Can support acidity predict sub-nanometer catalyst activity trends?

The effect of metal-oxide support on the activity of ethylene hydrogenation catalyzed by size-selected platinum clusters (8-20 atoms per cluster) is investigated. Size-selected clusters have been shown to possess unique catalytic properties which change as a function of the precise cluster size. The temperature-programmed reaction and isothermal pulsed molecular beam data both demonstrate a size-dependent reactivity on each support. However, predicted trends based on the acidic (SiO₂) or basic (MgO) properties of the support are more subtle, with the influence of the support also being a function of the cluster size. Only the average hydrogenation activity of all sizes measured, as well as the CO stretching frequency on clean clusters, were observed to follow predicted trends, but atomic-level resolution demonstrates a much more complicated picture. Pt₁₃ had the lowest hydrogenation activity of all sizes on amorphous SiO₂, while being the most active size on MgO, and trends between the same cluster sizes were observed to exhibit opposing behavior between the two supports. This demonstrates the potential of cluster materials for designing catalytic systems and also the difficulties encountered when formulating general concepts for reactivity in the nonscalable size regime.