Surface Chemistry of Methanol on Clean and Atomic Oxygen Pre

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J. Phys. Chem. C 2008, 112, 5501-5509

5501

Surface Chemistry of Methanol on Clean and Atomic Oxygen Pre-Covered Au(111) Jinlong Gong,† David W. Flaherty,† Rotimi A. Ojifinni,‡ John M. White,‡,§,| and C. Buddie Mullins*,†,‡ Department of Chemical Engineering, Department of Chemistry, and Center for Nano- and Molecular Science and Technology and Texas Materials Institute, UniVersity of Texas at Austin, 1 UniVersity Station C0400, Austin, Texas 78712-0231 ReceiVed: August 8, 2007; In Final Form: December 28, 2007

Desorption kinetics of methanol on Au(111) and the oxidation of methanol on atomic oxygen pre-covered Au(111) have been investigated employing temperature programmed desorption (TPD) and molecular beam reactive scattering (MBRS). On clean Au(111), methanol is weakly adsorbed and desorbs molecularly. Three adsorption states, which are assigned to methanol molecules desorbing from the monolayer (β phase), amorphous multilayers (R2 phase), and crystallized multilayers (R1 phase), are observed. For the β peak, detailed numerical analysis, based on mathematical inversion of the Polanyi-Wigner equation, leads to an optimized pre-exponential factor (log ν ) 11.3 ( 1.1 s-1) and coverage-dependent desorption energies, which are further used to accurately simulate TPD spectra for submonolayer initial methanol coverages. For methanol coverages of 0.2 and 1.0 ML, desorption energies of 40.6 ( 1.6 kJ/mol and 34.8 ( 1.4 kJ/mol are extracted. In the presence of atomic oxygen, TPD of adsorbed methanol exhibits desorption of only CH3OH, H2O, CO, and CO2. No other partial oxidation products or derivatives such as hydrogen, formaldehyde, formic acid, or methyl formate are detected during TPD or MBRS measurements. On the basis of our experimental results and previous similar studies on other coinage metals, we suggest that abstraction of hydrogen is the initial step in the surface decomposition of methanol and that the adsorbed/surface bound methoxy group [CH3O(a)] is the primary intermediate in the oxidation of methanol.

Introduction Alcohols, the simplest being methanol, have been intensively investigated in both model and practical catalytic systems.1,2 On the practical front, alcohols can be used as fuels, with certain environmental benefits, and feedstocks for preparation of chemical products (i.e., hydrocarbons).2,3 For model singlecrystal substrate systems, like the oxygen-Au(111) system used in this paper, adsorption, desorption, and reaction of methanol have been widely studied on both coinage and group VIII transition metals.4-11 For clean single-crystal coinage metals (i.e., Cu(111), Ag(111), and Au(110)), CH3OH adsorbs at cryogenic temperatures without dissociation, and during heating, there is no evidence for decomposition.7,9-11 When atomic oxygen is pre-dosed on single-crystal coinage metals8,11-19 and some other VIII transition metals (i.e., iron and platinum),20-25 methanol dissociates during heating by cleavage of the O-H or C-H bond to form, at least transiently, either methoxy and OH or formate and OH. In this case, the oxygen-induced reactions are commonly attributed to Bro¨nsted acid-base reactions.11 While bulk gold is typically not an active catalyst,26,27 nanoparticles of gold (i.e., with diameters of 2-5 nm) are very active for several reactions, for example, low-temperature CO oxidation,28-35 propylene epoxidation,36-38 and the water gas * Author to whom correspondence should be addressed. E-mail: [email protected]. † Department of Chemical Engineering. ‡ Center for Nano- and Molecular Science and Technology and Texas Materials Institute. § Department of Chemistry. | Deceased August 31, 2007.

shift reaction.39-41 The activity for oxidation reactions is typically attributed to O2 activation by Au nanoparticles. In turn, the catalytic activity of metal oxide supported Au clusters has spurred a renewed interest in investigating the chemistry of bulk gold,42-51 such as selective oxidation of ammonia and styrene. Insights obtained from studies of bulk gold surface reactions can assist in gaining a deeper understanding of the chemistry occurring on metal oxide supported Au nanoparticles (NPs). Although bulk gold does not measurably adsorb or dissociate molecular oxygen,52 it does readily chemisorb oxygen in its atomic form. Researchers have prepared atomic oxygen covered gold surfaces using a variety of approaches.44,45,49,53-55 Here, we have employed a radio frequency plasma jet source to prepare atomic oxygen for investigating the interaction and reactions between methanol and oxygen on Au(111) using temperature-programmed desorption (TPD) and isothermal reactive scattering with simultaneous product analysis.56,57 The only previous study of CH3OH on Au(111) under ultrahigh vacuum (UHV) conditions with or without oxygen atom pre-coverage is a brief report by Koel and co-workers as part of a temperature programmed reaction study of the reactivity of small molecules (i.e., CO, CO2, NO2, H2O, CH3OH, and C2H4) with oxygen.10 On clean Au(111), there was no evidence for dissociation of methanol (methanol was dosed at a surface temperature of 100 K) upon heating the sample. Regarding O-covered Au(111), CO2 and H2O were observed in the TPD measurements. No kinetic analysis was performed in either case. In the first section of this paper, we present results from investigation of the desorption kinetics of methanol on clean Au(111) and show evidence that reveals three distinct methanol adsorption states, similar to those shown previously on other

