Formation of an Oxametallacycle Surface Intermediate via Thermal

Apr 2, 2010 - 100 K. An initial chemical reaction is seen at around 180 K involving the sequential scission of the C-Cl and O-H bonds to produce hydro...
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J. Phys. Chem. C 2010, 114, 7913–7919

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Formation of an Oxametallacycle Surface Intermediate via Thermal Activation of 1-Chloro-2-methyl-2-propanol on Ni(100) Qing Zhao* Department of Physics, College of Science, Beijing Institute of Technology, Beijing 100081, People’s Republic of China

Rongping Deng Science DiVision, Beloit College, Beloit, Wisconsin 53511

Francisco Zaera Department of Chemistry, UniVersity of California RiVerside, RiVerside, California 92521 ReceiVed: January 11, 2010; ReVised Manuscript ReceiVed: March 17, 2010

The thermal chemistry of 1-chloro-2-methyl-2-propanol (CH2Cl(CH3)2COH) on a Ni(100) single-crystal surface, clean and after hydrogen preadsorption, was studied by temperature-programmed desorption and X-ray photoelectron spectroscopy. The 1-chloro-2-methyl-2-propanol adsorbs molecularly on the metal surface at 100 K. An initial chemical reaction is seen at around 180 K involving the sequential scission of the C-Cl and O-H bonds to produce hydroxyalkyl -CH2(CH3)2COH and oxametallacycle -CH2(CH3)2COintermediates. Hydrogenation of the first surface species produces tert-butyl alcohol, whereas further conversion of the oxametallacycle follows at least three reaction pathways, a cyclization to isobutene oxide, a hydrogenation to tert-butyl alcohol, and a dehydration reaction to produce isobutene, water, H2, and CO. It was also shown that hydrogen coadsorption on the surface enhances the production of tert-butyl alcohol and partially inhibits the decomposition to isobutene. 1. Introduction The conversion of hydrocarbons on metal surfaces often involves several competing reaction pathways.1,2 The reaction patterns of alcohol conversions, in particular, are often led by the initial scission of their O-H bond, which is quite weak,3-5 but can be followed by a complex combination of competing C-H, O-H, C-O, and C-C bond-scission steps. A good understanding of this surface chemistry is important for the design of efficient and selective processes in heterogeneous catalysis.6,7 In an effort to understand this chemistry, we have performed several surface science studies on the thermal chemistry of alcohols on single-crystal model surfaces under ultra-high-vacuum (UHV) conditions.8-16 On many metal surfaces, the scission of the O-H bond in alcohols may occur upon adsorption, or soon thereafter, and leads to the formation of a surface alkoxide intermediate.17-19 What happens next, though, depends strongly on the nature of the system being considered. In particular, the selectivity of the subsequent chemistry is likely to be dominated by the regioselectivity of the dehydrogenation step that follows. In many cases, elimination from the β carbon, that is, from the carbon adjacent to the original hydroxyl moiety, is favored, in which case, dehydrogenation to produce aldehydes or ketones dominates. However, in other systems, dehydrogenation may occur at the γ carbon instead. In that case, an oxametallacycle intermediate forms on the surface. For example, an experiment with ethanol on Rh(111) showed evidence of a methyl C-H scission and indicated that hydrogen elimination at this carbon site is key to * To whom correspondence [email protected].

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the formation of an oxametallacycle intermediate.20,21 Dehydrogenation at the γ carbon is kinetically less favorable compared with dehydrogenation at the beta position; thus, hydrogen elimination from the γ carbon occurs at higher temperatures and may be accompanied by other bond scissions.22 Oxametallacycles are key intermediates in epoxide ringopening processes on transition-metal surfaces23-26 and also play a central role in olefin epoxidations.27-30 Direct evidence for the latter reaction has been reported in surface science studies with Rh(111)21,22 and Ag(110).31,32 We have also reported evidence of oxametallacycle formation during the reaction of 2-iodoethanol on Ni(100).13 It was argued in that work that oxametallacycle formation could eventually lead to alcohol dehydration to olefins.33 However, that depends on the strength of the C-O bond. Typically, the C-O bond of alcohols is retained on many later transition-metal surfaces. For instance, it has been reported that tert-butyl alcohol decomposes into H2O, H2, and CO on clean Rh(111). On early transition metals, on the other hand, C-O bonds are easy to break, and many other reactions may be promoted, including dimerization of the remaining hydrocarbon moieties.3,34 It is worth mentioning, however, that C-O scission can also be promoted on late transition metals via modification of the surface by, for instance, coadsorbing oxygen. On Rh(111), this preconditioning was shown to induce C-O bond scission in tert-butanol at around 325 K.22 Our previous work has also shown that dehydration can be made to compete with dehydrogenation by adding electron-withdrawing substituents to the reacting alcohol: only dehydrogenation of 2-propanol to acetone is possible on Ni(100), but both acetone and propene are detected with 1,1,1-trifluoro-2-

