Oxygen Adlayer Research Articles

On the electrosorption of oxygen species and adlayer growth on noble metal surfaces

The surface chemistry, i.e. adsorption and surface phase formation, on Rh, Ir, Pt and Au single-crystal electrodes in 0.5 M H 2SO 4 has been studied in the potential range between 0 and 4 V vs. a standard hydrogen electrode by X-ray photoelectron spectroscopy and cyclic voltammetry. Both methods gave complementary results which were interpreted as potential-dependent changes from physisorption of water to adsorption of hydroxyl groups, hydroxide film formation, and eventual growth of thick adlayers of oxyhydroxide (Rh, Ir and Au) or hydroxide (Pt) with increasing applied voltage.

Characterization of oxygen, carbon, and sulfur adlayers on W(211)

Adlayers of oxygen, carbon, and sulfur on W(211) have been characterized by LEED, AES, TPD, and CO adsorption. Oxygen initially adsorbs on the W(211) surface forming p(2 × 1)O and p(1 × 1)O structures. Atomic oxygen is the only desorption product from these surfaces. This initial adsorption selectively inhibits CO dissociation in the CO( β 1) state. Increased oxidation leads to a p(1 × 1)O structure which totally inhibits CO dissociation. Volatile metal oxides desorb from the p(1 × 1)O surface at 1850 K. Oxidation of W(211) at 1200 K leads to reconstruction of the surface and formation of p(1 × n)O LEED patterns, 3 ⩽ n ⩽ 7. The reconstructed surface also inhibits CO dissociation and volatile metal oxides are observed to desorb at 1700 K, as well as at 1850 K. Carburization of the W(211) surface below 1000 K produced no ordered structures. Above 1000 K carburization produces a c(6 × 4)C which is suggested to result from a hexagonal tungsten carbide overlayer. CO dissociation is inhibited on the W(211)−c(6×4)C surface. Sulfur initially orders into a c(2 × 2)S structure on W(211). Increased coverage leads to a c(2×6)S structure and then a complex structure. Adsorbed sulfur reduces CO dissociation on W(211), but even at the highest sulfur coverages CO dissociation was observed. Sulfur was found to desorb as atomic S at 1850 K for sulfur coverages less than 7 6 monolayers. At higher sulfur coverages the dimer, S 2, was observed to desorb at 1700 K in addition to atomic sulfur desorption.

Characterization of oxygen, carbon, and sulfur adlayers on W(211)

Characterization of oxygen, carbon, and sulfur adlayers on W(211)

CO interaction with Fe(100): Effects of carbon and oxygen adlayers on co adsorption isotherms

The interaction of CO with clean Fe(100) at 300 K is shown to occur in two distinct stages. namely: (i) Dissociation of CO, leading to a carbon and oxygen coverage of approximately 0.25 monolayer respectively as indicated by AES and LEED, and (ii) reversible molecular CO adsorption which has been studied with ellipsometry. The isosteric heat of adsorption is dependent on the CO coverage and equals 100 ± 5 kJ mol at zero coverage. CO adsorption isotherms were recorded on a carbon covered Fe(100) surface and yielded a coverage independent isosteric heat of adsorption of 77 ± 4 kJ mol . No CO dissociation was observed on this surface. From CO adsorption isotherms studies on different adlayers, containing carbon and/or oxygen on Fe(100) at 300 K, it can be concluded that less CO is adsorbed when the surface contains more oxygen. A quantitative description of the CO isotherms can only be given by assuming the adsorbing surface to be heterogeneous.

Cesium ion desorption from oxygen and carbon adlayers on platinum surfaces with nanosecond time resolution: variation of desorption parameters with time available for surface diffusion and degree of surface heterogeneity

Cesium ion desorption from oxygen and carbon adlayers on platinum surfaces with nanosecond time resolution: variation of desorption parameters with time available for surface diffusion and degree of surface heterogeneity

Cesium ion desorption from oxygen and carbon adlayers on platinum surfaces with nanosecond time resolution: Variation of desorption parameters with time available for surface diffusion and degree of surface heterogeneity

Measurements of ionic cesium desorption from surfaces at temperatures above 1100 K have been made with the so-called field reversal method. Time constants down to 30 ns have been measured with very low surface densities of Cs. The prexponential factors for desorption from clean platinum and graphite covered platinum surfaces are close to 10 12 s −1, in agreement with simple desorption theory. However, for slightly oxygen covered surfaces and for carbon layers formed at low temperatures the preexponential and the desorption energy become significantly larger, while a strict Arrhenius form is retained. This is proposed to be due to the influence of surface inhomogeneity and surface diffusion, in line with the theoretical description of simultaneous diffusion and desorption recently given by Nordholm. In the oxygen case, the adsorbed oxygen is considered to be the origin of the heterogeneity, while in the carbon layer case the surface, which has been formed at low temperature, is disordered and may contain many strongly binding sites. The influence of bulk diffusion on the desorption is also described.