10.1021/jp0763735 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/19/2008

5502 J. Phys. Chem. C, Vol. 112, No. 14, 2008 metal surfaces (e.g., Ag,7 Cu,58 and Pt59). Furthermore, on the basis of a method previously proposed by Kay and coworkers,60,61 we mathematically invert temperature-programmed desorption spectra and obtain coverage-dependent kinetic parameters (e.g., the activation energy for desorption) using an optimized coverage-independent pre-exponential factor. In light of recent findings we have made regarding coadsorbed water and atomic oxygen,48 we revisit the chemistry of coadsorbed methanol and atomic oxygen on Au(111) and search for partial oxidation products such as hydrogen, formaldehyde, formic acid, or methyl formate. In addition to employing temperature programmed desorption, we have also used isothermal reactive scattering of methanol from an atomic oxygen covered Au(111) surface at various surface temperatures. We obtain similar conclusions to those of Koel and co-workers10 regarding the lack of formation of partial oxidation products. On the basis of the similarity of our study to others on coinage metals, we suggest that abstraction of the hydrogen atom bound to the oxygen in methanol is the initial step in its surface decomposition, and that a surface bound methoxy group [CH3O(a)] is the primary intermediate for the oxidation of methanol to H2O, CO, and CO2. Experimental Section The experiments reported in this paper were performed in a supersonic molecular beam surface scattering apparatus in which the ultrahigh vacuum scattering chamber has a base pressure of ∼2 × 10-10 Torr. The supersonic molecular beam source is quadruply differentially pumped in order to reduce the nonsupersonic component of the beam that effuses into the scattering chamber. More detailed descriptions of the chambers are reported elsewhere.29,62 Briefly, the scattering chamber is equipped with a reverse-view low-energy electron diffractometer (LEED) (Princeton Research Instruments RVL 8-120), an Auger electron spectrometer (AES, Perkin-Elmer C15-155 single pass CMA and 32-150 digital controller), a quadrupole mass spectrometer (QMS, Extrel C50-Q), and an ion gun (PerkinElmer 04-162). A customized six-way cross attached to the upper portion of the scattering chamber is individually pumped by a turbo pump and can be isolated from the scattering chamber by a gate valve. This portion consists of a home-built gold evaporator, a quartz crystal microbalance (QCM; Maxtek, BSH150), and a commercial nanocluster deposition system (Mantis, Nanogen50) placed in line with a quadrupole mass filter (Mantis, MesoQ) for sample preparation as well as for serving as the in situ high-pressure cell (HPC). The experiments are conducted on a Au(111) single crystal approximately 11 mm in diameter and 1.5 mm thick. The sample is secured to a tantalum plate (16.25 mm × 12.5 mm × 1 mm), and a TiO2 single crystal (10 mm × 10 mm × 0.5 mm thick, not used in this study) is mounted on the face of the tantalum plate opposite the Au crystal. Both samples are held in place with two 0.5 mm tungsten wire clips. Two tantalum wires are spot-welded to the tantalum plate to allow for computercontrolled, resistive heating of the sample. The tantalum heating wires are attached to two copper posts mounted on the sample probe that are in thermal contact with a liquid nitrogen reservoir. The sample probe is mounted on a precision manipulator that allows for positioning in the x, y, and z directions. The manipulator in turn is mounted on a differentially pumped rotary seal (Thermionics RNN-400) that allows for 360° of rotation with a precision of 0.1°. The sample temperature is measured by a chromel/alumel thermocouple (type K) spot-welded to the top edge of the tantalum plate. Temperature can be accurately

Gong et al. controlled by using a PID controller, which is connected to a programmable power supply, allowing for heating of the sample from 77 to 1200 K. The absolute temperature of the sample is calibrated with either the known multilayer desorption temperature for water or recombinative desorption temperature of atomic oxygen. Temperature-programmed desorption experiments are conducted in an angle-integrated fashion. The beams for the experiments were produced by expanding the gas through a 200 µm diameter alumina nozzle, which is part of a supersonic, RF plasma-jet atom source used for depositing atomic oxygen (Oa). Molecular beams result in a circular beam spot of diameter ∼3 mm on the surface at normal incidence; this is much smaller than the dimensions of the sample, thus minimizing gas interactions with other surfaces in the chamber. CH3OH (Mallinckrodt, 99.9%), CD3OH (Acros Organics, 99.5 atom % D), CH3OD (Acros Organics, 99.0 atom % D), O2 (Air Products, 99.9%), and Ar (Praxair, 99.9%) were used without further purification. An 8% (vol) O2 in argon mixture was used for generating atomic oxygen in the RF plasma-jet source with ∼40% dissociation fraction as determined via time-of-fight (TOF) measurements.63 Ionic species are deflected out of the beam line using a charged plate biased at 3 kV. The typical Oa flux was ∼2.6 × 1014 atoms cm-2 s-1. The same apertures and nozzle were used for beams of CH3OH and the O plasma to ensure that the dosed areas on the gold sample were co-incident (except that CH3OH was dosed with the RF power off). For all of the experiments, CH3OH was dosed with a nozzle pressure of 1 Torr. The crystal sample was cleaned by repeated Ar+ ion sputtering (1000 eV, ∼6 µA) for 30 min at a sample temperature of 300 K, followed by annealing at 850 K for 10 min. This procedure was repeated until the carbon coverage reached a small, constant value [(θC e 0.02 monolayer (ML)] as determined by AES. This final small amount of carbon was removed by exposing the sample to an atomic oxygen beam and then flashing to 700 K for 10 s. This technique removed all remaining carbon. Results and Discussion Adsorption and Desorption Kinetics of CH3OH on Clean Au(111). Methanol was dosed using a 300 K mildly supersonic molecular beam at a rate of ∼0.05 ML/s at normal incidence to the sample. The CH3OH coverages are defined relative to the area under the desorption curves of saturated monolayer states (peak at ∼155 K shown in Figure 1). While dosing, the sample was held at 77 K, well below the methanol multilayer desorption temperature (∼134 K). Methanol evolution is monitored using the m/e 31 signal, corresponding to the most intense fragment in the mass spectrum of gas-phase methanol. Methanol was the only desorption product detected in TPD experiments after dosing methanol on the clean Au(111) surface at 77 K. Repeated methanol exposures (e.g., three times) without cleaning the sample between exposures lead to identical CH3OH TPDs, and Auger electron spectra confirm that no dissociation products remain on the Au(111) surface following TPD, such as residual carbon or surface bound oxygen. Additionally, LEED measurements suggest that CH3OH is adsorbed randomly on clean Au(111) with no ordered structure in the overlayer. Methanol TPD spectra are shown in Figure 1 for initial coverages ranging from 0.24 to 2.86 ML. For each TPD, the sample was linearly heated at 0.5 K/s after exposing the clean Au(111) surface to the CH3OH beam at Ts ≈ 77 K. The TPD spectra show three distinct peaks, which we tentatively label as R1, R2, and β in order of increasing temperature. For submonolayer coverages, CH3OH is weakly adsorbed and

Surface Chemistry of Methanol on Au(111)

