Langmuir 1998, 14, 1411-1418
1411
Adsorption and Reaction of Ethyl Fragments on Reduced and Oxidized Silica-Supported Copper Particles M. D. Driessen and V. H. Grassian* Department of Chemistry, University of Iowa, Iowa City, Iowa 52242 Received July 3, 1997. In Final Form: November 11, 1997 Infrared spectroscopy and temperature programmed desorption have been used to investigate the adsorption and reaction of ethyl fragments, from the dissociative adsorption of ethyl chloride and ethyl bromide, on reduced- and oxidized-Cu/SiO2. On reduced-Cu/SiO2, ethyl chloride dissociates at room temperature to form adsorbed ethyl fragments on the copper particle surface. In addition, ethyl fragments can spill over onto the silica support where they undergo reaction with the SiOH groups to form SiOCH2CH3. The reaction chemistry of these adsorbed hydrocarbon fragments was followed as a function of temperature from 300 to 1100 K. Propane, ethylene, ethane, and methane evolve from reaction of ethyl fragments on the copper particle surface below 500 K. The distribution of these products on reducedCu/SiO2 is found to depend on sample preparation. On oxidized-Cu/SiO2, both CsBr and CsC bond dissociation in adsorbed ethyl bromide occur at room temperature. As determined by infrared spectroscopy, reaction of hydrocarbon fragments with surface oxygen atoms leads to the formation of adsorbed ethoxide, methoxide, and bidentate formate. Formaldehyde and acetaldehyde evolve from oxidized-Cu/SiO2 near 550 and 410 K, respectively; ethylene, CO, and CO2 form as well. Possible mechanisms for the formation of adsorbed and gas-phase products from C2H5X adsorption and reaction on reduced- and oxidized-Cu/SiO2 are discussed.
Introduction The chemistry of adsorbed hydrocarbon fragments on transition metal surfaces constitutes the basis for many industrially important catalytic reactions including the Fischer-Tropsch process.1,2 Hydrocarbon fragments can undergo several reactions on a metal surface, and the factors that affect the selectivity of one reaction over another are key to understanding catalytic processes. For these reasons, hydrocarbon reactions have been extensively studied on the surfaces of single crystal metal surfaces in ultrahigh vacuum.3-10 It has been shown that on single crystal metal surfaces adsorbed alkyl fragments can be selectively formed through the low temperature thermal or photochemical dissociation of the corresponding alkyl halide, as shown in reaction 1:9-12
Recently, it has been shown that the alkyl halide precursor approach is applicable to supported metal catalysts including Pt, Pd, and Cu supported on silica.13-17 In our studies of methyl iodide adsorption on supported metal catalysts, Cu/SiO2 is found to exhibit the most interesting behavior because the reaction of methyl fragments is found to depend on sample preparation.15 It was previously determined that several factors influence the surface chemistry of methyl iodide and methyl fragments on silica-supported copper particles. These factors include (i) the oxidation state of the copper, (ii) the hydroxyl group coverage on the silica support, and (iii) the surface roughness of the copper particles. Here we extend our studies to reactions of ethyl halides and ethyl fragments on reduced- and oxidized-Cu/SiO2 samples. Experimental Section
RX (a) f R(a) + X(a)
(1)
where X ) Cl, Br, or I). The alkyl halide approach has been used in many studies as a method for generating alkyl fragments. This approach has yielded hundreds of spectroscopic and chemical studies of alkyl fragments on metal surfaces. Bent has written an excellent review article on the importance of those surface science studies and their relevance to heterogeneous catalysis.7 * Author to whom correspondence should be addressed. (1) Anderson, R. B. The Fischer-Tropsch Synthesis Academic. New York, 1984. (2) Rofer-DePoorter, C. K. Chem. Rev. 1981, 81, 447. (3) Zaera, F. Chem. Rev. 1995, 95, 2651. (4) Lin, J.-L.; Chiang, C.-M.; Jenks, C. J.; Yang, M. X.; Wentzlaff, T. H.; Bent, B. E. J. Catal. 1994, 147, 250. (5) Paul, A.; Bent, B. E. J. Catal. 1994, 147, 264. (6) Zhou, X.-L.; White, J. M. J. Phys. Chem. 1991, 95, 5575. (7) Bent, B. E. Chem. Rev. 1996, 96, 1361. (8) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley and Sons: New York, 1994. (9) Zaera, F. Acc. Chem. Res. 1992, 25, 260. (10) Solymosi, F. Catal. Today 1996, 28, 193. (11) Zhou, X.-L.; Zhu, X.-Y.; White, J. M. Acc. Chem. Res. 1990, 23, 327. (12) Lin, J.-L.; Bent, B. E. J. Vac. Sci. Technol., A 1992, 10, 2202.
