J. Phys. Chem. 1996, 100, 18177-18182
18177
Isotope Exchange Reaction of Formate with Molecular Hydrogen on Ni(110) by IRAS Akira Yamakata, Jun Kubota, Junko N. Kondo, Kazunari Domen,* and Chiaki Hirose Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan ReceiVed: April 25, 1996; In Final Form: September 6, 1996X
Kinetics of the CH/CD isotope exchange reaction of formate (HCOO(a)/DCOO(a)) on Ni(110) with gaseous D2/H2 was investigated below the decomposition temperature (360 K) by infrared reflection absorption spectroscopy (IRAS). The apparent activation energies of HCOO(a) f DCOO (a) and DCOO(a) f HCOO(a) reactions were derived as 52 ( 5 and 58 ( 5 kJ‚mol-1, respectively. A half-order dependence of the reaction rate on hydrogen pressure was observed, suggesting that the rate-determining step was the reaction of the adsorbed formate with reversibly adsorbed hydrogen atom. The reaction of HCOO(a) with D2 was 2.1 ( 0.3 times faster than that of DCOO(a) with H2 at 300 K, and the origin of the isotope effect is discussed.
1. Introduction
SCHEME 1: H-D Isotope Exchange Reaction of Formate on Ni(110)
One of the promising approaches for understanding the details of surface chemical reactions including heterogeneous catalysis is in situ spectroscopic observation of adsorbed species under the reaction conditions. Although a large number of studies have been carried out to reveal the dynamic behavior of adsorbed molecules on powder catalysts by transmission IR spectroscopy,1,2 such studies are still limited on well-defined single-crystal surfaces under steady-state reaction conditions. Infrared reflection absorption spectroscopy (IRAS) is a powerful technique which enables the observation of vibrational spectra of the adsorbed species in the presence of molecules in the gas phase. Formate has been observed as an important intermediate in various heterogeneous catalytic reactions such as the watergas shift reaction and methanol synthesis reaction,3-5 and the reactivity of the formate adsorbed on metal surface is a subject worthy of careful study. The structure of adsorbed formic acid on Ni(110) was studied in detail regarding the decomposition process.6-8 The adsorbed species observed at around room temperature on Ni(110) was assigned to the formate on the short-bridge site forming an ordered c(2 × 2) layer by high-resolution electron energy loss spectroscopy (HREELS) and low-energy electron diffraction (LEED).9-12 The generation of formate from associatively adsorbed formic acid on Ni(110) was also studied by IRAS,13 and the orientation of the formate with the C-H bond being perpendicular to the surface was also confirmed. The decomposition of formate was found to be induced by atomic hydrogen on Ru(001),14,15 which suggests that the consideration of the presence of atomic hydrogen is a key factor in elucidating the decomposition mechanism of surface formate. In this work, the isotope exchange reaction of formate produced on Ni(110) under the atmosphere of hydrogen was studied to elucidate the dynamic behavior of the adsorbed species steadily interacting with H2 or D2. The hydrogen exchange reaction of the formate on Ni(110) as depicted in Scheme 1 was found to proceed at around room temperature in the presence of gas-phase hydrogen without any side reaction such as the decomposition reaction of formate itself. X
Abstract published in AdVance ACS Abstracts, October 15, 1996.
