Thermal Chemistry of Iodomethane on Ni(110). 2. Effect of

Sep 15, 2004 - Jarod Wilson , Hansheng Guo , Ricardo Morales , Egor Podgornov , Ilkeun Lee , Francisco Zaera. Physical Chemistry Chemical Physics 2007...
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J. Phys. Chem. B 2004, 108, 16226-16232

Thermal Chemistry of Iodomethane on Ni(110). 2. Effect of Coadsorbed Oxygen Hansheng Guo and Francisco Zaera* Department of Chemistry, UniVersity of California, RiVerside, California 92521 ReceiVed: July 6, 2004; In Final Form: July 30, 2004

The effect of coadsorbed oxygen on the thermal chemistry of iodomethane on Ni(110) single-crystal surfaces was studied by temperature-programmed desorption and X-ray photoelectron spectroscopy. It was found that extensive molecular desorption occurs at temperatures approximately 60 K higher than those seen for the desorption of multilayer CH3I on the clean surface, and that the activation of the C-I bond in the adsorbed iodomethane is deferred but not fully eliminated by the coadsorbed oxygen. The selectivity for the conversion of the surface methyl intermediates produced by the C-I bond scission is also affected by the oxygen precoverage. In broad terms, a decrease in methane production is accompanied by the formation of heavier hydrocarbons, mainly ethane and ethylene but also some propene and butene. Most of these hydrocarbons are presumably produced via a mechanism centered on a methylene insertion step, but additional low-temperature (∼190 K) ethylene and ethane are also made by direct coupling of methylene and methyl groups, respectively. Small amounts of formaldehyde, the only oxygenated hydrocarbon observed, and propenyl and butenyl radicals are seen as well.

1. Introduction The reactions of hydrocarbon fragments such as methyl (CH3) and methylene (CH2) on solid surfaces are central to many heterogeneous catalytic processes. In particular, the conversion of those C1 intermediates on oxides or oxygen-covered metal surfaces are connected with many partial oxidation,1-3 oxidative coupling,4 and oxidative dehydrogenation5,6 processes. On the basis of previous surface-science studies, it seems that the effect of oxygen on the thermal chemistry of these C1 species is dependent on the nature of the surface where it takes place. For instance, on Mo(100) and Ni(100) surfaces, oxygen appears to increase the rate of hydrocarbon formation and favor the production of higher molecular weight products.7,8 On Rh(111), oxygen modifies the properties of the surface in a similar way, except that only partial oxidation of methylene, not methyl, to formaldehyde is observed at high oxygen coverages.9-11 Similarly, on Ru(001), an excellent catalyst for the synthesis of higher hydrocarbons (C2+), adsorbed oxygen does not seem to significantly alter the conversion of CH2 surface species.12 On the other hand, the chain growth and/or coupling that takes place readily on clean copper,13,14 silver,15 and gold16,17 appears to not be significantly modified by predosed oxygen, except perhaps by adding the production of simple partial oxidation products such as CO.18 Also, the dominant reaction pathway for the combination of CH2I2 with oxygen on Cu(100) leads to the desorption of H2O and CO2,19 but on Ag(111) the formation of partial oxidized products (formaldehyde) is favored instead.20 On oxygen-treated nickel surfaces, Stair et al. have reported the formation of higher molecular weight products after very large (∼3000 langmuirs) doses of methyl radicals generated by pyrolysis of azomethane in the gas phase.7 These reactions appeared to be the most efficient on NiO thin films (compared to a Ni(100) surface covered with submonolayers of oxygen), and all C2-C4 products desorb at the same temperature. No detailed mechanistic studies were carried out on that system, but the suggestion was made that C-C bond formation occurs via bimolecular steps favored on surfaces with large amounts * To whom correspondence should be addressed. E-mail: [email protected].

of condensed CH3 radicals. This is to be contrasted with the production of hydrocarbons on oxygen-modified Mo(100), where results from similar experiments suggested chain propagation and termination steps requiring prior methylene formation.8 Our recent report on the coadsorption of CH2I2 with oxygen on Ni(110) also supports such a chain growth mechanism on nickel, at least in the monolayer range of methylene adsorbates.21 On the other hand, in the preceding paper in this issue, we show that chain growth starting from CH3 intermediates on Ni(110) is difficult because of the facile dehydrogenation and hydrogenation steps that lead to methane and hydrogen production.22 Nevertheless, in view of Stair’s results on Ni(100), it is conceivable that oxygen adsorbed on Ni(110) may modify the selectivity of the reactions available to the methyl intermediates, and therefore facilitate chain growth as in the (100) crystallographic plane of nickel. In line with this hypothesis, here we report on the results from our studies on the thermal chemistry of methyl groups prepared via thermal activation of adsorbed CH3I23,24 on oxygentreated Ni(110) surfaces. It was found that, indeed, preadsorbed oxygen inhibits the decomposition of both the original iodomethane and the methyl groups made after C-I bond scission, and favors the production of heavier hydrocarbons. Small amounts of oxygen (e0.15 langmuir) mainly block total dehydrogenation and enhance the production of methane, but higher coverages lead to significant chain growth, apparently by insertion of methylene moieties into nickel-alkyl bonds. Small amounts of formaldehyde and of alkenyl radicals are made as well. A mechanism is proposed to account for all these reactions. 2. Experimental Section Experiments were performed in an ultra-high-vacuum (UHV) chamber described previously.25,26 Briefly, the system is operated at a base pressure of approximately 1 × 10-10 Torr, and equipped with a UTI mass quadruple for temperature-programmed desorption (TPD), a dual-anode X-ray source and a concentric hemispherical analyzer (VG 100AX) for X-ray

