5828
J. Phys. Chem. B 1997, 101, 5828-5833
Adsorption of Hydrogen on Dispersed Copper-Rhodium Bimetallic Crystallites Shu-Chin Chou and Chuin-Tih Yeh* Department of Chemistry, Tsing Hua UniVersity, Hsinchu, Taiwan 300, Republic of China
Tsong-Huei Chang Department of Chemical Engineering, Ming-Hsin Institute of Technology & Commerce, Hsinfeng, Hsinchu, Taiwan 304, Republic of China ReceiVed: February 24, 1997; In Final Form: May 7, 1997X
Supported bimetallic Rh-Cu/A12O3 samples of different XCu [NCu/(NCu + NRh) atomic ratios] were prepared by impregnating a 3.8 wt % Rh/A12O3 sample with different amounts of Cu(NO3)2 solution and characterized with techniques of hydrogen chemisorption and 2H NMR spectroscopy. The irreversible hydrogen uptake of the bimetallic samples, measured from the chemisorption, increases upon impregnating a small amount of copper but decreases as the XCu becomes larger than 0.1. Deuterium atoms adsorbed on the bimetallic samples exhibit only a single 2H NMR peak. However, both the line width (∆υ) and the chemical shift (δ) of this peak vary significantly with the XCu ratio. Phenomena observed from chemisorption and NMR spectroscopy may be explained satisfactorily by the formation of two alloy phases, i.e., a rhodium-rich phase [(Rh), XCu ≈ 0.05] and a copper-rich phase, [(Cu), XCu ≈ 0.8], on the surface of bimetallic crystallites. Detected variations in the ∆υ and the δ suggest a fast chemical exchange of the deuterium atoms adsorbed on the surfaces of these two alloy phases. The rate constant of this exchange process is estimated from the variation of ∆υ to be kex ) (1.4 ( 0.6) × 106 exp(-8.6 ( 0.8 kJ mol-1/(RT)) s-1. From the average XCu calculated from the δ of 2H NMR and the XCu of the bulk composition estimated from ICP-MS, a surface enrichment of copper on supported Cu-Rh crystallites was indicated.
Introduction Effects of modifying supported rhodium catalysts by adding copper have been studied in many catalytic reactions,1-4 such as hydrogenation of CO, decomposition of methane, hydrogenolysis of alkane, and hydrogenation of benzene. Experimental results of these modification studies demonstrated that rhodium atoms were the active catalytic centers and that copper atoms played a role of dilution but had an insignificant effect on the electronic density of rhodium atoms. However, Goodman et al.5,6 found a transfer of electronic charge from rhodium to copper in a Cu-Rh(100) system from an X-ray photoelectron spectroscopy (XPS) study. Phase diagrams of the Cu-Rh binary alloy7 showed that these two metals were partially miscible and formed two alloy phases, i.e., a rhodium-rich phase [indicated as (Rh)] and a copper-rich phase [represented as (Cu)], at T < 1400 K. Irons et al.8 determined the fcc structures of these two alloy phases with the X-ray diffraction (XRD) and found a lattice constant of 3.78 Å for the (Rh) and 3.66 Å for the (Cu). Alloy clusters finely dispersed on support were usually difficult to characterize with the XRD technique.9 Sinfelt et al.l0 studied the cluster formation on Rh-Cu/SiO2 bimetallic samples by the EXAFS technique and found an apparent enrichment of copper on the surface but little miscibility in the bulk of Rh-Cu clusters. 1H NMR of chemisorbed hydrogen has been recently developed to probe alloy phases exposed on the surface of supported bimetallic crystallites. Wu et al.11,12 studied hydrogen chemisorbed on silica-supported Cu-Ru and Cu-Pt bimetallic crystallites using 1H NMR and demonstrated that copper tended to cluster with both Ru and Pt. 2H NMR spectroscopy has also X
Abstract published in AdVance ACS Abstracts, June 15, 1997.