S0022-3654(96)01198-7 CCC: $12.00
2. Experimental Section The experiment was carried out in an ultrahigh vacuum (UHV) chamber equipped with quadruple mass analyzer (QMS), LEED-AES optics. The base pressure of the chamber was below 2 × 10-10 Torr (1 Torr ) 133 Pa). The Ni(110) crystal (10 mm in diameter) was mounted on a manipulator by 0.3 mm tantalum wires spot-welded on the back of the crystal. The surface was cleaned by argon ion sputtering and annealing at 1023 K. For the IRAS measurement, a JEOL JIR-100 spectrometer was used with either a MCT or InSb detector at a resolution of 4 cm-1. The incident angle of the infrared beam through a NaCl window was about 84.5° from the surface-normal, and typically, 512 scans (12 min) were averaged for obtaining one spectrum. In the TPD measurement, five different mass numbers were simultaneously monitored with a heating rate of 3 K‚s-1. Formic acid was introduced onto the Ni(110) through a stainless steel nozzle (2 mm in diameter) located at 20 mm from the sample. The formic acid (99% purity) was dried over copper sulfate anhydride and purified by vacuum distillation and a freeze-pump-thaw cycle. The surface with saturation coverage of formate was obtained by dosing about 1 L (1 L ) 1 × 10-6 Torr‚s) of formic acid at 300 K, and the derived surface displayed the c(2 × 2) structure. As recently reported by Haq et al.,13 the coadsorbed CO was always observed at 2036 and 2073 cm-1 when formate was formed. The coverage of CO, however, was less than 5% of the total number of surface Ni atoms, and the surface prepared by the above method was used as a formate full-covered one (θHCOO ) 1) in the following experiments. The integrated intensity of CO bands did not change, although they slightly broadened during the subsequently performed isotope exchange reaction. The surface was exposed to D2 or H2 by back-filling the chamber under continuous pumping. IRA spectra were recorded under evacu© 1996 American Chemical Society
18178 J. Phys. Chem., Vol. 100, No. 46, 1996
Figure 1. Temperature-programmed desorption (TPD) spectra of formic acid adsorbed on Ni(110) surface at 113 K.
Figure 2. IRAS spectrum of formate adsorbed on a Ni(110) surface at 300 K.
ation after dosing a certain amount of hydrogen. Since the exchange reaction does not proceed in the absence of gaseous hydrogen, the change of IRA spectra gives the reliable rate of isotope exchange reaction. 3. Results and Discussion TPD of Formic Acid on Ni(110). The TPD spectra of H2, H2O, CO, CO2, and HCOOH from the formic acid adsorbed on Ni(110) at 110 K are shown in Figure 1. The desorption peak of HCOOH (m/e ) 46) around 185 K was assigned to the desorption from the second layer or multilayer of formic acid because this peak increased without saturation by increasing the exposure. The other peaks of H2, H2O, CO, and CO2 at 185 K were assigned to the cracking of HCOOH. A broad desorption peak of hydrogen was observed at 250∼320 K and was attributable to the dissociation of the hydroxyl group of formic acid to form formate. The desorption peaks of CO2 and H2 at around 360 K were due to the dehydrogenation of formate as was reported previously, while the desorption peak of CO at around 440 K was attributed to the dehydration of formate.6-8 H2O should be produced simultaneously in the dehydration but was not detected in our experiment presumably because of the high background signal of H2O from the wall of the chamber. The results indicated that the decomposition of formate started at 330 K. Therefore the CH/CD isotope exchange reaction was performed between 290 and 305 K. IRAS Spectra of Formate on Ni(110). The IRA spectrum of formate on Ni(110) surface at 300 K is shown in Figure 2. Four peaks observed at 779, 1365, 2854, and 2943 cm-1 are assigned to the OCO deformation band (δ(OCO)), the symmetric CO stretching band (νs(OCO)), the combination band of the inplane CH deformation band (δ(CH)) and the asymmetric CO stretching band (νa(OCO)), and the CH stretching band (ν(CH)), respectively. The assignments are summarized in Table 1 along with the values reported on other metal surfaces.16-20 The peaks at 2840 and 2944 cm-1 have been assigned to the ν(CH) and the combination band by Haq et al.,13 respectively, which is different from the present one. The combination band enhanced by Fermi resonance should not be more intense than the fundamental ν(CH)
