9820
J. Phys. Chem. C 2009, 113, 9820–9825
Annealing Effect on Reactivity of Oxygen-Covered Au(111) Rotimi A. Ojifinni,† Jinlong Gong,† David W. Flaherty, Tae S. Kim, and C. Buddie Mullins* Department of Chemical Engineering, Center for Nano and Molecular Science and Technology, and Texas Materials Institute, UniVersity of Texas at Austin, Austin, Texas 78712-0231 ReceiVed: March 11, 2009; ReVised Manuscript ReceiVed: April 7, 2009
We present results of an investigation into the effect of annealing on the reactivity of atomic oxygen adsorbed on Au(111) employing reactive molecular beam scattering (RMBS) and temperature-programmed desorption (TPD) techniques. Isotopically labeled water (e.g., H218O and D216O), carbon monoxide (CO), and oxygenlabeled carbon dioxide (C18O2) were used as probe molecules to investigate the reactivity of adsorbed oxygen. Our results show that the reactivity of atomic oxygen-precovered Au(111) is significantly altered by annealing. The annealed surfaces were prepared by depositing atomic oxygen (16O or 18O) at 77 K followed by annealing to temperatures ranging from 100-420 K before dosing probe molecules (H218O, CO, or C18O2) at 77 K. Without exception, annealing dramatically diminishes the reactivity of oxygen for all three probe reactions. In the case of the oxygen-water interactions, TPD indicates that annealing decreases the amount of oxygen isotope scrambling between oxygen and water. Additionally, the activity of the oxygen-precovered Au(111) surface for the CO oxidation reaction decreases monotonically as the surface is incrementally annealed to increasing temperatures. The decrease in activity is indicated both by diminishing CO2 production during reactive molecular beam scattering conducted at 77 K and by subsequent O2 TPD following the CO RMBS experiments. A similar loss of activity due to annealing is observed for the formation and decomposition of surface carbonate on Au(111) as detected by oxygen isotope exchange between adsorbed atomic oxygen (16Oa) and C18O2. These observations are attributed to the thermally induced stabilization of metastable oxygen species, suggesting that the metastable oxygen species are responsible for greater reactivity on the unannealed surface. A plausible explanation is that, at lower temperatures, the adsorbed atomic oxygen species reside in a metastable state from which the kinetic barrier to reaction is lower than when adsorbed or annealed at higher temperatures. Introduction Gold catalysis has drawn tremendous attention since Haruta discovered the exceptional catalytic activity of supported nanoparticles (NPs) 2-5 nm in diameter.1 Many surface chemical reactions have since been shown to be catalyzed on both Au NPs and single-crystal Au.2-6 Particular attention has been paid to low-temperature CO oxidation and other oxidation reactions on Au.2 However, several issues remain unclear regarding the mechanistic details of oxidative reactions. One outstanding question is the identity and nature of the reactive oxygen species. Some researchers report that atomic oxygen7-9 is the species responsible for reactivity on gold, whereas other groups propose that molecular oxygen is the reactive species.10,11 Stiehl et al. have shown unambiguously that both molecularly and atomically adsorbed oxygen are reactive in CO oxidation on Au/TiO2.12-16 One of the most effective approaches for characterizing oxygen states on metal surfaces is to exploit their chemical reactivity through the utilization of probe molecules.17 This article employs this methodology to study the effect of annealing on the reactivity of an atomic oxygen overlayer on Au(111) via simple reactions. The reactivity of oxygen can be changed by controlling its adsorption conditions, as shown in previous investigations on a variety of metal surfaces. For example, Gallagher demonstrated that an aluminum surface preadsorbed with oxygen at 295 K * To whom correspondence should be addressed. E-mail: mullins@ che.utexas.edu. † These authors contributed equally to this work.
