Catalysis by very small Au clusters

We review recent theoretical and experimental work on the catalytic properties of Au clusters that contain a few atoms and are supported on an oxide surface. The clusters are mass-selected and landed slowly on the oxide surface in ultra-high vacuum. STM measurements show that the clusters do not fragment and do not damage the surface when they are deposited nor do they coarsen after deposition. Their catalytic activity changes non-monotonically with the number of atoms and is sensitive to the nature of the support and to additives (hydroxyls, water, Na, Cl) present on the surface. Binary clusters (e.g. AunSr) can be more active than unary ones. Very recent work has managed to study catalysis by such clusters under realistic pressure conditions; their performance is very different from (and sometimes better than) that of large clusters.