Adsorption of oxygen and carbon dioxide on cesium-reconstructed Ag(1 1 0) surface

The adsorption of oxygen and carbon dioxide on cesium-reconstructed Ag(1 1 0) surface has been studied with scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). At 0.1 ML Cs coverage the whole surface exhibits a mixture of (1 × 2) and (1 × 3) reconstructed structures, indicating that Cs atoms exert a cooperative effect on the surface structures. Real-time STM observation shows that silver atoms on the Cs-covered surface are highly mobile on the nanometer scale at 300 K. The Cs-reconstructed Ag(1 1 0) surface alters the structure formed by dissociative adsorption of oxygen from p(2 × 1) or c(6 × 2) to a p(3 × 5) structure which incorporates 1/3 ML Ag atoms, resulting in the formation of nanometer-sized (10–20 nm) islands. The Cs-induced reconstruction facilitates the adsorption of CO₂, which does not adsorb on unreconstructed, clean Ag(1 1 0). CO₂ adsorption leads to the formation of locally ordered (2 × 1) structures and linear (2 × 2) structures distributed inhomogeneously on the surface. Adsorbed CO₂ desorbs from the Cs-covered surface without accompanied O₂ desorption, ruling out carbonate as an intermediate. As a possible alternative, an oxalate-type surface complex [OOC–COO] is suggested, supported by the occurrence of extensive isotope exchange between oxygen atoms among CO₂(a). Direct interaction between CO₂ and Cs may become significant at higher Cs coverage (>0.3 ML).     

Formation of Mo and MoSx nanoparticles on Au(1 1 1) from Mo(CO)6 and S2 precursors: electronic and chemical properties

Mo(CO)₆ can be useful as a precursor for the preparation of Mo and MoS_x nanoparticles on a Au(1 1 1) substrate. On this surface the carbonyl adsorbs intact at 100 K and desorbs at temperatures lower than 300 K. Under these conditions, the dissociation of the Mo(CO)₆ molecule is negligible and a desorption channel clearly dominates. An efficient dissociation channel was found after dosing Mo(CO)₆ at high temperatures (>400 K). The decomposition of Mo(CO)₆ yields the small coverages of pure Mo that are necessary for the formation of Mo nanoclusters on the Au(1 1 1) substrate. At large coverages of Mo (>0.15 ML), the dissociation of Mo(CO)₆ produces also C and O adatoms. Mo nanoclusters bonded to Au(1 1 1) exhibit a surprising low reactivity towards CO. Mo/Au(1 1 1) surfaces with Mo coverages below 0.1 ML adsorb the CO molecule weakly (desorption temperature<400 K) and do not induce C–O bond cleavage. These systems, however, are able to induce the dissociation of thiophene at temperatures below 300 K and react with sulfur probably to form MoS_x nanoparticles. The formed MoS_x species are more reactive towards thiophene than extended MoS₂(0 0 0 2) surfaces, MoS_x films or MoS_x/Al₂O₃ catalysts. This could be a consequence of special adsorption sites and/or distinctive electronic properties that favor bonding interactions with sulfur-containing molecules.     

Vanadium oxide nanostructures on Rh(1 1 1): Promotion effect of CO adsorption and oxidation

The adsorption of CO and the reaction of CO with pre-adsorbed oxygen at room temperature has been studied on the (2 × 1)ORh(1 1 1) surface and on vanadium oxideRh(1 1 1) “inverse model catalyst” surfaces using scanning tunnelling microscopy (STM) and core-level photoemission with synchrotron radiation. Two types of structurally well-defined model catalyst V₃O₉Rh(1 1 1) surfaces have been prepared, which consist of large (mean size of 50 nm, type I model catalyst) and small (mean size <15 nm, type II model catalyst) two-dimensional oxide islands and bare Rh areas in between; the latter are covered by chemisorbed oxygen. Adsorption of CO on the oxygen pre-covered (2 × 1)ORh(1 1 1) surface leads to fast CO uptake in on-top sites and to the removal of half (0.25 ML) of the initial oxygen coverage by an oxidation clean-off reaction and as a result to the formation of a coadsorbed (2 × 2)O + CO phase. Further removal of the adsorbed O with CO is kinetically hindered at room temperature. A similar kinetic behaviour has been found also for the CO adsorption and oxidation reaction on the type I “inverse model catalyst” surface. In contrast, on the type II inverse catalyst surface, containing small V-oxide islands, the rate of removal of the chemisorbed oxygen is significantly enhanced. In addition, a reduction of the V-oxide islands at their perimeter by CO has been observed, which is suggested to be the reason for the promotion of the CO oxidation reaction near the metal-oxide phase boundary.     