J. Phys. Chem. C, Vol. 112, No. 14, 2008 5503 of the methanol coverage on clean Au(111) at 77 K. This observation, combined with the fact that the formation of the multilayer starts only after the monolayer is entirely filled, suggests that an extrinsic precursor mechanism might be involved in the completion of the monolayer. To more accurately characterize this system, especially for methanol coverages of 1 ML or less, we utilize an inversion analysis method presented previously by Kay and co-workers60,61 to obtain coverage dependent desorption kinetic parameters. The coverage of methanol on the surface at any given temperature T is calculated by integrating the signals of desorption rate with respect to temperature/time from T to the limit T f ∞:

θ(T) )

Figure 1. TPD spectra for varying methanol coverages on clean Au(111) (heating rate β ) 0.5 K/s). Coverages are expressed in monolayer (ML) (1 ML corresponds to the amount of CH3OH that fills up peak β). For high coverages, the spectra show a three-peak feature. The peak with the highest desorption temperature is ascribed to methanol desorbed from the monolayer. The other two peaks, R1 and R2, correspond to methanol desorbed from multilayers with a crystalline or amorphous structure, respectively. Inset shows a curve from uptake measurement obtained from the TPD spectra. This uptake curve proves that the sticking coefficient of methanol on a clean Au(111) surface is coverage independent.

desorbs molecularly resulting in the β peak. A methanol coverage of 1 ML saturates the β peak at 155 K. Shifts to lower temperatures in desorption spectra with increasing coverage, like the one seen in the β peak, are frequently indicative of repulsive lateral interactions between adsorbate molecules. Similar results have been reported for methanol adsorbed on Ag(111), where the desorption peak shifted from 167 to 163 K with an increase in exposure from 0.7 to 4 L (1 Langmuir ) 1 × 10-6 Torr‚s).7 In addition to the β desorption feature, a second peak attributed to physisorbed multilayers appears in the TPD spectra at ∼143 K and is labeled as the R2 peak. The R2 feature saturates at ∼2.14 ML, and a third desorption feature (the R1 peak) appears at 134 K. The R1 peak is associated with multilayers adsorbed on top of the R2 state; however, these layers likely have a different structure. Previous studies on the adsorption of methanol on Pt(111) by Ehlers et al. have provided direct evidence for the difference in structure between R1 and R2 phases by employing UV photoelectron spectroscopy (UPS) and reflection absorption infrared spectroscopy techniques (RAIRS).59 Ehlers et al. report two phases of methanol on top of the monolayer at a sample temperature of 90 K. At deposition temperatures below 125 K, all multilayers have an amorphous structure. Upon annealing to 125 K, the majority of the multilayers convert into a crystalline R-ice phase leaving a few amorphous layers between the monolayer and the crystalline phase.59 Because of the similarities of crystal surface structure between Pt(111) and Au(111), we believe that the R1 peak can be attributed to desorption of a crystalline, R-ice phase, and the R2 peak can be attributed to desorption of the underlying amorphous phase. The uptake curve of methanol on clean Au(111), shown in the inset of Figure 1, is obtained by plotting the integral of the TPD curves against the methanol exposure time. Because the TPD area increases linearly with exposure time, it can be concluded that the sticking probability is nearly independent

1 β

dθ dT′ ∫T∞ dT′

(1)

where limTf∞ θ(T) ) 0 and β ) dT/dt. Each TPD spectrum provides the desorption rate r(θ, T) (which is a single-valued function of temperature and coverage) versus time for a given initial coverage and heating rate. As shown in Figure 1, the monolayer peak (which appears and saturates for increasing CH3OH coverages from 0 to 1.0 ML) produces a symmetric line-shape suggestive of a first-order desorption feature from the Au(111) surface. To extract the kinetic parameters of desorption, the Polanyi-Wigner equation is used,64 which provides the desorption rate r as a function of θ and T:

r(θ,T) ) -

dθ (θ,T) ) V(θ,T)θn exp[-Ed(θ)/RT] dt

(2)

in which, R is the gas constant, n is the order of desorption, Ed is the activation energy of desorption, and V is the preexponential factor of desorption. Assuming that V is independent of coverage, the PolanyiWigner equation is inverted to obtain the coverage-dependent desorption energy, Ed(θ), from each TPD spectra, as shown below.

[

Ed(θ) ) -RT ln -

dθ/dT βVθ

]

(3)

From Eq. 3, we calculated the coverage-dependent desorption energy of methanol as a function of methanol coverage from the Au(111) surface for 9 assumed values of the pre-exponential factor (log V ) 9, 10, 11, 12, 13, 14, 15, 16, and 17 s-1) using a TPD spectrum for an initial methanol coverage of 0.95 ML with a ramp rate of 0.5 K/s. On the basis of the coveragedependent desorption energy curves (not shown here), we find that the line shape of each curve is nearly independent of V; however, it is apparent that the desorption energy, Ed, shifts to higher values with increasing values of V. To further investigate the effect of the pre-exponential factor on simulated TPD spectra, we chose the experimental TPD data for three initial coverages of 0.48, 0.72, and 0.95 ML and obtained simulated spectra for each experimental TPD using the desorption energies acquired from Eq. 3. As shown in Figure 2, solid black dots represent the experimental data, while the dashed lines (of different colors) represent simulated TPDs. Each dashed line (detailed annotation is shown in the figure caption) corresponds to a different assumed pre-exponential factor. The highest initial coverage (0.95 ML) data set shown in Figure 2 was used to obtain the desorption energy values for each of the 9 preexponential factors (log V ) 9, 10, 11, 12, 13, 14, 15, 16, and 17 s-1). The simulated TPD spectra for this initial methanol coverage (0.95 ML) fit the experimental data very well for any

5504 J. Phys. Chem. C, Vol. 112, No. 14, 2008

Figure 2. Comparisons of experimental (solid black circles) and simulated TPDs for methanol desorption on Au(111) at 0.5 K/s and three initial coverages 0.48, 0.72, and 0.95 ML. Simulated data are shown for three initial methanol coverages by using those five energy curves and pre-exponential factors: 109 s-1 (dashed red lines), 1011 s-1 (dashed brown lines), 1013 s-1 (dashed blue lines), 1015 s-1 (dashed green lines), and 1017 s-1 (dashed purple lines). We have also simulated methanol TPDs using four other energy curves and pre-exponential factors (1010, 1012, 1014, and 1016 s-1). However, these data are not shown here for clarity of presentation. The best match to the experimental curve is the simulated data points based on a preexponential factor of 1011 s-1.

