High-Entropy-Stabilized Platinum Diborides for Poison-Resistant Catalysis

Alloying and solid-solution formation is a powerful technique that enhances and adds properties through elemental mixing, but unfortunately, some elements simply cannot mix as their chemical nature prevents a thermodynamically stable structure. For example, the inherent nobility of platinum group metals does not favor bond formation and precludes their incorporation into higher (boron-rich) metal borides. However, we demonstrate that when using five or more constituents, the higher mixing entropy will overcome these chemical limitations and form a stable high-entropy alloy, demonstrating the formation of new compounds with substituents that are seemingly impossible with a traditional metal alloying approach. The high-entropy boride (HEB) Al_0.2Nb_0.2Pt_0.2Ta_0.2Ti_0.2B₂ was synthesized, where platinum was forced to occupy a 12-coordinate site, sandwiched between honeycomb borophene sheets. In addition to the unusual coordination, the boron serves as a poison panacea. Pure platinum is strongly susceptible to sulfur poisoning by adsorption, rendering a platinum catalyst ineffective. Boron is known to be resistant to sulfur poisoning. The boron sheets present in the HEB shield the platinum from sulfur while maintaining high catalytic activity. This is confirmed with the facile hydrogenation of thiol-containing nitro compounds, where the HEB resists sulfur poisoning while retaining its high catalytic activity.