was unreactive toward CO at 80 K, whereas the same surface precovered with oxygen at 80 K led to the formation of carbonates and other products.18 Similar observations were reported regarding the reaction of surface-bound oxygen with water or ethylene on lead (Pb) surfaces.19 Sueyoshi et al. employed CO oxidation to probe the reactivity of oxygen adsorbed on Cu(110). They reported that oxygen species formed by adsorption at 100 K are 25 times more reactive with respect to CO2 formation than oxygen adsorbed at 300 K.20-22 All of these studies attributed the reactivity of oxygen adsorbed at low temperature to the presence of metastable oxygen (Oδ-) species. Numerous investigators have employed nanoparticles and single crystals to gain a better understanding of the nature of reactive oxygen species on gold.1,2,8,12,13,23-48 In a pioneering investigation, Outka and Madix showed that atomic oxygencovered Au(110) is reactive toward methanol, acetylene, and water.49 They attributed this reactivity to the Bro¨nsted base character of oxygen adatoms on gold as seen on other group 1B metals.49 Koel and co-workers studied the reactivity of atomic oxygen (using ozone to generate coverages up to 1 ML) with CO, CO2, NO2, H2O, CH3OH, and C2H4 on Au(111). They demonstrated that CO, NO2, and CH3OH spontaneously react with atomic oxygen species; however, H2O, CO2, and C2H4 are inert on O/Au(111).37 In a recent study, Min et al. utilized TPD, XPS, and STM to investigate oxygen adsorption states on Au(111) and their respective reactivity toward CO oxidation.42 They identified various coverage-dependent oxygen states, ranging from chemisorbed species to bulk oxide, when oxygen is deposited at 200 K. Interestingly, when oxygen is deposited
10.1021/jp9022019 CCC: $40.75 2009 American Chemical Society Published on Web 05/07/2009
Annealing Effect on O-Au(111) Reactivity at 400 K larger islands with well-ordered 2D structure indicative of surface oxide are formed. Chemisorbed oxygen shows the highest initial reaction rate in CO oxidation, which can be reduced by the formation of surface and bulk oxides.42 In addition, it has been demonstrated that Au(111) populated with atomic oxygen at 77 K is reactive toward water,50 carbon monoxide,29,50,51 carbon dioxide,52,53 ammonia,30 amines,54 and alcohols.6,31,55-57 Of particular relevance to this study, our group has shown that atomic oxygen-precovered Au(111) can activate water as demonstrated by oxygen exchange between 16O and H218O.50,51 Through reactive molecular beam scattering experiments utilizing isotopically labeled oxygen species, we have further established that water activated in such a manner can participate directly in CO oxidation.29,50,51 Recently, Friend et al. also investigated water and oxygen interactions on Au(111) using a combination of TPD and reflection absorption infrared spectroscopy.58 They determined that, on the Au(111) surface, water and atomic oxygen undergo reaction resulting in the transient formation and recombination of hydroxyl pairs. Very few investigations have addressed the effect of annealing on gold model catalysts. Gottfried et al. reported that annealing decreases the intensity of desorption features of CO from the clean Au(110) surface and attributed this effect to a reduction in the surface defect concentration.59 Paul and Bent investigated the relationship between surface structure and reactivity of gold using CH3I as a probe molecule.60 They observed that freshly sputtered Au(111) fully annealed promotes methyl coupling at ∼270 K, whereas the reaction does not occur until ∼350 K on the incompletely annealed surface.60 Here, we present experimental results demonstrating that the activity of the atomic oxygen-precovered Au(111) model catalyst for three separate probe reactions (oxygen isotope scrambling in water, CO oxidation, and carbonate formation and decomposition) is reduced as a result of annealing. We deduce that the atomic oxygen species adsorbed at low temperatures (e.g., 77 K in this work) are trapped in a metastable state from which the barrier to reaction with other adsorbates is lowered. Annealing stabilizes these oxygen species on the surface, thereby increasing the barrier to further reaction. Experimental Section All experiments reported here were performed in an ultrahigh vacuum (UHV) molecular beam surface scattering apparatus that has been previously described in detail16,29,36,50,61-63 but is briefly outlined here. The UHV chambers are comprised of a scattering/analysis chamber and a quadruply differentially pumped molecular beam source chamber. The scattering/analysis chamber has a base pressure of ∼1 × 10 -10 Torr and is equipped with standard surface analysis tools such as Auger electron spectrometry (AES), low energy electron diffraction optics (LEED), and a quadrupole mass spectrometer (QMS). The Au(111) single-crystal sample (11 mm in diameter, 1.5 mm thick) used for the experiments is mounted to a tantalum plate that can be resistively heated and is in thermal contact with a liquid nitrogen bath for cooling. A type-K thermocouple spot-welded to the tantalum plate is used to monitor surface temperature. Oxygen atoms are deposited on the Au(111) surface using a radio frequency (RF) generated plasma-jet source that produces a supersonic beam of O atoms from an 8% (vol.) O2 in Ar gas mixture.61,63,64 An oxygen dissociation fraction of ∼40%, as measured by time-of-flight techniques, is achieved. Ions are deflected from the O-atom beam by a charged plate (biased negatively at 3000 V) located below the beamline in one of the differential pumping stages. We have previously
