Kinetics of Redox Half-Cycles on Bi₂O₃ for the Selective Catalytic Combustion of Hydrogen

The kinetics of selective hydrogen combustion (SHC) in mixtures with hydrocarbons on Bi₂O₃ in the absence of gas-phase O₂, process conditions mimicking chemical looping processes and reduction half-cycles in Mars-van Krevelen mechanisms typical in hydrocarbon partial oxidation, are assessed by employing metrics for the rate and selectivity which account for the stoichiometric nature of oxygen abstraction in SHC. These assessments consider the kinetic relevance of surface reactions and bulk lattice oxygen diffusion in Bi₂O₃ particles. Low single-pass conversion enabled by operation in a recirculating batch reactor ensures that measured kinetics are not influenced by axial gradients in concentration or temperature. Dwell experiments alternating treatments of a reductant or an inert gas illustrate the facile nature of oxygen diffusion in Bi₂O₃ particles, enabling the accurate quantification of the rate of hydrogen combustion on Bi₂O₃ surfaces in the absence of fluid-phase oxidants. Anaerobic hydrogen combustion rates, normalized by the number of oxygen atoms exposed on the oxide surface, exhibit a first-order relationship with hydrogen partial pressure. An observed kinetic isotope effect of ∼3.4 demonstrates that hydrogen activation is the rate-limiting step in hydrogen combustion. The >97% selectivity of Bi₂O₃ lattice oxygen to combust hydrogen persists in systems with equimolar H₂ and CH₄, C₂H₄, C₂H₆, or C₃H₆ across varying extents of metal oxide reduction (O:Bi = 1.2-1.5) and, for the H₂/CH₄ system, upon repeated redox cycling in a gradient-less recirculating batch reactor. Density functional theory calculations suggest that Bi₂O₃ surfaces facilitate heterolytic bond activation, thus enabling selective hydrogen combustion despite the higher energetic requirement for thermal H-H bond cleavage relative to that of the C-H bonds of the hydrocarbon molecules examined herein. The selective heterolytic bond activation observed on Bi₂O₃ may provide strategies to preferentially cleave and combust molecules with stronger bonds in mixtures of organic compounds.