Yamakata et al. band. Therefore our assignment will be more reasonable. It should be noted that the peaks of δ(CH), the out-of-plane deformation band (π(CH)), and νa(OCO) were absent. Taking into account of the surface selection rule of IRAS, the C-H bond was regarded as being perpendicular to the surface. The presence of gaseous or adsorbed hydrogen did not lead to any noticeable change in the IRA spectrum of formate, suggesting that the coadsorption of hydrogen on formate-covered Ni(110) did not cause any significant change in IRA spectra. Isotope Exchange Reaction of Formate on Ni(110). When the HCOO-covered surface was exposed to D2 (7 × 10-6 Torr), the intensity of the ν(CH) (2943 cm-1) and the νs(OCO) (1365 cm-1) bands commenced to decrease, and new bands assigned to the CD stretching (ν(CD)) and the νs(OCO) bands of DCOO(a) species appeared at 2188 and 1327 cm-1, respectively, as shown in Figures 3 and 4. When the D2 gas was replaced by H2 after the ν(CH) band almost disappeared, the reverse phenomenon was observed; the ν(CH) band reappeared, and the initial peak intensity of ν(CH) was recovered completely at the expense of the ν(CD) band. The quantitative analysis of the process with normalization taking into account of the absorption coefficients (CD/CH ) 2.1) is shown in Figure 5. The total coverage of formate (HCOO(a) + DCOO(a)) was confirmed to be constant during the isotope exchange reaction, i.e., no side reactions such as decomposition or desorption occurred. Dipole-Dipole Interaction of the νs(OCO) Band. As shown in Figures 3 and 4, the peak position of the νs(OCO) band of HCOO(a) and DCOO(a) shifted as the reaction proceeded, while those of the ν(CH) and ν(CD) bands stayed at the same positions. Figure 6 is the plot against the coverage of the peak positions of the νs(OCO) band of HCOO(a) and DCOO(a), where the coverage was estimated from the intensities of the ν(CH) and ν(CD) bands shown in Figure 3. The peak positions shifted almost linearly with the coverage by about 7 and 10 cm-1 for HCOO(a) and DCOO(a), respectively. The frequency shift of the νs(OCO) mode of HCOO(a) and DCOO(a) was attributed to the dipole-dipole coupling among the same isotope species.21,22 The peak shift due to the dipoledipole coupling has been reported for the ν(CO) band of CO(a),21-23 νs(OCO) band of formate,16 and ν(CO) band of methoxy.24,25 The linear dependence of the presently observed peak shift on the coverage seems to suggest that the isotope exchange reaction on the Ni(110) surface proceeded homogeneously.21,22,26 In other word, islands of HCOO(a) and DCOO(a) were not formed during the exchange reaction. The intensities of the νs(OCO) bands were much larger than those of the ν(CH) and ν(CD) bands which have the linear relationship between them in intensity (not shown), and the distinct and linear shift of the νs(OCO) band with the coverage was used to evaluate the coverage in the subsequent analysis, as a more precise method than that calculated by the peak intensities. Kinetic Analysis of Isotope Exchange Reaction by IRAS. The kinetics of the isotope exchange reaction is discussed on the Ni(110) surface with full coverage of formate. The rates of the isotope exchange reactions are defined as follows:
rHD ) -
dθHCOO m PDn 2 ) k′′HDθHCOO dt
(1a)
rDH ) -
dθDCOO m PHn 2 ) k′′DHθDCOO dt
(1b)
where r, k′′, θHCOO, θDCOO, PD2, and PH2 are the rate of the exchange, apparent rate constant, coverage of HCOO(a) and DCOO(a), and pressure of D2 and H2, respectively. The
Isotope Exchange Reaction of Formate with H2
J. Phys. Chem., Vol. 100, No. 46, 1996 18179
TABLE 1: Vibrational Frequencies of Formate on Various Metal Surfacesa
a
Ni(110)
Ni(110)
Cu(100)
ν(CH)
2943
2840
2900
combination νs(OCO) δ(OCO)
2865 1365 779
2944 1352 770
2958 1358 -
method ref
IRAS c
IRAS 13
IRAS 16
Cu(100) 2910 2840
Ru(001)
Pt(110)
Pt(111)
2939
2950
2950
1330 760
2857b 1361 784
1340 785
1340 790
HREELS 17
IRAS 18
HREELS 19
HREELS 20
Frequencies are given in cm-1. b No explicit assignment by the author. c Present study.