S1089-5647(97)00668-8 CCC: $14.00
been used in our laboratory to study the effect of support on the adsorption of deuterium13,l4 and to characterize the alloy formation on Pd-Rh/Al2O3 samples.15 A 1:1 Pd-Rh alloy was found in a bimetallic sample with a Pd/Rh ratio of 1.0. Besides the 1:1 alloy of Pd-Rh, some Pd-rich alloy phases were additionally observed from samples with a Pd/Rh ratio larger than 1. Owing to abundant information established in this laboratory for the Rh/Al2O3 system, the same 2H NMR technique is used to characterize the alloy formation of the bimetallic Cu-Rh/Al2O3 samples. Experimental Section 1. Sample Preparation. Monometallic samples of 3.8 wt % Rh/γ-Al2O3 and 3.8 wt % Cu/γ-Al2O3 were prepared through the incipient wetness method by impregnating dried γ-Al2O3 support (Merck, 110 m2/g, BET surface area) with RhCl3‚H2O and Cu(NO3)2‚6H2O solution, respectively. The resulting pastes were dried for 4 h at 383 K and then calcined for 4 h at 673 K. Bimetallic Cu-Rh/Al2O3 samples with a wide range of copper loading were subsequently prepared from the calcined monometallic Rh/Al2O3 sample by an additional impregnation of Cu followed by similar drying and calcination treatments. Table 1 showed the atomic ratio of Cu [XCu ) NCu/(NCu + NRh)] of these samples measured from ICP-MS. 2. Sample Characterization. Hydrogen Chemisorption. Isotherms of hydrogen adsorption were measured at 300 K using the static volumetric technique. Prior to the adsorption measurement, each sample was pretreated with a 1 h reduction in a flow of hydrogen at 573 K and a 1 h evacuation at a vacuum of around 5 × 10-5 Torr at 573 K. The total uptake of hydrogen chemisorption was measured at 300 K on the sample after evacuating. The reversible uptake was a second uptake obtained © 1997 American Chemical Society
Adsorption of Hydrogen
J. Phys. Chem. B, Vol. 101, No. 30, 1997 5829
TABLE 1: Chacterizations of Samples Used in This Study
sample Rh 2Cu-Rh 5Cu-Rh 8Cu-Rh 15Cu-Rh 21Cu-Rh 41Cu-Rh 49Cu-Rh 59Cu-Rh 75Cu-Rh 83Cu-Rh Cu
metal loading N°H2 uptake (µmol g-1)b (wt %)a XCu Rh Cu NCu/(NCu + NRh) total rev irrevc 3.80 0.05 0.13 0.20 0.32 0.58 1.34 2.13 2.74 5.73 9.75 3.80
0 0.02 0.05 0.08 0.15 0.21 0.41 0.49 0.59 0.75 0.83 1
160.3 173.1 191.3 182.2 140.3 133.0 109.3 96.6 47.3 29.2 20.0 12.2
76.5 76.5 87.5 83.8 53.7 50.1 41.0 34.6 15.5 9.1 3.6 5.0
87.2 96.6 103.9 98.4 85.6 82.9 68.3 62.0 30.1 20.0 16.4 6.2
a
Measured by ICP-MS. b The value of uptake obtained by extrapolating to zero pressure for the total and the reversible adsorption isotherms. c The irreversible uptake was the difference between the total and the reversible uptake.
after the chemisorbed sample had been evacuated for 20 min at 300 K. The amount of hydrogen chemisorption (N°H2) in Table 1 was estimated by an extrapolation to zero pressure of the total and the reversible isotherm profiles. The difference between the total and the reversible uptake is regarded as the irreversible uptake. XRD Measurements. X-ray diffraction patterns determinations were made using a Shimadzu XD-5 diffractometer employing copper KR radiation. Copper crystallite sizes were calculated from X-ray diffraction patterns using the width of the (111) copper peak at half-height. Instrumental broadening was determined using a copper filament reference sample. 2H NMR Measurements. After a reduction-evacuation pretreatment similar to the chemisorption measurements, a portion of each sample was sealed with deuterium gas (Matheson, 99.999% purity) of the desired pressure (100, 300, and 500 Torr) and transferred to a Bruker MSL-300 spectrometer for 2H NMR measurements. An operating frequency of 46.05 MHz at a 7.05 T magnetic field was used for all measurements. Spectra of 2H NMR were obtained by employing a single 90° pulse technique with a pulse width of 7.6 µs, a delay time of 30 µs, and a repetition time of 0.1 s. A CH3CO2D/CH3CO2H solution was used as an external standard (δ ) 0 ppm) for chemical shift calibrations.