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Thermal Chemistry of CH3I on O2-Treated Ni photoelectron spectroscopy (XPS), and an ion sputtering gun for sample cleaning. The instrumentation for both TPD and XPS is interfaced to a personal computer for data acquisition and processing. The binding energy in XPS was calibrated by referring to signals from pure copper and gold metals.27 The mass spectrometer was retrofitted with an extendable nose cone with a 5.0 mm diameter aperture for the selective detection of species desorbing from the front surface in the TPD experiments. To minimize the decomposition of the adsorbates induced by stray electrons, a grounded grid was set in front of the ion gauge and the crystal was biased with a -50 V potential. Up to 15 different masses could be monitored in a single TPD run, and the raw data deconvoluted by referring to the cracking patters of the relevant compounds.22 Calibration of the hydrogen TPD yields was based on adsorption from the background,23 and the CH4 and CH3I TPD yields were then estimated using mass balance arguments together with relative sensitivity measurements using our mass spectrometer.28 The TPD traces are reported in arbitrary units, but scaling bars are provided in most figures to allow for relative comparisons of the different signal intensities. A Ni(110) single-crystal sample, 10 mm in diameter and 1 mm in thickness, was spot-welded to two Ta wires suspended on the ends of two copper posts directly connected via vacuum feedthroughs to a liquid nitrogen reservoir. This way the sample could be resistively heated to 1200 K and cooled with liquid nitrogen to 80 K. The sample temperature was measured using a K-type thermocouple spot-welded to the edge of the crystal, and a homemade temperature controller was used to provide linear temperature ramps or maintain the crystal to within (0.5 K of the specified temperatures. The heating rate for all TPD measurements was set to 10 K/s. The Ni(110) crystal was cleaned by repeated cycles of Ar+ ion sputtering and annealing until the surface was deemed clean and well ordered by XPS and TPD experiments with hydrogen and CO.26 Normal iodomethane and iodomethane-d3 (g99.5% purity, Aldrich) were used after purification via several freeze-pumpthaw cycles. H2 (99.998%) and D2 (99.5% D) were acquired from Matheson and used without further treatment. The exposures, uncorrected for ion gauge sensitivities, were all made by backfilling of the chamber via leak valves, and expressed in langmuir units (1 langmuir ) 1 × 10-6 Torr‚s). The hydrogen and deuterium were dosed at 180 K, while iodomethane was adsorbed at 90 K unless otherwise stated. Oxygen was deposited at ∼400 K to induce dissociative adsorption and drive the formation of the desired oxygen-induced reconstructed phases.29,30 3. Results The thermal chemistry of iodomethane on the oxygenpredosed Ni(110) surfaces was first characterized by TPD. A dose of 3.0 langmuirs of CH3I was chosen for these experiments to keep the coverage on the clean Ni(110) below saturation.22 Figure 1 shows the TPD spectra for H2 (left, a), CH4 (center, b), and CH3I (right, c) obtained as a function of oxygen predose. Molecular desorption was followed by recording the signal of the CH3+ cracking fragment (15 amu), but the identity of the desorbing species was corroborated by the concurrence of the signals for the iodine (127 amu) and molecular (142 amu) ions. This figure shows that a 3.0 langmuir CH3I dose on clean Ni(110) gives rise to a broad H2 desorption trace that starts around 250 K and peaks at 298, 321, 347, and 384 K. Upon preadsorption of as little as ∼0.02 langmuir of O2, the major desorption intensity shifts from the 321 K peak to a 298 K feature, but then switches back after a 0.3 langmuir O2 dose to

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Figure 1. Hydrogen (left, a), methane (center, b), and molecular (right, c) TPD spectra for 3.0 langmuirs of CH3I adsorbed on Ni(110) singlecrystal surfaces as a function of the oxygen predose. The O2 was dosed at 400 K to promote dissociation, while the iodomethane was deposited at 90 K. The molecular desorption traces presented here correspond to the signal for the CH3+ ion (15 amu), but additional experiments were carried out following the signals for 127 (I+) and 142 (CH3I+) amu to corroborate the identity of the desorbing species. The peaks around 205 K in the CH4 TPD spectrum above 1.0 langmuir of O2 and in H2 traces above 0.15 langmuir of O2 are due to recombination reactions of fragments from CH3I+ excitation in the mass spectrometer.