J. Phys. Chem. C, Vol. 113, No. 22, 2009 9821 shown that adsorbed O2,a is present on Au(111) after exposure to the plasma-jet source but the surface concentration is very small (less than 0.02 ML).12,13 Research-purity, isotopically labeled water [Isotec, 97.1% H218O and Spectra, 99.9% D216O] is used as received while atomic oxygen (16O and 18O) is generated from molecular oxygen (Matheson Trigas, 99.999% 16O2 and Isotec, 99.7% 18 O2). Water, oxygen and carbon monoxide are dosed via molecular beam and expanded through the same nozzle and apertures to ensure that the beam spots on the gold sample are identical and coincident. A typical value for the CO beam flux is ∼9 × 1013 molecules/cm2. The RF generator is switched on only when dosing atomic oxygen through the nozzle. The beam spot (∼3 mm in diameter) is much smaller than the sample size to minimize the effects of scattered gas interacting with other surfaces in the chamber. The C18O2 (Cambridge Isotopes Inc., 95% C18O2) used for the carbonate experiments is backfilled into the chamber using a leak valve. Au(111) is cleaned by argon ion sputtering (1 keV, 6 µA), followed by annealing in UHV (850 K for 10 min), which produces a carbon-free surface as verified by AES. Further cleaning with atomic oxygen ensures that the surface is free of all impurities. Surface crystallinity was verified by LEED. Oxygen coverages are estimated from the ratio of dN(E)/dE peak intensities of O(503 eV) to that of Au(239 eV) obtained via AES measurements. The O(503 eV)/Au(239 eV) ratio is compared to the O/Pt AES ratio of 0.3 observed for a p(2 × 2) oxygen adlayer on Pt(111), which corresponds to 3.9 × 1014 O atoms/cm2.29 Utilizing the standard Auger sensitivity factors for Au and Pt, we employ 0.95 as a conversion factor49 and determine that a O/Au AES ratio of 0.3 corresponds to 4.1 × 1014 oxygen atoms cm-2 (0.29 ML). Here, 1 ML of oxygen is defined as 1.39 × 1015 atoms/cm2 and refers to a single atomic layer of close-packed gold. Water coverages are determined from performing a mass balance during CO oxidation experiments involving coadsorbed water as described previously in detail.50 C18O2 exposures are reported in Langmuir (L) where 1 L corresponds to 1 × 10-6 Torr · s. For the experiments presented here, two distinct categories of surfaces are employed (i.e., unannealed and annealed). The unannealed surface is prepared by precovering Au(111) with atomic oxygen at 77 K followed immediately by dosing the relevant probe molecule (i.e., H2O, CO, or C18O2). The annealed surfaces are prepared by precovering Au(111) with atomic oxygen at 77 K followed by annealing to the indicated temperature, after which it is cooled to 77 K before dosing the desired probe molecule. Results and Discussion Reactivity of Co-Adsorbed Atomic Oxygen and Water. As mentioned previously, oxygen-precovered Au(111) can activate water leading to oxygen exchange between the atomic oxygen species (16O) and the water molecule (H218O). 50 This reaction was the first probe reaction that we employed to explore the affect of annealing on the reactivity of atomic oxygenprecovered Au(111). Temperature-programmed desorption (TPD) spectra of water (after dosing 0.53 ML of H218O on 0.18 ML of 16O-precovered Au(111) at 77 K) are displayed in parts (a) and (b) of Figure 1, whereas corresponding oxygen TPD spectra are shown in parts (c) and (d) of Figure 1, respectively. Note, the oxygen-precovered surface in part (a) of Figure 1 was unannealed prior to water exposure, whereas in part (b) of Figure 1 the oxygen-precovered surface was annealed to 300 K and then cooled to 77 K before dosing water.
9822
J. Phys. Chem. C, Vol. 113, No. 22, 2009
Ojifinni et al.
Figure 2. Fraction of initial 16O2 desorbing from the Au(111) surface after dosing 0.53 ML of H218O on each 16O-precovered surface (ΘO ) 0.18, 0.37, 0.50, 0.64, 0.84, and 1.30 ML). The unannealed surface is represented by (9, blue line), whereas the surface annealed to 300 K prior to H218O dose is represented by (b, red line). All isotopically labeled water and oxygen atoms were dosed at 77 K. Figure 1. TPD spectra of H218O after dosing (a) 0.18 ML of 16O on Au(111) directly followed by deposition of 0.53 ML of H218O; (b) 0.18 ML of 16O on Au(111), followed by annealing to 300 K (β ) 1 K/s) and finally deposition of 0.53 ML of H218O. Curves (c) and (d) show the corresponding oxygen TPD spectra for (a) and (b), respectively. TPD spectra of D216O (e) and (f) are from surface prepared in an analogous way to (a) and (b) respectively, with D216O and 18O. Curves (g) and (h) display the corresponding oxygen TPD spectra for (e) and (f), respectively. All doses are conducted at a surface temperature of 77 K and the sample is heated at rates of 1 and 3 K/s during water and oxygen desorption, respectively.
