In our previous work, Rosenberg et al. used the stabilization from high mixing entropy to force platinum to alloy into the high entropy boride (HEB): Al0.2Nb0.2Pt0.2Ta 0.2Ti0.2B2. This was the highest coordinated platinum-boride species reported thus far, a remarkable 12-coordinated platinum boride phase. Platinum borides prior to the discovery of this HEB, were only boron poor phases with low platinum coordination: Pt3B2 (platinum coordinated by 6 borons), PtB (platinum coordinated by 6 borons), and Pt2B (platinum coordinated by 3 borons). The fascination with forming a platinum high entropy diboride, comes from its unique crystal structure of alternating borophene sheets and metal (Figure ). Platinum based catalysts are poisoned by ppm concentrations of sulfur. Boron contributes sulfur-resistivity for poison-resistant catalysis allowing it to avoid sulfur poisoning, even at elevated sulfur concentrations and enable a longer catalyst lifetime. What made Al0.2Nb0.2Pt0.2Ta 0.2Ti0.2B2 remarkable was its ability to catalytically reduce 4-nitrothiophenol which contains a thiol group directly functionalized to the substrate. This would have been the worst possible test case for poison-resistant catalysts as the substrate itself is poisonous at ~1,000 ppm loadings, far exceeding the sulfur tolerance of previous poison-resistant catalysts. This remarkable tolerance must be derived from the borophene sheets' most important function; in that it shields platinum from sulfur, while maintaining its high catalytic activity. High entropy alloys were first introduced by both Yeh and Cantor who proposed that incorporating five or more principal elements could promote the formation of a single-phase structure. The high mixing entropy of the structure's five or more components thermodynamically stabilizes a single solid-phase compound, enabling them to overcome the tendency to separate into individual phases and form a single solid-solution phase at elevated temperatures. Here, the Gibbs free energy of ΔG = ΔH -TΔS (where G is the Gibbs free energy, H is enthalpy, T is temperature, and S is entropy) where the larger TΔS will cause ΔG to become negative and thus facilitate a spontaneous formation of the high entropy alloy. The four additional metals that were alloyed with platinum in our HEB were chosen diligently following the Hume Rothery rules for alloying; all these metals crystalize into diborides, their metallic radii (size) differ by less than 15% from that of platinum, and have a similar electronegativity and count of valence electrons. The Hume Rothery rules for alloying established a basic elemental scaffold to then begin to break the Hume Rothery rules to force platinum into this high entropy boride. Our previous work had not investigated the mechanism or route for how hydrogenation catalysis occurs. We hypothesize that the HEB follows a hybrid mechanism between the hydrogen spillover effect (HSPE), as found in heterogeneous catalysis and hydrogen atom transfer (HAT), as found in homogeneous catalysis. Both mechanisms produce mobile hydrogen radicals as the principal reducing agent.