Structural aspects of chemisorption at Cu(110) revealed at the atomic level
We illustrate the impact that scanning tunnelling microscopy (STM) has made on our understanding of chemisorption and catalysis at metal surfaces at the atomic level by considering four examples where information from surface sensitive techniques was also available. The advantages of STM and the limitations of some of the other experimental methods are discussed. (1) Through a combination of STM and X-ray photoelectron spectroscopy (XPS) we have established that a number of distinct oxygen chemisorbed states can exist at a Cu(110) surface. These are metastable and temperature dependent. Furthermore, the presence of chemisorbed sulphur is shown to promote a specific oxygen state – isolated oxygen strings – which are likely to be more chemically reactive than the oxygen overlayer present at Cu(110). In this sense the sulphur is a structural promoter. (2) The oxidation of ammonia under ammonia-rich conditions results in the growth of imide (NH) strings at a Cu(110) surface and this has been followed quantitatively by STM. The reactive surface oxygen state participates in an oxydehydrogenation reaction generating NH-radical species which undergo surface diffusion and result in string growth. (3) Nitric oxide dissociates at Cu(110) to generate a two-phase system of chemisorbed nitrogen and oxygen adatoms. The oxygen is present in a well ordered (2 × 1) structure and the nitrogen in a (3 × 2) structure. The limitations of an earlier LEED study are discussed. (4) Structural aspects of chemisorbed sulphur generated by the dissociative chemisorption of hydrogen sulphide and methyl mercaptan are discussed. In the latter case carbon–sulphur bond cleavage results in the formation of a sulphide overlayer at 450 K with complete removal of carbon as desorbed hydrocarbons. Various sulphur structures have been delineated over a wide temperature range.
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