Figure 3. Higher frequency region of IRA spectra of formate adsorbed on a Ni(110) surface at 300 K under D2 flow at 7 × 10-6 Torr.
subscripts of HD and DH are used to denote the reaction from HCOO(a) to DCOO(a) and that from DCOO(a) to HCOO(a), respectively, and the superscripts of m and n are the reaction orders with respect to the coverage of formate and pressure of D2 or H2, respectively. Since the coverage of HCOO(a) (θHCOO) apparently follows the exponential decay during the exchange reaction as shown in Figure 5, the parameter m was estimated to be unity. Then following equation was derived from (1a):
ln{θHCOO(t)/θHCOO(t ) 0)} ) -k′′DHPDn 2t
(2)
where t represents the reaction time, and θHCOO(t ) 0) and θHCOO(t) denote the coverage at t ) 0 and t, respectively. In Figure 7, the values of ln{θHCOO(t)/θHCOO(t ) 0) are plotted as a function of the reaction time when the pressure of the hydrogen was kept constant at 7 × 10-6 Torr and the temperature at 300 K. A good linear relationship between ln{θHCOO(t)/θHCOO(t ) 0) and t was obtained, which confirms that the reaction rate is in first order for θHCOO. The first-order dependence of the reaction rate on θHCOO again suggests that the reaction occurs homogeneously on the surface. To determine the dependence of the reaction rate on the hydrogen pressure, parameter n was substituted into eq 1a leading the relation
ln rHD ) ln k′′HD + ln θHCOO + n ln PD2
Figure 4. Lower frequency region of IRA spectra of formate adsorbed on a Ni(110) surface at 300 K under D2 flow at 7 × 10-6 Torr.
(3)
When the rate of the reaction is measured at a constant
Figure 5. Time course of the normalized peak intensity of the νCH and νCD bands during the isotope exchange reaction of formate on Ni(110) surface; O, 0, and 4 represent the quantities of HCOO(a), DCOO(a), and HCOO(a) + DCOO(a), respectively.
temperature and θHCOO, the parameter n is obtained by varying the D2 pressure. The D2 pressure was varied from 5 × 10-6 to 1.4 × 10-5 Torr at 300 K, and n was calculated to be 0.5 ( 0.1 as shown in Figure 8. Then, the following eqs 4a,b are derived from eq 1:
rHD ) k′′HDθHCOOPD0.52
(4a)
rDH ) k′′DHθDCOOPH0.52
(4b)
18180 J. Phys. Chem., Vol. 100, No. 46, 1996
Yamakata et al.
rHD )
k′′HD θ θ KD HCOO D
) kHDθHCOOθD rDH )
(7a)
k′′DH θ θ KH DCOO H
) kDHθDCOOθH
Figure 6. Coverage-dependent feature of the peak shift of νs(OCO) bands of formate on a Ni(110) surface during the isotope exchange reaction; O and 0 represent HCOO and DCOO, respectively.
(7b)
In eq 7, rHD or rDH and kHD or kDH represent the rate and rate constant of the surface reaction indicated by the suffixes (ratedetermining step). kHD and kDH are related to the apparent rate constants k′′HD and k′′DH as follows:
kHD )
k′′HD KD
(8a)
kDH )
k′′DH KH
(8b)
Denoting the apparent activation energy and the activation q q ′′ and EHD , left-hand energy of the rate-determining step by EHD and right-hand sides of eq 8a are rewritten as follows:
(
k′′HD ) A′′ exp Figure 7. Coverage dependence of the isotope exchange reaction on formate. The initial coverage of formate was set to be 1.
(
KD ) exp -
(
)
q EHD ′′ RT
)
(9a)
∆GD RT
) (
(9b)
)
q EHD ∆GD ′′ kHD ) A′′ exp exp RT RT
(
) A exp -
q q ) EHD ′′ - ∆HD EHD
Figure 8. Pressure dependence of the isotope exchange reaction of HCOO(a) on the deuterium. The initial coverage of formate was set to be 1, and the temperature of the surface was kept constant at 300 K.
