To gain a deeper understanding of how the crystal structure, or more specifically the morphology, affects its catalytic activity, we investigated how the crystallization of the HEB across synthetic methods. In our previous work, we had prepared the HEB Al0.2Nb0.2Pt0.2Ta 0.2Ti0.2B2 by crystallizing in molten aluminum; this modified flux growth method adapted from single crystal growth and thus we would expect to see tiny single crystals as a product. Likewise, arc melting from elemental Al, Nb, Pt, Ta, Ti, and B should result in an ingot that upon crushing should result in a poorly faceted sample with random morphology akin to crushed powder. When comparing the arc melted vs flux growth methods for synthesizing the HEB Al0.2Nb0.2Pt0.2Ta 0.2Ti0.2B2, we can see that the flux grown sample is more crystalline, containing abundantly clear facets, numerous hexagonal macrocrystals, which are reflective of its hexagonal microcrystalline structure (Figure ). The powder X-ray diffraction (pXRD) patterns for both samples confirm the crystal system as an MB2 (AlB2 type) of space group (191) P6/mmm, while the arc melted sample has some Al2O3 impurities present, this is a result of grinding the HEB ingot into a fine powder in a sapphire mortar and pestle for analysis by pXRD and SEM (Figure ). With identifying that the flux grown sample is more crystalline, what does this mean for their ability to perform hydrogen spillover on WO3? To that end, we mixed WO3 with our catalyst and then performed the hydrogenation with hydrogen gas. The flux grown HEB readily hydrogenates WO3 in 5% H2, 95% argon (Figure ), whereas the arc melted sample was unable to hydrogenate WO3 in the same gaseous reaction conditions. To confirm that the arc melted HEB was unable to catalyze WO3, first the time was increased to 24-hours in 5% H2, 95% argon (Figure ) and then the environment was changed to 100% H2 over 24-hours (Figure ), neither method was successful as the WO3 was left in its green non-hydrogenated form, whereas the flux growth HEB and platinum readily hydrogenated WO3 in a dilute hydrogen environment. This exemplifies the necessitation for flux growth of the HEB, giving support to the importance of platinum metallocenelike edges of the crystals for needed to drive catalytic activity. The arc melted HEB cannot have hydrogen bond to platinum, dissociate or transport hydrogen to the substrate. Here, facet expression of the [110] Miller Indices is key to catalytic performance; this lattice plane follows along the edge of the crystal, perpendicular to the c-axis, where platinum is exposed and available for catalysis. Given the hybrid nature of Al0.2Nb0.2Pt0.2Ta 0.2Ti0.2B2, the potential mechanism for hydrogenation can be viewed from the lens of a homogeneous, hydrogen atom transfer mechanism or a heterogeneous, hydrogen spillover effect mechanism. As the edges of the single crystal particles should resemble a platinum metallocene, each platinum atom can be thought of as an isolated atom surrounded by ligands. In effect, the edges should behave as a platinum complex which is known to release hydrogen radicals. We hypothesize that similar hydrogen atom transfer mechanisms can be explained by Scheme 1. Viewing the mechanism from the lens of a heterogeneous, hydrogen spillover effect mechanism, the borophene sheet separation can be thought of as a pore. Based on the pXRD pattern, we can determine the interlayer spacing between borophene sheets by using the lattice parameters: a = 3.073(2) Å and c = 3.278(1) Å. The [001] Miller Indices would represent the d-spacing of the borophene sheets representing the interlayer spacing. We can then subtract the Slater radii of two boron atoms (0.85 Å) to find the true pore size to be 1.578 Å. This is just large enough for hydrogen atoms (1.0 Å in diameter) to access the platinum atom, while being small enough to prevent any other species such as poisonous sulfur (2.0 Å in diameter) from binding to the catalytic active site. From there, the hydrogen dissociates to atomic hydrogen, leaves the "pocket", and then spills over the surface of the particle. It could be argued that within the high entropy boride particles, the surface serves as the non-reducible support for the entrapped platinum atom (Figure ). Scheme 1. The mechanism for a hydrogen atom transfer potentially involved for the HEB. The hydrogen gas dissociates onto the platinum complex, followed by a radical attack on the coordinated hydrogen from T (representing a reducible moiety). Note, the B12 represents the borophene layer as the ligands on platinum. Indeed, spillover has been the mechanism for previous attempts at poison-resistant catalysts where the restrictive pore size selectively ensures that atomic hydrogen is the mobile and active species. Yang et al implanted platinum nanoparticles inside zeolite cages with 3 Å pores, and was able to demonstrate sulfur tolerance by sequestering platinum and using spillover hydrogen as the reducing agent. Likewise, Calderone et al encapsulated platinum colloids in nanoporous silica with ~ 4 Å pores and was able to achieve some degree of sulfur tolerance. Regardless of the mechanism, both HAT and HSPE rely on mobile hydrogen radicals to serve as the principal reducing agent. It could be argued, what makes the HEB sulfur-resistant is that boron's role in the mechanism is more than just a steric shield against sulfur. To confirm whether a boronhydrogen bond is formed, we exposed the HEB catalyst to H2 under 1 atm for 24-hours and measured the sample using FTIR, which showed no peak around 2500 cm -1 , suggesting no boronhydrogen bond is formed in the catalytic process ( Figure ).