The Role of Impurities on the Low Temperature CO Oxidation on Au/TiO 2
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Catalysis Letters Vol. 109, Nos. 1–2, June 2006 (Ó 2006) DOI: 10.1007/s10562-006-0064-4
The role of impurities on the low temperature CO oxidation on Au/TiO2 Jorge M.C. Soares,a,b Matt Hall,a,b Marco Cristofolini,a,b and Michael Bowkera,b,* a School of Chemistry, Cardiff University, Cardiff CF10 3TB, UK School of Chemistry, University of Reading, Reading RG6 6AD, UK
b
Received 3 December 2005; accepted 13 February 2006
A number of Au/TiO2 catalysts have been prepared by a variety of methods in order to assess the affect of catalysts preparation methods on performance, catalyst contamination and the interplay between them. Their activity was studied in a pulse flow microreactor and it was found that preparation methods designed to eliminate impurities lead to more active samples. The effect of chlorine is often cited to be detrimental in the literature, but we have quantified it using XPS. It was found that the activity decreased in a nearly linear fashion with an increasing amount of this element at the surface. It is generally considered that catalysts prepared by the incipient wetness (IW) technique are ineffective for CO oxidation, but we show here that, by appropriate preparation methods, high activity IW catalysts can be made. KEY WORDS: Gold catalysis; CO oxidation; Cl poisoning; Au catalyst preparation; incipient wetness impregnation; high activity Au catalysts; low temperature catalysis.
1. Introduction For the preparation of gold catalysts, the literature [1] refers to two different well established methods: the impregnation-incipient wetness (IW) and the depositionprecipitation (DP) method. Both of these methods lead to gold catalysts with completely different catalytic properties, the former being much less active than the later. It is known that for the generation of catalytic activity on gold catalysts, a careful control of the experimental variables is required. In particular, a washing step to eliminate impurities from catalyst preparation, is crucial for the generation of activity. In fact, the original DP method as described by Haruta et al. [2,3], uses this experimental step for gold catalysts preparation. One of the most frequent impurities is chlorine because HAuCl4 is the most frequent gold precursor used. Although it is stated in the literature that chlorine species might have a detrimental effect [4–7], relatively little research has been carried out to assess this. An XRF study [8] by Vanice et al. has shown that Cl is a poison. DFT studies by Hammer et al. [9] described the possible negative role of Cl on the activity of Au/MgO samples. They claimed that Cl, as an electron acceptor, makes both the adsorption of O2 and the formation of the CO–O2 intermediate, more difficult. A XPS study by Wan et al. [10], has shown that the higher the Cl/Au ratio, the less active a Au/Y-zeolite is and it also confirmed that IW samples retain much more chlorine than DP samples, consistent with lower activity. The sodium *To whom correspondence should be addressed. E-mail: bowkerm@cardiff.ac.uk
effect is usually co
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