Both the unannealed surface as well as the annealed surface display two desorption features in the water TPD spectra. The lower-temperature peak (155 K) corresponds to desorption from isolated molecularly adsorbed water on Au(111), whereas the 175 K desorption peak is due to direct interaction between water molecules and oxygen atoms creating stable water-oxygen complexes (H2O-O).50 The intensity of the higher temperature feature provides a quantitative measure of the number of these complexes and thus the amount of interaction between coadsorbed water and oxygen. Integration of the 175 K feature for the unannealed surface [part (a) of Figure 1] corresponds to ∼29% of the total amount of water, whereas this feature accounts for only 18% in the case of the annealed surface [part (b) of Figure 1]. Similarly, as shown in part (c) of Figure 1, the coadsorption of water and oxygen on the unannealed surface leads to isotopic scrambling of ∼57% of the total amount of oxygen. However, the 300 K annealed surface [part (d) of Figure 1] induces isotopic scrambling of only ∼22% of the oxygen. These results indicate that the unannealed surface more readily induces reaction and isotope exchange between oxygen and water. A mass balance performed on the total amount of oxygen desorption demonstrates that no oxygen is lost during annealing. Similar experiments were performed employing D216O and 18 O. These results are displayed in parts (e)-(h) of Figure 1. As shown in parts (e) (the unannealed surface) and (f) (the annealed surface) of Figure 1, noticeably less water desorbs in the high-temperature feature on the annealed surface, ∼16%,
in comparison to the unannealed surface, ∼23%. Additionally, the annealed surface produces less isotopically scrambled oxygen, ∼38% of total adsorbed oxygen [part (h) of Figure 1], than the unannealed surface, ∼76% of total adsorbed oxygen [part (g) of Figure 1]. To further determine the effect of surface annealing on water reactivity with atomic oxygen, we measured the total amount of 16O2, which desorbs from surfaces with varying atomic oxygen (16O) precoverages to which a constant amount (0.53 ML) of water (H218O) is added. For example, the experiment shown in part (a) of Figure 1 has an initial coverage of 0.18 ML 16O to which 0.53 ML of H218O is added. The integral of the 16O2 (m/e ) 32) TPD spectrum in part (c) of Figure 1 divided by the integrated desorption of 16O2 (data not shown) from Au(111) solely covered with 0.18 ML 16O is equal to the fraction of oxygen that is not isotopically scrambled (subsequently referred to as unscrambled oxygen) on the unannealed surface. Similarly, on the annealed surface, the area beneath the mass 16 O2 (m/e ) 32) feature in part (d) of Figure 1 was divided by the integral of the 16O2 TPD from a surface solely covered with 0.18 ML 16O. These results provide a quantitative measurement of the extent of oxygen scrambling for a given oxygen coverage and annealing treatment. For example, a large fraction for 16O2 desorption indicates a small degree of exchange between water and surface oxygen adatoms, whereas conversely a low fraction of 16O2 desorption indicates a greater degree of oxygen scrambling and thus a more reactive surface. Figure 2 shows the relative fraction of unscrambled 16O2, which desorbs from surfaces prepared with various initial oxygen coverages, which then are annealed to 300 K (b, red line) or remain unannealed (9, blue line). Clearly, a lesser degree of isotope scrambling is observed for the surface annealed to 300 K prior to water adsorption. Studies of metastable oxygen on metal surfaces have been reviewed by Carley et al.17 Several examples have demonstrated that oxygen reactivity can be modified by changing the
Annealing Effect on O-Au(111) Reactivity conditions under which oxygen is adsorbed onto the metal surface. By modifying these conditions, oxygen can be kinetically trapped in a metastable state such that the barrier to reaction further reaction is decreased.17 We believe that such a metastable state of oxygen contributes to the increased reactivity of atomic oxygen toward water on Au(111). Annealing atomic oxygen-precovered Au(111) to 300 K prior to water adsorption stabilizes the oxygen species, which in turn decreases the probability of water dissociation and ultimately leads to less oxygen scrambling on the surface. As shown in Figure 2, for an oxygen precoverage of 0.18 and 0.53 ML of H218O, the fraction of unscrambled oxygen on the unannealed surface is about 0.18, whereas the comparable value is about 0.48 on the 300 K annealed surface. For both the annealed and unannealed surfaces, the fraction of unscrambled oxygen generally increases with increasing 16O coverage. This behavior suggests that atomic oxygen may be capable of blocking the active site required for the water-oxygen reaction. Presumably, as the atomic oxygen coverage increases, fewer sites are available for water to interact with the oxygen atoms, leading to less overall oxygen scrambling. Also, shown in Figure 2, for both the annealed and unannealed surfaces, increasing Oa coverage leads to reduced oxygen scrambling. Reactivity of Adsorbed Atomic Oxygen and Carbon Monoxide. Oxidation of CO to CO2 was utilized as the second reaction to probe the effect of annealing of oxygen on the gold model catalyst. As seen in the case of oxygen isotope scrambling in water, annealing induced oxygen atom stabilization also diminishes the reactivity for CO oxidation. One way to quantify the reactivity of oxygen-precovered Au(111) toward CO oxidation is by reactive molecular beam scattering (RMBS). In this case, impinging a molecular beam of CO onto the oxygenprecovered Au(111) surface at 77 K causes a fraction of the incident CO to react, thus forming CO2. We note that annealing the oxygen-precovered surface over the temperature range 77-400 K has no effect on the initial sticking probability of CO (S0 ≈ 0.7). Here, mechanistically, we envision the CO molecule first trapping on the oxygen-covered gold surface as a transiently, physically adsorbed species that can then either (i) react with adsorbed atomic oxygen, (ii) chemisorb, or (iii) desorb.65-73 The reaction yield can be calculated and reconciled though two techniques: direct measurement of the CO2 produced during the RMBS experiment, and comparison of the oxygen remaining on the surface after RMBS to that initially deposited evaluated by O2 TPD. The results of the RMBS experiments and the O2 TPD measurements are displayed in parts (a) and (b) of Figure 3, respectively. Prior to RMBS, the Au(111) surface is precovered with 0.37 ML 16O at 77 K, followed by annealing to the indicated temperature. After the sample is cooled to 77 K, a molecular beam of CO impinges upon the surface beginning at 10 s on the plot and extending for a total exposure of 10 s. Displayed in part (b) of Figure 3, the oxygen TPD spectra are collected immediately following RMBS of CO for each annealing temperature (note that some adsorbed oxygen will react with adsorbed CO during the TPD). These spectra represent the oxygen remaining on the surface after CO oxidation. Indeed, there is a clear correlation between CO2 production and annealing temperature. The greatest amount of CO2 is produced on the unannealed Au(111) surface, whereas a surface annealed to 400 K produces ∼75% less. For clarity, the insert of part (a) of Figure 3 shows the relative amount of CO2 produced as a function of annealing temperature, which is calculated by dividing the CO2 TPD area obtained for each
J. Phys. Chem. C, Vol. 113, No. 22, 2009 9823
Figure 3. (a) CO2 evolution at 77 K from 0.37 ML 16O-covered Au(111) surfaces annealed to varying temperatures (100, 150, 200, 250, 300, 350, 400, and 420 K) at β ) 1 K/s and cooled to 77 K before 10 s CO exposure. The lowest curve represents CO2 evolution from an unannealed surface. The insert at the upper portion of the panel shows the normalized amount of CO2 evolved as a function of annealing temperature. (b) TPD spectra showing unreacted O2 remaining on the Au(111) surface after each CO RMBS experiment. The lowest curve represents TPD of the remaining oxygen on the unannealed surface. The insert in the upper portion of panel shows the normalized amount of O2 remaining on the surface as a function of annealing temperature. We note that CO accumulates on the surface at 77 K, therefore while conducting the O2 TPD after CO RMBS the accumulated CO may scrub off a portion of the remaining oxygen from the surface. Thus, oxygen TPD shown in (b) and integrated O2 desorption plotted within the insert are smaller than can be accounted for from CO2 production exclusively during CO RMBS.