the 321 K temperature range. Overall, predosed oxygen leads to a marked decrease in H2 yield, with the low-temperature desorption states decreasing faster than the high-temperature ones, and pushes the leading edge of this H2 desorption to higher temperatures. In terms of CH4 production, the main TPD peak shifts downward, from 238 K on clean Ni(110) to an earlier 211 K feature as the oxygen exposure increases up to 0.15 langmuir, and the signal intensity in the 274 K shoulder decreases concurrently. Then, from 0.2 langmuir of O2 up, the yield of the methane that desorbs below 240 K declines, and the total CH4 desorption moves slowly upward in temperature. A small amount of water is produced around 470 K, with a yield that decreases monotonically as the O2 preexposure is increased from 0.3 to 10 langmuirs (data not shown). Another trend observed in this system is a decrease in the probability for iodomethane decomposition with increasing oxygen predose. The right panel of Figure 1 shows the corresponding increase in molecular desorption. In fact, the peaks seen around 205 K in the 2 (H2, Figure 1, left) and 16 (CH4, Figure 1, center) amu traces above 1.0 langmuir of O2 can also be attributed to recombination reactions of fragments after ionization of the CH3I molecules in the mass spectrometer (as corroborated by control experiments with a poisoned surface). On clean Ni(110), molecular iodomethane desorption occurs around 145 K but only for CH3I exposures above 5.0 langmuirs,22 so the molecular peak observed between 180 and 210 K in the series in Figure 1 (right) for 3.0 langmuirs of CH3I must result from inhibition of CH3I decomposition by the oxygen modification of the surface. Both the upward shift in peak temperature and the increase in molecular desorption yield observed with increasing O2 predose demonstrate the stabilization effect that atomic oxygen exerts on the adsorbed CH3I. The yields of all desorbing products from CH3I activation on Ni(110) surfaces are plotted in monolayer (ML) units against oxygen exposure in Figure 2. The H2, CH4, and CH3I yields were calculated by integration of the traces in Figure 1. The total uptake of iodomethane, denoted by CH3ITotal, was estimated

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Figure 2. TPD yields, in units of MLs, for the hydrogen, methane, and molecular iodomethane produced by activation of CH3I adsorbed on oxygen-predosed Ni(110). These yields were obtained by integration of the traces in Figure 1 after calibration of the H2 intensity using background adsorption and relative signal comparisons for the other species, and could be reproduced within (5%. Also reported in this figure are the total uptake of iodomethane (CH3ITotal), the sum of the heavier alkanes and alkenes produced (C2+), and the aggregate yield of the hydrocarbon radicals and the formaldehyde detected (Cn′). Both C2+ and Cn′ are reported as the amounts of CH3I consumed for their production.

by addition of the yields of all the TPD products, including higher molecular weight hydrocarbons and oxygenated compounds (formaldehyde and CO), and corroborated directly by I 3d5/2 XPS. The C2+ trace represents the total amount of C1 intermediates consumed to produce the heavier hydrocarbons (ethane, ethylene, propene, and butene), and the Cn′ data correspond to the amount of CH3I converted into hydrocarbon radicals and oxygenated compounds. It can be seen from Figure 2 that, for the given 3.0 langmuir CH3I exposure, an initial increase in total uptake of iodomethane with increasing oxygen predose is followed by a subsequent drop after a maximum is reached at about 0.15 langmuir of O2. On the other hand, the extent of the conversion of the adsorbed iodomethane decreases monotonically with oxygen coverage, and molecular desorption starts and grows at O2 doses above 0.1 langmuir. The variation of the relative yields of the TPD products throughout the oxygen exposure range studied here can be divided into three regions. In the first, for oxygen exposures below 0.15 langmuir, the majority (50-60%) of methyl groups produced by C-I bond activation of the adsorbed CH3I are hydrogenated to methane, while the rest decompose all the way to atomic hydrogen and carbon. From 0.15 to 3.0 langmuirs, the extent of the hydrogenation of the methyl groups decreases, perhaps because of a reduction in hydrogen availability, but the production of higher hydrocarbons increases markedly. Finally, above 3.0 langmuirs of O2, about 40% of the adsorbed CH3I desorbs molecularly, while the rest desorbs mostly in the form of higher hydrocarbons. The TPD spectra of the higher hydrocarbons, as well as those of the oxygenated species (formaldehyde), are compiled for the case of a 0.3 langmuir O2 predose in Figure 3. The left panel of Figure 3 reproduces the raw TPD spectra, while the right frame shows the processed data after deconvolution of the relevant compounds. It is clearly seen that both ethylene and ethane are produced in this case. The high-temperature (244 K) peaks for those display similar shapes and positions, indicating their relation to the same ethyl intermediates. The low-temperature features, on the other hand, show different behavior, and may therefore represent different surface pro-

Guo and Zaera

Figure 3. TPD spectra for 3.0 langmuirs of CH3I adsorbed on Ni(110) predosed with 0.3 langmuir of O2. The left panel (a) reports the raw traces obtained for a selection of masses representative of the heavy (C2+) hydrocarbons produced, while the right frame (b) displays the traces obtained for the different products after deconvolution. Ethylene, ethane, propene, butene, formaldehyde, and a couple of alkenyl radicals are all detected in these experiments.

