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J. Phys. Chem. B 1999, 103, 4562-4565
In-Situ Observation of Hydrogenation of Ethylene on a Pt(111) Surface under Atmospheric Pressure by Infrared Reflection Absorption Spectroscopy Toshiaki Ohtani, Jun Kubota, Junko N. Kondo, Chiaki Hirose, and Kazunari Domen* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ReceiVed: March 1, 1999
Hydrogenation of ethylene on a Pt(111) surface in the presence of gaseous ethylene and hydrogen in the 103 Pa range was investigated by infrared reflection absorption spectroscopy (IRAS). The IRA peaks assigned to di-σ-bonded ethylene and ethylidyne were observed on Pt(111) under the flow of 1.3 × 102 Pa of ethylene and 6.5 × 103 Pa of hydrogen at 200-300 K, and the apparent activation energy of ethylene hydrogenation was estimated to be 37 ( 5 kJ mol-1 from the rate of temperature-dependent ethane production. The ethylene hydrogenation on ethylidyne-covered Pt(111) was also examined under the same condition. The peak due to the di-σ-bonded ethylene was not observed on ethylidyne-covered Pt(111), although the similar rate of hydrogenation and the similar activation energy (39 ( 5 kJ mol-1) were obtained as those on clean Pt(111). The π-bonded ethylene, which has been previously proposed as an intermediate by the sum-frequency generation (SFG) experiment under the catalytic hydrogenation condition and by IRAS under the low-temperature condition, was not detected in the present study. The similarity of the rate and activation energy for clean and ethylidyne-covered Pt(111) surfaces suggests that the rate of hydrogenation does not depend on either coverage of di-σ-bonded ethylene or ethylidyne. The role of the observed species in the hydrogenation of ethylene is discussed.
Introduction Hydrogenation of ethylene is one of the simple catalytic reactions over metal and oxide catalysts, and numerous studies have been carried out on the mechanism of the hydrogenation.1,2 A mechanism of ethylene hydrogenation over supported metal catalysts was first proposed by Horiuti and Polanyi as that the activation of hydrogen and/or hydrogenation of the ethyl intermediate was proposed to be the rate-determining step.3 An ethyl intermediate is considered to be in equilibrium with adsorbed ethylene,3 and great attention has been given to the nature of adsorbed ethylene.4 The in-situ and postreaction analyses of single-crystal metal surfaces have been well established in the past several years, and spectroscopic techniques to monitor the surfaces during the reaction have become one of the most powerful methods to determine the intermediate and elemental steps as well as to carry out the kinetic investigation.5,6 Sum-frequency generation (SFG) spectroscopy has been applied by Cremer and Somorjai to the observation of the surface species during ethylene hydrogenation.7 They revealed that di-σ- and π-bonded ethylenes and ethylidyne (tCCH3) are present on Pt(111) under the flow of ethylene and hydrogen mixture7 and proposed that π-bonded ethylene is an intermediate on the basis of the correlation existing between the amount of the π-bonded ethylene and the rate of hydrogenation.7,8 They also suggested the presence of π-bonded propylene during hydrogenation of propylene.9 The present authors have, on the other hand, examined the behavior of π-bonded ethylene in the presence of ∼103 Pa of ethylene using infrared reflection absorption spectroscopy (IRAS), and revealed the presence of * Corresponding author. Fax: 81-45-924-5276. E-mail: kdomen@ res.titech.ac.jp.
