(675g) Descriptors for Rates and Selectivities of H2 and CxHy Combustion on Bi2O3

We report that selective combustion of H₂ on Bi₂O₃ in binary H₂/CH₄, H₂/C₂H₄, H₂/C₂H₆, H₂/C₃H₆ mixtures occurs with lattice oxygen selectivity exceeding 98% in stoichiometric reduction cycles, a result which is inconsistent with selectivity arguments based solely on molecular descriptors for H-H and C-H bond strengths. We employ a gradient-less recirculating batch reactor to obtain rates and selectivities which depend intrinsically on extent of oxide conversion. Oxygen selectivity to H₂ on Bi₂O₃ exceeding 98% persists even when >50% of lattice oxygen atoms have been removed from the oxide, and upon repeated redox cycling, suggesting that the chemical characteristics of Bi₂O₃ which enable SHC are innate to Bi-O bonds and do not depend on morphological characteristics which inherently are affected during partial reduction of the metal oxide. We synthesize Bi₂O₃ samples exhibiting unique crystallographic unit cells at ambient conditions and again illustrate unchanging selectivities to hydrogen combustion in stoichiometric experiments. We probe the relevance of oxygen diffusion and surface reactions present in redox cycles via dwell experiments to illustrate that oxygen diffusion does not limit H₂ combustion rates during SHC on Bi₂O₃ particles. Applying metrics for rates which depend instantaneously on density of exposed surface oxygen species during stoichiometric reduction cycles, we evaluate the kinetic dependence of observed H₂ combustion rates on gas phase reductant partial pressures. We expound on the utility of Bi₂O₃ as a selective combustion catalyst by probing its ability to selectively combust one hydrocarbon molecule in a mixture with multiple hydrocarbons, specifically to examine the molecular features which dictate hydrocarbon combustion rates on Bi₂O₃.