cesses. Propene desorption starts at 200 K and peaks at 235 K, and butene desorbs at still higher temperatures, around 298 K. No detectable propane or butane desorption was seen in these experiments. The TPD yields for the C2, C3, and C4 hydrocarbons were found to follow approximate 1.00:0.45:0.05 ratios, and the total combined consumption of C1 intermediates for the production of all of the C2+ species was found to amount to about 40% of the methane production and ∼20% of all the adsorbed CH3I (Figure 2). In addition to the alkanes and alkenes discussed in the previous paragraph, production of some formaldehyde was also detected around 220 K. The identification of this product was based on a comparison of the TPD spectra obtained after adsorption of 3.0 langmuirs of CH3I on surfaces predosed with 5.0 langmuirs of 18O2 versus 5.0 langmuirs of 16O2. The data from those experiments are displayed in Figure 4. In the raw data with 18O2, signals for both 31 and 32 amu were detected around ∼210 K, with relative intensities in good agreement with the cracking pattern of CH218O. In an analogous way, the formaldehyde desorption from the surface treated with 16O2 was isolated from the 29 and 30 amu traces. Ethylene, ethane, propene, and butene desorption could all be seen in these experiments as well. The identification of the other unknown traces for 41 and 55 amu, which show peaks around 320-345 and 300-325 K, respectively (depending on the O2 preexposure), also relied on this isotope-labeling experiment. Specifically, given that similar desorption features are seen for these masses with 16O and 18O, and that no signals were observed for 43 or 57 amu after 18O2 predosing, it was inferred that those fragments do not contain oxygen atoms. More generally, it was concluded that the formation of oxygenated hydrocarbons other than formaldehyde is not significant in this system. Additional TPD experiments with allyl iodide (a precursor for allyl radicals) carried out on clean and oxygen-modified Ni(110) yielded similar peaks for 41 and 55 amu above 320 K (data not shown). On the basis of all these observations, the 41 and 55 amu traces in the present coadsorption systems were attributed to unsaturated free radicals, to C3H5• and C4H7• species, respectively. The same species were observed previously in TPD experiments with CH2I2 coadsorbed with oxygen.21

Thermal Chemistry of CH3I on O2-Treated Ni

Figure 4. Oxygen-isotope-labeling TPD data for 3.0 langmuirs of CH3I on Ni(110) predosed with 5.0 langmuirs of either 16O2 (left, a) or 18O2 (right, b). Appropriate deconvolution for the spectra with 18O or 16O, taking into account the expected changes in molecular mass, was used to confirm the formation of formaldehyde around ∼210 K. Also, the similarity of the peaks seen for the 41 and 55 amu traces in both cases indicates that they correspond to species (alkenyl radicals) with no oxygen atoms.

The identification of the mechanistic steps responsible for the formation of the various hydrocarbons was also aided by experiments with deuterium labeling. Key results from these studies are shown in Figure 5. It should be noted that during sample cooling and CD3I dosing some background hydrogen always adsorbs on the surface, hence the detection of some normal hydrogen. In experiments where 3.0 langmuirs of CD3I was dosed on Ni(110) pretreated with 0.3 langmuir of O2, both D2 and HD were seen to desorb around 320 K, in the same way as H2 desorbs in TPDs with 3.0 langmuirs of CH3I on 0.3 langmuir of O2/Ni(110) (compare Figure 5 and trace g in Figure 1). Also, as discussed in Figure 1, the 3 (HD) and 4 (D2) amu signals that appear around 204 K are likely to be due to CD3I molecular desorption; otherwise, all H2, HD, and D2 traces display similar shapes, indicating thorough isotope scrambling. In terms of methane production, monohydrogenated methane (CD3H) forms around 208 K by incorporation of background hydrogen, and CD4 desorption appears around 260 K, about 50 K above the CD3H trace, because of the rate-limiting decomposition of CD3,ads needed to provide the extra D atoms for deuteriation. The CD3H and CD4 methane isotopologues display TPD features similar to those observed with CD3I on clean Ni(110).22 No CD2H2 desorption is observed in these data, indicating negligible H-D exchange. Desorption of both ethene and ethane are initiated above 150 K, but while the intensities of the peaks for the ethane isotopologues are all centered around 207 K, the ethylene traces display broad peaks with most of their intensities around 258 K. These results differ significantly from those obtained with CH3I (Figure 3), since in the deuterium-labeling case all of the ethane formation is seen at low temperatures and the intense C2H4 feature at 189 K is not reproduced. Also, with CD3I there is a smaller desorption signal for propene, and no detectable butene production (data not shown). The TPD spectra of the alkanes, alkenes, formaldehyde, and C3 and C4 free radicals that form from 3.0 langmuirs of CH3I coadsorbed with varying amounts of predosed oxygen, from 0 to 10.0 langmuirs, are depicted in Figures 6 and 7. Regarding the data in Figure 6, ethylene production is first seen with 0.1

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Figure 5. TPD data for 3.0 langmuirs of CD3I adsorbed on a Ni(110) surface predosed with 0.3 langmuir of O2. Since adsorption of normal hydrogen from the background could not be completely avoided, signals are detected for H2 and HD, as well as for hydrocarbons resulting from H incorporation or H-D exchange. The early production of CD3H compared to CD4 provides an estimate for the activation of CD bonds in adsorbed CD3 species. Also, the ethane and ethylene produced by both methylene insertion and direct coupling reactions show a limited amount of H-D scrambling.