the π-bonded ethylene on the surface at only below 180 K.10,11 Thus, the adsorption of π-bonded ethylene in a considerable amount is questionable under the reaction conditions of hydrogenation above 200 K. On Pt(111) between 240 and 300 K, di-σ-bonded ethylene was found to be present in equilibrium with the gas-phase ethylene.12 The molecular beam study has revealed the presence of weakly adsorbed ethylene which is regarded as an important species in the hydrogenation on Pt(111).13 It now seems reasonable to suppose that the weakly adsorbed species in equilibration with gaseous molecules plays a significant role during the catalytic reactions, although the issue of which specific species determines the rate of hydrogenation is still debatable. In the present study, the authors have utilized the in-situ IRAS for the observation of surface species on Pt(111) in the flow of ethylene and hydrogen mixture at 1.3 × 102 and 6.5 × 103 Pa, respectively. Under these conditions, formation of ethane was found by product analysis and the occurrence of steady-state hydrogenation was confirmed. Relation between the observed species by IRAS and the rate of hydrogenation is discussed. Experimental Section Experiments were performed in an UHV system consisting of sample preparation and IRAS chambers (base pressure of ∼10-8 Pa).11,12,14 The preparation chamber was equipped with an Ar ion gun, an Auger electron spectroscope (AES) and lowenergy electron diffraction (LEED) composite analyzer, and a quadrupole mass spectrometer (QMS). A small IRAS cell equipped with BaF2 windows was positioned inside the IRAS chamber. The IRAS cell was connected to a gas flow system to expose the sample up to atmospheric pressure of reactant gas. Commercial grade ethylene and hydrogen gases were used
10.1021/jp9907241 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/18/1999
Letters
Figure 1. In-situ IRA spectra of ethylene hydrogenation on Pt(111) at various temperatures. The mixture of 1.3 × 102 Pa ethylene and 6.5 × 103 Pa hydrogen flowed through the cell with the flow rates of 0.09 and 4.5 cm3 min-1 STP, respectively. Trace (a) was obtained without hydrogen in the atmospheric pressure of ethylene at 6.3 × 102 Pa and 150 K.10
after purification; passing over an activated copper catalyst and silica gel for ethylene, and passing through a Pd blind tube for hydrogen. The flow rates of the gases were controlled by electrical mass flow controllers and the pressure inside the IRAS cell was controlled by an electrical pressure regulator and a rotary pump. One part of the product gas from the cell was continuously introduced to the preparation chamber to analyze the contents by QMS. IRA spectra were obtained by a JEOL JIR-100 Fourier transform infrared spectrometer at 4 cm-1 resolution. A liquid nitrogen cooled InSb/MCT composite detector was used: InSb for 1800-4000 cm-1 and MCT for 800-1800 cm-1. A wire grid polarizer which was revolved by stepping motor, was used to obtain the ratio spectra for p- and s-polarized beams to cancel the absorptions by the sample gases. The spectra for p- and s-polarized beams were alternately collected by every 20 scans, and each 1040 scans were accumulated. The Pt(111) sample (10 mm diameter) was prepared by the cycle of Ar+ bombardment at 300 K and annealing at 1100 K. The cleanliness and long-range structure were checked by AES and LEED. Results and Discussion The temperature-dependent IRA spectra of Pt(111) under the flow of a mixture of 1.3 × 102 Pa of ethylene and 6.5 × 103 Pa of hydrogen at the flow rates of 0.09 and 4.5 cm3 min-1, respectively, where the flow rates were converted into the volumes for standard temperature and pressure (STP: 1.01 × 105 Pa, 273 K), are shown in Figure 1. At 220 K, a peak was observed at 2918 cm-1, which was assigned to the C-H symmetric stretching mode of di-σ-bonded ethylene.15,16 The peak due to di-σ-bonded ethylene in the presence of gaseous ethylene was reported to be positioned at 2906 cm-1.12 The shift