Photoelectron diffraction study of a catalytically active overlayer: C2H2 on Pd{111}

A quantitative structure determination of a newly discovered (2×2) adsorption phase of acetylene chemisorbed on Pd{111} has been performed by scanned-energy mode photoelectron diffraction: this phase corresponds to the threshold coverage for the catalytic conversion of acetylene to benzene. The carbon atoms in the C₂H₂ molecule are located almost over bridge sites with a C–C bond length of 1.34+0.10 Å, the centre of the molecule being positioned almost over a hollow site. Of the two hollow sites the hcp site (directly above a second layer Pd atom) is favoured, particularly by a subset of the data most sensitive to this aspect of the structure, but the full analysis indicates that the fcc site (above a third layer Pd atom) cannot formally be excluded. The adsorption site adopted by acetylene in the higher coverage phase on Pd{111} is essentially identical. This is the dominant structure in the coverage regime which is catalytically active for the conversion of acetylene to benzene. The implications of these findings for acetylene coupling reactions over Pd{111} are discussed.     

Adsorption and dissociation of CO on Fe(1 1 0) from first principles

We employ spin-polarized periodic density functional theory (DFT) to characterize CO adsorption and dissociation on the Fe(1 1 0) surface. We investigate the site preference for CO on Fe(1 1 0) at θ_CO = 0.25 and 0.5 monolayer (ML), for different functional forms of the generalized gradient approximation (GGA) to electron exchange and correlation within DFT. At 0.25 ML, we predict the existence of a new ordered structure comparable in stability to one proposed previously. At 0.5 ML, we confirm the preference of a distorted on-top adsorption configuration suggested by experiment. The calculated heats of adsorption, CO stretching frequencies, and work function changes agree well with experiment. When dissociating from the on-top site, we predict that CO first moves off the on-top site and then goes through a lying-down transition state with a barrier of 1.52 eV. Diffusion of CO on Fe(1 1 0) from the on-top site to the long-bridge site is predicted to have a very small barrier of 0.1 eV. Dissociation of CO from the long-bridge site goes through the same transition state as from the on-top site, but the former has a slightly lower barrier. After dissociation, O atoms remain on the surface while C atoms are embedded into Fe(1 1 0), indicating C atoms may readily diffuse into Fe(1 1 0).     

Interaction of CO2 with Cs-promoted Fe(110) as compared to Fe(110)/K+CO2

The interaction of CO₂ with Cs-promoted Fe(110) at 85 K as well as temperature-dependent reactions between 100 and 700 K have been studied by means of ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Several surface species could be detected at 85 K, i.e. carbon monoxide (CO), carbonate (COⁿ⁻₃), physisorbed linear carbon dioxide (CO^lin₂) and very small amounts of oxidic oxygen (O_ox). An oxalate species (C₂O^m−₄) could not be identified definitively, but from comparison with the literature there is evidence that C₂O^m−₄ is present. Increasing the temperature after saturation with CO₂ leads to a complicated reaction behaviour. CO₂ either desorbs or dissociates into CO and COⁿ⁻₃ or forms C₂O^m−₄ at temperatures between 85 and 160 K. Above 160 K C₂O^m−₄, decomposes in parallel reactions into CO₂, COⁿ⁻₃ and CO. Above 320 K, adsorbed CO either desorbs into the gas phase or dissociates into C and O. In the temperature region between 500 and 700 K a recombination of C and O to CO and the desorption of Cs take place. As in the case of Fe(110)/K+CO₂, at high alkali coverages two carbonate species could be detected which dissociate upon heating at different temperatures. The system Fe(110)/Cs+CO₂ is proved to be very similar to the system Fe(110)/K+CO₂.     