Figure 6. Ethylene (a), ethane (b), propene (c), and butene (d) TPD data for 3.0 langmuirs of iodomethane adsorbed on Ni(110) predosed with varying amounts of oxygen. The low-temperature peaks seen in the traces for both ethylene and ethane are attributed to the direct coupling of methylene and methyl groups, respectively. The hightemperature peaks in those data, as well as the production of propene and butene, are the result of a mechanism that starts with a methylene migratory insertion and is followed by β-hydride and reductive elimination steps.

langmuir of O2, but develops a second peak by 0.3 langmuir of O2. After that, the low-temperature (190 K) peak remains at the same value and displays a comparable desorption intensity all the way to the 10.0 langmuir exposure, but the hightemperature ethylene signal shifts upward (from 240 to 300 K) while its intensity first grows and then declines above 1.0 langmuir. For ethane, a double-peak structure appears at 0.3 langmuir of O2 with maxima at 200 and 240 K, and the hightemperature peak parallels the behavior of the high-temperature feature for ethylene in terms of both peak temperature and intensity (the low-temperature peak disappears at oxygen predoses as low as 0.5 langmuir). The desorption temperature for propene increases with oxygen exposure in a way similar to those seen with ethane and ethene, but the desorption intensity

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Figure 7. Corresponding formaldehyde (a), C3H5• (b), and C4H7• (c) TPD data for the experiments reported in Figure 6. No significant changes are seen in the desorption behavior of any of these products with varying oxygen coverages.

Figure 8. Comparison of methane, ethylene + ethane, propene, butene, and formaldehyde TPD yields, in MLs, for 3.0 langmuirs of CH3I on O/Ni(110) after oxygen preexposures of 0, 0.3, 1.0, 3.0, and 10.0 langmuirs. Selectivity toward the production of both formaldehyde and the heavier hydrocarbons is seen at intermediate oxygen coverages.

does not change significantly throughout (a maximum in yield is seen around 3.0 langmuirs of O2). Finally, a small amount of butene production is detected around 295 K in most cases, but that does not exhibit a strong dependence on oxygen exposure. Continuing with the data in Figure 7, formaldehyde desorbs in one single peak around 225 K on Ni(110) predosed with 0.3 langmuir of O2, at about 205 K with 3.0 langmuirs of O2, and at approximately 212 K with 10.0 langmuirs of O2, and the yield goes through a maximum around 5.0 langmuirs of O2. Small amounts of water are detected around 230 K, possibly from coadsorption of background gases, but the water production around 450 K due to disproportionation of hydroxyl groups31 is insignificantly small (data not shown). The peaks for the desorption of the C3H5• and C4H7• radicals are seen around 320345 and 300-325 K, respectively, and do not change much with oxygen exposure. The yields calculated from the data in Figures 6 and 7 for the alkanes, alkenes, and formaldehyde are compared in Figure 8 for 0, 0.3, 1.0, 3.0, and 10.0 langmuir oxygen predoses. As stated above, no high-molecular-weight compounds were detected for 3.0 langmuirs of CH3I on clean Ni(110), but significant quantities are seen for all oxygen-predosed surfaces.

Guo and Zaera

Figure 9. I 3d5/2 XP spectra for 3.0 langmuirs of CH3I on Ni(110) predosed with 0.3 (left, a) and 10.0 (right, b) langmuirs of O2. A shift in peak position is seen in both cases around ∼150 K indicative of the scission of the C-I bond in the adsorbed iodomethane. Gaussian fits to the two peaks associated with CH3I and adsorbed I are indicated in the data for the intermediate temperatures by the light gray solid lines to better estimate the extent of the conversion. The loss seen in the total intensity for the 10.0 langmuirs of O2 case is due to molecular iodomethane desorption.

The combined yield of all the hydrocarbons produced reaches a maximum for oxygen predoses around 0.3-0.4 langmuir (see also Figure 2), but the surface modified with 1.0 langmuir of O2 selectively produces more C2+ hydrocarbons. XPS was used next to estimate the temperature at which the C-I bond breaks during the thermal activation of CH3I adsorbed on oxygen-pretreated Ni(110) surfaces. The I 3d5/2 XP spectra for 3.0 langmuirs CH3I dosed on surfaces predosed with 0.3 langmuir (Figure 9, left) and 10.0 langmuirs (Figure 9, right) of O2 are reported as a function of annealing temperature in Figure 9. There, the raw data, shown as dots, were fitted to Gaussian peaks (solid lines) to facilitate the analysis.28 According to the left panel of Figure 9, on a surface preexposed to 0.3 langmuir of O2, the C-I bond appears to be activated around 150 K as indicated by the shift in the I 3d5/2 XPS peak from 620.4 to 619.7 eV binding energy,21,22 but a small amount of CH3I (∼10%) is still intact after annealing at 190 K. This correlates well with the molecular desorption seen in TPD around 200 K. There is also a corresponding shift in the C 1s XPS peak during this transition, from 284.8 eV at 90 K to 283.5 eV above 190 K (Figure 10). This scission of the C-I bond is more clearly identified on the surface preexposed with 10.0 langmuirs of O2 (Figure 9b), where the onset of the CH3I dissociation is indicated by the broadening of the I 3d5/2 peak that occurs around 110 K, and its completion manifested by the final binding energy value of 618.7 eV observed at 230 K. In addition, a decrease in overall XPS signal intensity above 150 K indicates some (∼40%) molecular desorption, in reasonable agreement with the TPD data (∼45%; see Figure 2). The fact that the broadening of the I 3d5/2 XPS feature is seen throughout a wide range of annealing temperatures implies that CH3I dissociation is significantly affected by oxygen coverage. Notice also that between 110 and 230 K the adsorbed iodine atoms produced by CH3I dissociation exhibit a binding energy 1.0 eV below that observed on the clean surface,22 a shift that is partially reversed after annealing to 400 K. Finally, the carbon 1s XPS signal can be tracked upon