CO oxidation on Pd(100): A study of the coadsorption of oxygen and carbon monoxide

Oxygen adsorption, and coadsorption and reaction with CO were studied with temperature programmed reaction spectroscopy (TPRS), low energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (EELS). Oxygen adsorption at 300 K was studied for coverages up to 0.5, while coadsorption with CO was studied in the temperature range of 80 to 450 K, and O and CO coverages of 0 to 0.25 and 0 to 0.8, respectively. Oxygen adsorbed into p(2 × 2) islands for coverages in excess of 0.05 and yielded a fully developed p(2 × 2) structure for 1 4 monolayer coverage at 300 K. The p(2 × 2)O pattern was gradually replaced by a c(2 × 2)O structure as the oxygen coverage was increased to 0.5. Oxygen desorption occurred in three temperature programmed desorption states at 840, 730, and 695 K. The highest temperature state was populated at all coverages, whereas the 730 and 695 K states appeared as the oxygen coverage exceeded 0.25 and 0.36, respectively. The 695 K state was unusually narrow suggesting attractive interactions in the oxygen adlayer for coverages of 0.36 to 0.5. For the surface precovered with 0.25 monolayer of oxygen, CO adsorption at 80 K caused a disordering of the p(2 × 2)O structure. At lower oxygen coverages CO initially adsorbed at sites apart from the oxygen domains, but also adsorbed within oxygen islands upon filling of the exterior sites. CO adsorption in the interior of the islands produced a directly interacting COPdO complex characterized by a CO stretching frequency of 2125 cm −1. Reaction-limited CO 2 evolution occurred in three states; viz. reaction of CO with (1) isolated oxygen atoms or p(2 × 2)O islands at 420 K, (2) disordered oxygen islands at 360 K, and (3) by a low temperature reaction between 100 and 310 K due to the COPdO complexes. No evidence of molecularly adsorbed CO 2 or carbonate (CO 3) reaction intermediates was observed by EELS at 80 K.

Surface EXAFS of the (2x1) oxygen adlayer on Ag(110)

Surface EXAFS of the (2x1) oxygen adlayer on Ag(110)

Surface EXAFS of the (2 × 1) oxygen adlayer on Ag(110)

Azimuthal and polar angular dependent surface extended X-ray absorption fine structure (SEXAFS) studies have been performed on Ag(110)-O(2 × 1) using the synchrotron radiation of the new electron storage ring BESSY. Independent measurements with the electric field vector E parallel to the [100] and [110] directions yield first and second nearest-neighbour distances of 2.06 and 2.17 Å, respectively. They show unambiguously that the adsorbed atomic oxygen is located on the two-fold bridge site in the [100] direction 0.2 Å above the surface Ag atoms. This result is supported by a comparison of SEXAFS amplitudes.

Lateral interaction effects on the reaction of CO2 and oxygen adsorbed on Ag(110)

The effect of oxygen adatom coverage and order upon the formation of CO3 intermediates via CO2+O(a)→CO3(a) was examined on the Ag(110) surface. The reaction probability for CO2 was found to be strongly dependent upon the oxygen adlayer. Adsorption of CO2 upon an ordered oxygen adlayer at 300 K occurred with a net reaction probability of the order of 10−2 or less, and resulted in the formation of (1×2) domains of CO3 which caused compression of the adsorbed (n×1) oxygen atom chains as determined by LEED experiments. Adsorption of CO2 upon a disordered oxygen adlayer at 170 K occurred with an initial sticking probability of unity; however, the saturation coverage of CO3 intermediates formed was less than the initial oxygen atom coverage. Adsorption of CO2 upon an ordered oxygen adlayer at 170 K occurred with lower reaction probability than at 300 K, indicating that the order of the oxygen adlayer strongly affected the CO2 reaction kinetics. In all cases the reaction of CO2 was hindered by increasing the initial oxygen atom coverage.

