X-ray absorption spectroscopy and CO oxidation activity of Au/Al 2 O 3 treated with NaCN
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Catalysis Letters Vol. 99, Nos. 1–2, January 2005 (Ó 2005)
X-ray absorption spectroscopy and CO oxidation activity of Au/Al2 O3 treated with NaCN Jason T. Calla and Robert J. Davis* Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22094-4741, USA
Received 10 August 2004; accepted 13 October 2004
Gold nanoparticles were supported on Al2 O3 by a deposition–precipitation method followed by heating in He. The resulting catalyst was then treated with NaCN solutions to remove most of the gold. X-ray absorption spectroscopy at the Au LIII edge was used to monitor the oxidation state and atomic structure of Au throughout the procedure. The gold or a Au/Al2 O3 sample after deposition–precipitation was in a +3 oxidation state and was subsequently reduced to metal after heating to 623 K in He. Treatment of a reduced sample with either 1 or 2 wt.% NaCN in water removed 79% or 86% of the Au, respectively. Although the remaining Au was cationic, it was also reduced to metal after heating to 623 K in He. The rate of CO oxidation per gold atom, the apparent activation energy, and the orders of reaction were unaffected by the NaCN treatments. These results show that alumina does not stabilize cationic Au species and that metallic Au appears to be important for the CO oxidation reaction. The presence of water or H2 increased the rate of CO oxidation over all of the Au/Al2 O3 catalysts. However, the promotional effect of co-fed water was greater over the NaCN treated samples. KEY WORDS: gold, alumina, carbon monoxide, oxidation, water, hydrogen, carbon dioxide, EXAFS, XANES, sodium cyanide, oxygen.
1. Introduction Although bulk gold is the noblest of metals [1], supported gold nanoparticles are very active in a variety of catalytic reactions. It was not until Haruta et al. [2] demonstrated the ability of gold particles to effectively catalyze the oxidation of CO and H2 that intense study has been focused on this unusual system. Many explanations for the source of gold’s reactivity have been proposed, however, the underlying principles of this phenomenon have been elusive [3]. Indeed, high catalytic activity has been attributed to anionic, metallic, and cationic Au species. One school of thought proposes that anionic gold is the key feature of an active CO oxidation catalyst. A combination of work on gas-phase, anionic clusters [4], MgO supported Au clusters [5,6], and quantum chemical calculations [4–6] suggests that oxygen vacancy F-center defects at the metal–support interface facilitate electron transfer to the Au metal particle and activate it for catalysis. Others indicate that metallic Au is necessary for high activity. Haruta and coworkers concluded that Au nanoparticles with strong support interactions are essential for high catalytic activity [7,8]. Several research groups have used density functional theory to explore the barriers to adsorption and reaction of CO and O2 on different gold surfaces [9,10]. Their results indicate that low-coordinated gold atoms provide energetically favo