The infrared experiments were done in an IR cell that has been described previously.14,15,18,19 The cell consists of a 2.75inch stainless steel cube with two differentially pumped barium fluoride windows and a sample holder through which thermocouple and power feedthroughs are connected to a tungsten sample grid. The temperature of the sample is monitored by thermocouple wires spot welded to the top of the sample grid. The sample holder design is such that the sample may be cooled to near liquid nitrogen temperatures and heated resistively up to 1300 K. The cell is attached to an all stainless steel vacuum chamber through a 2-ft bellows hose. The vacuum system is rough pumped using a turbomolecular pump and then pumped with an 80 L/s ion pump. (13) (a) McGee, K. C.; Driessen, M. D.; Grassian, V. H. J. Catal. 1996, 159, 69; (b) McGee, K. C.; Driessen, M. D.; Grassian, V. H. J. Catal. 1995, 157, 730. (14) (a) Driessen, M. D.; Grassian, V. H. J. Catal. 1996, 161, 810; (b) Driessen, M. D.; Grassian, V. H. Langmuir 1995, 11, 4213. (15) Driessen, M. D.; Grassian, V. H. J. Am. Chem. Soc. 1997, 119, 1697. (16) Rasko, J.; Bontovics, J.; Solymosi, F. J. Catal. 1993, 143, 138. (17) Solymosi, F.; Rasko, J. J. Catal. 1995, 155, 74. (18) Miller, T. M.; Grassian, V. H. J. Am. Chem. Soc. 1995, 117, 10969. (19) Basu, P.; Ballinger, T. H.; Yates, J. T., Jr. Rev. Sci. Instrum. 1988, 59, 1321.
S0743-7463(97)00721-X CCC: $15.00 © 1998 American Chemical Society Published on Web 02/07/1998
1412 Langmuir, Vol. 14, No. 6, 1998 Samples are made by spraying a slurry of copper(II) nitrate trihydrate (Strem Chemicals, 99.999%) and silica (Cabosil, M-5, 200 m2/g) suspended in acetone and water onto a tungsten grid (Buckbee-Mears). A template is used to mask one half of the tungsten grid, allowing one side to be coated with ∼30 mg of Cu/SiO2 and the other with ∼30 mg of SiO2 (to monitor reactions on the silica support without the presence of the copper particles). Copper loadings on the order of 15% by weight are typically used in these experiments. The copper surface area is ∼13% of the total catalyst surface area. After the sample is prepared, it is mounted inside the IR cell, wrapped in heating tape, and processed by one of three methods. The three different processing procedures are denoted as reducedCu/SiO2(473 K), reduced-Cu/SiO2(673 K), and oxidized-Cu/SiO2. The reduced-Cu/SiO2(473 K) and reduced-Cu/SiO2(673 K) are samples that have been reduced under different conditions. The processing for all three samples begins with a 12 h, 473 K bakeout. The copper is then reduced with hydrogen. Hydrogen (Air Products, Research Grade) is introduced into the sample cell in 400 Torr quantities for 15 min followed by a 15-min evacuation. Hydrogen is introduced for increasingly longer periods of time; that is, 30, 60 and then 120 min, with each reduction period followed by a 15-min evacuation. Following this initial reduction, the samples are oxidized in 100 torr of oxygen (Air Products, 99.6%) for 10 min followed by evacuation and a 30-min reduction in hydrogen. This oxidation/reduction cycle is repeated if necessary to remove residual organics. Nothing further is done for the Cu/SiO2(473 K) sample, but preparation of the Cu/SiO2(673 