid line) and calculated Pd-Pd contribution (dashed line); (E) residual spectrum illustrating the Pd-Mo contributions: imaginary part and magnitude of phase- and amplitude-corrected Fourier transform (k3-weighted,Ak = 4.50-12.00 A-I) of raw data minus calculated Pd-Pd Pd-0, Pd-01 EXAFS (solid line) and calculated Pd-Mo contribution (dashed line).
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[Pd2Mo2(Cp)2(C0)6(PPh3)2]was molecularly (or almost molecularly) adsorbed in the MgO surface. Evidently this cluster is stable enough to remain nearly intact on the surface. The comparison of the infrared spectrum of the cluster in solution and that on the surface implies that the cluster-surface interaction was relatively weak. This result is important because it indicates that the cluster is stable enough to be adsorbed essentially intact and that it might be anchored to the support without loss of the strong interactions between the two metals. Formation of Supported Palladium Particles from [PdClz(PhCN)2] and from Mixtures of IPdC12(PhCNhl and IMo(co)6]on MgO. The chemisorption- data indicating the amounts of H:! and CO taken up by the sample prepared from
[PdC12(PhCN)2]were used in the conventional wayI9 to estimate the dispersion of the supported palladium. These data indicate an average palladium particle size of about 35 A. This result is in good agreement with the TEM results. The TEM results also show that palladium particles about this same size formed from the mixture of [PdClz(PhCN)z] and [Mo(C0)6] on MgO following treatment in H2. The EXAFS results obtained for one of these samples c o n f m the conclusion that aggregated palladium formed. The Pd-Pd first-shell coordination number was found to be 10.7 (Table 5). These results are typical for noble metals on metal oxides following treatment in H2 at elevated temperatures. Evidently I
Highly Dispersed MgO-Supported Model Pd-Mo Catalysts I
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J. Phys. Chem., Vol. 99, No. 18, 1995 6933 40
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r, A Figure 7. Results of EXAFS analysis near the Pd K edge obtained with the best calculated coordination parameters characterizing the MgOsupported sample prepared from [P~~Moz(C~)~(CO)~(PP~~)~] following treatment in HZas described in the text: (A) experimental EXAFS (solid Pd-0, Pd-O1 Pd-Mo contributions (dashed line); (B) imaginary part and magnitude of Fourier line) and sum of the calculated Pd-Pd transform &'-weighted, Ak = 4.50-12.30 A-') of experimental EXAFS (solid line) and sum of the calculated Pd-Pd rd-0, Pd-01 Pd-Mo contributions (dashed line); (C) imaginary part and magnitude of Fourier transform (k3-weighted, Ak = 4.50-12.30 A-]) of experimental Pd-0, Pd-Ol Pd-Mo contributions (dashed line); (D) imaginary part and magnitude EXAFS (solid line) and sum of the calculated Pd-Pd of phase- and amplitude-corrected Fourier transform (k3-weighted, Ak = 4.50- 12.00 A-1) of raw data minus the calculated Pd-0, Pd-01 Pd-Mo contributions (solid line) and calculated Pd-Pd contribution (dashed line); (E) residual spectrum illustrating the Pd-Mo contributions: imaginary part and magnitude of phase- and amplitude-corrected Fourier transform (k3-weighted, Ak = 4.50-12.00 A-1) of raw data minus calculated Pd-0, Pd-O1 EXAFS (solid line) and calculated Pd-Mo contribution (dashed line). Pd-Pd
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cluster markedly hindered the aggregation of palladium on the the molybdenum in these bimetallic samples was relatively MgO surface. ineffective in minimizing the aggregation of the noble metal. Consistent with this conclusion, the EXAFS data provide Formation of Highly Dispersed Supported Particles from evidence of interactions of the palladium and the molybdenum Supported [Pd2Mo2(cp)2(co)6(PPh3)2]. In contrast, when the in this sample. The EXAFS results are discussed in combination inorganometallic precursor was [P~~Mo*(C~)~(CO)~(PP~~)~I with the chemisorption and TPD data in the following parastead of the mixture of palladium and molybdenum complexes, graphs. only very small palladium clusters formed on the MgO, as shown by the EXAFS results (Table 6). The Pd-Pd first-shell Chemisorption of H2 and CO on the Supported Metals. The chemisorption data indicate that the amounts of H2 and of coordination number was found to be only 4.5, indicating the CO chemisorbed on the samples made from [Pd2Moz(Cp)2(C0)6presence of small clusters. These clusters were evidently too small to observe by TEM. We conclude that the initial close (PPh&] were markedly less than the amounts chemisorbed on the sample prepared from [PdClz(PhCN)2] and that prepared proximity of the palladium and the molybdenum in the precursor
Kawi et al.
