Nanoalloying MgO-Deposited Pt Clusters with Si To Control the Selectivity of Alkane Dehydrogenation

Nanoalloying MgO-deposited Pt clusters with eight main-group elements was computationally screened for potential selectivity toward partial dehydrogenation of small alkanes. Si was revealed as a promising dopant for Pt across two cluster sizes, as it appears to favor stopping dehydrogenation of alkanes at alkenes. Pure Pt clusters on MgO(100) tend to strongly adsorb ethane and activate one of the C-H bonds but then bind ethylene in an activated di-sigma fashion and even spontaneously dehydrogenate it. They also strongly bind C atoms, as the first step of deactivation by coking. PtSi clusters also bind and activate ethane but, in contrast to pure Pt clusters, bind ethylene preferentially in a weaker pi fashion and do not spontaneously dehydrogenate it. Additionally, the affinity of C atoms to PtSi is reduced by ca. 2 eV in comparison to pure Pt clusters of the same size. Finally, Si stabilizes the clusters against dissociating single Pt atoms and thus is predicted to slow down catalyst sintering by Ostwald ripening. Thus, Si is predicted to improve both the selectivity and stability of Pt cluster catalysts. The cluster behavior appears to be linked to its spin multiplicity: isomers with unpaired electrons exhibit high activity and, while they are abundant for pure Pt clusters, Si tends to quench spins. Si also causes clusters to become more globular, with fewer valence-unsaturated Pt sites. The discovered properties of Si potentially exceed those of B, the recently proposed doping agent for Pt clusters whose effect on selectivity was confirmed experimentally.