10.1021/jp100243t  2010 American Chemical Society Published on Web 04/02/2010

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propanol (CF3CH(OH)CH3).14 It seems that the stability of the C-O bond in alcohols and, with that, the selectivity of its thermal surface conversion can be tuned by changes in the electronic properties of the surface and/or reactants. In a continuation of our studies on the factors that affect selectivity in the thermal conversion of alcohols adsorbed on metal surfaces, here, we report results from our studies on the thermal reactions of 1-chloro-2-methyl-2-propanol on Ni(100). Halohydrocarbons are commonly used as precursors to produce surface intermediates because the carbon halogen bond scission can be activated on surfaces at temperatures below those where the C-H bond scission starts,35,36 and in this case, the 1-chloro2-methyl-2-propanol was used to produce the appropriate oxametallacycle intermediate. It should be noted, however, that, with haloalcohols, the scission of the carbon-halogen bond may compete with the activation of the O-H bond, which is also quite weak. Consequently, alkoxide, hydroxyalkyl, and/or oxametallacycle intermediates could all form during their conversion on the metal surface. The surface chemistry of this system may also change upon modification of the surface by, for instance, preadsorbing hydrogen or oxygen. All these possibilities were explored and are discussed below. 2. Experimental Section The experiments were conducted in an ultra-high-vacuum chamber described in detail in previous reports.8,10,37 The base pressure of the chamber was maintained under 1 × 1010 Torr. The chamber is equipped with appropriate instrumentation for temperature-programmed desorption (TPD) measurement, X-ray photoelectron spectroscopy (XPS), ion-scattering spectroscopy (ISS), secondary-ion mass spectrometry (SIMS), and Auger electron spectroscopy (AES). TPD experiments were carried out by using an Extrel quadrupole mass spectrometer (QMS) capable of detecting signals in a mass range between 1 and 800 amu. Up to 15 individual masses could be detected simultaneously in each TPD experiment thanks to the use of an interfaced personal computer and home-written software. The ionizer of this spectrometer is located inside an enclosed compartment with a 7 mm diameter frontal aperture used for gas sampling. In the experiments reported here, the Ni crystal was placed within 1 mm of the front aperture in order to detect the molecules that desorbs from the surface. A linear heating rate of 10 K/s was used in all experiments. The choice of specific masses to follow the evolution of the different desorbing products and the analysis of those signals to account for potential cracking pattern overlaps were done following criteria discussed in detail elsewhere.38 The XPS spectra were obtained using a 50 mm radius hemispherical electron energy analyzer set at the constant pass energy of 50 eV, which corresponds to a total resolution of approximately 1.2 eV full width at half-maximum (fwhm). An aluminum anode was used as the X-ray source. The Cu 2p3/2 at 932.7 eV, Ni 2p3/2 at 852.7 eV, and Ni 3p3/2 at 66.2 eV peaks were used for energy scale calibration.39 The Ni(100) crystal was cut and polished using standard procedures and spot-welded to two tantalum rods attached to a manipulator capable of cooling to liquid nitrogen temperature and of resistively heating to temperatures above 1300 K. Prior to each experiment, cycles of oxygen treatment, ion sputtering, and annealing were performed to clean the crystal surface until no impurities were detected by XPS and until standard CO and H2 TPD could be reproduced. The 1-chloro-2-methyl-2-propanol ((CH3)2C(OH)CH2Cl) was purchased from Aldrich (purity ) 97%) and subjected to several

Zhao et al.