Figure 1. Isotherms of hydrogen adsorption for the alumina support and the 3.8% Cu/Al2O3 sample at 300 K, where the lines a, b, and c represent the total uptake for alumina, the total uptake for Cu/Al2O3, and the reversible uptake for Cu/Al2O3, respectively.
Figure 2. Isotherms of hydrogen adsorption for 3.8% Rh/Al2O3 sample at 300 K, where the lines a and b are the total hydrogen uptake and the reversible hydrogen uptake, respectively.
Results and Discussion 1. Chemisorption Measurements. Figure 1 compares isotherms of hydrogen adsorption on the 3.8% Cu/Al2O3 monometallic sample and the alumina support. The total uptake of H2 on pure alumina support (curve a) was negligible at low H2 pressure but increased linearly with PH2. Both the total and the reversible uptakes (curves b and c) on the 3.8% Cu/Al2O3 displayed a dramatic uptake at low pressure (PH2 < 10 Torr) and a gentle increase in the uptake after PH2 > 10 Torr. All three isotherms in Figure 1 (curves a-c) exhibited parallel asymptotes in the high-pressure region (PH2 > 50 Torr). The difference between the total and the reversible uptake in the region of parallel asymptotes suggested an irreversible uptake (NHir) of 6.2 µmol g-l for the 3.8% Cu/Al2O3 sample. The calculated uptake ratio of NHir/NCu ) 0.02 is much smaller than the copper dispersion (DCu ) NCus/NCu ) 0.2)l6,l7 estimated from XRD for the Cu/Al2O3 sample. This low stoichiometry (NHir/ NCus ) 0.1) of hydrogen chemisorption reflects the filled 3d band of copper atoms. High hydrogen uptakes have been found from the monometallic sample of 3.8% Rh/Al2O3 (Figure 2). A dispersion of
Figure 3. Effect of copper content (XCu) for the irreversible hydrogen uptake on Cu-Rh/Al2O3.
DRh ) 0.46 was estimated for the rhodium crystallites on this sample from a NHir ) 83.8 µmol g-1 calculated from Figure 2 and an assumption of NHir/NRhs )1.0.18 Figure 3 shows the profile of irreversible hydrogen uptake (NHir) obtained from all the rhodium-containing samples prepared. The NHir obtained from these samples showed the following features when XCu was increased: (a) NHir ) 83.8 µmol g-1 at XCu ) 0; (b) a steep increase of NHir to 104 µmol g-1 (the maximum uptake in Figure 3) in 0 < XCu < 0.05; (c) a steady, slow decline of NHir to 60 µmol g-1 within an XCu
5830 J. Phys. Chem. B, Vol. 101, No. 30, 1997
Chou et al.
Figure 5. XRD results obtained from high rhodium content samples after prereduction at 300 K.
Figure 4. Speculative diagram for the variation in surface composition of bimetallic crystallite with various XCu: (striped portion) support; (clear portion) pure rhodium; (dotted portion) (Rh) alloy phase; (solid portion) (Cu) alloy phase.