The effects of carbon, oxygen, sulfur and potassium adlayers on CO and H 2 adsorption on Fe(100)

The adsorption and desorption of CO and hydrogen was studied on clean Fe(100) and Fe(100) with adiayers of carbon, oxygen, sulfur, and potassium using X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption. Hydrogen was found to be dissociatively adsorbed on Fe(100) with a binding energy of 86 kJ/mole. CO absorption was found to be complex, involving several molecular binding states as well as dissociated CO. The activation energy for dissociation of CO on Fe(100) was estimated to be 105 kJ/mole. Sulfur, oxygen, and carbon adlayers all reduced the binding energies for CO and hydrogen and inhibited CO dissociation. Potassium was observed to enhance the CO and hydrogen binding strengths and increase the amount of CO dissociation relative to the clean surface. These results have been compared to LCAO calculations for CO adsorption on an Fe(100) surface with adiayers. The model calculations showed that adiayers of carbon, oxygen, and sulfur reduced the CO binding energy on Fe(100) by bonding with the same Fe(3d) orbitals as the CO(2π ∗) orbitals would. Potassium was found to enhance the CO binding energy on Fe(100) due to interactions between the K(4s) orbital and the CO(2π ∗) orbitals.

The effects of carbon, oxygen, sulfur and potassium adlayers on CO and H 2 adsorption on Fe(100)

The effects of carbon, oxygen, sulfur and potassium adlayers on CO and H 2 adsorption on Fe(100)

Interaction of cesium and oxygen on W(110): I. Cesium adsorption on oxygenated and oxidized W(110)

Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10 15 cm −2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (∼1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10 15 oxygen atoms per cm 2. Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10 15 cm −2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.

Interaction of cesium and oxygen on W(110) I. Cesium adsorption on oxygenated and oxidized W(110)

Abstract Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10 15 cm −2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (∼1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10 15 oxygen atoms per cm 2 . Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10 15 cm −2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.

The adsorption, desorption, and exchange reactions of oxygen, hydrogen, and water on platinum surfaces: IV. Field emission studies on the adsorption of water, hydrogen and the reaction between hydrogen and adsorbed oxygen

Field electron emission microscopy has been used to study the adsorption of water on platinum and its production by the reaction of hydrogen with adsorbed oxygen. Techniques are described whereby clean reproducible Pt field emitters can be produced without irreversible contamination or excessive tip blunting. Direct water adsorption always led to emitter failure, consequently adsorbed water was produced by the reaction of hydrogen with adsorbed oxygen. This reaction occurred from 100 to 175 K with a decrease in work function from Δφ = +0.6 eV, for oxygen adsorption, to a minimum work function, Δφ = −0.55 eV, at 175 K. From 175 to 300 K the work function increases and reattains a positive shift, Δφ ∼ + 0.2 eV, and a field emission pattern characteristic of adsorbed oxygen which are uninfluenced by further H 2 interaction up to 600 K. There is little change in emission anisotropy, but enhanced emission from Pt(110) planes suggests that the reaction is particularly facile on this plane. The work function shifts indicate the formation of adsorbed water up to 175 K, which therefore forms an electropositive adsorbed state, followed by desorption of water in the interval 175 to 300 K from an adlayer stabilised by the presence of unreacted oxygen. These observations are in agreement with thermal desorption results, although the observation that part of the oxygen adlayer could not be reduced was surprising since thermal desorption studies suggested that this efficient reaction proceeded to completion. Rationalisation requires that the unreactive oxygen is undetected in the thermal desorption experiments and supporting evidence is given from oxygen adsorption, desorption and exchange, field emission and Auger electron spectroscopy experiments.

The adsorption, desorption, and exchange reactions of oxygen, hydrogen, and water on platinum surfaces IV. field emission studies on the adsorption of water, hydrogen and the reaction between hydrogen and adsorbed oxygen

Abstract Field electron emission microscopy has been used to study the adsorption of water on platinum and its production by the reaction of hydrogen with adsorbed oxygen. Techniques are described whereby clean reproducible Pt field emitters can be produced without irreversible contamination or excessive tip blunting. Direct water adsorption always led to emitter failure, consequently adsorbed water was produced by the reaction of hydrogen with adsorbed oxygen. This reaction occurred from 100 to 175 K with a decrease in work function from Δφ = +0.6 eV, for oxygen adsorption, to a minimum work function, Δφ = −0.55 eV, at 175 K. From 175 to 300 K the work function increases and reattains a positive shift, Δφ ∼ + 0.2 eV, and a field emission pattern characteristic of adsorbed oxygen which are uninfluenced by further H2 interaction up to 600 K. There is little change in emission anisotropy, but enhanced emission from Pt(110) planes suggests that the reaction is particularly facile on this plane. The work function shifts indicate the formation of adsorbed water up to 175 K, which therefore forms an electropositive adsorbed state, followed by desorption of water in the interval 175 to 300 K from an adlayer stabilised by the presence of unreacted oxygen. These observations are in agreement with thermal desorption results, although the observation that part of the oxygen adlayer could not be reduced was surprising since thermal desorption studies suggested that this efficient reaction proceeded to completion. Rationalisation requires that the unreactive oxygen is undetected in the thermal desorption experiments and supporting evidence is given from oxygen adsorption, desorption and exchange, field emission and Auger electron spectroscopy experiments.

