(582cq) Effects of Controlled Crystalline Plane of Hydroxyapatite on Methane Conversion Reactions

Hydroxyapatite (HAP, Ca₁₀(PO₄)₆(OH)₂) is a type of calcium phosphate with a hexagonal crystal structure, commonly studied in biomaterials, electrolyte membranes and catalysts. The orientation of HAP, for example, along the crystallographic c-axis direction with the (002) plane exposed as end face, significantly enhances the ion conductivity of HAP electrolyte membranes and wear resistance of HAP-containing teeth and bones. The effect of HAP orientation on catalytic reactions over HAP-based catalysts, however, has not been explored. This work reports the influences of controlled HAP crystal orientation (along c-axis direction) and exposed crystalline plane ((002) as end face) on methane conversion reactions over HAP and lead-substituted HAP (Pb-HAP) catalysts. The alignment of HAP-based crystals in the catalysts was achieved via an electrochemical deposition of HAP seeds on a titanium substrate, the secondary hydrothermal crystallization to promote the c-axis growth normal to the substrate, and peeling off of the HAP-based crystals from the substrate. Two catalytic reactions, methane combustion and oxidative coupling of methane to C₂ (ethylene and ethane) were tested over the oriented HAP-based catalysts. The results show that oriented HAP-based catalysts with exposed (002) crystalline facet promoted oxygen adsorption which facilitated the reaction between methyl radicals and adsorbed oxygen species to form CO₂ product when compared to randomly oriented HAP-based catalysts. The present analysis explored for the first time the effects of HAP crystal orientation and controlled crystalline plane exposure in methane conversion reactions, which may also be applied to other apatite materials and catalytic reactions.