range 0.05-0.50; (d) a steep decrease of NHir from 60 to 20 µmol g-1 at XCu ≈ 0.50, remaining at ∼20 µmol g-1 after XCu > 0.50. The steep increase of the feature b in the uptake profile is rather surprising. Some extra adsorption sites must have been created at initial additions of copper to the Rh/Al2O3 monometallic sample. From the isosteric heats of hydrogen adsorption on a sample of Cu-Rh/SiO2, Frennet et al.16 suggested that new active sites were formed from an electronic interaction between rhodium and copper and have a chemisorption stoichiometry larger than 2. The phase diagram of Cu-Rh bulk alloys in the literature showed two alloy phases, i.e., a rhodium-rich phase [XCu ) 0.06, denoted as (Rh)] and a copper-rich phase [XCu ) 0.8, denoted as (Cu)]. The maximum hydrogen uptake in the Cu-Rh/Al203 samples appears at XCu ) 0.05 in Figure 3. If the composition of (Rh) in the bulk alloy is XCu ) 0.06 and the dispersion of rhodium on the Rh/Al203 is DRh ) 0.46, the maximum NHir uptake should have occurred when the surface of the bimetallic crystallites was completely covered by a (Rh) phase. Obviously, the (Rh) phase on the bimetallic sample of XCu ) 0.05 has a NHir/NRhs > 1.0. Upon XCu > 0.05 in feature c, a copper-rich alloy phase, (Cu), may be additionally formed in the bimetallic samples. Since Cu atoms have a lower surface energy (1.35 J m-2) than Rh (2.00 J m-2),17 the (Cu) phase might stay on the surface of bimetallic crystallites. The irreversible hydrogen uptake on the bimetallic crystallites starts to decline because the (Cu) phase has a hydrogen-chemisorption stoichiometry (∼0.1) much smaller than that (>1.0) of the (Rh) phase. When the amount of added copper is large enough (XCu ≈ 0.5), the fraction of the (Cu) phase becomes high enough to completely cover the (Rh) surface and the hydrogen uptake decreases dramatically [feature d]. Figure 4 presents a physical model for the consecutive variation in the structure of supported bimetallic crystallites in
increasing XCu. The four features observed in chemisorption are described by two views, a top view that describes distributions of different metal phases exposed on the surface and a vertical section view that presents depth profiles of these phases. Figure 4a illustrates a rhodium crystallite on the monometallic Rh/Al2O3 sample (XCu ) 0). The white region and the slant line region in the top view of Figure 4a represent the rhodium crystallite and the alumina support, respectively. The average particle size of rhodium crystallites is ∼20 Å according to the measured dispersion (DRh ) 0.46).18,19 Small white circles in the vertical section view of Figure 4a indicate rhodium atoms in the dispersed crystallite. It has eight rhodium atoms in its diameter and displays a hemispheric shape. Figure 4b presents a typical bimetallic crystallite on the 5CuRh/Al2O3 sample from which a maximum NHir/NRh ratio among all bimetallic samples is found from the chemisorption. Two phases, i.e., a pure rhodium phase (white region) and a (Rh) alloy phase (dotted region), are found in the vertical section view. The surface of the crystallite is covered mainly by the (Rh) phase. When XCu of the bimetallic sample is over 0.05, excessive copper may either diffuse into the bulk of the rhodium particle and thereby thicken the (Rh) phase or add to the (Rh) phase to form a (Cu) phase (vertical section view of Figure 4c). The surface of the bimetallic crystallite (Figure 4c) is thus composed of two different alloy phases, i.e., the (Rh) and the (Cu) (black region). The fraction of (Cu)-phase coverage (designated in this report as S(Cu)) on the surface therefore increases with XCu. A situation of S(Cu) ) 1 should be reached (Figure 4d) before XCu of bimetallic samples reaches 0.8. In the depth profile (vertical section view) of Figure 4, an enrichment of Cu on the surface of Cu-Rh bimetallic crystallites is suggested. This enrichment resulted from a surface energy of Cu that is lower than that of Rh. An enrichment of Cu on the surface of (Cu) and (Rh) phases in the alloy crystallites may also be expected from thermodynamics. Unfortunately, we cannot confirm this prediction from our experimental results. 2. XRD Characterizations. In an attempt to prove the formation of alloys [i.e., (Rh) and (Cu)] on supported Cu-Rh/ Al203 bimetallic samples, we specially prepared an additional series of bimetallic samples with a high metal loading (>23 wt %) for X-ray diffraction (XRD) examinations. The monometallic 23% Rh/Al2O3 and the 10% Cu/Al2O3 samples displayed a (111) diffraction peak at 2θRh ) 41.2° and 2θCu ) 43.3° (traces a and d of Figure 5), respectively . The 2θ value of alloy phases should vary with the XCu value according to a linear relationship:
2θ ) 2θRh(1 - XCu) + 2θCuXCu
(1)
Adsorption of Hydrogen
J. Phys. Chem. B, Vol. 101, No. 30, 1997 5831
Figure 7. Effect of Cu loading on the observed (a) chemical shift and (b) line width at 300 K under PH2 ) 300 Torr.
