This novel catalyst, which consists of alumina-supported, size-selected, sub-nanometer clusters of platinum (Pt8-10), was synthesized and found to have an activity of 40-100 times that of other state-of-the-art catalysts for this reaction, including larger Pt monoliths. At the same time, the catalyst successfully suppressed undesired side reactions and maintained high selectivity (~65%) to the desired product, propylene. Concurrent first-principles computations probed the molecular-level basis of the remarkable catalytic properties of these Pt clusters.
With density functional theory (DFT) calculations, the thermodynamics and kinetics of the relevant elementary reaction pathways were determined on simple models of the size-selected clusters. For comparison, the same pathways were then investigated on single-crystal models of larger Pt nanoparticles. The cluster calculations revealed much lower activation barriers for the cleavage of propane C-H bonds than were calculated on the single-crystal models, implying significantly faster conversion of propane to propylene.
Further, these low barriers were found to correlate well with simple structural properties of the catalysts: the more highly under-coordinated the Pt atoms on the cluster or surface, the lower the calculated activation barrier. These correlations, in turn, suggest that coordination and bond-counting effects play a first-order role in determining the catalytic properties of sub-nanometer Pt clusters.
This work is significant for two primary reasons. First, it introduces a novel catalyst for an important petrochemical reaction that may, ultimately, have significant industrial applications. Second, it proposes an important fundamental principle for understanding the catalytic properties of subnanometer metal clusters. This principle may, in turn, be of use in the design of still better catalysts for this and related heterogeneous catalytic reactions.