The above-derived value 0.5 ( 0.1 of parameter n and unity of m indicate that the rate-determining step is the reaction between dissociatively adsorbed hydrogen and surface formate. Incidentally, for the activation of molecular hydrogen to be the ratedetermining step, the order n should be unity, and a null value of n should lead to the saturation in the amount of dissociatively adsorbed hydrogen under the reaction condition. The adsorbed deuterium (D(a)) and deuterium in the gas phase (D2(g)) are under equilibrium condition as expressed as follows: 1
/2D2(g) a D(a)
(5)
The equilibrium constant KD for the above is defined as
KD )
θD PD0.52
(6)
where θD represent the coverage of D(a). Then eq 7 follows from eqs 4 and 6
)
q EHD ′′ - ∆HD RT
(9c) (9d)
Here, ∆GD or ∆HD and A′′ or A represent the change of free energy on adsorption or heat of adsorption of D(a) and apparent pre-exponential factor or pre-exponential factor of the ratedetermining step including the entropy term of the adsorption of hydrogen, respectively. The energy diagram of the isotope exchange reaction of formate is represented as depicted in Scheme 2. Equations 4a and 9a lead to the relation for the apparent activation energy EHD′′ expressed as follows:
ln rHD ) -
q EHD ′′ + (ln A + ln θHCOO + ln PD0.52 ) (10) RT
In eq 10 the last three terms on the right-hand side are constant at constant values of PD2 and θHCOO. The apparent activation energy was obtained by measuring the rate of the reaction at 7 × 10-6 Torr of H2 or D2 in the temperature range from 290 to 305 K. The Arrhenius plots (the plots of ln(rHD) and ln(rDH) versus (1/T) are shown in Figure 9, and the apparent activation energies of the HCOO(a) to DCOO(a) and the DCOO(a) to HCOO(a) reactions were estimated to be 52 ( 5 and 58 ( 5 kJ‚mol-1, respectively. The temperature of the gas-phase H2 or D2 was not changed on the grounds that the dissociated hydrogen atoms play a key role as described above. The activation energy of the rate-determining step (Eq) can be estimated by the eq 9d. Because there are no data for ∆HD on formate-covered Ni(110), the value of the clean surface was used as a first approximation. The heat of adsorption of H2 on
Isotope Exchange Reaction of Formate with H2
J. Phys. Chem., Vol. 100, No. 46, 1996 18181 SCHEME 3: Mechanism of the Isotope Exchange Reaction
Figure 9. Arrhenius plots for the isotope exchange reaction of formate; O and 0 represent the reactions of HCOO(a) f DCOO(a) and DCOO(a) f HCOO(a), respectively. The pressure of D2 or H2 was kept constant at 7 × 10-6 Torr.
SCHEME 2: Energy Diagram of the Isotope Exchange Reaction
clean Ni(110) was reported to be about 45 kJ‚mol-1 (per H atom),27 and the activation energy of the rate-determining step of the isotope exchange reaction would be about 100 [)52∼58 + 45] kJ‚mol-1. Isotope Effect. The rate of the reaction from CH to CD (rHD) is 2.1 ( 0.3 times faster than that from CD to CH (rDH) at 300 K with PD2 ) PH2. The rate-determining step was found to be the reaction between adsorbed hydrogen and formate. Referring to eq 8, it consists of the thermodynamical and kinetic isotope effects (KIEs) as expressed by the following relation:
rHD KD kHD ) rDH KH kDH
(11)
The thermodynamical isotope effect (KD/KH) is evaluated by the difference of the zero-point energies between molecular hydrogen and an adsorbed one, which is equivalent to the difference of the heat of adsorption of a D atom from that of a H atom. It was reported that the heat of adsorption of a D atom is 1.3∼2.1 kJ‚mol-1 (per H and D atom) larger than that of H atom on Ni film.28,29 Thus KD/KH is estimated by eqs 6 and 9b as 1.7∼2.3 at 300 K. This estimation indicates a very small KIE in the isotope exchange reaction, i.e., kHD/kDH Z 1. The microscopic reaction mechanism is then discussed on the basis of the rate-determining step with negligible KIE. The isotope exchange reaction is usually considered to proceed either through an “abstraction-addition” mechanism in which a C-H bond of formate dissociates first and the resulting complex accepts a D atom (Scheme 3a) or through an “additionabstraction” mechanism (Scheme 3b).30 The two mechanisms depict the exchange reaction as consisting of two steps occurring in a different order; C-H (C-D) bond cleavage and C-D (CH) bond formation. The KIE of C-H (C-D) bond cleavage was studied for the decomposition of the adsorbed species by means of TPD.31,32 For example, the C-H bond is cleaved 4.4 or 5.5 times faster than the C-D is at 300 K in the case of formate31 or methoxy.32 These results indicate that the transition state involved has very little C-H bond character, and the KIE is dominated by the difference of the zero-point energies between the reactants (product-like transition state). It implies