TABLE 2: Effect of Temperature on the Line Width of 2H NMR at PD2 ) 300 Torr line width (∆ν)/kHz
Figure 6. Effect of Cu loading on 2H NMR spectrum of XCu-Rh/ Al2O3 obtained at 300 K under PH2 ) 300 Torr.
according to eq 1, the (Rh) [XCu ) 0.06] and the (Cu) [XCu ) 0.8] should have a 2θ diffraction peak at 41.3° and 42.8°, respectively. These two 2θ values have been confirmed by XRD spectra of unsupported alloy samples in the literature.4,8 Diffraction peaks for alloy Cu-Rh phases were expected from bimetallic samples. Traces b and c in Figure 5 display the XRD spectra obtained from the Cu-Rh/Al2O3 samples prepared by adding copper to the 23% Rh/Al2O3 sample (trace b for XCu ) 0.1 and trace c for XCu ) 0.4). The observed diffraction peaks indeed confirmed a formation of alloy phases in the bimetallic samples. Unfortuntely, their peak widths were too broad to deconvolute reasonably into (Rh) and (Cu) peaks. 3. 2H NMR Measurements. Figure 6 compares 2H NMR spectra for deuterium (300 Torr) adsorbed on Cu-Rh/Al203 samples with different XCu. Deuterium atoms adsorbed on the 3.8% Rh/Al2O3 sample show a pressure and temperature independent chemical shift of δRh ) -150 ppm. The observed upfield signal on comparing to δ ) 0 for deuteriums in CH3COOD was due to a shift of electron density from the rhodium crystallite to adsorbed deuterium atoms, i.e.,
sample
F(Cu)
T ) 250 K
T ) 300 K
T ) 350 K
Rh 8Cu-Rh 15Cu-Rh 21Cu-Rh 41Cu-Rh 49Cu-Rh 51Cu-Rh 75Cu-Rh 83Cu-Rh
0 0.03 0.06 0.17 0.42 0.57 0.93 0.95 1
7.6 7 7.3 7.9 7.2 6.9 4.5 3.5 3
1.9 2.2 2.5 3.0 4.2 4.0 2.4 1.7 1.3
0.9 0.9 1.1 1.3 1.4 1.4 0.9 0.7 0.5
bimetallic crystallites covered by a pure Cu surface layer. Accordingly, a (Cu) phase instead of a pure Cu phase should have covered the surface of bimetallic crystallites on samples with a high XCu ratio. This same conclusion has been proposed in the chemisorption section. Obviously, two distinctive kinds of adsorption site, i.e., a (Rh) site and a (Cu) site, probably existed on the surface of bimetallic Cu-Rh/A12O3 samples for XCu > 0.05. The observed single peak from the bimetallic samples therefore indicates a fast chemical exchange of adsorbed deuterium atoms on these two kinds of sites, i.e., k1
D(Rh) y\ z D(Cu) k
(2)
2
where D(Rh) and D(Cu) represent the deuterium adsorbed on the (Rh) phase and the (Cu) phase, respectively, and k1 and k2 are the exchange rate constants. The observed chemical shift can be related to chemical shifts from the 8Cu-Rh/A1203 (δ(Rh) ) -142 ppm) and the 83Cu-Rh/A1203 (δ(Cu) ) -14 ppm) according to
D2 + 2Rhs f 2Rhsδ+Dδ-
δobs ) [1 - F(Cu)]δ(Rh) + F(Cu)δ(Cu)
The observed δCu ) 100 ppm for deuterium adsorbed on 3.8% Cu/Al2O3 is consistent with a 1H NMR report for hydrogen adsorbed on a Cu/SiO2 sample. The downfield peak of deuteriums adsorbed on Cu was attributed to the Knight shift.20 Only a single 2H NMR peak was found from each bimetallic sample. The observed peak position in Figure 7a shifted gradually downfield from the -150 ppm of XCu ) 0 to -14 ppm at the XCu > 0.59 on increasing XCu. The latter shift deviates significantly from the +100 ppm expected from