Adsorption of carbon monoxide on Pt{100} surfaces: dependence of the CO stretching vibrational frequency on surface coverage

We have used the ab initio cluster model approach to study the dependence of the CO stretching frequency on CO surface coverage. We have also investigated the relative importance of the various factors that can affect the position of the CO stretching band as coverage increases. Two effects can change the CO stretching frequency: the adsorbate–adsorbate dipole coupling, which is a purely physical effect, and the changes in the 2π^* CO molecular orbitals, due to the different chemical environment at higher coverages. From our vibrational analysis, we conclude that CO–CO dipole coupling is the main cause of the upward shift of the CO stretching band when the CO coverage is increased. The population of the 2π^* CO molecular orbitals does not change at any coverage within the region considered. We have also estimated the ¹²CO–¹³CO dipole coupling, which previous studies have assumed to be weak. Our results demonstrate that the ¹²CO–¹³CO dipole coupling is indeed weak compared with the ¹²CO–¹²CO dipole coupling. At a CO surface coverage of 0.5 monolayers (ML), we have calculated a band shift of 40 cm⁻¹ to higher frequency. However, we should point out that when one ¹²CO molecule is surrounded by a ¹³CO environment, the ¹²CO stretching band shifts 10 cm⁻¹ upwards. We have also computed the heat of adsorption of CO on Pt{100}-(1×1) as a function of CO coverage. The initial heat of adsorption is calculated to be about 192 kJ mol⁻¹ and then drops to 180 kJ mol⁻¹ at 0.5 ML. These results agree quite well with recent calorimetric measurements. Besides that, we have estimated that the CO–CO interaction energy at 0.5 ML is repulsive and has a value of 5 kJ mol⁻¹.     

Cluster and periodic DFT calculations of adsorption and activation of CO2 on the Cu(hkl) surfaces

The adsorption behavior and thermal activation of carbon dioxide on the Cu(1 1 1), Cu(1 0 0), and Cu(1 1 0) surfaces have been investigated by means of density functional theory calculations and cluster models and periodic slabs. According to the cluster models, the optimized results indicate that the basis set of C and O atoms has a distinct effect on the adsorption energy, but an indistinct one on the equilibrium geometry. For the CO₂/Cu(hkl) adsorption systems studied here, the final structure of adsorbed CO₂ is near linear and the preferred modes for the adsorption of CO₂ onto the Cu(1 1 1), Cu(1 0 0), and Cu(1 1 0) surfaces are the side-on adsorption at the cross bridge site with an adsorption energy of 13.06 kJ/mol, the side-on adsorption at the short bridge site (13.54 kJ/mol), and the end-on adsorption on the on-top site with C–O bonds located along the short bridge site (26.01 kJ/mol), respectively. However, the calculated adsorption energies from periodic slabs are lower as compared to the experimental data as well as the cluster model data, indicating that the periodic slab approach of generalized gradient approximation in the density function theory may be not suitable to obtain quantitative information on the interaction of CO₂ with Cu(hkl) surfaces.     

CO2 adsorption and reaction on Fe(111): An angle resolved photoemission (ARUPS) study

Molecular CO₂ adsorption is observed on an Fe(111) surface at 85 K. For the main fraction of molecules the relative binding energies of the valence ion states as determined by ARUPS are consistent with those in the gas as well as in the condensed phase, and indicate that the electronic structure of that fraction of adsorbed molecules is only slightly distorted upon adsorption. There is a fraction of adsorbed molecules at 85 K that can be identified as bent, anionic CO₂⁻ species. While the weakly adsorbed, linear CO₂ molecules desorb at low temperature, the CO₂⁻ species is stable up to 160–180 K. The latter is proposed to be a precursor to dissociation. Above this temperature adsorbed carbon monoxide and oxygen are observed on the surface, and at room temperature the CO₂⁻ signals have disappeared. Heating above room temperature dissociates the CO molecules into carbon and oxygen.     

Interaction between CO and NH3 coadsorbed on Ru(001): its effects on the ordering in mixed adlayers and the ammonia dissociation

The coadsorption of CO and ammonia on Ru(001) has been investigated by low-energy electron diffraction (LEED), temperature-programmed desorption (TPD) and high-resolution electron energy-loss spectroscopy (HREELS). The main focus has been on the interaction between different admolecules on the surface and its important role in surface reaction. Exposing CO-precovered Ru(001) to ammonia at 100 K leads to the formation of mixed ordered layers with a (2 × 2) periodicity. It was found that two types of (2 × 2) structures are formed depending on the CO precoverage. One of the (2 × 2) structures (-phase) contains one CO and two ammonia molecules per (2 × 2) unit cell and the other (β-phase) contains two CO and one ammonia. Structure models for the two phases are proposed based on vibrational spectra measured for the coadsorbed phases of CO and ammonia (¹⁵NH₃ or ND₃). TPD results suggest that the ammonia dissociation takes place on clean and CO-precovered Ru(001). The amount of dissociated ammonia decreased initially with increasing CO precoverage, passed a minimum at θ_CO = 0.25, increased with a further increase of CO coverage, and eventually reached a saturation value above θ_CO = 0.5. The dissociation of ammonia in the β−(2 × 2) structure was found to be enhanced by a factor of 4–6 as compared with the dissociation in the −(2 × 2) structure. The HREEL spectra indicated that the C_3v molecular axis of ammonia is tilted in the coadsorbed layers, the tilting being most pronounced in the β−(2 × 2) phase with a high CO partial coverage. This observation suggests that the tilting of ammonia due to the interaction with CO facilitates electron donation from Ru 4d to LUMO of ammonia, leading to the N-H bond dissociation. The microscopic model for the CO-NH₃ interaction on metal surfaces is presented.     