that the transition state of the hydrogenation does not differ too much from the dehydrogenated state,33 and it will be a reactantlike transition state. The KIE in hydrogenation is, therefore, expected to be very small in dehydrogenation reactions. If the rate-determining step of the exchange reaction in Scheme 3a or 3b is the C-H (C-D) bond cleavage, the value of the KIE should be larger than unity, i.e., kHD/kDH > 1. On the contrary, assuming that the rate-determining step is the C-D or C-H bond formation in Scheme 3a or 3b, the KIE appears to be very small, which could account for the present result. However, in Scheme 3a, it is unlikely that the intermediate OCO species exist stably; it would rather desorb as CO2. The absence of the decomposition of formate species during the isotope exchange reaction in the present study may exclude the possibility of Scheme 3a. Because the intermediate in Scheme 3b is known to exist as oxymethylene on oxide surfaces,34 one may be tempted to take the “addition-abstraction” mechanism as a likely one. The recent theoretical calculation, however, has shown that the oxymethylene on Cu surface is unstable.35 In this respect, the Scheme 3b might be questionable. As another possibility, the exchange reaction does not necessarily proceed by two steps but by one step. In this case, the absence of the KIE implies that the difference of the zeropoint energies between the reactants and the transition state are almost the same. The present study leads us to propose a concerted mechanism (Scheme 3c) in which both abstraction and addition proceed simultaneously in the transition state. At the present stage, we prefer Scheme 3c to Scheme 3b as a reaction mechanism of the isotope exchange reaction. For more detailed discussions, further experiments and theoretical calculations would be required. 4. Conclusion The isotope exchange reaction of formate (HCOO(a)/DCOO(a)) on Ni(110) with gaseous D2/H2 was investigated below the decomposition temperature (330 K) by means of IRAS. When D2 was admitted on the HCOO(a)-covered surface at 300 K, the peak intensity of the ν(CH) band was decreased and that of the ν(CD) band was increased with the D2 dosing time. The feature reversed when D2 was replaced by H2 after HCOO(a) was exchanged completely to DCOO(a). In the course of the exchange reaction, any side reactions such as decomposition or desorption were not observed. The apparent activation energies for the HCOO(a) to DCOO(a) and the DCOO(a) to HCOO(a) reactions were obtained as 52 ( 5 and 58 ( 5
18182 J. Phys. Chem., Vol. 100, No. 46, 1996 kJ‚mol-1, respectively. The half-order dependence of the reaction on hydrogen (deuterium) pressure suggested that the rate-determining step was the reaction of the formate with reversibly adsorbed atomic hydrogen. The activation energies of the rate-determining step were roughly estimated to be ca. 100 kJ‚mol-1 by taking account of the heat of adsorption of hydrogen. The observation that the reaction rate of HCOO(a) with D2 was 2.1 ( 0.3 times faster than that of DCOO(a) with H2 at 300 K was attributed to the difference of the amounts of adsorption of deuterium and hydrogen. The kinetic isotope effect was very small in the present exchange reaction, and a possible reaction mechanism was proposed. Acknowledgment. A.Y. is grateful to Dr. K. Onda and T. Ohtani for valuable discussions and suggestions. References and Notes (1) Urban, M. W. Vibrational Spectroscopy of Molecules and Macromolecules on Surfaces; John Wiley and Sons: New York, 1993. (2) Anderson, J. R.; Boudart, M. Catalysis Science and Technology; Springer-Verlag: Berlin, Heidelberg, New York, Tokyo, 1984. (3) Campbell, I. M. Catalysis at surfaces; Chapman and Hall: London, 1988. (4) Keim, W. Catalysis in C1 Chemistry; D. Reidel Publishing Co.: Dordrect, The Netherlands, 1983. (5) Rodrigusz, J. A.; Goodmann, D. W. Surf. Sci. Rep. 1991, 14, 1. (6) McCarty, J. G.; Falconer, J. L.; Madix, R. J. J. Catal. 1973, 30, 235. (7) Benziger, J. B.; Madix, R. J. Surf. Sci. 1979, 79, 394. (8) Falconer, J. L.; Madix, R. J. Surf. Sci. 1974, 46, 473. (9) Madix, R. J.; Gland, J. L.; Mitchell, G. E.; Sexton, B. A. Surf. Sci. 1983, 125, 481. (10) Jones, T. S.; Richardson, N. V.; Joshi, A. W. Surf. Sci. 1988, L948, 207. (11) Jones, T. S.; Richardson, N. V. Surf. Sci. 1989, 211/212, 377.
