Gold is the noblest among metals and suffers no oxidation in air. While this chemical inertness may be welcome to the jewellery, dental and electronic industries, it has been the main reason for relatively little chemical research over the decades. In the field of heterogeneous catalysis, gold was regarded as the odd metal until the 1980s when supported nanoparticles of gold were discovered to be highly active in reactions such as the oxidation of carbon monoxide (M. Haruta et al. J. Catal. 115 (1989) 301). The number of papers dealing with catalysis by gold has grown exponentially ever since.
Considerable effort is currently being devoted to determine the mechanism of CO oxidation on metal-oxide supported Au catalysts. The activation of the oxygen molecule is generally regarded as a key step and various scenarios have been suggested, including unique electronic properties of small supported Au particles. Charged states (Auδ+ vs Auδ-) due to catalyst preparation/calcination or interaction with F-centers of the support and special site requirements are only met in the interface between the Au particle and the support.
a) Gold catalysts are capable of providing high performance at low temperatures in certain reactions (for CO oxidation, 50% conversion of CO was reported at temperatures as low as ~240 K). In other reactions such as HC-SCR (selective catalytic reduction of NOx by hydrocarbons) the catalyst light-off temperature is still too high. The reasons for these dramatically different performances are not yet clear.
Both the CO oxidation and lean deNOx reactions are studied in our laboratory using various gold catalysts supported on "reducible" oxides such as TiO2, Co3O4 or CeO2. Care is taken to prepare catalysts with well-defined composition and to avoid contamination by sources able to influence the catalytic performance. All supports are home-made and follow the oxalate route of precipitation (US patent 6,362,239 B1 Mar. 26,2002, P. Buess, R. F. I. Caers, A. Frennet, E. Ghenne, C. Hubert, N. Kruse). Catalysts are selectively promoted or poisoned by adding wanted amounts of foreign cations or anions (around 10-2 wt%).
Catalysts are characterised by BET, TPD/TPR and XPS-SIMS. Catalytic activities at steady state are measured by mass spectrometry and chemiluminescence (for NOx) Reaction mechanisms are studied by Chemical Transient Kinetics.
b) The chemical reactivity of metastable Au2O3 * xH2O is studied in reactions with CO, NO and hydrocarbons.
A large range of conditions from UHV to ambient pressures are applied in these studies. In all cases, reduction to metallic gold is observed; the temperature-dependent kinetics differ, however. No or very little partial catalytic oxidation is observed when using hydrocarbons: CO2 and water are the only products.
After reaction between CO and gold oxide, reduced gold is visible.
c) To demonstrate the occurrence or absence of specific support effects, reaction studies with pure Au metal are performed using field emission techniques (FIM, PFDMS). The samples in this case are given the form of tips so that the effect of steps and kinks can be elucidated. In this way we have demonstrated that bulk Au metal, in the absence of a support, cannot dissociate the oxygen molecule. In particular, low-coordinated Au sites do not break the O-O bonds.