| dc.description.abstract | We have hitherto discussed the features of adsorbed oxygen on a variety of metal surfaces as characterised by the techniques of electron spectroscopy. Specifically, the characteristics of the different oxygen species O²?, O?? and O? in UPS, XPS and EELS are discussed. While the adsorbed molecular oxygen on most metal surfaces is of peroxo-type, the presence of superoxo-type oxygen on some metal surfaces is indicated. Molecularly adsorbed oxygen seems to sequentially go through a number of different adsorption states, increasing the metal-oxygen bond strength and weakening the O–O bond in each step, until it finally undergoes dissociation. A potential energy diagram as depicted in Fig. I.8 describes this situation. For purposes of convenience, the electron states of the chemisorbed molecular oxygen obtained from UPS studies are summarised in Table 1.1. The O(1s) binding energies of the molecular species on different metal surfaces are presented in Table 1.2. In Table 1.3 the vibrational frequencies of the dioxygen species on metal surfaces are summarised. In Fig. I.11 we show an (O–O) stretching frequency versus bond order plot prepared by making use of the vibrational frequencies of free O?, O?? and O?²?. The information given in Tables 1.1–1.3 and Fig. I.11 are directly helpful to characterise adsorbed oxygen species on metal surfaces using the electron spectroscopic techniques.
Concluding Remarks
Adsorbed molecular oxygen exists in a peroxo (O?²?) form with a filled antibonding ?* level on metal surfaces such as single crystal Cu(110), polycrystalline Cu and Ag. This species is characterised by a three-peak UPS spectrum due to ?g, ?u and ?g levels. In EELS the ?(O–O) band of the peroxo species is observed in the 600–800 cm?¹ region. The O(1s) band of this species is seen in the region 530.5–533 eV. On a polycrystalline Ag surface, besides the peroxo species, there is evidence for the presence of a superoxo (O??) species which is characterised by a ?(O–O) band around 1300 cm?¹. The peroxo species disappears from the surface at 230 K whereas the superoxo species is stable only up to 200 K. Molecularly adsorbed oxygen is selectively stabilised when the surface of polycrystalline Cu is modified with chlorine and dichloroethane.
The present study shows the presence of a peroxo-type oxygen species characterised by an O(1s) binding energy of 532.6 eV in addition to the O²? species with a binding energy of 528.5 eV in YBa?Cu?O? at 300 K. The concentration of the peroxo-type oxygen species is found to increase when the sample is in the superconducting state. The surface conductivity, monitored by employing electron energy loss spectroscopy, shows a temperature-dependent variation which parallels that of the concentration of the peroxo species. This suggests a close link between the formation of the peroxo species and changes in surface conductivity. Non-superconducting YBa?Cu?O? does not show these temperature-dependent variations in O(1s) or surface conductivity.
Formation of surface hydroxyl groups by interaction of molecules is shown to occur on a Cu(110) surface. The characteristic bands of the surface hydroxyl group formed from the various molecules are listed in Table IV.7. Such proton abstraction reactions leading to the formation of surface hydroxyl groups are found to be universal. Temperature of formation of the surface hydroxyl group is found to depend on the nature of the molecule.
The present study has provided useful information on the nature of intermediate species formed by the transformation of organic molecules molecularly adsorbed on Cu surfaces. On a clean Cu(110) surface, the adsorption of carbonyl compounds such as HCHO, CH?CHO and (CH?)?CO is found to be molecular at low temperatures. However, these molecules undergo dissociation on a Cu(110) surface modified by depositing aluminium. This has been explained as due to enhanced electron transfer from the metal to the antibonding orbitals of the adsorbate. On an atomic oxygen-covered surface we are able to monitor the oxidation of acetaldehyde to yield adsorbed acetate and acetic acid species. CH?OH and CH?–O–CH? adsorbed on Cu surface transform to the C?H?O species. | |