A molecular beam investigation of the catalytic oxidation of CO on Pd (111)

A detailed investigation of the steady-state and nonsteady-state reaction CO+1/2O2→CO2 on Pd (111) has been carried out with the molecular beam technique. It could be shown conclusively that the reaction proceeds between two adsorbed species (Langmuir–Hinshelwood mechanism) throughout the temperature and pressure range investigated. For low CO coverages, the activation energy of the reaction was determined to be 25 kcal/mole, whereas at moderate CO coverages, a rearrangement of the oxygen adlayer takes place resulting in a reduction of the activation energy to 14 kcal/mole. It is not possible to formulate a simple kinetic expression for the reaction rate which is valid over the entire range of temperatures and pressures due to changes in the adsorption rate for O2, coadsorption of CO and O2, to diffusion in the adlayer, and to changes in the geometrical arrangement within the adlayer with varying coverage.

Electron Spectroscopic Study of Nitric Oxide Adsorbed on Copper

The interaction of NO with copper has been investigated by electron spectroscopy. Nitric oxide is shown to be molecularly adsorbed in a linearly bonded configuration at 85 K; this adlayer is unstable above 120 K, resulting in an exclusively oxygen adlayer at 295 K, the nitrogen being desorbed. At 85 K N(1s) peaks observed at 401 and 406 eV are associated with two different surface species. We suggest that the former dissociates while the latter desorbs at a binding energy of 406 eV on thermal activation of the adlayer.Interaction of nitric oxide with Cu at 295 K results in dissociative chemisorption with both oxygen and nitrogen retained at the surface. The surface concentration suggests that each nitrogen and oxygen adatom is bridge-bonded to two surface copper atoms. Estimates of absolute surface coverages are made by comparing the intensities of the N(1s) and O(1s) peaks with the Cu(2p) substrate intensity, while valence level spectroscopy (He I and II radiation) supplements core-level spectroscopy for discriminating between molecular and dissociated surface species.

Electron spectroscopic study of nitrogen species adsorbed on copper

The interaction of a number of nitrogen containing molecules (NO, NH3 and N2H4) with copper has been investigated by electron spectroscopy. Nitric oxide is shown to be molecularly adsorbed in a linearly bonded configuration at 85 K; this adlayer is unstable above 120 K, resulting in an exclusively oxygen adlayer at 295 K, the nitrogen being desorbed. At 85 K N(1s) peaks observed at 401 and 406 eV are associated with NOδ–(ads) and NOδ+(ads) species, the former dissociating and the latter desorbing on thermal activation of the adlayer.Interaction of nitric oxide with Cu at 295 K results in dissociative chemisorption with both oxygen and nitrogen retained at the surface. The surface concentration suggests that each nitrogen and oxygen adatom is bridge-bonded to two surface copper atoms. Estimates of absolute surface coverages are made by comparing the intensities of the N(1s) and O(1s) peaks with the Cu(2p) substrate intensity, while valence level spectroscopy (He I and II radiation) supplements core-level spectroscopy for discriminating between molecular and dissociated surface species.Ammonia dissociates at 295 K on copper while with oxidized copper the surface oxide is replaced by an imide-type surface. With hydrazine, adsorption is molecular over the temperature range 85 to 295 K, in contrast to our observations with iron.

Angular dependence of uv photoelectron distributions for oxygen adsorbed on W(100)

Recent theoretical work has suggested the possibility of determining chemisorption bond geometry by the use of uv photoemission. This possibility is being investigated experimentally in an apparatus developed at NBS. The apparatus is described, and preliminary results are presented which show energy distributions as a function of emission angle for electrons photoemitted from W(100), both clean and with adsorbed oxygen. The results obtained at hν = 21.2 eV for several emission angles indicate substantial angular dependence of the photoelectron distributions, not only for the clean (100) surface, but also, more dramatically, with an oxygen adlayer.