Figure 1. Cl 2p XPS spectra from 6.0 L of 1-chloro-2-methyl-2propanol adsorbed on Ni(100) at 100 K after annealing at 100, 140, 180, and 260 K.

freeze-pump-thaw cycles before being introduced into the vacuum chamber. Hydrogen (purity ) 99.99%), deuterium (isotopic purity > 99.5%), and oxygen (purity ) 99.99%) were purchased from Matheson and used as supplied. The purity of all gases was periodically checked by mass spectrometry. No impurities were detected in the mass spectra. All gas exposures were measured in langmuir (1 L ) 10-6 Torr · s), uncorrected for ion gauge sensitivities. 3. Results The thermal reactivity of 1-chloro-2-methly-2-propanol on the Ni(100) surface was first tested by using XPS. Figure 1 shows the Cl 2p XPS spectra obtained for a 6.0 L 1-chloro-2methyl-2-propanol dose of the clean surface at 100 K after annealing to different surface temperatures. At 100 K, the Cl 2p peak is centered at 200.6 eV, and a similar result was seen after annealing to 140 K, indicating no significant surface chemistry at that temperature. The chemical state of the chlorine atom at this stage is most probably due to bonding to the alcohol moiety in the original 1-chloro-2-methyl-2-propanol. After annealing to 180 K, on the other hand, the Cl 2p XPS peak position shifts to 199.3 eV, about 1.3 eV lower than the peak position before the annealing. No additional peak position changes are detected after annealing to 260 K, but the peak intensity does get reduced, suggesting the desorption of some chlorine-containing species. The Cl 2p binding energy change seen when increasing the annealing temperature from 140 to 180 K indicates a chemical change most likely associated with the dissociation of the C-Cl bond. It should be said that the Cl 2p peak in the spectrum at 100 K is broad and can be fitted to two components, a main feature at 200.2 eV and a smaller contribution at 202.0 eV. These are not a reflection of two different species and can easily assigned to the expected Cl 2p3/2 and Cl 2p1/2, respectively, instead. Similarly, the Cl 2p trace for 180 K can be fitted to a pair of primary peaks at 198.4 and 200.1 eV and a second smaller set at 200.2 and 202.0 eV. These are assigned to surface atomic Cl and to Cl in the haloalcohol, respectively.

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Figure 3. TPD spectra from 6.0 L of 1-chloro-2-methyl-2-propanol adsorbed on clean Ni(100) at 100 K. Figure 2. O 1s XPS spectra from 6.0 L of 1-chloro-2-methyl-2propanol adsorbed on Ni(100) at 100 K after annealing at 100, 140, 180, and 260 K.

The O 1s XPS spectra obtained from the same set of experiments are shown in Figure 2. At 100 K, the spectrum shows a peak at 532.8 eV, a value characteristic of oxygen atoms in the hydroxyl group of alcohols. Indeed, this peak position matches that determined for ICH2CH2OH/Ni(100) in an earlier report from our laboratory.13 Also, as with the Cl 2p XPS data, annealing at 140 K leads to no appreciable changes in the O 1s XPS data. It seems that the oxygen state remains in the same chemical state after the sample is annealed to this temperature. However, after annealing at 180 K, a clear shift to a value of 531.7 eV is observed. The R-OH state disappears at this temperature, and a new metal-oxygen bond is made, associated with the formation of an alkoxide species on the surface. By 260 K, the O 1s peak becomes weaker and shifts to 530.2 eV, a value more typical of atomic oxygen on the surface of the metal. We will discuss the nature of these chemical changes in more detail in the next section. More information on the surface chemistry of the 1-chloro2-methyl-2-propanol was obtained from results from TPD experiments, such as those shown in Figure 3. Desorption of a combination of molecular 1-chloro-2-methyl-2-propanol and tert-butyl alcohol can be observed in the trace for 59 amu, which corresponds to a ((CH3)2COH)+ fragment from either CH2Cl(CH3)2COH or (CH3)3COH, between 160 and 260 K. The first peak in that trace, centered at 178 K, is likely due to molecular desorption of 1-chloro-2-methyl-2-propanol; decomposition of halohydrocarbons on metal surfaces is typically initiated by the scission of the carbon-halogen bond,7 and according to our XPS data, that starts at temperatures above 180 K (Figure 1). The second feature at 202 K, on the other hand, may be associated with the formation of tert-butyl alcohol instead, via an initial scission of the C-Cl, followed by hydrogenation of the resulting hydroxyalkyl intermediate, -CH2(CH3)2COH, on the surface. The formation of isobutene oxide was also identified in the TPD experiments. Indeed, a weak, but clearly distinguishable, peak is seen in the trace for 72 amu at around 200 K. Isobutene is another species produced from this system, in this case, at around 250 K. Smaller hydrocarbon species could not be unambiguously identified due to the overlap of their mass spectra with fragments from larger hydrocarbon species.