J. Phys. Chem. B, Vol. 103, No. 22, 1999 4563 of 12 cm-1 in the present result is possibly caused by the effect of the presence of hydrogen on the surface. With heating the sample above 300 K, new peaks appeared at 2883 and 1338 cm-1. These peaks were assigned to the C-H symmetric stretching and CH3 symmetric deformation modes of ethylidyne.17 It is noteworthy that di-σ-bonded ethylene was present on the surface even at 320 K, although the di-σ-bonded ethylene is desorbed or decomposed below 250 K in a vacuum.18-20 The presence of di-σ-bonded ethylene at 320 K suggests that di-σbonded ethylene is equilibrated with gaseous ethylene with suppression of the formation of ethylidyne under the conditions.12 The π-bonded ethylene has a very strong IRAS peak assigned to the out-of-plane bending (CH2 wagging) mode at 954 cm-1, and it is actually present on Pt(111) below 180 K under 1.3 × 103 Pa of ethylene.10 Trace (a) in Figure 1 is the IRA spectrum of adsorbed ethylene on Pt(111) at 150 K in 6.3 × 102 Pa of ethylene.10 The peaks at 3090 and 954 cm-1 were assigned to the C-H antisymmetric stretching and out-of-plane bending modes of π-bonded ethylene, respectively.10,11 The peak at 2906 cm-1 is due to di-σ-bonded ethylene.10,11 The coverage of π-bonded ethylene is close to the saturation coverage under the condition,10 and it may be approximately 0.25 with respect to the number of Pt atoms. Although some signals may be present around 1000 cm-1 under the hydrogenation conditions, the authors tentatively judged them due to the noise. The absence of the band due to π-bonded ethylene under the hydrogenation conditions at 220-320 K clearly indicates that the coverage of π-bonded ethylene under the conditions is less than one tenth of the saturation coverage if it is present on the surface. Any peaks assignable to the π-bonded ethylene were thus not identified. The heat of adsorption of π-bonded ethylene has been derived as 35 ( 10 kJ mol-1 in the low-temperature region.10,11 With the extrapolation from the low temperature data, it is reasonably expected that the extreamely high pressure of more than 107 Pa is necessary for the adsorption of π-bonded ethylene with coverage of ∼0.1 at 300 K. The reason the π-bonded ethylene has been detected by SFG7,8 but not by IRAS under similar condition is probably attributed to the observations of different species. The characteristic band at 954 cm-1 due to the out-of-plane bending mode of π-bonded ethylene in IRA spectra directly indicates that the ethylene molecules have the complete CdC double bond with the molecular plane parallel to the surface.10,11 If the CdC bond is partially cleaved on the surface, the deformation band corresponding to the out-of-plane bending mode (CH2 wagging) loses the absorption coefficient similar to the di-σ-bonded ethylene. The π-bonded ethylene proposed by the present authors has the IRAS peak at 3090 cm-1 due to the C-H antisymmetric stretching mode,11 and this fact apparently contraradicts the observation of the symmetric stretching mode at ∼3000 cm-1 by SFG.7,8 It suggests that the molecular structure of each species which was assigned to π-bonded ethylene by SFG and IRAS should be somewhat different. The in-situ observation during hydrogenation of ethylene on ethylidyne-covered Pt(111) was next performed. The ethylidynecovered Pt(111) was prepared by exposure of Pt(111) to 1.3 × 102 Pa of ethylene at 320 K. Figure 2 shows the temperaturedependent IRA spectra of the ethylidyne-covered Pt(111) surface under the flow of ethylene and hydrogen at the same conditions as those in Figure 1. At any temperatures, only the peaks assigned to ethylidyne were observed at 2881, 1340, and 1132 cm-1 (the peak at 1132 cm-1 was assigned to the C-C stretching mode).17 Any peaks due to di-σ-bonded ethylene
4564 J. Phys. Chem. B, Vol. 103, No. 22, 1999
Figure 2. In-situ IRA spectra of ethylene hydrogenation on ethylidynecovered Pt(111) as a function of temperature. The mixture of 1.3 × 102 Pa ethylene and 6.5 × 103 Pa hydrogen flowed through the cell with the flow rates of 0.09 and 4.5 cm3 min-1 STP, respectively. The Pt(111) surface was first exposed to 1.3 × 102 Pa of ethylene at 320 K to form the ethylidyne layer.