6934 J. Phys. Chem., Vol. 99, No. 18, 1995 I
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r, A Figure 8. Results of EXAFS analysis near the Mo K edge obtained with the best calculated coordination parameters characterizing the MgOsupported sample prepared from [Pd2Mo2(Cp)2(C0)6(PPh3)21following treatment in HZas described in the text: (A) experimental EXAFS (solid
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Mo-0, Mo-01 Mo-Pd contributions (dashed line); (B) imaginary part and magnitude of phaseline) and sum of the calculated Mo-Mo and amplitude-corrected Fourier transform (kl-weighted, Ak = 4.50- 12.30 kl) of experimental EXAFS (solid line) and sum of the calculated Mo-0, Mo-01 Mo-Pd contributions (dashed line); (C) imaginary part and magnitude of phase- and amplitude-corrected Fourier Mo-Mo transform @-weighted, Ak = 4.50-12.30 k') of experimental EXAFS (solid line) and sum of the calculated Mo-Mo Mo-0, Mo-01 Mo-Pd contributions (dashed line); (D) imaginary part and magnitude of phase- and amplitude-corrected Fourier transform (k3-weighted, Ak = 4.50-12.00 kl) of raw data minus the calculated Mo-0, Mo-0, Mo-Pd contributions (solid line) and calculated Mo-Mo contribution (dashed line); (E) residual spectrum illustrating the Mo-Pd contributions: imaginary part and magnitude of phase- and amplitude-corrected Fourier transfonn (k3-weighted, Ak = 4.50-12.00 A-') of raw data minus calculated Mo-Mo Mo-0, Mo-01 EXAFS (solid line) and calculated Mo-Pd contribution (dashed line).
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from a mixture of [PdC12(PhCN)2] and [Mo(C0)6]. The low chemisorption capacity of the sample made from the bimetallic cluster is thought to be a consequence of a modification of the reactivity of the palladium caused by its interaction with molybdenum. Evidently, the interactions between palladium and molybdenum in the sample made from [PdC12(PhCN)2]
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[Mo(C0)6] were weaker than those in the sample made from
[Pd2Mo2(Cp)2(CO)6(PPh3)2].Because H2 and CO chemisorption experiments following treatment of the former sample in 0 2 at 200 "C showed increased uptakes, we suggest that the interactions between the Pd and the Mo decreased as a result of this 0 2 treatment. Because H2 chemisorption measurements made
Highly Dispersed MgO-Supported Model Pd-Mo Catalysts
J. Phys. Chem., Vol. 99, No. 18, 1995 6935
following treatment of the sample made from [PdzM02(Cp)2(C0)6(PPh3)2] in 0 2 at 500 “C gave about the same results as Pd those made before this 0 2 treatment, we conclude that the PdMo interactions in this sample were maintained even after the sample had been subjected to stringent oxidizing conditions. Thus, the bimetallic interactions are strong in this sample, M0 suggesting that such materials might be stable under harsh operating conditions. Desorption of CO from Pd-Mo Sites. The TPD measureW ments show that CO was desorbed from the Pd-Mo samples only at high temperatures (about 600 “C) (Figure 4). HighFigure 9. Simplified structural model of a bilayer consisting of molybdenum cations interacting with the MgO support and providing temperature CO desorption peaks have also been reported for a platform for the highly dispersed palladium. According to this model, supported Ni-Ti catalysts and attributed to desorption from the molybdenum helps to maintain the dispersion of the palladium. bimetallic sites.20 Thus it is plausible that the high-temperature CO desorption peak resulted from desorption of CO from Pdconfirm that the Pd-Mo interactions were strong. The evidence Mo sites. It has been suggested that CO may be adsorbed on is the following. First, the EXAFS results indicate an average bimetallic sites in a “tilted” form,21whereby the carbon atom Pd-Pd coordinationnumber of only 4.7, which is much smaller is bonded to the noble metal and the oxygen atom is bonded to than the value obtained for the other bimetallic sample. Second, the oxophilic metal. Because molybdenum cations are strongly there are substantial Pd-Mo interactions, with each Pd atom, oxophilic, the interaction between molybdenum and the oxygen on average, being surrounded by about three nearest-neighbor atom of CO may be strong enough that a high temperature is Mo atoms at an average distance of 2.66 A. Third, the Pd-0 required to desorb CO from these bimetallic sites. Again, the contribution was found to be small, consistent with the sugresults are consistent with the postulate that the palladium and gestion that most of the near neighbors of Pd were Pd and Mo. molybdenum remained in close proximity in the samples even Additional information about the Pd-Mo structures is after treatment at high temperatures, under both reducing and provided by the Mo edge EXAFS data for the sample prepared oxidizing conditions. from [Pd2Mo2(Cp)2(C0)6(PPh3)2]. First, the