K) sample continues from this point. The Cu/SiO2(673 K) sample is then resistively heated to 673 K in the presence of 400 Torr of hydrogen for periods of 12 h or longer. Preparation of oxidized-Cu/SiO2 samples follow the exact same procedure as for the Cu/SiO2(473K) samples; however, after the last reduction, the sample is then oxidized in 5 torr of oxygen at 473 K for 120 min. After processing, the IR cell is placed on a linear translator inside the spectrometer sample compartment. Either the Cu/ SiO2 or SiO2 side of the tungsten grid can be translated into the IR beam for data acquisition. This design enables us to examine the chemistry of both Cu/SiO2 and SiO2 during the course of a particular experiment. A Mattson RS-1 Fourier transform infrared spectrometer equipped with a narrowband mercury cadmium telluride detector was used for the IR measurements. Spectra were recorded by averaging 1000 scans at an instrument resolution of 4 cm-1. Absorbance spectra shown represent single beam spectra referenced to the appropriate single beam spectrum of the Cu/SiO2 or SiO2 sample prior to reaction. The transmission range of SiO2 goes down to ∼1300 cm-1. Temperature-programmed desorption (TPD) experiments were done in a high vacuum chamber equipped with a 400 L/s ion pump, sample cell, and quadrupole mass spectrometer (DetecTorr II, UTI Instruments). The temperature was ramped by interfacing a programmable power supply to a PC. A heating rate of 1 K/s was used. A maximum of 12 different masses were monitored simultaneously. After introduction of high pressures (∼15 Torr) of CH3I in the sample cell, the cell was pumped with a turbomolecular pump for 2 h before opening the valve to the mass spectrometer chamber and acquiring TPD data. Ethanol (CH3CH2OH; Pharmco) and ethyl chloride, (CH3CH2Cl, Aldrich, 99.7%) were transferred to a glass bulb and subjected to several freeze-pump-thaw cycles before use. CD3CH2Cl (CDN Isotopes, 99.7%) was used without further purification.
Results and Discussion 1. Sample Characterization. As discussed in the Experimental Section, three different procedures for preparation of the Cu/SiO2 samples were employed. These
Driessen and Grassian
three samples, denoted as reduced-Cu/SiO2(673 K), reduced-Cu/SiO2(473 K), and oxidized-Cu/SiO2, have been previously characterized by transmission electron microscopy (TEM) and CO adsorption.15 A full discussion of how these samples were characterized can be found in ref 15. Briefly, it has been determined from TEM that the Cu/SiO2 samples used in this study are composed of copper nanoparticles with diameters between 2 and 5 nm. CO adsorption data in conjunction with IR spectroscopy revealed that samples reduced in H2 for long time periods of time and at high temperatures [i.e., reduced-Cu/SiO2(673 K) samples] consist of copper nanoparticles that are reduced and have surfaces that are atomically smooth. Reduced samples processed in H2 at lower temperatures [i.e., reduced-Cu/SiO2(473 K) samples] consist of copper nanoparticles that are reduced and have surfaces that are atomically rough. Another difference between reducedCu/SiO2(673 K) and reduced-Cu/SiO2(473 K) is that there is a lower hydroxyl group coverage on the silica support for reduced-Cu/SiO2(673 