The desorption of CO, H2O, and H2 is also reported in Figure 3b. CO desorption, the result of 1-chloro-2-methyl-2-propanol decomposition, is quite typical with hydrocarbons on metal surfaces. In this case, CO desorption occurs at 380 K; the first peak seen in the 28 amu trace at 255 K correlates with the desorption of isobutene also seen in the 56 amu trace. Water desorption is observed between 150 and 250 K. The desorption at 160 K occurs at too low a temperature to be associated with thermal dehydration of the original haloalcohol and is more likely the result of water adsorption from the background. The higher-temperature shoulder of that water feature, on the other hand, reflects water formation during the thermal conversion of the original haloalcohol and perhaps originates from decomposition of the hydroxyalkyl intermediate, -CH2C(CH3)2OH, or even from a C-O bond scission in the oxametallacycle intermediate that also forms on the surface. Hydrogen desorption occurs in a wide range from 250 to 450 K and is due to the recombination of surface hydrogen atoms, both from background adsorption and from decomposition of the adsorbed molecules (especially the high-temperature features). Additional TPD experiments were conducted with 1-chloro2-methyl-2-propanol dosed on hydrogen and deuterium preexposed Ni(100) surfaces. In these experiments, the clean Ni(100) was first dosed with 50 L of H2 or D2 and then with 6.0 L of 1-chloro-2-methyl-2-propanol, all at 100 K. The data from these experiments are shown in Figure 4. It was determined that preadsorption of hydrogen or deuterium does not significantly change the molecular desorption of 1-chloro-2-methyl-2-propanol, but does enhance tert-butyl alcohol formation. This can be seen in Figure 4a, in which the second desorption peak, at 202 K, grows and dominates the spectra. This is seen in the 59 amu trace in the case of H/Ni(100) and in the traces for both 59 and 60 amu in the case of D/Ni(100). The total amount of tert-butyl alcohol desorption increases by a factor of about 4 upon hydrogen (or deuterium) preadsorption. It is also noted in Figure 4b that an increase in isobutene oxide desorption is also seen with hydrogen preadsorption: although this species remains a minor product, its desorption intensity increases by a factor of 2 with the coadsorbed hydrogen. In terms of the production of isobutene on the clean versus hydrogen-predosed Ni(100) surfaces, it can be clearly seen that isobutene formation is less favored under the existence of surface atomic H, as shown in Figure 4c. This may be correlated with the fact that tert-butyl alcohol formation is more favored in the latter because both isobutene and tert-butyl alcohol are likely

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Figure 5. Tert-butyl alcohol (59 amu) and deuterium tert-butyl alcohol (60 and 61 amu) TPD spectra from 6.0 L of 1-chloro-2-methyl-2propanol adsorbed to clean and H- and D-predosed Ni(100) surfaces at 100 K; 50 L doses were used for the H2 and D2 preexposures of the Ni(100) surface.

Figure 4. Tert-butyl alcohol (59 amu), isobutene oxide (72 amu), isobutene (56 amu), and water TPD spectra from 6.0 L of 1-chloro2-methyl-2-propanol adsorbed on clean and H- and D-predosed Ni(100) surfaces at 100 K; 50 L doses were used for the H2 and D2 preexposures of the Ni(100) surface.

to originate from the same precursor. The enhanced isobutene oxide production may also be correlated to the reduction of isobutene formation. No obvious changes were seen for CO desorption among the three samples, the clean Ni(100), H/Ni(100), and D/Ni(100) surfaces (data not shown). Comparable amounts of extensive decomposition may occur in all three cases, and CO detection here seems to be limited by the kinetics of its desorption, not by that of the surface reactions with the hydrocarbon intermediates. However, the desorption peaks in the samples with H and D preadsorption shift from 380 to 430 K, perhaps reflecting the effect that surface H exerts on the desorption of CO.40 Finally, a significant change in water desorption is seen with hydrogen preadsorption in the form of an enhancement in the peak at 205 K. This peak can be clearly attributed to decomposition of the surface hydroxylalkyl or oxametallacycle species. The low-temperature peak, which is assigned to the background desorption, remains roughly the same. In the case of the deuterium preadsorption, the detection of signals in the 60 and 61 amu traces, representative of the tertbutyl alcohol produced, indicates some deuterium incorporation during the formation of the alcohol, most likely in the hydrogenation step that must follow the C-Cl bond-breaking step. This result is shown in Figure 5. The TPD signal of the 60 amu trace shows a similar feature as the hydrogen preadsorption sample. The lower-temperature peak at 178 K is likely due to H/D exchange at the hydroxyl group rather than in the methyl group because the activation energy of the O-H bond on the surface is lower. It is also possible to identify the incorporation of deuterium atoms into both the hydroxyl group