pre-exponential factors, since these experimental data were mathematically inverted to yield the coverage-dependent desorption energy. The calculated desorption energy, in turn, produces a desorption rate and thus establishes the function Ed(θ), which can be employed to simulate the same TPD spectra for any values of the pre-exponential factor. On the basis of the results shown in Figure 2, it can be seen that the extent of the match between the experimental data and the simulated spectra for the two lower initial coverages (0.48 and 0.72 ML) has a strong dependence on the value of the pre-exponential factor. Accurate reproduction of the TPD spectrum for any submonolayer initial methanol coverages requires the appropriate combination of the pre-exponential factor and the coveragedependent desorption energy. For the nine pre-exponential factors used to simulate the spectra (we have only shown five of them in Figure 2 for clarity of presentation), the best match to the experimental curve is the simulated data points based on a pre-exponential factor of 1011 s-1. To determine the best-fit pre-exponential factor, we used a least-square error (chi squared, χ2) between the simulated and experimental data for four initial methanol coverages (0.24, 0.48, 0.72, and 0.95 ML, the inverted experimental set), where the χ2 error is the sum of the squares of the difference between each experimental TPD data point and the corresponding simulated one. Figure 3 shows the χ2 errors between the experimental and the simulated TPD data shown in Figure 2 as a function of the assumed pre-exponential factor. A fourth-order polynomial function as shown by a solid line has been used to fit these χ2 errors. The minimum value of χ2 error is observed (illustrated by an open symbol in the inset of Figure 3) at log ν ) 11.3 s-1. It has been shown in a study by Tait et al. that this minimized value of the χ2 error is statistically accurate to within (10% in log ν,61 namely, log ν )11.3 ( 1.1 s-1. We have also examined the potential sources of experimental errors (e.g., thermocouple and QMS measurements) that may cause a larger error for the pre-exponential factor and have found that these effects typically contribute far less than (10% in log ν. On the basis of this best-fit pre-exponential factor (log ν ) 11.3 s-1), we have calculated Ed(θ) of methanol from Au(111)

Gong et al.

Figure 3. The χ2 errors between the experimental and the simulated TPD data shown in Figure 2 as a function of an assumed pre-exponential factor used in the inversion analysis for each prefactor in Figure 2 (solid black circles). The solid black diamonds represent extra pre-exponential factors which are not used in Figure 2 but only used in this figure to acquire more precise data for the χ2 errors. The solid line in this figure is a fourth-order polynomial fit, where the minimum error is obtained at ν ) 1011.3 s-1 (as shown by an open circle).

Figure 4. Comparisons of experimental (solid black circles) and simulated (solid lines) TPDs for methanol desorption on Au(111) at a ramp rate of 0.5 K/s and four initial coverages 0.24, 0.48, 0.72, and 0.95 ML. Inset is methanol desorption energy vs. coverage for methanol desorption on Au(111) from the best-fit pre-exponential factor, 1011.3 s-1. The desorption energy is obtained from the TPD spectrum shown in Figure 1 (θmethanol ) 0.95 ML, β ) 0.5 K/s) by inversion of the Polanyi-Wigner equation. First-order desorption and a constant preexponential factor of 1011.3 s-1 are used in the inversion procedure. Simulations are based on the coverage-dependent desorption energy shown in the inset.

as shown in the inset in Figure 4. The desorption energy is relatively high at low methanol coverages and decreases with increasing methanol coverage in two regions: in the range of 0-0.38 ML, the desorption energy decreases rapidly with increasing coverage, and in the range of 0.38-0.95 ML, the desorption energy nearly stays constant with increasing coverage. The changes in both regions suggest repulsive interactions between the neighboring methanol molecules. Specifically, desorption energies of 40.6 ( 1.6 kJ/mol and 34.8 ( 1.4 kJ/ mol are found for methanol coverages of 0.2 and 1.0 ML, respectively. (We estimate the uncertainty in the activation energy to be ∼4%.) Using this pre-exponential factor and the calculated Ed(θ), we simulate a number of desorption curves (TPDs) for different initial coverages (0.24, 0.48, 0.72, and 0.95 ML) that are shown as solid lines in Figure 4. The solid black circles in Figure 4 are corresponding experimental data for the four initial coverages. It can be seen that the simulated results are in close agreement (the fit is less good for θ ) 0.24 ML)

Surface Chemistry of Methanol on Au(111)

J. Phys. Chem. C, Vol. 112, No. 14, 2008 5505

TABLE 1: Temperature-Programmed Desorption Parameters for CH3OH on Different Transition Metal Single-Crystal Surfacesa

substrate Au(111) (current work) Au(110)11 Ag(111)7 Ag(110)65 Cu(110)58 Cu(100)66 Cu(111)9 Pt(111)66,67 Pt(110)68 Pd(100)69

monolayer desorption temperature (K)

multilayer desorption temperature (K)

monolayer desorption energy, Ed (kJ/mol)

155

134

34.8

200 163 165 185 168 210 190 160-220 170

175 143 140 140 150

50.2 39.6 40.5 46.1 41.5 54.0 47.3 38.6-54.0 45.3

145 147 145

a Multilayer desorption temperature is based on the first multilayer, and a monolayer desorption temperature is for a saturated layer. The desorption temperatures are obtained from TPD spectra with varying ramp rates from 0.5-15 K/s.