Interaction of oxygen with the Pt(1 1 1) surface in wide conditions range. A DFT-based thermodynamical simulation

We present the results of ab initio calculations of oxygen atomic adsorption in a wide range of coverage on Pt(1 1 1). At θ = 0.25 ML, the O adsorption at fcc hollow site is clearly favoured over the hcp site. At θ = 0.5 ML, the O adsorption energy decreases but the same site is favoured. When experimental or theoretical previously reported data are available, the calculated adsorption energies and site preferences are in good agreement. Among the various configurations and coverages investigated in the present work, no adsorption is stable beyond θ = 0.5 ML, except by occupation of a subsurface tetrahedral site. In that case, a total O coverage of 0.75 ML could be achieved, which is only slightly less stable than the θ = 0.5 ML configuration.

The use of thermodynamics permitted to explore the temperature–pressure stability domain corresponding to 0.25 ML, 0.5 ML and 0.75 ML. From this, we conclude that subsurface O species could be stable at temperatures lower than 700 K, with O₂ pressures of 1 bar or less.     

TPD, HREELS and UPS study of the adsorption and reaction of methyl nitrite (CH3ONO) on Pt(111)

The adsorption and reaction of methyl nitrite (CH₃ONO, CD₃ONO) on Pt(111) was studied using HREELS, UPS, TPD, AES, and LEED. Adsorption of methyl nitrite on Pt(111) at 105 K forms a chemisorbed monolayer with a coverage of 0.25 ML, a physisorbed second layer with the same coverage that desorbs at 134 K, and a condensed multilayer that desorbs at 117 K. The Pt(111) surface is very reactive towards chemisorbed methyl nitrite; adsorption in the monolayer is completely irreversible. CH₃ONO dissociates to form NO and an intermediate which subsequently decomposes to yield CO and H₂ at low coverages and methanol for CH₃ONO coverages above one-half monolayer. We propose that a methoxy intermediate is formed. At least some C–O bond breaking occurs during decomposition to leave carbon on the surface after TPD. UPS and HREELS show that some methyl nitrite decomposition occurs below 110 K and all of the methyl nitrite in the monolayer is decomposed by 165 K. Intermediates from methyl nitrite decomposition are also relatively unstable on the Pt(111) surface since coadsorbed NO, CO and H are formed below 225 K.     

Evidence for bicarbonate formation on vacuum annealed TiO2(110) resulting from a precursor-mediated interaction between CO2 and H2O

The reaction of CO₂ and H₂O to form bicarbonate (HCO⁻₃) was examined on the nearly perfect and vacuum annealed surfaces of TiO₂(110) with temperature programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS) and high resolution electron energy loss spectrometry (HREELS). The vacuum annealed TiO₂(110) surface possesses oxygen vacancy sites that are manifested in electronic EELS by a loss feature at 0.75 V. These oxygen vacancy sites bind CO₂ only slightly more strongly (TPD peak at 166 K) than do the five-coordinated Ti⁴⁺ sites (TPD peak at 137 K) typical of the nearly perfect TiO₂(110) surface. Vibrational HREELS indicates that CO₂ is linearly bound at the latter sites with a ν_a(OCO) frequency similar to the gas phase value. In contrast, oxygen vacancies dissociate H₂O to bridging OH groups which recombine to liberate H₂O in TPD at 490 K. No evidence for a reaction between CO₂ and H₂O is detected on the nearly perfect surface. In sequentially dosed experiments on the vacuum annealed surface at 110 K, CO₂ adsorption is blocked by the presence of preadsorbed H₂O, adsorbed CO₂ is displaced by postdosed H₂O, and there is little or no evidence for bicarbonate formation in either case. However, when CO₂ and H₂O are simultaneously dosed, a new CO₂ TPD state is observed at 213 K, and the 166 K state associated with CO₂ at the vacancies is absent. SSIMS was used to tentatively assign the 213 K CO₂ TPD state to a bicarbonate species. The 213 K CO₂ TPD state is not formed if the vacancy sites are filled with OH groups prior to simultaneous CO₂+H₂O exposure. Sticking coefficient measurements suggest that CO₂ adsorption at 110 K is precursor-mediated, as is known to be the case for H₂O adsorption on TiO₂(110). A model explaining the circumstances under which the proposed bicarbonate species is formed involves the surface catalyzed conversion of a precursor-bound H₂O–CO₂ van der Waals complex to carbonic acid, which then reacts at unoccupied oxygen vacancies to generate bicarbonate, but falls apart to CO₂ and H₂O in the absence of these sites. This model is consistent with the conditions under which bicarbonate is formed on powdered TiO₂, and is similar to the mechanism by which water catalyzes carbonic acid formation in aqueous solution.     