K) relative to reduced-Cu/SiO2(473 K) because the silica support becomes more dehydroxylated at the higher temperatures. It has been previously determined that the hydroxyl group coverage on the reduced-Cu/SiO2(673 K) is ∼25-50% less than that of reduced-Cu/SiO2(473 K) and that hydroxyl groups in close proximity to the Cu particles are preferentially removed.14a,15 The oxidized-Cu/SiO2 samples consist of copper particle surfaces with adsorbed oxygen atoms with a surface stoichiometry of Cu2O. 2. Adsorption and Reaction of Ethyl Chloride on Reduced-Cu/SiO2. Ethyl chloride reaction on a pure silica sample, reduced-Cu/SiO2(673 K), and reduced-Cu/ SiO2(473 K) was investigated by introducing ethyl chloride at an equilibrium pressure of 10.8 Torr for 60 min at room temperature. Following evacuation of unreacted ethyl chloride, IR spectra were collected and are shown in Figure 1. Figure 1a shows the IR spectrum of silica following reaction with ethyl chloride. It can be seen that there are no new absorptions in the SiO2 spectrum. The IR spectrum following reaction of ethyl chloride with reduced-Cu/SiO2(673 K) is shown in Figure 1b. Several absorptions are observed in the CH stretching region near 2967, 2927, 2873, and 2830 cm-1, and in the CH2 and CH3 deformation region near 1453 and 1379 cm-1. The spectrum taken following adsorption of ethyl chloride on reduced-Cu/SiO2(473 K) also contains several IR absorptions in the CH stretching region although they are distinctly different from those observed on the reduced-Cu/SiO2(673 K) sample. Absorptions are observed in the 3000-2800 cm-1 region near 2982, 2932, 2903, and 2878 cm-1 and in the 1300-1500 cm-1 region near 1483, 1477, 1394, 1381, and 1369 cm-1. It is evident from the spectrum shown in Figure 1a that silica does not react with ethyl chloride under the reaction conditions used here; therefore, any reaction observed on the Cu/SiO2 samples is due to the presence of the copper particles. The IR spectrum after adsorption of CH3CH2Cl on reduced-Cu/SiO2(673 K) at room temperature (Figure 1b) is very similar to the IR spectrum of ethyl fragments formed from the partial hydrogenation of adsorbed ethylene, on Pt/SiO2 and Ni/SiO2 (see Table 1).20,21 The spectrum in Figure 1b is therefore assigned to ethyl fragments adsorbed on the copper surface of the reducedCu/SiO2 (673 K) sample. Figure 2 displays the IR spectrum following adsorption of CH3CH2Cl onto reduced-Cu/SiO2(673 K) as a function (20) De la Cruz, C.; Sheppard, N. J. Mol. Structure 1991, 247, 25. (21) Primet, M.; Sheppard, N. J. Catal. 1976, 41, 258.
Adsorption/Reaction of Ethyl Fragments on Copper
Figure 1. Infrared spectra taken following the adsorption of CH3CH2Cl on (a) SiO2, (b) reduced-Cu/SiO2(673 K), and (c) reduced-Cu/SiO2(473 K) at room temperature. The spectrum labeled d was taken following adsorption of CH3CH2OH on SiO2 at room temperature. Table 1. Vibrational Assignment of Adsorbed Ethyl Fragments mode descriptiona
CH3CH2-Cu/SiO2a (CD3CH2-Cu/SiO2)
CH3CH2-Pt/ SiO2b
CH3CH2-Ni/ SiO2c
νas(CH3) νs(CH2) νs(CH3) 2δ(CH3) δas(CH3) δs(CH3)
2967 (2222) 2927 (2936) 2873 (2120) 2830 (2078) 1453 1379
2957 2939 2870
2958 2925 2878
1468 1383
1452 1379
a
This work. b Reference 20. c Reference 21.