with the experimental counterparts regarding the positions of desorption peaks (Tdes) and the shape of desorption curves, such as the peak temperature shift due to the methanol-methanol interactions. It is notable that the desorption energy of 34.8 kJ/mol for 1 ML methanol on Au(111) is comparable to values found on other transition metal single-crystal surfaces, for example, silver, copper, and platinum (as listed in Table 1). These desorption energy values were obtained using a Redhead analysis70 with an assumed pre-exponential factor log ν ) 13 s-1. (A desorption energy of 38.7 kJ/mol is obtained if this value of ν is used in our analysis.) Adsorption and Reaction of CH3OH on Atomic Oxygen Pre-Covered Au(111). Oa/Au(111). Great efforts have been made to investigate the nature of oxygen on both gold single-crystal surfaces and nanoclusters to gain a better understanding of the unique properties of gold catalysts.11,26,28,31,32,38-40,42-45,47,53,54,71 However, gold singlecrystal surfaces do not chemisorb molecular oxygen via molecular or dissociative chemisorption under UHV conditions.52,53 Although some early work reported the chemisorption of oxygen on gold,53 later investigation has ascribed such results to the dissociative adsorption of oxygen on silicon and/or calcium impurities. The difficulty of populating gold surfaces with oxygen via exposure of gaseous, molecular oxygen has motivated researchers to develop other ways of creating atomic oxygen pre-covered gold surfaces. These approaches include thermal dissociation of gaseous O2 on hot filaments,53 exposure to ozone,54 co-adsorption of NO2 and H2O,55 O+ sputtering,44 electron-induced adsorption of oxygen,45 and electron bombardment of condensed NO2.49 The Madix group has shown that cracking molecular oxygen on a hot filament can be an effective method for covering a bulk gold surface with atomic oxygen.53 Madix and co-workers have also shown that the surface oxide formed can be decomposed completely by heating which leads to the recombinative desorption of atomic oxygen producing a single desorption peak in TPD spectra at ∼650 K. Recently, Koel and co-workers have employed ozone to create an oxygen pre-covered Au(111) surface.54 Their TPD experiments showed a single peak appearing at ∼550 K which shifted to lower temperature at very low coverages. Similar experimental results were obtained later by Davis and Goodman for oxygen desorption from the Au(111) and Au(110) surfaces, in which a

Figure 5. O2 TPD spectra for varying oxygen atom coverages on Au(111) (heating rate β ) 3 K/s). Atomic oxygen was dosed at 77 K using a radio frequency-generated plasma jet.

single desorption peak was found for each substrate at ∼ 535 K and ∼ 470 K, respectively.43 Wang and Koel examined the co-adsorption of NO2 and water-ice on Au(111) at 86 K and found that adsorbed atomic oxygen could be reliably generated from their reaction.55 They noticed in TPD measurements that the produced atomic oxygen adlayer desorbs recombinatively at ∼530 K. Recently, Gottfried et al. produced chemisorbed atomic oxygen on Au(110) from irradiation of physisorbed O2 layers with low-energy electrons or UV photons.45 The chemisorbed oxygen species desorbs at ∼550 K upon heating the sample. More recently, Deng et al. employed electron bombardment of condensed NO2 at 100 K to deposit atomic oxygen on clean Au(111).49 During TPD, they observed that the oxygen recombinatively desorbed with a peak temperature of 550 K. In the experiments presented here, chemisorbed atomic oxygen was generated and dosed via a supersonic, RF-generated plasma jet. No evidence of oxygen adsorption, either dissociative or molecular, is observed by AES or TPD measurements if a thermal beam of O2 (equivalent exposure of ∼50 L) is dosed on our Au(111) sample with the RF power off or if the sample is given a 100 L exposure of room-temperature oxygen by backfilling. For completeness, we include Figure 5 in this paper, which shows a representative series of oxygen TPD spectra, taken with a heating rate of 3 K/s for various initial oxygen atom coverages on Au(111). Oxygen coverages were typically determined by comparing the integrated area under the corresponding O2 TPD spectrum with the area under the TPD curve with a coverage of ∼2.1 ML (1 ML is defined as 1.387 × 1015 molecules/atoms cm-2).46 In Figure 5, desorption of the lowest oxygen coverage (0.16 ML) produces a peak around 520 K, and with increasing oxygen coverages, this peak shifts to higher temperatures. An oxygen coverage of 0.61 ML produces a peak around 535 K which shifts more gradually with further increases in oxygen coverage. The TPD spectra at high coverages in Figure 5 show evidence indicative of first-order kinetics even though the recombination of O adatoms to form O2 is expected to be a second-order process. Similarly, Koel et al. reported first-order desorption kinetics from a Au(111) surface populated with atomic oxygen by ozone decomposition.54 The desorption activation energy of oxygen from Au(111) based on a Redhead analysis is ∼30 kcal/mol at 0.16 ML oxygen coverage, increasing to ∼32 kcal/mol at 0.85 ML. These values are in good agreement with previous work by Goodman et al. 43 and Koel et al.54

5506 J. Phys. Chem. C, Vol. 112, No. 14, 2008

Figure 6. Thermal desorption of CH3OH following CH3OH adsorption on Au(111) at 77 K with atomic oxygen pre-coverages of (a) θO ) 0.08 ML; and (b) θO ) 0.84 ML. The spectra were taken with a ramp rate of 1 K/s. Atomic oxygen was dosed at 77 K.

CH3OH/Oa/Au(111). Methanol was dosed on the atomic oxygen pre-covered Au(111) surface, and temperature programmed desorption was employed to investigate the interaction of methanol with oxygen. Selected CH3OH spectra with coverages ranging from 0.24 to 1.90 ML from these experiments are shown in Figure 6. For methanol exposures on the Au(111) surface with an oxygen pre-coverage of 0.08 ML (Figure 6a), two desorption peaks are observed near 135 and 161 K, respectively. It appears that a relatively small amount of surface oxygen shifts the most tightly bound CH3OH (0.24 ML) to a peak around 200 K compared with the CH3OH TPD peak on clean Au(111) (167 K). On a Au(111) surface with an oxygen pre-coverage of 0.84 ML (Figure 6b), two methanol desorption peaks are observed upon heating the sample, and increased oxygen pre-coverages further shift the desorption peaks to higher temperatures. The desorption features we observed are in good agreement with the methanol desorption features observed by Koel et al. on Au(111) pre-covered with atomic oxygen, although they generated atomic oxygen by ozone exposure.10 As presented earlier, low-temperature desorption from multilayers shows zero-order desorption kinetics and is ascribed to a physisorbed methanol multilayer. On the other hand, it appears that the position of the monolayer desorption peak of methanol shifts slightly to higher temperature (compared to clean Au(111)), which can be ascribed to either atomic-oxygen-stabilized methanol or possibly recombination and disproportionation of surface methoxy groups (generated on Au(111) by de-protonating methanol with pre-adsorbed atomic oxygen). Indeed, the effect of adsorbates (e.g., oxygen) on the electronic properties of metal surfaces has been investigated computationally and experimentally, and it has been shown that even small amounts

