Nanocrystalline gold catalysts: A reflection on catalyst discovery and the nature of active sites

Abstract

complex as we deal with highly complex mixtures of structures that co-exist in the active catalyst. There may in many cases not just be a single structure for an active site, rather there may be a range of structures which display a broad spectrum of activities and specificities. For example in heterogeneously catalysed oxidation catalysis different sites are considered responsible for the formation of the selective and non-selective products. A further complication is that the active sites might be present in exceeding low concentrations on the surface of the catalyst. As most of the atoms present in the solid sample are located in the bulk this presents a further challenge in understanding the nature of the active site in heterogeneously catalysed reactions. This has led over the preceding decades to the design of model systems using single crystals and well defined surface as these can be interrogated using modern surface spectroscopy. The understanding of gold catalysed heterogeneously catalysed reactions has been hampered by all these constraints. One of the most well studied reactions concerns the oxidation of CO using nanocrystalline gold, discovered by Haruta (4). The identification of the mechanism of reaction and the nature of the active site has instigated numerous studies (5,6). Bond and Thompson have so far given one of the best descriptions of the possible reaction mechanism based on their detailed analysis of the available literature (2). However, the mechanism has not been further refined as the nature of the active site remains a matter of debate. Indeed, there is the added complexity that different gold may have different active sites on different oxide supports, as defect formation is more facile with some oxides. A number of proposals emphasize the size and morphology of the gold particle and its interface with the support (1, 8), whereas others postulate that the metal oxidation state (9, 10) and support material can have strong effects (1, 11). Most of the fundamental insights into this reaction have come from well defined model studies (12–15) in which specific structures are tailor-made (14).The difficulty is always Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff, CF10 3AT, UK Dedicated to the memory of David T Thompson