A reactive oxygen state at a barium promoted Au (100) surface: the oxidation of ethene at cryogenic temperatures
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Catalysis Letters Vol. 101, Nos. 3–4, June 2005 ( 2005) DOI: 10.1007/s10562-005-4879-1
A reactive oxygen state at a barium promoted Au (100) surface: the oxidation of ethene at cryogenic temperatures A.F Carleya, P.R Daviesa, M.W Robertsa, and A.M Shaha,b a
School of Chemistry, Cardiff University, Cardiff, CF10 3TB b University of the Punjab, Lahore, Pakistan
Received 1 February 2005; accepted 9 February 2005
Although Au (100) does not adsorb oxygen at either 295 K or 80 K, a barium modified Au(100) surface is active in oxygen dissociation resulting, through surface diffusion of oxygen adatoms, in the formation of a chemisorbed oxygen adlayer. This oxygen species is inactive for ethene oxidation, as is the oxygen species pre-adsorbed at an Au(100)–Ba surface at 80 K, and the clean Au(100)–Ba surface. However, when molecularly adsorbed ethene present at a Au(100)–Ba surface at 80 K is exposed to dioxygen and warmed to 140 K, surface carbonate is observed. We conclude that a transient oxygen species is the oxidant. KEY WORDS: gold; barium; oxygen transients; ethene.
Much of our understanding of oxidation catalysis at metal surfaces is based on the spectroscopic characterisation of oxygen states under static experimental conditions, although in the case of oxidation catalysis on gold, more attention has been given to the structural characteristics of the surface [1]. However, it is now well established that oxygen adsorbed at low temperatures (80 K), or transiently present under dynamic coadsorption conditions, can activate (oxidise) molecules under conditions where the stable chemisorbed oxygen overlayer at metal surfaces is unreactive, and this has led to new insights into oxidation catalysis [2–4]. In this paper, we establish that a transient oxygen state present at a barium modified Au (100) surface is effective in the facile oxidation of ethene at low temperatures. The X-ray photoelectron spectra were obtained using a VG ESCA3 spectrometer and the surface concentration data determined from an analysis of the O(1s), Au(4f) and Ba(3d) spectra using the method of Carley and Roberts [5]. The barium was deposited on the Au(100) crystal by means of an electrically heated getter (SAES). Although the clean Au (100) surface is unreactive to dioxygen [6], extensive adsorption occurs when the surface doped with barium (rBa ¼ 0:61015 cm)2) is exposed to dioxygen (figure 1), characterised by an O(1s) binding energy of 531.0 eV; the surface oxygen adatom concentration is calculated to be 3 · 101 cm)2. This is analogous to what was observed in previous studies of the Zn(0001)–Ba surface (rBa ¼ 0:06 1015 cm)2), where the rate of dioxygen dissociation was increased by a factor of nearly 103 compared with that at the atomically clean Zn(0001) surface [7] and resulted in the formation of the complete ‘‘final state’’ chemisorbed layer. In both
cases, Au(100) and Zn(0001), the oxygen uptake is significantly greater than that expected for the formation of an oxygen state associated directly with the dopant Ba atoms: following oxygen
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