annealing temperature by the CO2 TPD area of the unannealed surface. The insert in part (b) of Figure 3 provides the relative amount of oxygen left on the surface after the reaction is obtained by dividing the integral of each oxygen TPD by that from a Au(111) surface precovered with 0.37 ML 16O at 77 K without CO exposure. The amount of unreacted oxygen increases with increasing annealing temperature prior to CO dose, which is consistent with the measurements shown in part (a) of Figure 3. Reactivity of Adsorbed Atomic Oxygen and Carbon Dioxide. Finally, the formation and decomposition of carbonate was utilized to investigate the effect of annealing on oxygen reactivity.52 As previously reported on oxygen-covered Au(111), scrambling of isotopically labeled oxygen can be employed to determine the rate of carbonate formation and decomposition.52 Surface carbonate is formed when atomic oxygen (16O)precovered Au(111) is exposed to oxygen-labeled carbon dioxide (C18O2). Upon heating, the surface carbonate decomposes and three distinct carbon dioxide isotopes desorb (C18O2, C18O16O, and C16O2). At higher temperatures, atomic oxygen remaining on the surface (18O a and 16Oa) recombinatively desorbs forming 16O2 and 16O18O (18O2 presumably forms also, however it is not observed due to the small surface concentrations of 18Oa). The reaction probability is then determined for the given experimental conditions by a comparison of the quantity of 16O 2 and 16O18O observed in TPD. Figure 4 shows reaction probabilities for surface carbonate formation and
9824
J. Phys. Chem. C, Vol. 113, No. 22, 2009
Figure 4. Reaction probability of carbonate formation as a function of annealing temperature on Au(111) precovered with 0.64 ML 16O at 77 K. The surface was annealed to the indicated temperatures (e.g., 150, 300, and 400 K) before dosing C18O2. Following annealing, the surface was exposed to 30 L C18O2 at 77 K. The leftmost point represents carbonate formation probability from a surface left unannealed prior to C18O2 exposure. All surface doses and exposures were done at 77 K, and a heating rate of 3 K/s was used.
decomposition on atomic oxygen (16O)-precovered Au(111) surface as a function of the annealing temperature. In each experiment, the Au(111) surface is precovered with 0.64 ML of 16O, annealed to the indicated temperature (150, 300, and 400 K) and later exposed to 30 L C18O2 after cooling to 77 K. For comparison, the same experiment was performed on a Au(111) surface precovered with 0.64 ML of 16O at 77 K without annealing prior to 30 L C18O2 exposure at 77 K. The reaction probability decreases notably with increasing annealing temperature as shown in Figure 4. Specifically, the unannealed surface is 9.2 times more reactive than a surface annealed to 400 K. Annealing to 300 K results in a reduction in the carbonate formation probability by a factor of 4 compared to the unannealed surface. Under the conditions used, no oxygen is lost during annealing or reaction as determined by integrating the TPD spectra of 16O2 (mass 32) and 18O2 (mass 34). It is evident from these results that the surface reactivity of atomic oxygen is maximized when adsorbed at low temperatures (77 K in current work). At low temperatures, adsorbed atomic oxygen exists in a metastable, reactive state from which the barrier to further reaction is relatively low. By annealing, the metastable oxygen becomes thermally stabilized and trapped in a chemisorbed state from which the barrier to further reaction is higher than the unannealed state. Therefore, this work provides experimental evidence that atomic oxygen is a reactive species on gold and that its reactivity can be altered by changing the adsorption and annealing conditions of the surface. Our results could provide insights into the nature of oxygen on gold surfaces related to the catalytic oxidative reactivity. Considering the similar trends [regarding the annealing-dependent reactivity of the oxygen-covered Au(111)] observed for all three of the probe reactions, we attribute the effect of annealing to the relaxation of highly reactive (e.g., metastable oxygen species at low temperatures) into a less reactive state (at high temperatures). Our results suggest that at least two discrete atomic oxygen species reside on the surface. Indeed,
