(93g) Exploring Cluster-Size Effect in Catalysis: DFT Insights on Hcooh Decomposition on H-BN Supported Pd6 Sub-Nanometer Clusters

Sub-nanometer metallic clusters were recently demonstrated to be promising, highly selective catalysts for propylene epoxidation [1]. The presence of extremely low-coordinated metal sites as well as effects arising from the interaction with supports are among the reasons of their unexpected catalytic properties.

In this study, we focus on the case of a boron nitride (h-BN) supported Pd₆ cluster as potential catalyst for formic acid (HCOOH) decomposition to hydrogen. Periodic density functional theory (DFT) calculations were used to shed light on the factors controlling the catalyst activity and selectivity at the sub-nanometer level. The competition between formate (HCOO) and carboxyl (COOH) decomposition pathway 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 contrast to what is expected from Pd nanoparticles, exposing the (111) facet [2-3]. We demonstrate that the presence of defects drastically changes the energetic landscape of HCOOH decomposition. In particular, in presence of a B monovacancy, cluster reconstruction phenomena and electronic charge transfer occurring at the interface between the defective h-BN support and Pd₆ were correlated with an increase in selectivity towards CO production.

This study well illustrates how the 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.

[1] Lei Y., Mehmood F., Lee S., Greeley J., Lee B., Seifert S., Winans R. E., Elam J. W., Meyer R. J., Refdern P. C., Teschner D., Schlögl R., Pellin J. M., Curtiss L. A., Vajda S., Science 328, 5975, 224–228 (2010).

[2] Schimmenti R., Cortese R., Duca D., Mavrikakis M., ChemCatChem 9, 9, 1610–1620 (2017)

[3] Herron J. A., Scaranto J., Ferrin P., Li S., Mavrikakis M., ACS Catal., 4, 12, 4434–4445 (2014).