Continued progress in catalyst synthesis and characterization is allowing a more quantitative evaluation of the effect of cluster size on catalytic properties, as we are approaching the single atom limit [1]. At the atomic size regime, catalysts have properties often closer to molecules than to large nanoparticles. The presence of extremely low-coordinated metal sites as well as effects arising from the interaction with supports are among the reasons of their unusual catalytic properties. Density functional theory (DFT) is a powerful tool that can be used to shed light on the factors controlling the catalyst activity and selectivity at the sub-nanometer scale. We focus on the case of a boron nitride (h-BN) supported Pd6 cluster as a potential catalyst for formic acid (HCOOH) selective decomposition to hydrogen. The competition between formate (HCOO) and carboxyl (COOH) decomposition pathways was studied on different sites and the effect of support defects – B and N monovacancies – was explicitly addressed. DFT calculations suggest that the reaction mainly follow the HCOO-mediated pathway, while the formation of COOH, a well-known CO precursor, is not energetically favored, in striking contrast to what is expected from Pd nanoparticles, exposing the (111) facet [2-3]. However, we demonstrate that the presence of support-defects drastically changes the energetic landscape for HCOOH decomposition. For example, when a B monovacancy is present on the support, non-negligible CO production is expected, due to cluster reconstruction and charge transfer effects. The example of Pd6/h-BN nicely illustrates how reaction energetics can be controlled by the interplay of various factors at the sub-nanometer scale, such as electronic charge transfer and the intrinsic cluster fluxionality, which can be enhanced by the presence of defects on the support.

Cluster-size effect in catalysis: From sub-nano to extended surfaces

2019 Fall National Meeting

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