Ojifinni et al. in an investigation employing STM, Friend and co-workers studied the surface morphology of oxygen-covered Au(111) and its correlation with reactivity as a function of oxygen coverage and deposition temperature.42 Three types of oxygen on Au were identified as chemisorbed metastable oxygen (indicating oxygen bound to the Au (111) surface or small disordered gold islands), a surface oxide (corresponding to a well-ordered 2D Au-O phase), and a bulk oxide (ordered 3D structures containing Au and O). They showed that upon increasing deposition temperature of oxygen on Au(111) (i.e., from 200 to 400 K) the adsorption state of oxygen changes from chemisorbed species to oxide.42 They further classified the order of reactivity (in CO oxidation) of atomic oxygen as: chemisorbed (metastable) oxygen > oxygen in a surface oxide (i.e., well-ordered 2D Au-O phase) > oxygen in a bulk gold oxide (i.e., 3D structures containing Au and O).42 In our case, although the oxidized Au(111) is prepared in a different way (O-plasma vs ozone decomposition42), the structure of the annealed oxidized Au(111) could be similar to those observed in previous STM measurements. That is, upon annealing the O-Au(111) surfaces from 77 to 400 K, the state of oxygen may undergo a gradual evolution from chemisorbed (metastable) oxygen (i.e., 77-200 K) to Au-O oxides (e.g., 200-400 K) leading to a decrease in the reactivity. On the basis of a literature survey and our experimental data, it is suggested that mobility of oxygen species on the surface is crucial to the reactivity. Compared to metastable oxygen, oxygen in ordered structures (i.e., Au-O oxides) is certainly less mobile and would react more slowly with our probe molecules. Additionally, a similar observation was drawn from UPS and XPS experiments on atomic oxygen-covered Ag(111) by Felter et al.74 They established the presence of two distinct types of atomic oxygen (active and inactive) on Ag(111). Active oxygen was characterized by an O1s peak at 528.5 eV, whereas the inactive specie was observed at 530 eV. CO exposure followed by XPS demonstrated that the oxygen with a peak at 528.5 eV was more closely correlated with CO oxidation activity than the oxygen species located at 530 eV.74 Whereas their study did not investigate the effect of annealing on the relative population of each oxygen species, it is notable that the reactivity of the individual oxygen states can be distinguished. Conclusions In our investigation of the effect of annealing on the reactivity of atomic oxygen on Au(111), we utilized temperatureprogrammed desorption and reactive molecular beam scattering measurements to study three probe reactions; isotope scrambling between atomic oxygen and water, oxidation of carbon monoxide, and carbonate formation and decomposition. The reactivity of surface-bound oxygen species is significantly reduced for all three probe reactions by annealing the surface. These results are consistent with the few previous studies on metal surfaces concerning the effect of adsorption conditions on the reactivity of oxygen. On Au(111), TPD results of water-oxygen interactions on an annealed surface indicate that the reactive oxygen state is obtained in the greatest concentrations when the sample remains unannealed. Annealing the oxygen-precovered surface to 300 K results in a reduction in the intensity of the higher temperature (∼175 K) water desorption peak, which is related to the decomposition of H2 O-O complexes. Diminished reactivity of the oxygen due to annealing was also evident in the corresponding oxygen TPD spectra, which displayed a lesser degree of oxygen isotope scrambling between adsorbed water