and the methyl group in the tert-butyl alcohol in the TPD trace for the 61 amu. The 202 K peak still remains in the spectrum but the 178 K peak does not, which further indicates the different nature of these two peaks; the 178 K peak in the 59 and 60 amu spectra is associated with molecular 1-chloro-2-methyl-2propanol. In the spectra for the deuterated tert-butyl alcohol species (60 and 61 amu), we see both the 202 and the 270 K peaks, which indicates that at least two mechanisms are involved in the formation of tert-butyl alcohol. The desorption peak at 202 K is probably related to the hydrogenation of the hydroxyalkyl intermediate, -CH2(CH3)2COH, whereas the desorption peak at 270 K is likely from hydrogenation of a different intermediate, as will be discussed below. The higher-temperature desorption peak is weak in the TPD spectra and cannot be resolved from the molecular adsorption signal in the data obtained on the clean and hydrogen predosed Ni(100). However, the extended shoulder on the high-temperature side of those TPD spectra does suggest the existence of this high-temperature desorption feature. Moreover, the shoulder is more obvious in the spectrum for H/Ni(100) than in the spectrum without the hydrogen preadsorption on surface. 4. Discussion In this brief paper, we discuss the surface chemistry of 1-chloro-2-methyl-2-propanol on Ni(100) single-crystal surfaces, as evidenced by TPD and XPS studies. Molecular adsorption appears to occur at temperatures below 140 K. The value for the Cl 2p XPS binding energy of 200.6 eV determined here at 100 and 140 K is likely to represent the molecular state of the 1-chloro-2-methyl-2-propanol, as the Cl 2p binding energy in organic compounds has been identified to be around 200.5 eV in other XPS experiments,41 and a synchrotron-based XPS experiment identified the doublet Cl 2p3/2 and Cl 2p1/2 at 200.6 and 202.2 eV, respectively, for 1-chloro-2-methyl-2-propanol on Ag(111).32 On the other hand, the shift in Cl 2p XPS binding energy seen in our experiments at 180 K, to a value of 198.1 eV for the Cl 2p3/2 peak, indicates the formation of surface absorbed chlorine atoms. This assignment can also be supported with