Letters activation energy was estimated as 39 ( 5 kJ mol-1. It should be noted that the reaction rates and activation energies of hydrogenation over the ethylidyne-covered Pt(111) surface are almost the same as those over the ethylidyne-free Pt(111) surface. The ethylidyne species is known to be inactive for hydrogenation of ethylene8,22 and the rate of hydrogenation is not directly controlled by the coverage of ethylidyne. If di-σbonded ethylene is a precursor of the rate-determining step, the coverage of di-σ-bonded ethylene may reflect the reaction rate. The rate of hydrogenation over Pt(111) is not related to the coverage of di-σ-bonded ethylene and the di-σ-bonded ethylene is not a precursor in front of the activation barrier of the hydrogenation reaction.7,8 The present results of the product analysis also indicate that neither ethylidyne nor di-σ-bonded ethylene is a precursor for the activation barrier of the hydrogenation. The reaction order for ethylene pressure is known to be almost zero in the studied pressure range, and the coverage of adsorbed ethylene in the precursor state has been thus considered to be saturated.23 Absence of the peak due to the π-bonded ethylene in the present spectra in contrast to the observation of the π-bonded ethylene at 180 K10 indicates that the π-bonded ethylene cannot adsorb to a considerable extent under the catalytic conditions. The adsorbed ethylene which was proposed as a π-bonded ethylene by Cremer and Somorjai from the SFG spectra7,8 could not be detected by IRAS. They have probably observed some other kind of species which is somewhat different from the π-bonded species found by IRAS.10 In the present study, the coverage of neither di-σ-bonded nor π-bonded ethylene gave reasonable understanding of the rate of hydrogenation and the reaction order. Both di-σ-bonded and π-bonded ethylene are concluded not to be the precursor of the ratedetermining step of hydrogenation, although the authors cannot completely exclude the possibility of the presence of IRASundetectable species on the surface. The activation of hydrogen for the hydrogenation of ethylene may play an important role in the ethylene hydrogenation under the catalytic condition,24 and the rate of hydrogenation is probably independent of the coverage of any ethylene species on the mechanism. Further experiments should be required to clarifiy this point. Conclusion
Figure 3. Arrhenius plots of the rates of ethane production over (a) Pt(111) and (b) ethylidyne-covered Pt(111). The mixture of 1.3 × 102 Pa ethylene and 6.5 × 103 Pa hydrogen flowed through the cell with the flow rates of 0.09 and 4.5 cm3 min-1 STP, respectively. The derived activation energies were (a) 37 ( 5 and (b) 39 ( 5 kJ/mol.
which was observed on clean Pt(111) as shown in Figure 1 did not emerge. It is obvious that the ethylidyne blocked the sites for the adsorption of di-σ-bonded ethylene.12 The saturation coverage of ethylidyne is known to be 0.25 with respect to the number of surface Pt atoms, and no vacant site is present on the surface for the adsorption of di-σ-bonded ethylene.20 The suppression of adsorption of di-σ-bonded ethylene by ethylidyne in the present study was in good agreement with that proposed by the SFG results under the catalytic conditions.7,8 The partial pressure of produced ethane, which corresponds to the rate of hydrogenation of ethylene over Pt(111), was monitored by QMS, and the results are shown in Figure 3 as Arrhenius plots. The apparent activation energy of 37 ( 5 kJ mol-1 was derived from the slope, and it was in good agreement with the previous work.21 The Arrhenius plots for ethylidynecovered Pt(111) are also shown in Figure 3, and the apparent
The Pt(111) surface during hydrogenation of ethylene under the flow of a mixture of ethylene and hydrogen was examined by in-situ IRAS. The di-σ-bonded ethylene was observed on Pt(111) during the hydrogenation below 320 K, and ethylidyne formed under the hydrogenation above 280 K. On the ethylidyne-covered Pt(111) surface, only ethylidyne was observed under the reaction condition indicating that the sites for the adsorption of di-σ-bonded ethylene were blocked by ethylidyne. No bands assignable to π-bonded ethylene were detected on either surface. The rates of ethane production were measured on Pt(111) and ethylidyne-covered Pt(111) and activation energies were derived as 37 ( 5 and 39 ( 5 kJ mol-1, respectively. Similarity of the rates and activation energies of hydrogenation over both surfaces indicates that the reaction rate is not correlated with the coverage of either di-σ-bonded ethylene nor ethylidyne. References and Notes (1) Bond, B. C.; Wells, P. B. AdV. Catal. 1964, 15, 91. (2) Bertolini, J. C.; Massardier, J. The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 3; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1984. (3) Horiuti, I.; Polanyi, M. Trans. Faraday Soc. 1934, 30, 1164.