of temperature. As the spectra show, there is a drastic decrease in the intensity of the IR bands associated with adsorbed ethyl fragments between 373 and 423 K. By 473 K it is clear that the absorptions due to ethyl fragments adsorbed on the copper particles are gone and IR absorptions due to another adsorbed species, which were apparently masked by ethyl absorptions are evident. The intensity pattern and frequencies of the remaining bands match the spectrum of SiOCH2CH3. This second species is stable to higher temperatures, which is consistent with the assignment of this species to SiOCH2CH3 as will be discussed in more detail. In addition to the adsorption of CH3CH2Cl, CD3CH2Cl was also adsorbed onto reduced-Cu/SiO2(673 K). The IR spectrum taken following adsorption is shown in Figure 3a, along with the IR spectrum taken after heating the sample to 573 K and cooling to room temperature (Figure 3c). The spectrum shown after heating to 573 K is due to SiOCH2CD3 because ethyl fragments react and desorb at temperatures below 573 K (vide supra). It is also evident that upon heating, CsD bond dissociation in
Langmuir, Vol. 14, No. 6, 1998 1413
Figure 2. Temperature-dependent IR spectra following the adsorption of CH3CH2Cl on reduced-Cu/SiO2(673 K). Each spectrum was recorded at 298 K after heating to the indicated temperature. Two species are present. Cu-CH2CH3 has intense absorption bands at 2967, 2927, 2873, 2830, 1453, and 1379 cm-1 in the spectrum labeled 298 K. After heating to 473 K, the absorption bands present in the spectrum at 2972, 2941, 2906, 2884, 1463, 1447, and 1383 cm-1 are due to SiOCH2CH3. SiOCH2CH3 absorptions in the room temperature spectrum are masked by the more intense absorptions of Cu-CH2CH3 in the 298 K spectrum.
adsorbed ethyl yields adsorbed D atoms. The band near 2760 cm-1 is evidence that these D atoms can migrate to the silica support and undergo an exchange reaction with SiOH to give SiOD. The absorption bands in the spectrum shown before heating, Figure 3a, are due to a combination of SiOCH2CD3 and ethyl groups adsorbed on the copper particle surface. To obtain the spectrum of CH2CD3 adsorbed on the copper particles, the spectrum of SiOCH2CD3 (Figure 3b) and SiOD was subtracted from that of the spectrum containing both SiOCH2CD3 and ethyl groups adsorbed on the copper particles. This difference spectrum is shown in Figure 3c. The IR absorptions due to Cu-CH2CD3 are observed at 2936 cm-1 in the CH stretching region and at 2222, 2120, and 2078 cm-1 in the CsD stretching region. Because there is only a single band observed in the CH stretching region for CH2CD3, it is obviously associated with the CH2 group. In the same manner, the bands in the C-H stretching region (2967 and 2873 cm-1) that are not present in the partially deuterated ethyl fragment, but are present in the spectrum of the perhydro ethyl fragment, are assigned to CH3 modes. The bands in the C-D stretching region are correlated to the bands that were observed in the C-H stretching region due to methyl stretching modes (2222 and 2120 cm-1). Using the frequency shifts observed upon deuteration and the vibrational assignment for adsorbed ethyl fragments on Pt and Ni surfaces given by
1414 Langmuir, Vol. 14, No. 6, 1998
Driessen and Grassian Table 2. Vibrational Assignment of SiOCH2CH3 C2H5Cl-Cu/ mode descriptiona SiO2b (473 K) C2H5OH-SiO2b C2H5OH-SiO2a νas(CH3) νas(CH2) νs(CH3) νs(CH2) δs(CH2)-scissor δas(CH3)-int. bend δs(CH3)-wag δ(CH2)-rock δs(CH2)-wag a
2982 2932 2903 2878 1483 1477 1394 1381 1369
2986 2940 2911 2882 1492 1452 1399 1384 1348
2986 2950 2909 1489 1459 1397 1373 1299
Reference 23. b This work.