Thermal Activation of CH2Cl(CH3)2COH on Ni(100) evidence from previous reports. For instance, the Cl 2p3/2 binding energy was identified as 198.5 eV in NiCl2,42 and the binding energies of the Cl 2p3/2 and Cl 2p1/2 XPS peaks from absorbed atomic chlorine on Ag (110) have been reported as 198.1 and 199.7 eV, respectively.32 Although the TPD experiment shows some 1-chloro-2-methyl-2-propanol molecular desorption below 180 K, the XPS data reveal that a fraction of the adsorbed 1-chloro-2-methyl-2-propanol decomposes via a C-Cl bond scission step, most likely to produce an initial hydroxyalkyl species on the surface. Carbon-halogen bonds are typically weak, which is why many halohydrocarbons have been used as precursors for the preparation of surface species in modern surface science work.1,2,36 However, compounds with different halogens display different reactivities. Specifically, C-I bond scissions in alkyl groups occurs at temperatures below 200 K on many transitionmetal surfaces,35,43,44 but the C-Cl bond scission in CH3Cl requires a higher activation energy. Early experiments did not report C-Cl bond scission for CH3Cl on Pt(111),45 Ag(111),46 Al(111),47 or Ni(100).48 Nevertheless, the C-Cl scission activation energy is expected to be lower in larger alkyl chloride molecules.49 A study on the carbon-halogen bond scission in alkyl halides on Cu(100) shows that the C-Cl bond dissociation rate is smaller than those of C-Br and C-I bond dissociation and that the reaction rate is higher in larger alkyl species.44 This was also observed on Pd(111).50 Extensive reviews of the chemistry of hydrocarbons produced from halohydrocarbons on transition-metal surfaces can be found in Zaera’s and Bent’s reports.1,35,36 Earlier experiments on Ag(110) failed to detect significant reactivity of 1-chloro-2-methyl-2-propanol on the clean surface but indicated isobutene oxide formation, which does require C-Cl bond scission, on a surface predosed with oxygen.31,32 The observations from our experiment show that C-Cl bond scission can occur on the clean Ni(100) at low temperatures without the aid of coadsorbed oxygen. In terms of the information reflected by the O 1s XPS data in Figure 2, the asymmetric shape of the peak in the 100 K spectrum indicates more than one oxygen state on the surface. That peak could be fit with two components, a dominant peak at 532.8 eV and a minor peak at 531.7 eV. The 532.8 eV peak is characteristic of the R-OH in its molecular state. On the other hand, the small peak at 531.7 eV in our spectrum for O 1s can be assigned to surface hydroxyls51 or perhaps attributed to pre-existing surface water on the surface (from background adsorption). Regardless, by 180 K, there is no trace of the R-OH species on the surface, and the O 1s XPS trace only shows one single peak around 531.7 eV. In this case, that new feature is associated with an alkoxide species13 and indicates the scission of the O-H bond and the formation of an oxametallacycle intermediate. By 260 K, the O 1s XPS signal shows up as a weak peak at around 530.2 eV, somewhere between the O 1s binding energy of CO on Ni(111) (531.1 eV)52 and that of nickel oxide (529.5 eV),16,53-56 and the TPD data reported in Figure 3b show CO desorption at temperatures above 300 K. The O 1s peak at 260 K can, therefore, be attributed to the CO that forms during an earlier decarbonylation reacton. However, the formation of some surface nickel oxide cannot be fully excluded; the O 1s peak at 260 K can be due to a mixture of CO and nickel oxide states. Scission of the C-Cl and O-H bonds in the 1-chloro-2methyl-2-propanol adsorbed on Ni(100) by 180 K leads to the formation of new surface species, most likely hydroxylalkyl, -CH2(CH3)2COH, and oxometallacycle, -CH2(CH3)2CO-, intermediates. Further conversion of these species is likely to

J. Phys. Chem. C, Vol. 114, No. 17, 2010 7917 require the activation of C-H bonds, and those reactions normally display higher activation energies. In fact, hydrogenation of the initial intermediates may take precedent, hence the detection of tert-butyl alcohol in the TPD experiments (Figures 3 and 4). Such hydrogenation steps are clearly enhanced by any preadsorbed hydrogen or deuterium on the surface (Figure 4). Tert-butyl alcohol is made via hydrogenation of the hydroxylalkyl surface intermediate. The surface oxametallacycle intermediate, -CH2(CH3)2CO-, on the other hand, is not easily hydrogenated and, therefore, becomes the precursor for the remaining surface chemistry seen on the Ni(100) surface at temperatures above 180 K. In particular, a small amount of isobutene oxide is detected, although this is not a dominant reaction pathway. Such a reaction has also been reported on Ag(110), in which case, the reaction was shown to be promoted by surface oxygen.31 Competing with this epoxidation reaction is a deoxygenation of the oxametallacycle to isobutene, which desorbs around 250 K (Figures 3 and 4). Considering that both C-Cl and O-H bonds cleave before the formation of the isobutene oxide and isobutene products, both those species probably originated from the same precursor, most likely the oxametallacycle -CH2(CH3)2CO- mentioned before. Hydrogen preadsorption suppresses the production of isobutene but appears to promote some isobutene oxide formation. The primary reaction pathways for the oxametallacycle intermediate have been indentified to be an epoxidation, a deoxgenation, and an extensive decomposition. On the other hand, the reversible hydrogenation reaction of the oxametallacycle was not observed. Nevertheless, although the oxametallacycle is not easily hydrogenated, evidence of the reversible hydrogenation of this intermediate is provided by our TPD experiment with deuterium coadsorption on the surface, in which a second tert-butyl alcohol desorption peak at 270 K was observed. Two reaction pathways can be proposed leading to tert-butyl alcohol formation. The first is through the -CH2(CH3)2COH intermediate, which leads to tert-butyl alcohol desorption at the lower temperature of 202 K. By contrast, the second pathway is more likely to involve the -CH2(CH3)2COoxametallacycle, which leads to the tert-butyl alcohol desorption at 270 K (Figure 5). Hydrogenation of an oxametallacycle to the corresponding alcohol is a multistep process requiring the reductive elimination of hydrogen with both end Ni-C and end Ni-O bonds. The most likely pathway in this case may be the initial hydrogenation of the end carbon to produce an alkoxide intermediate. Regardless, though, both carbon and oxygen ends need to be hydrogenated, and that requires a higher activation energy and displays a lower reaction rate than the hydrogenation of the -CH2(CH3)2COH intermediate. There is precedent for isobutene formation from tert-butyl alcohol on oxygen-predosed Rh(111).22 In that experiment, isobutene desorbs at about 350 K and is also likely to originate from decomposition of an oxametallacycle intermediate, the alleged product of an earlier γ hydride elimination step. In our experiment, the oxametallacycle intermediate requires a C-Cl bond activation step instead, and that occurs at a much lower temperature than the γ hydride elimination in the Rh(111) case. As a consequence, isobutene desorbs at a lower temperature here compared with the case of O/Rh(111). It should also be said that the surface chemistry of 1-chloro-2-methyl-2-propanol has only been reported previously on a Ag(110) surface, and no observable isobutene oxide formation was observed there without the addition of oxygen promoters.31,32 The most likely reason for this difference is that alcohols are much more difficult