Gong et al. of adsorbates can significantly alter the electronic characteristics of the surface atoms and the binding strength of coadsorbates.72 The types of oxygen on Au can be defined as chemisorbed oxygen (indicating oxygen bound to the Au (111) surface or small disordered gold islands), a surface oxide, (corresponding to a well-ordered two-dimensional Au-O phase), and a bulk oxide, defined as ordered (3D structures containing Au and O). Regarding the classifications of oxygen on Au and reactivity (i.e., CO oxidation), Friend and co-workers concluded the order of reactivity of oxygen as chemisorbed oxygen greater than oxygen in a surface oxide that is greater than oxygen in a bulk gold oxide.73 This result also implies that less positively charged Au is more reactive than more positively charged Au on the basis of the assumption that more charge transfer from gold to oxygen takes place in the gold oxide. In our case, higher coverages of oxygen appear to stabilize adsorbed methanol to a greater extent than lower coverages. As mentioned earlier, while this may be partly a coverage effect (i.e., more O-methanol interactions), it could also be influenced by the chemical state of the oxygen. In particular, oxide-like (Au-O) domains may exist only at the higher coverage, while chemisorbed oxygen on Au(111) may play a dominate role at the lower coverages. Additionally, previous investigations of atomic oxygen covered bulk gold have shown that chemisorbed atomic oxygen can alter the electronic structures of Au surfaces. Measurements carried out by Saliba et al. demonstrate that the adsorption of 1.0 ML of oxygen atoms on Au(111) increases the work function of the Au surface by +0.80 eV, indicating electron transfer from the Au substrate into the oxygen adlayer (i.e., the oxygen-induced formation of Auδ+ sites).54 Similar oxygen-induced work function changes have also been observed on Au(110) (1 × 2).47 Decomposition/oxidation of CH3OH was activated on the Au(111) surface in the presence of atomic oxygen. Figure 7a shows the temperature programmed reaction spectrum (TPRS) following an exposure of 1.43 ML of methanol on a 0.08 ML atomic oxygen pre-covered Au(111). For θO ) 0.08 ML, oxidation of CH3OH consumes all of the oxygen adatoms as shown by the lack of recombinative desorption of oxygen near the expected desorption temperature of 520 K (shown in the inset of the top panel). Similarly, methanol at a coverage of 1.43 ML on a Au(111) surface at 77 K with θO ) 0.84 ML was studied as shown in Figure 7b in which ∼16% of adsorbed methanol reacted. A significant quantity of oxygen adatoms (∼0.32 ML) remains on the surface and desorbs producing a peak at ∼520 K (shown in the inset of the bottom panel) via recombination. For both oxygen coverages, oxidation of some of the methanol was also indicated by the detection of water and other oxidation products such as CO and CO2. No other partial oxidation products or derivatives such as hydrogen (H2), formaldehyde (HCHO), formic acid (HCOOH), and methyl formate (HCOOCH3) were detected either during the CH3OH impingement phase of the experiment or during the subsequent TPDs. Regarding methanol oxidation on other single-crystal coinage metals (i.e., Cu and Ag), Wachs and Madix, in an early influential investigation, reported measurement of methanol oxidation on Cu(110).12 On the basis of temperature programmed desorption (TPD) experiments, they concluded that methanol reacts readily with adsorbed oxygen at 180 K to form a surface methoxy species. The methoxy decomposed at ∼330 K producing formaldehyde with the majority of it desorbing. Surface-bound formaldehyde underwent further oxidation to formate and was detected by the simultaneous desorption of CO2 and H2 at ∼440 K.12 Similar phenomena have also been reported on Ag(110)8 by the same

Surface Chemistry of Methanol on Au(111)

J. Phys. Chem. C, Vol. 112, No. 14, 2008 5507

Figure 8. Evolution of H216O from atomic oxygen (16Oa) pre-covered Au(111) at 200 K. The Au(111) sample is exposed to oxygen atoms at Ts ≈ 77 K to produce an atomic oxygen coverage of θO ≈ 0.36 ML. The sample is then heated to 200 K prior to impinging a continuous CH3OH beam (starts at t ) 10 s and ends at t ) 40 s) on the surface.

Figure 7. TPD spectra for CH3OH, CO2, H2O, and CO following adsorption of 1.43 ML CH3OH on Au(111) at 77 K with atomic oxygen pre-coverages of (a) θO ) 0.08 ML and (b) θO ) 0.84 ML. The spectra were taken with a ramp rate of 1 K/s. Atomic oxygen was dosed at 77 K. Inset shows the subsequent TPD spectra of O2 after the adsorption of CH3OH on atomic oxygen pre-covered Au(111) at 77 K (β ) 1 K/s).

group and on Cu(111)9 by Yates and co-workers. However, unlike on copper and silver, formaldehyde (HCHO) is not a product of methanol oxidation on Au(111) or Au(110).11 To further explain the major pathway of oxidation of CH3OH, a possible elemental reaction scheme is shown below in Eqs. 4-8. For all oxygen coverages:

CH3OH(a) + O(a) f H2O(g) + CH3O(a)

(4)

CH3O(a) + O(a) f H2O(g) + CO(a)/CO(g)

(5)

CO(a) + O(a) f CO2(g)

(6)

At high oxygen coverages, the mechanism shown immediately below, as well as the mechanism depicted above, is applicable.

CH3O(a) + O(a) f HCOO(a) + H2O(g)

(7)

HCOO(a)+ O(a) f CO2(g) + H2O(g)

(8)

As shown above, the initial step in the reactions is the dissociation of methanol. In order to further investigate the initial step in the surface decomposition of methanol and to search for possible partial oxidation products such as hydrogen (m/e