Thermal Chemistry of CH3I on O2-Treated Ni

Figure 10. C 1s XPS data for the 3.0 lanmuirs of CH3I/0.3 langmuirs of O2/Ni(110) case evidencing both the C-I bond scission and the depletion of surface hydrocarbon species with increasing temperature.

heating to 300 K (Figure 10), but disappears after annealing at 400 K because, according to the TPD data, most of the hydrocarbon species desorb by that temperature. 4. Discussion On the basis of our extensive past experience with the generation of hydrocarbon surface species via the thermal activation of halohydrocarbons,24,32 CH3I was chosen here to generate methyl groups on oxygen-predosed Ni(110) surfaces. It was found that C-I bond scission is inhibited by adsorbed oxygen, hence the higher yield of molecular desorption detected, but that surface methyl groups could still be generated for oxygen preexposures all the way up to 10.0 langmuirs. In the following discussion we highlight the thermal chemistry identified in this study for those methyl groups on the O/Ni(110) surfaces. Using O 1s XPS data (not shown), together with a previous calibration of this system,25 the oxygen exposures employed in this study (e10 langmuirs) can be roughly divided into three regimes, i.e., below e0.3 ML for O2 exposures of e1.0 langmuir, 0.3-0.5 ML after a 1.0-3.0 langmuir O2 dose, and 0.5-0.67 ML for the 3.0-10.0 langmuir of O2 range. Thus, oxygen dosing on Ni(110) leads to reconstruction of the surface into an O-(3 × 1)i periodical lattice below 1.0 langmuir of O2, an O-(2 × 1) phase between 1.0 and 3.0 langmuirs of O2, and the O-(3 × 1)f structure above 3.0 langmuirs.29 Also, according to STM studies, doses on the order of 0.1 langmuir of O2 give rise to a layer of about 0.1 ML of oxygen in isolated -Ni-Ostrings,29 and exposures around 0.2 langmuir of O2 (∼0.15 ML) produce added -Ni-O- rows with quite high mobility.33 These O/Ni(110) structures can be used to explain the changes in the surface chemistry of coadsorbates, including the trends seen for molecular adsorption. Thus, both the decrease in total uptake and the increase in molecular desorption temperature within the 0.1-1.0 langmuir oxygen exposure range (Figure 1, right) can be ascribed to a diminishing number of Ni atoms available for adsorption and an increase in CH3I-Ni bonding strength upon increasing electron withdrawal from neighboring oxygen atoms in the -Ni-O- rows. Analogous behavior has been previously reported by us for carbon monoxide,25 water,31 and ammonia.34 Regarding the subsequent thermal chemistry of the methyl groups formed upon C-I bond activation, their dehydrogenation

J. Phys. Chem. B, Vol. 108, No. 41, 2004 16231 appears to slow down on the Ni(110) surfaces obtained after oxygen exposures below 0.15 langmuir at the expense of more self-hydrogenation to methane. The methane production starts at ∼210 K and peaks above ∼240 K, a temperature range associated with the start of the methyl decomposition that produces the surface hydrogen required for hydrogenation, since methyl recombination with hydrogen adsorbed from the background starts at temperatures as low as 170 K. At oxygen exposures above 0.15 langmuir, dehydrogenation occurs at lower temperatures, hence the lower temperature for CH4 production in the TPD data. It should be pointed out, however, that the details of this behavior depend somewhat on the initial coverage of CH3I, and show a normal kinetic isotope effect that makes the conversion harder with CD3I. One of the most interesting observations from the experiments reported here is the fact that, besides methane desorption, large yields are seen for the production of ethane and ethylene, and also for heavier hydrocarbons. The chain growth that leads to the formation of C2+ hydrocarbons most likely requires the production of a second C1 intermediate on the surface, a methylene moiety. In fact, the production of significant amounts of methylene intermediates is evidenced by the 190 K ethene peak in the TPD reported in Figure 6, which has been assigned, on the basis of isotope-labeling experiments, to the direct coupling of two adsorbed methylenes.21 A small amount of methyl coupling is also detected for ethane at that temperature for the 0.3 langmuir O2 predose case (Figure 3), and more prominently in the experiments with CD3I (Figure 5). Both the desorption of the methylene coupling product and the limited isotope exchange seen in the ethane and ethylene produced with CD3I set an upper limit for methylene formation on the Ni(110) surface at approximately 170 K. It appears that the dehydrogenation of the methyl surface intermediates to methylene groups takes place concurrently with the C-I bond scission step that produces the CH3,ads moieties in the first place. The results from the experiments reported here with methyl groups, together with those with methylene species from diiodomethane activation,21 strongly suggest that methylene is more stable than methyl on the oxygen-treated Ni(110) surfaces. Only if sufficient methylene species are present on the surface is it possible to promote the CH2 migratory insertion steps into metal-alkyl groups necessary for chain growth.28,35 It also seems that further dehydrogenation of methylene into methylidyne and/or surface carbon is effectively slowed by the oxygen predosed on the surface, presumably because of a decrease in the availability of the large ensembles of nickel surface atoms needed for such reactions.14,35,36 This is clearly indicated by the diminishing hydrogen yields in the TPD experiments with increasing oxygen predose. Instead, the methylene surface species are mainly taken for the formation of heavier hydrocarbons. Initially, methylene insertion into a nickel-methyl bond produces an ethyl surface intermediate. This rate-limiting step is then followed by competing β-hydride and reductive elimination reactions to produce ethylene and ethane, respectively. Indeed, the similar kinetics of formation of both of those products above ∼220 K attests to their common origin, and their comparable yields attest to the ease with which a second ethyl group hydrogenates to ethane once the first releases a hydrogen atom via the β-H elimination step. Note, however, that the upward shift of the desorption temperatures of these C2 products with increasing oxygen coverage indicates not only the stability of the relevant hydrocarbon intermediates against decomposition, but also an increase in difficulty for the C-C chain propagation reactions above 3.0 langmuirs of O2.