Letters (4) Kemball, C. J. Phys. Chem. 1956, 60, 735. (5) Surface Science of Catalysis; Dwyer, D. J., Hoffmann, F. M., Eds.; American Chemical Society: Washington, DC, 1992. (6) Rodriguez, J. A.; Goodman, D. W. Surf. Sci. Rep. 1991, 14, 1. (7) Cremer, P. S.; Somorjai, G. A. J. Chem. Soc., Faraday Trans. 1995, 91, 3671. (8) Cremer, P. S.; Su, X.; Shen, Y. R.; Somorjai, G. A. J. Am. Chem. Soc. 1996, 118, 2942. (9) Cremer, P. S.; Su, X.; Shen, Y. R.; Somorjai, G. A. J. Phys, Chem. 1996, 100, 16302. (10) Kubota, J.; Ohtani, T.; Kondo, J. N.; Hirose, C.; Domen, K. Appl. Surf. Sci. 1997, 121/122, 548. (11) Kubota, J.; Ichihara, S.; Kondo, J. N.; Hirose, C.; Domen, K. Surf. Sci. 1996, 357/358, 634. (12) Ohtani, T.; Kubota, J.; Kondo, J. N.; Hirose, C.; Domen, K. Surf. Sci. 1998, 415, L983. (13) O ¨ fner, H.; Zaera, F. J. Phys. Chem. B 1997, 101, 396. (14) Kubota, J.; Ohtani, T.; Kondo, J. N.; Hirose, C.; Domen, K. Vacuum., in press.
J. Phys. Chem. B, Vol. 103, No. 22, 1999 4565 (15) Hoffmann, H.; Griffiths, P. R.; Zaera, F. Surf. Sci. 1992, 262, 141. (16) Fan, J.; Trenary, M. Langmuir 1994, 10, 3649. (17) Ransley, I. A.; Ilharco, L. M.; Bateman, J. E.; Sakakini, B. H.; Vickerman, J. C.; Chesters, M. A. Surf. Sci. 1993, 298, 187. (18) Somorjai, G. A.; Van Hove, M. A.; Bent, B. E. J. Phys. Chem. 1988, 92, 973. (19) Steininger, H.; Ibach, H.; Lehwald, S. Surf. Sci. 1982, 117, 685. (20) Kesmodel, L. L.; Dubois, L. H.; Somorjai, G. A. J. Chem. Phys. 1979, 70, 2180. (21) Zaera, F.; Somorjai, G. A. J. Am. Chem. Soc. 1984, 106, 2293. (22) Davis, S. M.; Zaera, F.; Gordon, B. E.; Somorjai, G. A. J. Catal. 1985, 92, 240. (23) Cortright, R. D.; Goddard, S. A.; Rekoske, J. E.; Dumesic, J. A. J. Catal. 1991, 127, 342. (24) Rekoske, J. E.; Cortright, R. D.; Goddard, S. A.; Sharma, S. B.; Dumesic, J. A. J. Am. Chem. Soc. 1991, 96, 1880.
