Electron spectroscopic studies of the adsorption and reactivity of oxygen on transition metal surfaces covered with alkali and alkaline earth metals

Abstract

Characteristic Cl(2p) and O(1s) peaks are seen by XPS attributable to distinct adsorbed chlorine and oxygen species on potassium and barium dosed Ag surfaces: a) There is selective stabilization of molecular dioxygen species on both potassium and barium dosed Ag surfaces at 300 K in the presence of adsorbed chlorine. When chlorine is adsorbed prior to oxygen, only molecular dioxygen stabilizes on excess O? dosage. b) On chlorination, the intensity of pre-adsorbed oxygen (O and O?) on the surface is found to decrease appreciably at 300 K. This could possibly be due to chlorine-induced diffusion of atomic oxygen into the sub-surface region. c) For 8 × 10¹? atoms/cm², preferential chlorination of Ag takes place as shown by a single Cl(2p) peak associated with Ag. For higher potassium coverages, a high binding energy Cl(2p) peak is seen due to adsorbed chlorine associated with potassium. d) In the presence of either potassium or barium, the saturation intensity of Cl(2p) associated with Ag is considerably enhanced. The saturated coverage of Cl(ad) is independent of potassium coverage in the system, while it does increase with increase in barium concentration. e) On the barium dosed surface, two characteristic Cl(ad) species: Cl(a) associated with Ag and Cl(b) associated with barium are observed even for relatively low barium coverages. f) The Cl(a) species associated with Ag is found to occupy a preferred site on both potassium and barium dosed surfaces, resulting in chemisorptive replacement of O into the sub-surface region. Several studies of the oxidation of CO have shown primarily a 'clean-off' reaction which is considered to result in the formation of CO? [7]. Engelhardt et al. [8] examined the interaction of CO with oxygen adsorbed on Ag(110); the incoming CO molecules were found to react rapidly with the adsorbed oxygen atoms to form CO? which desorbed immediately. It was also shown that small quantities of CO as a contaminant in the oxygen supply to a silver surface considerably diminished the surface concentration of oxygen adatoms. Albers et al. [9] studied the oxidation of CO in more detail using ellipsometry. The oxidation process was found to vary with the amount of precovered oxygen, but invariably led to a 'clean-off' reaction. In a more recent study, a similar result was seen on Ag(111) when the surface was precovered with active oxygen [4]. However, when CO was exposed at low temperatures, a new reaction pathway leading to the formation of CO? and CO? was observed. Kitson et al. [10] have shown in their thermal desorption studies that the oxide (O²?) and atomic oxygen species on the potassium-covered Ag surface can react with CO. On exposure of CO to potassium-, cesium-, or barium-covered surfaces saturated with oxygen at 300 K, a distinct C(1s) peak at 289 eV is observed. This peak together with an O(1s) peak at 531.5 eV can be identified as arising from the same surface entity. The concentration ratio of oxygen to the 289 eV carbon is ? 3:1 indicating a surface carbonate. This assignment is in agreement with the C(1s) line position observed for several metal carbonates such as nickel carbonate [11], cadmium carbonate and silver carbonate [12]. The He II spectra (Fig. IV.4) lend further credence to this assignment; the peaks observed at 4.6 and 9.3 eV after exposure to CO agree with those observed for metal carbonate at 4.6 eV (1e? and 4e?) and 9.0 eV (3e? and 1a?) [13]. The 4.6 eV peak is due to non-bonding O(2p) orbitals. The second intense band at 9.3 eV is produced by the ionization of the 3e? and 1a? molecular orbitals, both of which are C(2p)-O(2p) bonding in character [13]. The small shoulder due to the 4a? molecular orbital of carbonate at 10.6 eV [13] seems to be merged with the second main band [see Fig. IV.4 (curve D)]. As mentioned above, the formation of carbonate is expected to occur in the oxidation of CO on Ag precovered with active oxygen at low temperatures. While there is good agreement for the C(1s) peak position, the He II spectra here are, however, in better agreement with the calculated and experimentally verified values for carbonate molecular orbital levels of Connor et al. [13, 14]. The third peak in the He II spectrum at 12.8 eV [Fig. IV.4 (curve D)] can be assigned to the 4a band of molecularly adsorbed CO. Adsorption of molecular CO has been seen on several transition metals and (1? + 5?) bands have been identified in the 8-10 eV range and the 4a band between 11 and 13 eV [15]. Since a peak is seen at around 9 eV from the carbonate species, the (1? + 5?) band of CO(ads) cannot be isolated. In XPS, a C(1s) peak is observed at 284.7 eV on potassium- and barium-covered surfaces in addition to the carbonate C(1s) peak at around 289 eV. However, it is not possible to distinguish between molecular and dissociated CO on the basis of the 284.7 eV C(1s) peak position. The O(1s) peak from the carbonate species at 531.5 eV also overlaps with that of adsorbed molecular CO. The highly positive enthalpy of dissociative chemisorption of CO on Ag (334 kJ mol?¹) and the small negative value for molecular chemisorption (?27 kJ mol?¹) [16] strongly suggest chemisorption of CO in its molecular form. The peak at 531.5 eV, however, is sharp in all three cases, indicating that carbonate is the major oxygen-containing species. In the cesium-covered surface, there is no peak at 284.7 eV, suggesting almost total conversion of CO to CO?. Note also that an O(1s) peak at 535 eV and C(1s) peak at 292 eV attributable to CO?(ads) [4] were not seen in any of the above experiments, even at low temperatures. The O(1s) spectra after adsorption of CO on all three surfaces clearly indicate that both oxide (O²?) and atomic oxygen types react with CO in the formation of the CO? species. Although the oxygen-to-total carbon concentration ratio in the case of potassium- and barium-covered surfaces is 2:1, the ratio of oxygen to 289 eV carbon is 3:1. From the above observations, the following reaction sequence for potassium- and barium-covered Ag surfaces is suggested.