where F(Cu) is the fraction of deuterium atoms adsorbed on the (Cu) phase. The δobs values observed in Figure 7 may therefore be used to estimate F(Cu) through eq 3 (in Table 2). Interestingly, the line width (∆ν) of the 2H NMR peak (shown in Figure 7b) varied also with the XCu ratio. The line width was 2.2 kHz for the monometallic Rh/A12O3 sample, gradually increased in the bimetallic samples with the XCu ratio, reached a maximum ∆ν () 4.2 kHz) at XCu ) 0.41, and then narrowed to 1.25 kHz at the XCu ) 0.83. The 1.25 kHz is considerably narrower than that of the ∼3.3 kHz observed from the Cu/A1203
(3)
5832 J. Phys. Chem. B, Vol. 101, No. 30, 1997
Chou et al. TABLE 3: Effect of Temperature on Exchange Rate Constant at Different Deuterium Pressure PD2 ) 100 Torr PD2 ) 300 Torr PD2 ) 500 Torr T/K (k1 + k2) × 10-4/s-1 (k1 + k2) × 10-4/s-1 (k1 + k2) × 10-4/s-1 250 300 350
Figure 8. Variation of line width with the fraction of deuterium adsorbed on copper-rich alloy [(Cu)], where the open circles are the experimental data points and the solid line is the simulation line (eq 4).
2.71 5.10 10.41 Ea ) 9.7 ( 0.1 kJ/mol
3.03 4.70 9.93 Ea ) 8.4 ( 0.3 kJ/mol
2.91 5.36 8.54 Ea ) 7.8 ( 0.1 kJ/mol
process is thus calculated from the Arrhenius equation. Chang et al.l4 found three kinds of adsorbed deuterium on the Rh/SiO2 samples, i.e., rigid (Dr), mobile (Dm), and weakly adsorbed (Dw). Dm and Dw also proceeded by a fast exchange, which has an activation energy of Ea ) 16.6 ( 1.9 kJ mol-l. Evidently, the energy for Cu-Rh/Al2O3 is smaller than Rh/SiO2 samples. Probably, the deuterium adsorbed on the (Cu) surface has a higher mobility than the weakly adsorbed deuterium on the Rh/ SiO2 sample. Measured activation energy of exchange process did not significantly vary with the deuterium pressure and showed only a small variation (∼ 2 kJ) from PD2 ) 100 Torr to 500 Torr (Table 3). Conclusions
Figure 9. Effect of temperature on 2H NMR spectrum of a 49CuRh/Al2O3 sample.
sample. Conceivably, the (Cu) exhibited a weaker adsorptive strength (due to interactions between both copper and rhodium) than monometallic Cu/A12O3. According to the theory of chemical exchange,21 ∆ν [)(πT2, -1] may vary with the molar fraction of the exchanging obs) species as shown in eq 4:
l/T2, obs ) [1 - F(Cu)]/T2,(Rh) + F(Cu)/T2,(Cu) + 4π2 [1 - F(Cu)]F(Cu)[υ(Rh) - υ(Cu)]2(kl + k2)-l (4) where [υ(Rh) - υ(Cu)] is the chemical shift difference (in Hz) that depends on the strength of the magnetic field of the spectrometer. T2,(Rh) and T2,(Cu) are the spin-spin relaxation time of D(Rh) and D(Cu), respectively, in the absence of exchange. Figure 8 displays an optimum simulation curve for ∆ν calculated from eq 4 for kl + k2 ) 4.73 × 104 s-l. The decent fitting of the simulation curve with the data points indicates the exchange rate constant has a value of 4.73 × 104 s-l at 300 K. Furthermore, the line width of the deuterium adsorbed on the bimetallic samples was shown in Figure 9 to depend on temperature. Simulated exchange rate constants are 3.03 × 104 and 9.93 × 104 s-l (see Table 3) at 250 and 350 K, respectively. An activation energy (Ea) of 8.4 ( 0.3 kJ mol-l for the exchange