Figure 3. Infrared spectra taken following the adsorption of (a) CD3CH2Cl on reduced-Cu/SiO2(673 K) at room temperature (the spectrum labeled a contains contributions from CD3CH2 groups adsorbed on the copper particles and SiOCH2CH3), and (b) after heating to 573 K, this spectrum contains absorption bands of SiOCH2CH3 and SiOD. (c) Difference spectrum obtained after subtracting spectrum b from spectrum a. The spectrum shown in c is that of CD3CH2 groups adsorbed on the surface of the copper particles.
Sheppard and co-workers,20,21 the IR absorption bands for CH2CH3 and CH2CD3 adsorbed on copper particles are assigned in Table 1. One of the bands (2078 cm-1) in the deuterated fragment is assigned to an overtone of the CD3 deformation as has been done for condensed CD3CH2I.22 The two bands below 1500 cm-1 have been assigned by Sheppard and co-workers to the asymmetric and symmetric deformation modes of the methyl groups.20,21 This assignment is confirmed by the absence of these two bands in the spectrum of CH2CD3. The spectrum recorded after adsorption of CH3CH2Cl on reduced-Cu/SiO2(473 K) (Figure 1c) is quite different from the IR spectrum taken following ethyl chloride adsorption on reduced-Cu/SiO2(673 K). The bands observed in the reduced-Cu/SiO2(473 K) spectrum are similar to that of an ethoxy species bound to silica. If the bands in the spectrum shown in Figure 1c are compared with literature spectra of SiOCH2CH3, it is evident that the predominant species present on the reduced-Cu/SiO2(473 K) sample following the room temperature adsorption of ethyl chloride is SiOCH2CH3.23 The spectrum of SiOCH2CH3 was reproduced in our laboratory after adsorption and reaction of C2H5OH on SiO2, this spectrum is shown in Figure 1d. The reaction between ROH and SiOH groups is known to produce the alkoxide, SiOR.24 The intensity pattern and frequencies of the IR absorptions for SiOCH2CH3 from ethanol adsorption match most of the bands in the IR spectrum taken following ethyl chloride adsorption on reduced-Cu/SiO2(473 K) very well (see Table 2). The temperature-dependent IR spectra for the reducedCu/SiO2(473 K) sample following adsorption of CH3CH2Cl are shown in Figure 4. The 298 K spectrum is identical to the one shown in Figure 1c. As the sample is heated, there is little obvious change in the spectrum up to T ) 473 K. However, if a difference spectrum is taken between (22) Hoffman, H.; Griffiths, P. R.; Zaera, F. Surf. Sci. 1992, 262, 141. (23) Tedder, L. T.; Lu, G.; Crowell, J. E. J. Appl. Phys. 1991, 69, 7037. (24) Morrow, B. A. J. Chem. Soc., Faraday Trans. 1 1974, 1527.
Figure 4. Temperature-dependent IR spectra taken following the adsorption of CH3CH2Cl on reduced-Cu/SiO2(473 K). Each spectrum was recorded at 298 K after heating to the indicated temperature.