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Zhao et al. the formation of tert-butyl alcohol and isobutene oxide and effectively suppresses the production of isobutene. Surface hydrogen also enhances the hydrogenation of both hydroxyalkyl and oxametallacycle intermediates. Unlike earlier experimental results with 1-chloro-2-methyl-2-propanol on Ag(110) and tertbutyl alcohol on Rh(111), our work demonstrates that surface oxametallacycles can be formed without the need of any oxygen promotion on Ni(100) and that such oxametallacycles can be hydrogenated on the surface. The formation of an oxametallacycle at low temperatures is attributed to the easy rupture of the C-Cl and O-H bonds on Ni(100) surfaces. Acknowledgment. This work was supported by the U.S. National Science Foundation under Grant No. NSF-CHE 0742414. Additional support was provided by the Ministry of Science and Technology of China (2009IM033000) and the National Natural Science Foundation of China (50935001).

Figure 6. Proposed reaction pathways for the thermal conversion of 1-chloro-2-methyl-2-propanol on Ni(100).

to activate on silver surfaces19,57 and typically require the presence of coadsorbed oxygen to extract the hydroxo hydrogen. Once that step takes place, oxametallacycle formation is possible there as well. On the basis of the above experimental results, we propose the reaction scheme of 1-chloro-2-methyl-2-propanol conversion on Ni(100) shown in Figure 6. Thermal conversion in this system is initiated by the scission of the C-Cl bond, which occurs at temperatures around 180 K and leads to the formation of a hydroxyalkyl intermediate, -CH2(CH3)2COH. Hydrogenation of this intermediate produces some tert-butyl alcohol, while dehydrogenation at the hydroxyl group, which occurs at roughly the same temperature as the C-Cl bond scission, leads to the formation of an oxametallacycle intermediate, -CH2(CH3)2CO-. That second intermediate undergoes further thermal chemistry, including epoxidation to produce isobutene oxide, deoxygenation to yield isobutene, and hydrogenation to tert-butyl alcohol. The surface chemistry of oxametallacycles, in particular, their ability to produce epoxides, has been studied in some detail on Rh(111)21,22 and Ag(110).31,32 Such a reaction is less likely on earlier transition metals though, hence, the novelty of the results reported here on Ni(100). We have also in the past reported evidence of oxametallacycle formation during the reaction of 2-iodoethanol on Ni(100),13 but in that case, the chemistry did not prove as rich as what is discussed in this report. The relative strengths of the C-I versus C-Cl bonds appear to affect the selectivity of the relevant reactions and, with that, allow for the formation of the epoxide in the latter case. 5. Conclusions The surface chemistry of 1-chloro-2-methyl-2-propanol was investigated on Ni(100) single-crystal surfaces. Molecular adsorption occurs at 100 K, but both O-H and C-Cl bonds are broken at temperatures below 180 K. The rupture of the C-Cl bond leads to the initial formation of a hydroxyalkyl intermediate, a small amount of which hydrogenates to produce tert-butyl alcohol. The remaining surface species dehydrogenate at the hydroxo position to produce an oxametallacycle species, -CH2(CH3)2CO-. This intermediate is central in the surface chemistry of the conversion of 1-chloro-2-methyl-2-propanol on the clean Ni(100) surface and explains the formation of both isobutene, the main product, and the small amount of isobutene oxide seen at higher temperatures. Surface hydrogen enhances

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