Similar to the unsupported Cu-Rh system, copper atoms introduced into a Rh/Al2O3 sample tends to alloy with its rhodium crystallites. Both the hydrogen chemisorption and 2H NMR of chemisorbed deuterium indicate a formation of alloy phases on the surface of crystallites. The alloy phase formed on the surface varies with the fraction of copper added. A (Rh) phase with a high capability for hydrogen chemisorption was found at XCu < 0.05. For XCu > 0.05, a (Cu) phase was additionally formed. The abundance of the (Cu) phase on the surface of bimetallic crystallites increases with XCu. It should be noted that 2H NMR is proposed in the study as a versatile technique to characterize the alloy formation in supported bimetallic samples. The chemical shift of the adsorbed deuterium atoms determines the abundance of the (Rh) and the (Cu) phases on the surface of Cu-Rh bimetallic crystallites. The variation of the peak width determines also the exchange rate of adsorbed atoms between these two alloy phases. In Figure 4, a speculated depth profile of the distribution of different alloy phases was suggested. A surface enrichment of Cu on the Cu-Rh bimetallic crystallites was proposed on the basis of a difference in the surface composition estimated from 2H NMR and the bulk composition estimated by ICP-MS. Acknowledgment. We appreciate the National Science Council of the Republic of China for financial support of this study under Contract No. NSC 86-2113-M-007-029 and NSC 86-2113-M-159-001. References and Notes (1) Krishnamurthy, R.; Chuand, S. S. C.; Ghosal, K. Appl. Catal. 1994, 114, 109. (2) Solyrnosi, F.; Cserenyi, J. Catal. Lett. 1995, 34, 343. (3) Coq, B.; Dutartre, R.; Fjgueras, F.; Rouco, A. J. Phys. Chem. 1989, 93, 4904. (4) Clarke, J. K. A.; Peter, A. J. Chem. Soc., Faraday Trans. 1 1975, 72, 1201. (5) Rodriguez, J. A.; Campbell, R. A.; Goodman, D. W. J. Phys. Chem. 1990, 94, 6936. (6) Rodriguez, J. A.; Campbell, R. A.; Goodman, D. W. J. Phys. Chem. 1991, 95, 477. (7) Massalski, T. B.; Okamoto, H.; Subramanian, P. R.; Kacprazak, L. Binary Alloy Phase Diagrams, 2nd ed.; ASM: Metals Park, OH, 1992. (8) Irons, L.; Mini, S.; Brower, W. E., Jr. Mater. Sci. Eng. 1988, 98, 309.
Adsorption of Hydrogen (9) van’t Blik, H. F. J.; Prins, R. J. Catal. 1986, 97, 188. (10) Meitzner, G.; Via, G. H.; Lytle, F. W.; Sinfelt, J. H. J. Chem. Phys. 1983, 78, 882. (11) Wu, X.; Gerstein, B. C.; King, T. S. J. Catal. 1990, 121, 271. (12) Wu, X.; Bhatia, S.; King, T. S. J. Vac. Sci. Technol. A 1992, 10, 2729. (13) Chang, T. H.; Cheng, C. P.; Yeh, C. T. J. Phys. Chem. 1991, 95, 5239. (14) Chang, T. H.; Cheng, C. P.; Yeh, C. T. J. Phys. Chem. 1992, 96, 4151. (15) Chang, T. H.; Cheng, C. P.; Yeh, C. T. J. Chem. Soc., Faraday Trans. 1994, 90, 1157.
J. Phys. Chem. B, Vol. 101, No. 30, 1997 5833 (16) Zauwen, M. N.; Crucq, A.; Degols, L.; Lienard, G.; Frennet, A.; Mikhalenko, N.; Grange, P. Catal. Today 1989, 5, 237. (17) Anderson, J. R.; Preatt, K. C. Introduction to Characterization and Testing of Catalysts; Academic Press: Sydney, 1985. (18) Ho, Y. S.; Yeh, C. T. J. Mol. Catal. 1990, 59, 53. (19) Satterfield, C. N. Heterogeneous Catalysis in Industrial Practices, 2nd ed.; McGraw-Hill: New York, 1991; Chapter 5. (20) Khanra, B. C.; King, T. S. J. Phys. Chem. 1993, 97, 4164. (21) Jackrnan, L. M.; Cotton, F. A. Dynamic Nuclear Magnetic Rsonance; John Wiley: New York, 1975.