16232 J. Phys. Chem. B, Vol. 108, No. 41, 2004 The rate of the methylene migratory insertion step, which determines the final yields of the heavier hydrocarbons, is primarily controlled by the methylene:methyl ratio on the surface. That ratio is expected to be higher in CH2I2/O/Ni(110) than in CH3I/O/Ni(110), as the former demands some dehydrogenation of the initial methylene fragments as the source of surface hydrogen for methyl formation. In the latter case, conversely, a dehydrogenation step of the initial methyl moieties is required to produce methylene. The different starting points in the two systems also affect the population of surface hydrogen. These differences in methylene:methyl ratio and surface hydrogen availability in turn modify the relative rates of the reaction steps involved in the hydrocarbon conversion pathways. The end results are lower alkene:alkane ratios and heavier hydrocarbon total yields in CH3I/O/Ni(110) compared to CH2I2/O/Ni(110).28 It appears that methylene insertion is faster when starting with diiodomethane, and that β-hydride elimination dominates over reductive elimination with surface hydrogen in the cases of diiodomethane on O/Ni(110)21 and of iodomethane on clean Ni(110)22 but not for iodomethane on oxygen-pretreated nickel surfaces. A few other products desorb from thermal activation of iodomethane on O/Ni(110), formaldehyde in particular (Figures 3, 4, and 7). Two general mechanisms can be envisioned for that process: (1) an initial dehydrogenation of methyl moieties to methylene followed by oxygen insertion into the Ni-CH2 bond or (2) an oxygen insertion into a nickel-methyl bond to methoxide followed by β-hydride elimination. However, the first of these mechanisms can be easily discarded, because it is known that formaldehyde production with CH2I2, a direct precursor to methylene, occurs at higher temperatures (>260 K)21 than with CH3I (210-250 K, Figure 7). On the other hand, β-hydride elimination from alkoxides such as the methoxide proposed here is typically a facile reaction.3,37,38 It can therefore be concluded that it is the second mechanism, involving methoxide formation, that makes this system applicable for formaldehyde production. Note also that no other oxygenates were detected in these systems. This rules out any chain growth mechanism starting with a methoxide, and also any oxygen insertion into nickel-carbon bonds for alkyls other than methyl. Finally, the chain growth mechanism proposed here for alkane and alkene formation is also applicable to the production of the C3H5• and C5H7• radicals reported in Figures 3, 4, and 7 as long as those start from a 1-iodomethyl (CH2Iads) intermediate.21 The production of these unsaturated radicals is undoubtedly related to the presence of oxygen on the surface, as they could not be observed with CH3I on clean Ni(110). As the oxygen stabilizes the C-I bond, CH2Iads intermediates may be produced via the direct dehydrogenation of the CH3I adsorbed on the surface. However, this is only a minor process, as the total yield of C3H5• and C5H7• radicals in the TPD experiments is quite small. 5. Conclusions TPD and XPS experiments with CH3I on oxygen-predosed Ni(110) single-crystal surfaces indicate that the thermal chemistry of methyl groups on nickel surfaces can be markedly affected by the coadsorbed oxygen. The sticking coefficient, and with that the total coverage obtained by dosing 3.0 langmuirs of CH3I at 90 K, increases with oxygen exposure up to ∼0.15 langmuir and then decreases in the intermediate 0.22.0 langmuir regime; the layer becomes saturated above 3.0 langmuirs. The activation of the C-I bond in adsorbed CH3I is also inhibited but not fully blocked by the coadsorbed oxygen.