data indicate The TPD results for the sample containing palladium but not substantial Mo-Mo interactions, with a short Mo-Mo distance. molybdenum (Figure 4) show that the CO was much more This short distance might indicate the presence of molybdenum weakly bound than the CO in the sample made from [Pd2Mo2in a high oxidation state resulting from the high-temperature (cp)2(co)6(PPh3)2],consistent with the interpretation of strong treatment. Second, each Mo atom is shown to interact with, bimetallic interactions in the latter sample. Furthermore, the on average, about 3 Pd atoms at an average distance of 2.66 A. TPD data characterizing the sample made from the combination This Mo-Pd bond distance determined from analysis of the of [PdC12(PhCN)2] and [MO(C0)6] (Figure 4)show that the CO Mo edge data is the same as the Pd-Mo distance determined was not bonded as strongly as on the sample made from the from the Pd edge data (Tables 6 and 7). This result confirms bimetallic precursor. The spectrum of the sample made from the conclusion that this bimetallic interaction is strong. Third, [PdC12(PhCN)2] and [MO(C0)6] is characterized by a lowthere were substantial interactions between molybdenum and temperature peak that may be associated with CO adsorbed on support oxygen atoms. Each Mo atom was observed to interact palladium (although it is not clear why it appears at a with, on average, about one support oxygen atom at a distance temperature different from that observed in the TPD spectrum of 2.16 8, (this distance suggests bonding between Mo cations of the sample containing palladium and no molybdenum, Figure and 0 anions22)and with, on average, about two support oxygen 4). There is also a peak at about 580 “C in the spectrum of the atoms at a longer distance, namely, 2.55 A. Metal-support sample made from [PdC12(PhCN)2] and [MO(C0)6]; we suggest oxygen distances of 2.6-2.7 A have been observed frequently that this peak is indicative of some bimetallic structures, but for reduced group 8 metals on the surfaces of metal oxides.23 far fewer than are present in the sample made from the bimetallic Bilayer Model. The EXAFS results provide the basis for cluster. The latter conclusion is consistent with the TPR and suggestion of a simplified model of an average structure of the catalysis data of Halasz et a1.,l1 who made bimetallic catalysts bimetallic clusters in the sample made from [ P ~ ~ M o ~ ( C P ) ~ ( C O ) ~ from a mixture of palladium and molybdenum salts. (PPh3)2]. The data indicate a bilayer structure similar to that Structure of Supported Pd-Mo Clusters. The EXAFS proposed by F ~ n for g ~y-Al203-supported ~ Re-Pt catalysts data provide a basis for evaluation of the structure of the prepared from [PtRe2(C0),2]; Fung’s model is similar to that bimetallic clusters in the sample prepared from [Pd2Mo2(Cp)2proposed by Yermakov et al.25 for SiO2-supported Re-Pt (C0)6(PPh3)2] as well as a basis for comparing the bimetallic catalysts. The simplified structural model is shown in Figure interactions in the sample made from [PdC12(PhCN)2] [Mo9. According to this model, monolayers of molybdenum cations (co)6] with those in the sample made from [Pd2Mo2(Cp)2(C0)6are located atop oxygen ions of the support, with each Mo cation (PPh3)2]. The EXAFS results characterizing the former sample being coordinated with three support oxygen ions and with three indicate an average Pd-Pd coordination number of 10.7. This Pd atoms atop the molybdenum monolayer. Each Pd atom is result means that each Pd atom in the sample had about 11 near also coordinated with three Mo cations. The coordination Pd neighbors, consistent with the presence of relatively large parameters are approximately equal to those observed by Pd particles (the value for bulk palladium is 12). Consistent EXAFS spectroscopy (Table 6). We recognize that this model with this conclusion, only a small Pd-Mo contribution was is greatly simplified, and a variety of structures is expected to indicated by the EXAFS data; the Pd-Mo coordinationnumber have been present. We emphasize also that other structural (Table 5 ) was found to be only 0.5. Thus the EXAFS results models can be constructed that will agree about as well as this confirm the conclusion from the TPD data that bimetallic one with the EXAFS data. For example, a structure with two interactions in the sample made from the two metal complexes layers of Pd atoms, rather than one, and with a smaller were weak. monolayer of Mo cations than the 23-atom layer shown in Figure In contrast, the EXAFS results obtained at the Pd edge for 9, gives about as good a fit to the data. A significant limitation the sample made from [P~~Mo~(CP)~(CO)~(PP~~)~] of the structural model is that it does not account for the Pd-0 (Table 6)
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Kawi et al.