Annealing Effect on O-Au(111) Reactivity and oxygen after annealing to 300 K in comparison to the unannealed sample. Reactive molecular beam scattering of CO on oxygenprecovered Au(111) surfaces demonstrate that annealing decreases the reactivity of the system toward CO oxidation. For identical oxygen precoverages, the greatest amounts of CO2 are produced on the unannealed surface and CO2 production decreases monotonically with increasing annealing temperature. Subsequent oxygen TPD spectra after each CO RMBS experiment demonstrate that less oxygen is consumed by reaction with CO as the annealing temperature is increased. Experiments of surface carbonate formation and decomposition also show that annealing the atomic oxygen-precovered Au(111) surface significantly lowers the reaction probability. We hypothesize that for all three cases the observed reduction in reactivity is due to thermal stabilization of metastable oxygen species as a result of annealing. Therefore, the metastable oxygen species are responsible for higher levels of reactivity observed on the unannealed surfaces. Acknowledgment. The authors thank the Department of Energy (DE-FG02-04ER15587), Welch Foundation (F-1436), and National Science Foundation (CTS-0553243) for their generous support of this research. J.L.G acknowledges the David & Mary Miller Fellowship through the University of Texas at Austin for financial support. References and Notes (1) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 405. (2) Bond, G. C.; Thompson, D. T. Catal. ReV. Sci. Eng. 1999, 41, 319. (3) Meyer, R.; Lemire, C.; Shaikhutdinov, S. K.; Freund, H. Gold Bull. 2004, 37, 72. (4) Min, B. K.; Friend, C. M. Chem. ReV. 2007, 107, 2709. (5) Gong, J. L.; Mullins, C. B. Acc. Chem. Res. 2009, DOI: 10.1021/ ar8002706. (6) Madix, R. J.; Friend, C. M.; Liu, X. Y. J. Catal. 2008, 258, 410. (7) Bondzie, V. A.; Parker, S. C.; Campbell, C. T. Catal. Lett. 1999, 63, 143. (8) Grunwaldt, J. D.; Baiker, A. J. Phys. Chem. B 1999, 103, 1002. (9) Schubert, M. M.; Hackenberg, S.; van Veen, A. C.; Muhler, M.; Plzak, V.; Behm, R. J. J. Catal. 2001, 197, 113. (10) Hakkinen, H.; Landman, U. J. Am. Chem. Soc. 2001, 123, 9704. (11) Konova, P.; Naydenov, A.; Venkov, C.; Mehandjiev, D.; Andreeva, D.; Tabakova, T. J. Mol. Catal. A 2004, 213, 235. (12) Stiehl, J. D.; Gong, J. L.; Ojifinni, R. A.; Kim, T. S.; McClure, S. M.; Mullins, C. B. J. Phys. Chem. B 2006, 110, 20337. (13) Stiehl, J. D.; Kim, T. S.; McClure, S. M.; Mullins, C. B. J. Am. Chem. Soc. 2004, 126, 13574. (14) Stiehl, J. D.; Kim, T. S.; McClure, S. M.; Mullins, C. B. J. Am. Chem. Soc. 2004, 126, 1606. (15) Stiehl, J. D.; Kim, T. S.; McClure, S. M.; Mullins, C. B. J. Phys. Chem. B 2005, 109, 6316. (16) Stiehl, J. D.; Kim, T. S.; Reeves, C. T.; Meyer, R. J.; Mullins, C. B. J. Phys. Chem. B 2004, 108, 7917. (17) Carley, A. F.; Davies, P. R.; Roberts, M. W. Curr. Opin. Solid St. Mat. Sci. 1997, 2, 525. (18) Gallagher, D. E. Photoelectron Spectroscopic Studies of Aluminum and Chromium Surfaces; University College, Cardiff, UK, 1987. (19) Carley, A. F. R., S.; Roberts, M. W. Surf. Sci. 1983, 135, 35. (20) Sueyoshi, T.; Sasaki, T.; Y., I. Chem. Phys. lett. 1995, 241, 189. (21) Sueyoshi, T.; Sasaki, T.; Y., I. J. Phys. Chem. 1996, 100, 1048. (22) Sasaki, T.; Sueyoshi, T.; Y., I. Surf. Sci. 1994, 316, L1081. (23) Boyen, H. G.; Kastle, G.; Weigl, F.; Koslowski, B.; Dietrich, C.; Ziemann, P.; Spatz, J. P.; Riethmuller, S.; Hartmann, C.; Moller, M.; Schmid, G.; Garnier, M. G.; Oelhafen, P. Science 2002, 297, 1533. (24) Canning, N. D. S.; Outka, D. A.; Madix, R. J. Surf. Sci. 1984, 141, 240. (25) Choudhary, T. V.; Goodman, D. W. Top. Catal. 2002, 21, 25. (26) Daniells, S. T.; Makkee, M.; Moulijn, J. A. Catal. Lett. 2005, 100, 39. (27) Date, M.; Haruta, M. J. Catal. 2001, 201, 221. (28) Davis, K. A.; Goodman, D. W. J. Phys. Chem. B 2000, 104, 8557.