the spectrum recorded after heating to 473 K and the spectrum recorded before heating, loss in intensity of IR absorption bands near 2970, 2932, 2872, and 1453 cm-1 is observed. These bands can be assigned to ethyl fragments adsorbed on the surface of copper particles. These bands are masked by the more intense bands of SiOCH2CH3. The IR bands due to SiOCH2CH3 do not begin to decrease in intensity until 573 K and are gone after heating to 1073 K, as seen in Figure 4. It is concluded from the aforementioned data that both Cu-CH2CH3 and SiOCH2CH3 form on the two reducedCu/SiO2 samples, reduced-Cu/SiO2(673 K) and reducedCu/SiO2(473 K), although the relative amounts of the two species differ on the two samples. On reduced-Cu/ SiO2(473 K) the IR spectrum at 298 K is primarily due to SiOCH2CH3 absorptions, and on reduced-Cu/SiO2(673 K), the infrared spectrum is primarily due to Cu-CH2CH3 absorptions. Although the CusCl stretch is too low in frequency to be detected in these experiments, the chlorine atoms are most likely bonded to the copper surface, as is found for single crystal surfaces following CsCl bond
Adsorption/Reaction of Ethyl Fragments on Copper
Langmuir, Vol. 14, No. 6, 1998 1415
dissociation of adsorbed alkyl chlorides.9-12 Reactions 2 and 3 show these two reaction pathways: Cu-C2H5 + Cu-Cl
(2)
SiOCH2CH3 + Cu-Cl
(3)
Scheme 1. Concerted Mechanism for Alkyl Spillover
Reduced-Cu/SiO2 + C2H5Cl
The difference in the amount of SiOCH2CH3 produced on reduced-Cu/SiO2(673 K) and reduced-Cu/SiO2(473 K) can be attributed to the difference in hydroxyl group coverage on the silica support for these two samples. Previously, we have shown that methyl fragments from methyl iodide dissociation can spill over onto the silica support where they react with SiOH groups to form SiOCH3.14,15 The extent of SiOCH3 formation was found to be dependent on the SiOH coverage with a greater amount of SiOCH3. Similar processes are observed for ethyl fragments (i.e., ethyl fragments can migrate to the silica support and react with SiOH groups to form SiOCH2CH3 to an extent that depends on the SiOH coverage). Spillover of hydrogen is a well-documented phenomenon in catalysis that occurs over macroscopic distances.25-27 The mechanism for this process is stepwise. Initially, molecular hydrogen dissociates on the surface of the metal particles. Hydrogen atoms then diffuse on the metal particle surface and migrate on to the support. For example, when D2 is adsorbed on Rh/Al2O3, D2 dissociates on the Rh surface followed by D atom spillover to the alumina support. The D atoms then diffuse across the support and can exchange with OH groups to form OD.27 Initially, it was thought that the mechanism for methyl spillover was similar,14 however, experiments to verify that methyl and ethyl migrate to the support after adsorption on the metal surface were unsuccessful. In addition, the importance of hydroxyl group coverage on the extent of spillover suggests a different mechanism may be operative. It is proposed that the spillover of alkyl groups from alkyl halide dissociation is a concerted process as shown in Scheme 1. (Scheme 1 shows only the spillover process, Reaction 3, and not Reaction 2, which leads to ethyl bonded to the copper particles). As the RsX bond dissociates, the SiOsR bond is formed. Therefore, only hydroxyl groups in very close proximity to the metal particle will participate in the spillover process. The adsorbed hydrogen atom product has not been characterized spectroscopically but it has been shown that it is available for further reaction.15 One difference between the results previously reported for methyl reaction and the results presented here for ethyl reaction is that some SiOCH2CH3 forms after adsorption of ethyl chloride on reduced-Cu/SiO2(673 K). The SiOCH3 species was not observed on reduced-Cu/SiO2(673 K) after adsorption of methyl iodide. This difference may be attributed to a sensitivity factor as the SiOCH2CH3 IR bands are of greater intensity compared with the SiOCH3 bands. A comparison of ethyl halide adsorption and reaction on single crystal surfaces with that on supported metal catalysts shows there are both similarities and differences. One significant difference is the formation of products on the support. Another difference is the thermal stability of adsorbed ethyl fragments on the copper particle surface. (25) Conner, W. C., Jr., Pajonk, G. M.; Teichner, S. J. Adv. Catal. 1986, 34, 1. (26) Studies in Surface Science and Catalysis, Vol. 77. New Aspects of Spillover Effect in Catalysis; Inui, T.; Fujimoto, K.; Uchijama, T.; Masai, M. Eds.; Elsevier: Amsterdam, 1993. (27) Cavanaugh, R. R.; Yates, J. T., Jr. J. Catal. 1981, 68, 22.
Bent and co-workers28-30 found that ethyl fragments on Cu(111) undergo β-hydride elimination to evolve ethene at temperatures