6936 J. Phys. Chem., Vol. 99, No. 18, 1995 interactions indicated by the EXAFS data. It is evident from these data that a significant fraction of the Pd atoms were in contact with the support. According to the model, the oxophilic molybdenum, which bonds to the oxygen of the MgO and also interacts strongly with the palladium, provides the key to the stabilization of the clusters. The results show that the close proximity of the two metals in the precursor [Pd2Mo2(Cp)2(Co)6(PPh3)2]is essential to the formation and stability of structures such as that shown in the model. We suggest that the use of such bimetallic clusters may be of some general value in the preparation of highly dispersed bimetallic clusters. Supported bimetallic catalysts are important in naphtha reforming applications and have been investigated extensively.26 Pd-Re supported on y-Al2O3 and prepared from salt precursors was characterized by Meitzner et al.,27who found from EXAFS spectroscopy that the Pd-Pd coordination number was about 7, and the Re-Re coordination number was about the same. Pt-Re catalysts were also investigated by these authors, but the data were not sufficient for estimation of coordination numbers. In contrast, F ~ n g used * ~ an organometallic cluster of Pt and Re to prepare a y-Al203-supported catalyst, finding a Pt-Pt coordination number of only 2.3. The small platinum clusters were shown to be stabilized by the rhenium, which was strongly bonded to the y-A1203. Thus the results observed in the work summarized here are similar to those of Fung and different from those of Meitzner et al. The comparisons reinforce the conclusion that the close proximity of the two metals in the bimetallic precursor was essential for the maintenance of interactions between them in the treated sample. The EXAFS data characterizing this sample indicate a PdPd distance of 2.85 A, which is longer than the distance representative of bulk palladium; the Pd-Pd distance in tbe sample prepared from [PdClz(PhCN)2] [Mo(C0)6] (2.76 A) is about the same as the bulk value. Thus the results indicate that the presence of molybdenum in the vicinity of the palladium influenced the Pd-Pd bonding. EXAFS data have been presented2* showing that hydrogen present (as palladium hydride) in palladium clusters about 10 8, in average diameter in zeolite Y increases the average PdPd distance from to 2.73 to 2.80 8,; the distances correspond to the evacuated sample on the one hand and that treated in HZat 120 mbar at 27 "C on the other. Because our samples were treated in H2 under similar conditions prior to the EXAFS experiments, these data suggest that our samples might be most appropriately regarded as palladium hydrides (or mixed metal hydrides). The EXAFS results are not sufficient to lead to a conclusion about this point; however, the comparisons of the EXAFS data presented above leave little room for doubt about the significant differences between the structures formed from [P~~Mo~(CP)~(CO)~(PP~~)~] and those formed from the mixture of [PdC12(PhCN)2] and [Mo(C0)61. The Pd-Mo distance of 2.66 8, characteristic of the sample prepared from [P~~Moz(C~)~(CO)~(PP~~)~] is short enough to suggest strong interactions between the two metals, consistent with the maintenance of the high dispersion of the palladium even after treatment of the sample under forcing conditions.