Guo and Zaera The stabilizing effect of surface oxygen on the CH3I adsorbates gives rise to a distinctive molecular desorption from the first layer at temperatures up to 60 K higher than those for CH3I multilayers on the clean surface. The scission of the C-I bond in adsorbed iodomethane leads to methane production, with a yield that first grows up to ∼0.15 langmuir of O2 and then decreases monotonically at higher oxygen coverages. Hydrogen production also decreases monotonically throughout the oxygen exposure range explored (0-10 langmuirs), but that is in part compensated by a significant production of higher molecular weight compounds above 0.2 langmuir O2 predoses, including the ethylene, ethane, propylene, and butene made by a chain growth mechanism that starts with a methylene migratory insertion step. Additional ethylene and ethane desorption from coupling of methylene and methyl surface species, respectively, are also seen at low temperatures, prior to the formation of any of the other hydrocarbons. Formaldehyde is the only oxygenated hydrocarbon observed in these coadsorption systems, and is most likely produced via a methoxy intermediate from oxygen insertion into the nickel-methyl bond, perhaps at the end of -Ni-O- rows. Finally, small amounts of alkenyl radicals are produced around 300 K. Acknowledgment. Financial support for this work was provided by the U.S. Department of Energy. References and Notes (1) Hickman, D. A.; Schmidt, L. D. Science 1993, 259, 343. (2) Lunsford, J. H. Catal. Today 2000, 63, 165. (3) Zaera, F. Catal. Today 2002, 81, 149. (4) Lunsford, J. H. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, Germany, 1997; Vol. 4, pp 1843-1856. (5) Kung, H. H. AdV. Catal. 1994, 40, 1. (6) Argyle, M. D.; Chen, K. D.; Bell, A. T.; Iglesia, E. J. Catal. 2002, 208, 139. (7) Dickens, K. A.; Stair, P. C. Langmuir 1998, 14, 1444. (8) Kim, S. H.; Stair, P. C. J. Phys. Chem. B 2000, 104, 3035. (9) Bol, C. W. J.; Friend, C. M. J. Am. Chem. Soc. 1995, 117, 11572. (10) Bol, C. W. J.; Friend, C. M. Surf. Sci. 1995, 337, L800. (11) Bol, C. W. J.; Friend, C. M. J. Am. Chem. Soc. 1995, 117, 8053. (12) Kis, A.; Kiss, J.; Solymosi, F. Surf. Sci. 2000, 459, 149. (13) Lin, J. L.; Chiang, C. M.; Jenks, C. J.; Yang, M. X.; Wentzlaff, T. H.; Bent, B. E. J. Catal. 1994, 147, 250. (14) Bent, B. E. Chem. ReV. 1996, 96, 1361. (15) Zhou, X.-L.; White, J. M. J. Phys. Chem. 1991, 95, 5575. (16) Paul, A.; Yang, M. X.; Bent, B. E. Surf. Sci. 1993, 297, 327. (17) Paul, A. M.; Bent, B. E. J. Catal. 1994, 147, 264. (18) Ali, A. K. M.; Saleh, J. M.; Hikmat, N. A. J. Chem. Soc., Faraday Trans. 1 1987, 83, 2391. (19) Kova´cs, I.; Solymosi, F. J. Mol. Catal. A 1999, 141, 31. (20) Scheer, K. C.; Kis, A.; Kiss, J.; White, J. M. Top. Catal. 2002, 20, 43. (21) Guo, H.; Zaera, F. Surf. Sci. 2003, 547, 299. (22) Guo, H.; Zaera, F. J. Phys. Chem. B 2004, 108, 16220. (23) Tjandra, S.; Zaera, F. Langmuir 1992, 8, 2090. (24) Zaera, F. Acc. Chem. Res. 1992, 25, 260. (25) O ¨ fner, H.; Zaera, F. J. Phys. Chem. B 1997, 101, 9069. (26) Chrysostomou, D.; Flowers, J.; Zaera, F. Surf. Sci. 1999, 439, 34. (27) Handbook of X-Ray Photoelectron Spectroscopy; Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. F., Muilenberg, G. E., Eds.; PerkinElmer Corp.: Eden Prairie, MN, 1978. (28) Guo, H.; Zaera, F. Surf. Sci. 2003, 547, 284. (29) Eierdal, L.; Besenbacher, F.; Laegsgaard, E.; Stensgaards, I. Surf. Sci. 1994, 312, 31. (30) Dastoor, H. E.; Gardner, P.; King, D. A. Surf. Sci. 1993, 289, 279. (31) Guo, H.; Zaera, F. Catal. Lett. 2003, 88, 95. (32) Zaera, F. Prog. Surf. Sci. 2001, 69, 1. (33) Sprunger, P. T.; Okawa, Y.; Besenbacher, F.; Stensgaard, I.; Tanaka, K. Surf. Sci. 1995, 344, 98. (34) Guo, H.; Chrysostomou, D.; Flowers, J.; Zaera, F. J. Phys. Chem. B 2003, 107, 502. (35) Zaera, F. Chem. ReV. 1995, 95, 2651. (36) Zhou, X.-L.; White, J. M. Surf. Sci. 1988, 194, 438. (37) Gleason, N. R.; Zaera, F. J. Catal. 1997, 169, 365. (38) Zaera, F.; Gleason, N. R.; Klingenberg, B.; Ali, A. H. J. Mol. Catal. A 1999, 146, 13.