2), formaldehyde (m/e 29), formic acid (m/e 46), and methyl formate (m/e 60), we used reactive scattering of methanol from atomic oxygen covered Au(111) at various surface temperatures (200-400 K) under transient conditions. These temperatures are above the desorption peak temperatures of methanol (to avoid accumulation of methanol on the surface) and of water (180 K) but well below the onset of desorption of oxygen. However, no formation of partial oxidation products was detected in our reactive scattering measurements at different surface temperatures. Figure 8 shows an example of a reactive scattering experiment performed at 200 K on Au(111) with 0.36 ML coverage of atomic oxygen pre-dosed at ∼77 K. At time t ) 10 s, a beam of methanol is impinged on the oxygen-covered sample, and the water production is monitored with the QMS. As shown, there is an initial rapid rise in the H2O production upon impingement of the CH3OH beam (10 s) followed by a decay in the H2O production as the oxygen on the sample is consumed. The hydroxyl group in methanol (resulting from the cleavage of the C-16O bond in methanol) cannot be responsible for the formation of water (from disproportionation of 16OH) observed here since only mass 20 (H218O) [and no mass 18 (H216O)] was observed in a reactive scattering experiment (Figure 9) in which the Au(111) surface was pre-covered with 18O . Indeed, in recent studies, we have also shown that atomic a oxygen on the Au(111) surface can abstract hydrogen from water and ammonia at low temperature,42,57 which can be ascribed to the Bro¨nsted base character of oxygen adatoms on all of the group 1B metals.11 One of the first studies of methanol oxidation on gold was performed by Outka and Madix on a Au(110) single crystal.11 They also showed evidence suggesting that abstraction of the hydroxyl hydrogen of methanol by oxygen adatoms is the first step in the surface reaction, which leads to the formation of surface hydroxyls and subsequently water. To further distinguish between the cleavage of the methyl or hydroxyl hydrogen bonds (i.e., which reaction is the initial step), we have conducted additional experiments using isotopically labeled methanol (both CD3OH and CH3OD used to distinguish the hydroxyl hydrogen from the methyl hydrogens). First, we examined the desorption of D2O and H2O from Au(111) precovered with CD3OH and atomic oxygen [CD3OH/Oa/Au(111)] as well as CH3OD and atomic oxygen [CH3OD/Oa/Au(111)] upon heating the sample to 700 K. We also performed reactive scattering of labeled methanol (CD3OH and CH3OD) from atomic oxygen covered Au(111) at elevated surface temperature

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Gong et al. 18O

we only observed mass 28 (C16O) instead of both mass 28 and mass 30 (C18O) in the subsequent TPD spectra (not shown). Upon heating the surface, the CO either desorbs from Au(111) or further reacts with oxygen adatoms to produce CO2 at 165 K. The CO2 desorption feature at 165 K can be ascribed to a balance between the surface reaction rate constant increasing and the steady-state CO surface population decreasing with increasing substrate temperature.47 Recently, White and co-workers showed that methanol is transformed, because of the pre-adsorbed oxygen atoms, to methoxy species on the Pt(111) surface with the subsequent formation of H2O and CO2 upon heating.23 They proposed that the methoxy species further decomposes to form CO and H2, part of which are oxidized to CO2 and H2O by surface oxygen adatoms.23 Notably, as shown in Figure 7b and reaction 8, besides the CO2 desorption feature at 165 K, we also observe another small CO2 desorption feature around 310 K because of the decomposition of surface bound formate formed in reaction 7. Similarly, in their study of methanol oxidation on Au(110),11 Outka and Madix also showed that CO2 was desorbed at 340 K because of the decomposition of surface formate formed from partial oxidation of methanol. Additionally, employing IRAS Hirose et al. employed infrared reflection absorption spectroscopy (IRAS)24 showing that the methanol undergoes partial oxidation to formate intermediate on the molecular oxygen covered Pt(111) surface and further decomposes to form CO2 at higher temperatures.25 Finally and interestingly, we were unable to detect methyl formate (HCOOCH3, m/e 60) formed from the reaction between surface bound formaldehyde and methoxy species as reported on Au(110) by Outka and Madix (produced from coadsorbed oxygen atoms and methanol). The difference in the surface chemistry of methanol on Au(110) and Au(111) strongly suggests that there is a significant structural sensitivity in these reactions. Such structural sensitivity has also been observed in the dissociation of methanol on the clean Au(310)74 surface whereas no dissociation occurs on the Au(110)11 and Au(111)10 clean surfaces. a),

(C16O)

Figure 9. Evolution of H218O (m/e ) 20) and H216O (m/z ) 18) from atomic oxygen (18Oa) pre-covered Au(111) at 200 K. The Au(111) sample is exposed to oxygen atoms (18Oa) at Ts ≈ 77 K to produce an atomic oxygen coverage of θO ≈ 0.36 ML. The sample is then heated to 200 K prior to impinging a continuous CH3OH beam (starts at t ) 10 s and ends at t ) 40 s) on the surface.

(i.e., 200 K) under transient conditions to study the evolution of D2O and H2O. In both types of experiments, we observe the production of both D2O and H2O, and thus, we could not conclude that O-H bond cleavage occurs more easily than C-H bond cleavage. Second, we tried to employ a possible kinetic isotope effect (C-H vs C-D or O-H vs O-D) to confirm that O-H bond cleavage occurs more easily than C-H bond cleavage or vice versa. In order to do this, we dosed the same amount (i.e, 0.5 ML) of methanol (CH3OH, CD3OH, and CH3OD) on atomic oxygen with same coverage (i.e., 0.4 ML) on Au(111) and then monitored the percentage of unreacted oxygen desorbed from Au(111) upon heating. If O-H bond cleavage occurred more easily than C-H bond cleavage, we would expect that less oxygen would react with CH3OD/Oa/Au(111) than with CH3OH/Oa/Au(111) [we have observed this trend with D2O/ Oa/Au(111) and H2O/Oa/Au(111)]. On the other hand, if C-H bond cleavage occurs more readily than O-H bond cleavage, we anticipate that more adsorbed oxygen will remain on the CD3OH/Oa/Au(111) surface than on the CH3OH/Oa/Au(111) surface. However, for all three cases [CD3OH/Oa/Au(111), CH3OD/Oa/Au(111), and CH3OH/Oa/Au(111)], we observed the same amount of unreacted oxygen within our experimental uncertainties (i.e., ∼5%). Thus, these experiments with isotopically labeled methanol neither confirm nor disprove that surface methoxy is the initial intermediate in the reaction sequence. However, O-H bond cleavage as a first step (resultant CH3Oad) is fairly well-established in methanol oxidation on other single-crystal coinage metals [i.e., Cu, Ag, and Au(110)]. On the basis of these facts and our experimental observations (including our work with coadsorbed water with atomic oxygen) on Au(111), we believe that abstraction of hydroxyl hydrogen from methanol by atomic oxygen present on Au(111) is the initial step in the surface decomposition of methanol and that the adsorbed/surface bound methoxy group (CH3Oad) is the primary intermediate in the oxidation of methanol. As shown in reactions 5 and 7, hydrogen atoms in the methoxy group [CH3O(a)] left on the surface are further abstracted in the presence of atomic oxygen, leading to the formation of water and either CO and/or surface bound formate group [HCOO(a)] dependent on oxygen coverage (as mentioned earlier on, we were not able to detect any formaldehyde). It appears that no C-O bond is broken during the reactions since, in a similar isotopic experiment (where we used CH3OH and