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Acknowledgment. This research was supported by Ford Motor Co. The EXAFS data were analyzed with the Eindhoven University EXAFS Data Analysis Program, developed by M. Vaarkamp and D. C. Koningsberger. We acknowledge the support of the U.S. Department of Energy, Division of Materials Sciences, under Contract No. DE-FG05-89ER45384, for its role in the operation and development of beam line X- 11A at the National Synchrotron Light Source. The NSLS is supported by the Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DEAC02-76CH00016. We are grateful to the staff of beam line X-1 1A for their assistance. References and Notes (1) Shelef, M. Catal. Rev. Sci. Eng. 1975, 11, 1. (2) Taylor, K. C. In Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, 1984; Vol. 5, p 119. (3) Wei, J. Adv. Catal. 1975, 11, 57. (4) Kummer, J. T. J . Phys. Chem. 1986, 90, 4747. (5) Pfefferle, L. D.; Pfefferle, W. C. Catal. Rev. Sci. Eng. 1987, 29, 219. (6) Shelef, M.; Graham, G. W. Catal. Rev. Sci. Eng. 1994, 36, 433. (7) Ismagilov, Z. R.; Kerzhentsev, M. A,; Susharina, T. L. Russ. Chem. Rev. 1990, 59, 973. (8) Gandhi, H. S.; Yao, H. C.; Stepien, H. K. In Catalysis Under Transient Conditions; Bell, A. T., Hegedus, L., Eds.; ACS Symposium Series 178; American Chemical Society: Washington, 1982; p 143. (9) DeVries, J. E.; Yao, H.C.: Baird, R. J.; Gandhi, H. S. J . Caral. 1983, 84, 8. (10) Adams, K. M.; Gandhi, H. S. Ind. Eng. Prod. Res. Dev. 1983, 22, 207. (1 1) Halasz, I.; Brenner, A,; Shelef, M.; Ng, K. Y. S. Appl. Catal. A: General 1992, 82, 51. (12) Halasz, I.; Brenner, A,; Shelef, M. Catal. Lett. 1992, 16, 311. (13) Fung, A. S.; Tooley, P. A,; McDevitt, M. R.; Gates, B. C. Polyhedron 1988, 7, 2421. (14) (a) Braunstein, P.; Bender, R.; Jud, J.-M. Inorg. Syn. 1989, 26, 341. (b) Bender, R.; Braunstein, P.; Jud, J.-M.; Dusausoy, Y. Inorg. Chem. 1983, 22, 3394. (15) Rehr, J. J.; Mustre de Leon, J.; Zabinsky, S. I.; Albers, R. C. J . Am. Chem. SOC.1991, 113, 5135. (16) Kirlin, P. S.; van Zon, F. B. M.; Koningsberger, D. C.; Gates, B. C. J. Phys. Chem. 1990, 94, 8439. (17) van Zon, J. B. A. D.; Koningsberger, D. C.; van't Blik, H. F. J.; Sayers, D. E. J . Chem. Phys. 1985, 82, 5742. (18) Koningsberger, D. C.; Prins, R. X-Ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES: Wiley: New York, 1988. (19) Via, G. M.; Sinfelt, J. H.; Lytle, F. W. J . Chem. Phys. 1979, 71, 690. (20) Alekseev, 0. S.; Shmachkov, V. A.; Ryndin, Yu. A,; Yermakov, Yu. I. Kinet. Katal. 1987, 28, 908. (21) Sachtler, W. M. H. Proceedings of the 8th International Congress on Catalysis; Dechema: Frankfurt, 1984; Vol. 1, p 151. (22) Chang, J.-R.; Gron, L. U.; Honji, A.; Sanchez, K. M.; Gates, B. C. J. Phys. Chem. 1991, 95, 9944. (23) Koningsberger, D. C.; Gates, B. C. Catal. Lett. 1992, 14, 271. (24) Fung, S. L. A. Ph.D. Thesis, University of Delaware, Newark, 1989. (25) Yennakov, Yu. I.; Kuznetsov, B. N.; Zakharov, V. A. Catalysis by Supported Complexes; Elsevier: Amsterdam, 1981; Chapter 10. (26) Sinfelt. J. H. Bimetallic Catalysts. Discoveries. ConceDts. and A p & z t i o n s ;Wiley: New York, 1983.' (27) Meitzner, G.; Via, G. H.; Lytle, F. W.; Sinfelt, J. H. J . Chem. Phys. 1987, 87, 6354. (28) Moraweck, B.; Clugnet, G.; Renouprez, A. J . China. Phys. (France) 1986, 83, 265. JE942340A
