Only a supporting role

J. Am. Chem. Soc. 135, 12634–12645 (2013)

Using smaller particles in heterogeneous catalytic systems is a logical step forward for those trying to achieve greater activity and decrease the amount of expensive metals needed. Amongst other effects, reducing the particle size leads to a higher number of low-coordination active sites per atom of catalyst. Recent improvements in synthesis and characterization techniques have pushed this effort to the extreme, leading to the development of the smallest heterogeneous 'particles' possible — supported single-atom catalysts. As well as better activity, single-atom catalysts could also improve selectivity because, unlike for many other heterogeneous catalysts, they do not suffer from a broad distribution of particle sizes and thus chemistries.

Previous single-atom systems, however, have relied on supports that play an active role in the chemistry, which occurs through the Langmuir−Hinshelwood mechanism whereby the reactants first bind to adjacent sites on the surface before reacting with each other. Now Chaitanya Narula and colleagues at Oak Ridge National Laboratory, Tennessee, have observed the catalytic oxidation of carbon monoxide occurring exclusively on single atoms of platinum supported on inert θ-alumina. The materials were prepared by reacting alumina powder with aqueous chloroplatinic acid, evaporating off the water and pyrolysing the resulting powder. Several samples with varying amounts of platinum were prepared and characterized. Using electron microscopy it was observed that when platinum loading was low (0.18 wt% Pt), it existed on the surface as single atoms only, however, when the loading was increased to 1 or 2 wt%, 10–20-atom agglomerates were also observed.

Narula and colleagues confirmed that the supported single atoms of platinum were active for the oxidation of CO and used density functional theory calculations to investigate the mechanism. Their calculations revealed that, in preference to CO, the bare platinum atoms adsorb O₂ (in a side-on geometry). CO subsequently binds to the oxygenated platinum atom and reacts with the bound O₂ to form a carbonate. This species then dissociates to give CO₂ and a bound O atom that can react with a further CO molecule.