Yamakata et al. (12) Jones, T. S.; Ashton, M. R.; Richardson, N. V. J. Chem. Phys. 1989, 90, 7564. (13) Haq, S.; Love, J. G.; Sanders, H. E.; King, D. A. Surf. Sci. 1995, 325, 230. (14) Sun, Y. K.; Weinberg, W. H. J. Chem. Phys. 1991, 94, 4587. (15) Xie, J.; Mitchell, W. J.; Lyons, K. J.; Weinberg, W. H. J. Chem. Phys. Lett. 1994, 101, 9195. (16) Hayden, B. E.; Prince, K.; Woodruff, D. P.; Bradshow, A. M. Surf. Sci. 1983, 133 , 589. (17) Sexton, B. A. Surf. Sci. 1979, 88, 319. (18) Weisel, M. D.; Chen, J. G.; Hoffmann, F. M.; Sun, Y.-K,; Weinberg, W. H. J. Chem. Phys. 1992, 97, 9396. (19) Hoffmann, P.; Bare, S. R.; Richardson, N. V.; King, D. A. Surf. Sci. 1983, 133, L459. (20) Columbia, M. R.; Crabtree, A. M.; Thiel, P. A. J. Am. Chem. Soc. 1992, 114, 1231. (21) Hoffmann, F. M. Surf. Sci. Rep. 1983, 8, 211. (22) Chabal, Y. J. Surf. Sci. Rep. 1988, 3, 107. (23) Ortega, A. Thesis, Freie Universitat Berlin, 1983. (24) Ryberg, R. Phys. ReV. Lett.. 1982, 49, 1579. (25) Bare, S. R.; Stroscio, J. A.; Ho, W. Surf. Sci. 1985, 150 , 399. (26) Burrows, V. A.; Sundaresan, S.; Chabal, Y. J.; Christman, S. B. Surf. Sci. 1985, 160, 122. (27) Christmann, K.; Schober, O.; Ertl, G. ; Neumann, M. J. Chem. Phys. 1974, 60, 4528. (28) Weldler, G.; Broker, F. J.; Fisch, G.; Schroll, G. Z. Phys. Chem.: N.F. 1971, 76, 212. (29) Ozaki, A. Isotopic Studies of Heterogenious Catalysis; Kodansha Academic Press: Tokyo, 1977. (30) King, D. A.; Woodruff, D. P. The chemical Physics of Solid Surfaces and Heterogenious Catalysis: Elsevier: Amsterdam, 1982; Vol. 4., Chapter 8. (31) Madix, R. J.; Telford, S. G. Surf. Sci. 1992, 277, 246. (32) Madix, R. J.; Telford, S. G. Surf. Sci. 1995, 328, L576. (33) Campbell, C. T.; Campbell, J. M.; Dalton, P. J.; Henn, F. C.; Rodriguez, J. A.; Seimanides, S. G. J. Phys. Chem. 1989, 93, 806. (34) Busca, G.; Lamotte, J.; Lavalley, J.C.; Lorenzelli, V. J. Am. Chem. Soc.. 1987, 109, 5197. (35) Kakumoto, T. Energy ConVers. Manage. 1995, 661, 36(6-9).
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