Tailoring high-entropy borides for hydrogenation: crystal morphology and catalytic pathways

Abstract

The high-entropy boride (HEB) Al_0.2Nb_0.2Pt_0.2Ta_0.2Ti_0.2B₂, with its unique crystal structure and high coordination (platinum coordinated to 12 boron atoms), 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-melting 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 exhibits 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 hydrogenation through a suspended solid solution in addition to a dissolved solution. We propose that the HEB Al_0.2Nb_0.2Pt_0.2Ta_0.2Ti_0.2B₂ follows a hybrid HAT/HSPE mechanism, where H₂ binds to the platinum atoms on the edges of the HEB and dissociates, and then the radical hydrogen departs to the substrate.