Theoretical study of CO2 activation on Pt(111) induced by coadsorbed K atoms

The adsorption of CO₂ on the clean and potassium-precovered Pt(111) surface has been studied by means of the cluster model approach within the hybrid B3LYP density functional theory-based method. On the clean surface, CO₂ is undistorted and weakly bound. The equilibrium position of this physisorbed species appears at a rather large distance from the surface. However, when coadsorbed K atoms are included in the model, a chemisorbed, bent CO₂ species on top of a surface Pt atom is found. The presence of the coadsorbed K is found to be necessary to promote CO₂ chemisorption and activation, the key step in activating the CO₂ molecule being a direct interaction with the coadsorbate. In addition, the calculated vibrational frequencies for this chemisorbed species are in agreement with available experimental data.     

Chemisorption and reactivity of furan on Pd{111}

XPS, HREELS, ARUPS and Δø data show that furan chemisorbs non-dissociatively on Pd{111} at 175 K, the molecular plane being significantly tilted with respect to the surface normal. Bonding involves both the oxygen lone pair and significant π interaction with the substrate. The degree of decomposition that accompanies molecular desorption is a strong function of coverage: 40% of the adsorbate desorbs molecularly from the saturated monolayer. Decomposition occurs via decarbonylation to yield CO_a and H_a followed by desorption rate limited loss of H₂ and CO. It seems probable that an adsorbed C₃H₃ species formed during this process undergoes subsequent stepwise dehydrogenation ultimately yielding H₂ and C_a.     

Defect induced surface chemistry: A comparison of the adsorption and thermal decomposition of C2H4 on Rh{111} and Rh{331}

The adsorption and thermal decomposition of C₂H₂ on Rh{111} is compared to the atomically stepped Rh{331} surface over a temperature range of 300 to 800 K. Using X-ray photoelectron spectroscopy (XPS) we find that the C 1s spectra as a function of C₂H₄ exposure exhibit a shift in binding energy (E_b) from 283.5 eV at 1 L C₂H₄ exposure on both surfaces to 283.8 eV on Rh{33 and to 284.1 eV on Rh{111} at saturation coverage (4 L). Careful analysis of the C 1s E_b value and full width at half maximum as a function of surface temperature after a 10 L exposure of C₂H₄ at 300 K reveals that a species consistent with a C₂H adsorbate composition is formed between 400 and 450 K on Rh{111}. This species is also observed on Rh{331} although at the lower temperature of 375 K. Computer peak deconvolution of the C 1s spectra between 500 and 700 K suggests that a CH_ads or C_ads surface fragment is formed and increases in concentration at the expense of the C₂H species as the surface temperature increases. Above 750 K a graphite overlayer is formed on both surfaces. This overlayer, however, exhibits a low degree of carbon π-character bonding on Rh{331}. The adsorption and decomposition mechanisms suggest that the 300 K C₂H₄ adsorbate on Rh{331} is ethylidyne and that the stepped surface is more thermally reactive than the flat Rh{111} surface.     

