Tailoring High Entropy Borides for Hydrogenation: Crystal Morphology and Catalytic Pathways

Abstract

The high entropy boride (HEB) Al0.2Nb0.2Pt0.2Ta0.2Ti0.2B2, with its unique crystal structure and high coordination (platinum coordinated to 12 borons), has been shown in our previous work to exhibit exceptional catalytic properties, especially in sulfur-rich environments, where traditional platinum catalysts would succumb to sulfur-poisoning. In this work, we investigate the mechanism of the HEB catalyst, first by comparing the synthesis by flux growth, as previously reported, to an arc melted preparation. It is evident that the aluminum flux growth synthesis encourages the growth of single crystals, with clear and defined crystal facets, whereas the arc melted sample results in poorly defined facets with non-uniform morphology. Here, we explore two potential mechanisms: hydrogen spillover effect (HSPE) and hydrogen atom transfer (HAT) by which the catalytic pathway is performed. Hydrogenation reactions were performed using WO3 and 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO), which highlight the ability of the heterogeneous HEB catalyst to perform the hydrogenation through a suspended solid solution in addition to a dissolved solution. We propose that the HEB Al0.2Nb0.2Pt0.2Ta0.2Ti0.2B2 goes through a hybrid HAT/HSPE mechanism, where H2 bonds to the platinum atoms on the edges of the HEB, dissociate, then the radical hydrogen departs to the substrate.