J. Phys. Chem. C, Vol. 113, No. 22, 2009 9825 (29) Gong, J. L.; Ojifinni, R. A.; Kim, T. S.; Stiehl, J. D.; McClure, S. M.; White, J. M.; Mullins, C. B. Top. Catal. 2007, 44, 57. (30) Gong, J. L.; Ojifinni, R. A.; Kim, T. S.; White, J. M.; Mullins, C. B. J. Am. Chem. Soc. 2006, 128, 9012. (31) Gong, J. L.; Flaherty, D. W.; Ojifinni, R. A.; White, J. M.; Mullins, C. B. J. Phys Chem. C 2008, 112, 5501. (32) Gong, X. Q.; Hu, P.; Raval, R. J. Chem. Phys. 2003, 119, 6324. (33) Gottfried, J. M.; Elghobashi, N.; Schroeder, S. L. M.; Christmann, K. Surf. Sci. 2003, 523, 89. (34) Gottfried, J. M.; Schmidt, K. J.; Schroeder, S. L. M.; Christmann, K. Surf. Sci. 2002, 511, 65. (35) Gottfried, J. M.; Schmidt, K. J.; Schroeder, S. L. M.; Christmann, K. Surf. Sci. 2003, 525, 184. (36) Kim, T. S.; Stiehl, J. D.; Reeves, C. T.; Meyer, R. J.; Mullins, C. B. J. Am. Chem. Soc. 2003, 125, 2018. (37) Lazaga, M. A.; Wickham, D. T.; Parker, D. H.; Kastanas, G. N.; Koel, B. E. ACS Symp. Ser. 1993, 523, 90. (38) Liu, Z. P.; Hu, P.; Alavi, A. J. Am. Chem. Soc. 2002, 124, 14770. (39) Liu, Z.-P.; Jenkins, S. J.; King, D. A. Phys. ReV. Lett. 2005, 94, 196102. (40) Lopez, N.; Norskov, J. K. J. Am. Chem. Soc. 2002, 124, 11262. (41) Mills, G.; Gordon, M. S.; Metiu, H. J. Chem. Phys. 2003, 118, 4198. (42) Min, B. K.; Alemozafar, A. R.; Pinnaduwage, D.; Deng, X.; Friend, C. M. J. Phys. Chem. B 2006, 110, 19833. (43) Outka, D. A.; Madix, R. J. Surf. Sci. 1987, 179, 351. (44) Parker, D. H.; Koel, B. E. J. Vac. Sci. Technol., A 1990, 8, 2585. (45) Pireaux, J. J.; Chtaib, M.; Delrue, J. P.; Thiry, P. A.; Liehr, M.; Caudano, R. Surf. Sci. 1984, 141, 211. (46) Saliba, N.; Parker, D. H.; Koel, B. E. Surf. Sci. 1998, 410, 270. (47) Schrader, M. E. Surf. Sci. 1978, 78, L227. (48) Trapnell, B. M. W. Proc. Royal Soc., A. 1952, 218, 566. (49) Outka, D. A.; Madix, R. J. J. Am. Chem. Soc. 1987, 109, 1708. (50) Ojifinni, R. A.; Froemming, N. S.; Gong, J.; Pan, M.; Kim, T. S.; White, J. M.; Henkelman, G.; Mullins, C. B. J. Am. Chem. Soc. 2008, 130, 6801. (51) Kim, T. S.; Gong, J.; Ojifinni, R. A.; White, J. M.; Mullins, C. B. J. Am. Chem. Soc. 2006, 128, 6282. (52) Ojifinni, R. A.; Gong, J. L.; Froemming, N. S.; Flaherty, D. W.; Pan, M.; Henkelman, G.; Mullins, C. B. J. Am. Chem. Soc. 2008, 130, 11250. (53) Gong, J. L.; Mullins, C. B. J. Phys. Chem. C 2008, 112, 17631. (54) Gong, J.; Yan, T.; Mullins, C. B. Chem. Commun. 2009, 761. (55) Gong, J.; Mullins, C. B. J. Am. Chem. Soc. 2008, 130, 16458. (56) Gong, J. L.; Flaherty, D. W.; Yan, T.; Mullins, C. B. ChemPhysChem 2008, 9, 2461. (57) Xu, B.; Liu, X.; Haubrich, J.; Madix, R. J.; Friend, C. M. Angew. Chem., Int. Ed. 2009, 48, 1. (58) Quiller, R. G.; Baker, T. A.; Deng, X.; Colling, M. E.; Min, B. K.; Friend, C. M. J. Chem. Phys. 2008, 129, 064702. (59) Gottfried, J. M.; Schmidt, K. J.; Schroeder, S. L. M.; Christmann, K. Surf. Sci. 2003, 536, 206. (60) Paul, A. M.; Bent, B. E. J. Catal. 1994, 147, 264. (61) Wheeler, M. C.; Reeves, C. T.; Seets, D. C.; Mullins, C. B. J. Chem. Phys. 1998, 108, 3057. (62) Wheeler, M. C.; Seets, D. C.; Mullins, C. B. J. Chem. Phys. 1996, 105, 1572. (63) Wheeler, M. C.; Seets, D. C.; Mullins, C. B. J. Chem. Phys. 1997, 107, 1672. (64) Pollard, J. E. ReV. Sci. Instrum. 1992, 63, 1771. (65) Mullins, C. B.; Rettner, C. T.; Auerbach, D. J.; Weinberg, W. H. Chem. Phys. Lett. 1989, 163, 111. (66) Mullins, C. B.; Weinberg, W. H. J. Vac. Sci. Technol. 1990, 8, 2458. (67) Mullins, C. B.; Weinberg, W. H. J. Chem. Phys. 1990, 92, 3986. (68) Mullins, C. B.; Weinberg, W. H. J. Chem. Phys. 1990, 92, 4508. (69) Rettner, C. T.; Mullins, C. B.; Bethune, D. S.; Auerbach, D. J.; Schweizer, E. K.; Weinberg, W. H. J. Vac. Sci. Technol. 1990, 8, 2699. (70) Seets, D. C.; Reeves, C. T.; Ferguson, B. A.; Wheeler, M. C.; Mullins, C. B. J. Chem. Phys. 1997, 107, 10229. (71) Seets, D. C.; Wheeler, M. C.; Mullins, C. B. J. Chem. Phys. 1997, 107, 3986. (72) Seets, D. C.; Wheeler, M. C.; Mullins, C. B. Chem. Phys. Lett. 1997, 266, 431. (73) Ferguson, B. A.; Reeves, C. T.; Mullins, C. B. J. Chem. Phys. 1999, 110, 11574. (74) Felter, T. E.; Weinberg, W. H.; Lastushkina, G. Y.; Boronin, A. I.; Zhdan, P. A.; Boreskov, G. K.; Hrbek, J. Surf. Sci. 1982, 118, 369.
JP9022019