The IR spectra of Mg5C(CO) complexes on the (0 0 1) surfaces of polycrystalline and single crystal MgO

The FTIR spectroscopy of carbon monoxide adsorbed on polycrystalline MgO smoke has been investigated as a function of the CO equilibrium pressure at constant temperature (60 K) (optical isotherm) and of the temperature (in the 300–60 K range) at constant CO pressure (optical isobar). In both cases the spectra fully reproduce those of CO adsorbed on the (0 0 1) surface of UHV cleaved single crystals [Heidberg et al., Surf. Sci. 331–333 (1995) 1467]. This result, never attained in previous investigations on dispersed MgO, contribute to bridging the gap which is commonly supposed to exist between surface science and the study of “real” (defective) systems. Depending on the surface coverage θ the main spectral features due to the CO/MgO smoke interaction are a single band shifting from 2157.5 (at θ→0) to 2150.2 cm⁻¹ (at θ=1/4) or a triplet, at 2151.5, 2137.2 and 2132.4 cm⁻¹ (at θ>1/4). These manifestations are due to the ν(CO) modes of Mg_5C²⁺· · · CO adducts formed on the (0 0 1) terminations of the cubic MgO smoke microcrystals. The formation of the CO monolayer is occurring in two different phases: (i) a first phase with CO oscillators perpendicularly oriented to the surface (2157–2150 cm⁻¹) and (ii) a second phase constituted by an array of coexisting perpendicular and tilted species (triplet at 2151.5, 2137.2 and 2132.4 cm⁻¹). A much weaker feature at 2167.5–2164 cm⁻¹ is assigned to Mg_4C²⁺· · · CO adducts at the edges of the microcrystals. The heat of adsorption of the perpendicular Mg_5C²⁺· · · CO complex in the first phase has been estimated from the optical isobar and results to be 11 kJ mol⁻¹.     

Structure and NO reactivity of Zr-deposited Pd surfaces

The structure and NO reactivity of Zr-deposited Pd surfaces were investigated by X-ray photoelectron spectroscopy, low-energy electron diffraction, infrared reflection absorption spectroscopy, and temperature-programmed desorption. Zr on Pd(1 0 0) was oxidized to ZrO₂ by exposure to O₂ at 773 K. Heating at 823 K in a vacuum led to decomposition of ZrO₂ to Zr metal and O₂. The activation energy for ZrO₂ decomposition changed remarkably at Θ_Zr = 0.4. For Θ_Zr > 0.4, a hexagonal structure was observed for ZrO₂/Pd(1 0 0); no ordered structure was observed for Θ_Zr < 0.4. Deposited Zr had no significant effect on the adsorption and decomposition of NO on Pd(1 0 0) but resulted in a creation of new NO dissociation sites on Pd(3 1 1). Zr on Pd(3 1 1) was oxidized to ZrO_X by oxygen produced from NO dissociation. Heating at 823 K in a vacuum led to decomposition of ZrO_X to Zr metal and O₂.     

Adsorption of oxygen on Au(111) by exposure to ozone

Atomic oxygen coverages of up to 1.2 ML may be cleanly adsorbed on the Au(111) surface by exposure to O₃ at 300 K. We have studied the adsorbed oxygen layer by AES, XPS, HREELS, LEED, work function measurements and TPD. A plot of the O(519 eV)/Au(239 eV) AES ratio versus coverage is nearly linear, but a small change in slope occurs at Θ_O=0.9 ML. LEED observations show no ordered superlattice for the oxygen overlayer for any coverage studied. One-dimensional ordering of the adlayer occurs at low coverages, and disordering of the substrate occurs at higher coverages. Adsorption of 1.0 ML of oxygen on Au(111) increases the work function by +0.80 eV, indicating electron transfer from the Au substrate into an oxygen adlayer. The O(1s) peak in XPS has a binding energy of 530.1 eV, showing only a small (0.3 eV) shift to a higher binding energy with increasing oxygen coverage. No shift was detected for the Au 4f_7/2 peak due to adsorption. All oxygen is removed by thermal desorption of O₂ to leave a clean Au(111) surface after heating to 600 K. TPD spectra initially show an O₂ desorption peak at 520 K at low Θ_O, and the peak shifts to higher temperatures for increasing oxygen coverages up to Θ_O=0.22 ML. Above this coverage, the peak shifts very slightly to higher temperatures, resulting in a peak at 550 K at Θ_O=1.2 ML. Analysis of the TPD data indicates that the desorption of O₂ from Au(111) can be described by first-order kinetics with an activation energy for O₂ desorption of 30 kcal mol⁻¹ near saturation coverage. We estimate a value for the Au–O bond dissociation energy D(Au–O) to be 56 kcal mol⁻¹.