Conclusions An investigation of the desorption kinetics and oxidation of methanol on clean and atomic oxygen pre-covered Au(111) was conducted under UHV conditions. We have carried out temperature programmed desorption experiments for methanol on the clean Au(111) surface, which reveal three different adsorption states. These states are assigned to methanol molecules desorbing from the crystallized multilayers, amorphous multilayers, and the monolayer, listed in order of increasing desorption temperature. Using the Polanyi-Wigner equation, we have inverted the TPD data to obtain coverage-dependent kinetic parameters for the desorption of methanol on Au(111). It was shown that the choice of pre-exponential factor significantly affects the quality of simulated methanol TPD. An optimized pre-exponential factor (log ν ) 11.3 ( 1.1 s-1) and coverage-dependent desorption energy were obtained. These values were used to accurately simulate TPD spectra for a range of submonolayer initial methanol coverages. The desorption energy for methanol clearly decreases with increasing coverage, and we attribute this trend to repulsive interactions between neighboring methanol molecules. For methanol coverages of 0.2 and 1.0 ML, desorption energies of 40.6 ( 1.6 kJ/mol and 34.8 ( 1.4 kJ/mol are extracted. Because of the difficulties inherent to populating the Au(111) with gaseous molecular oxygen under UHV conditions,

Surface Chemistry of Methanol on Au(111) a RF plasma-jet source was used to populate the sample with atomic oxygen. The monolayer desorption peak of methanol shifts to higher temperature in the presence of oxygen adatoms, which is due to either atomic-oxygen-stabilized methanol or recombination and disproportionation of surface methoxy groups. Upon heating the oxygen pre-covered surface populated with methanol, the adsorbed methanol was oxidized to form H2O, CO, and CO2. No other partial oxidation products or derivatives such as hydrogen, formaldehyde, formic acid, or methyl formate were detected during the reactions. On the basis of the reactive scattering measurements at various surface temperatures under transient conditions and the similarity of our study to others on coinage metals, we suggest that abstraction of hydrogen is the initial step in the surface decomposition of methanol and that the methoxy group [CH3O(a)] is the primary intermediate for complete oxidation of methanol. These results further demonstrate that atomic oxygen precovered bulk gold surfaces can be catalytically active. Acknowledgment. The authors would like to thank the Welch Foundation (F-1436), Department of Energy (DE-FG0204ER15587), and National Science Foundation (CTS-0553243) for their generous financial support and also acknowledge with pleasure the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. J.L.G. would like to acknowledge the International Precious Metals Institute (IPMI) for a student award received in June 2007. References and Notes (1) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley & Sons: New York, 1994; Chapter 7. (2) Tatibouet, J. M. Appl. Catal. A 1997, 148, 213. (3) Wu, J. C. S.; Fan, Y. C.; Lin, C. A. Ind. Eng. Chem. Res. 2003, 42, 3225. (4) Levis, R. J.; Jiang, Z.; Winograd, N. J. Am. Chem. Soc. 1989, 111, 4605. (5) Street, S. C.; Liu, G.; Goodman, D. W. Surf. Sci. 1997, 385, L971. (6) Desai, S. K.; Neurock, M.; Kourtakis, K. J. Phys. Chem. B 2002, 106, 2559. (7) Jenniskens, H. G.; Dorlandt, P. W. F.; Kadodwala, M. F.; Kleyn, A. W. Surf. Sci. 1996, 357-358, 624. (8) Wachs, I. E.; Madix, R. J. Surf. Sci. 1978, 76, 531. (9) Russell, J. J. N.; Gates, S. M.; Yates, J. J. T. Surf. Sci. 1985, 163, 516. (10) Lazaga, M. A.; Wickham, D. T.; Parker, D. H.; Kastanas, G. N.; Koel, B. E. ACS Symp. Ser. 1993, 523, 90. (11) Outka, D. A.; Madix, R. J. J. Am. Chem. Soc. 1987, 109, 1708. (12) Wachs, I. E.; Madix, R. J. J. Catal. 1978, 53, 208. (13) Ammon, C.; Bayer, A.; Held, G.; Richter, B.; Schmidt, T.; Steinruck, H. P. Surf. Sci. 2002, 507-510, 845. (14) Chen, A. K.; Masel, R. Surf. Sci. 1995, 343, 17. (15) Zhou, L.; Gunther, S.; Moszynski, D.; Imbihl, R. J. Catal. 2005, 235, 359. (16) Carley, A. F.; Davies, P. R.; Mariotti, G. G.; Read, S. Surf. Sci. 1996, 364, L525. (17) Felter, T. E.; Weinberg, W. H.; Lastushkina, G. Y.; Zhdan, P. A.; Boreskov, G. K.; Hrbek, J. J. Vac. Sci. Technol. 1982, 20, 887. (18) Andreasen, A.; Lynggaard, H.; Stegelmann, C.; Stoltze, P. Surf. Sci. 2003, 544, 5. (19) Wachs, I. E. Surf. Sci. 2003, 544, 1. (20) Rufael, T. S.; Batteas, J. D.; Friend, C. M. Surf. Sci. 1997, 384, 156. (21) Chen, Y. X.; Miki, A.; Ye, S.; Sakai, H.; Osawa, M. J. Am. Chem. Soc. 2003, 125, 3680. (22) Wang, J.; DeAngelis, M. A.; Zaikos, D.; Setiadi, M.; Masel, R. I. Surf. Sci. 1994, 318, 307. (23) Akhter, S.; White, J. M. Surf. Sci. 1986, 167, 101. (24) Endo, M.; Matsumoto, T.; Kubota, J.; Domen, K.; Hirose, C. J. Phys. Chem. B 2000, 104, 4916. (25) Endo, M.; Matsumoto, T.; Kubota, J.; Domen, K.; Hirose, C. J. Phys. Chem. B 2001, 105, 1573.

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