Exploring the roles of oxygen species in H₂ oxidation at β-MnO₂ surfaces using operando DRIFTS-MS

Understanding of the roles of oxygen species at reducible metal oxide surfaces under real oxidation conditions is important to improve the performance of these catalysts. The present study addresses this issue by applying a combination of operando diffuse reflectance infrared Fourier transform spectroscopy with a temperature-programmed reaction cell and mass spectrometry to explore the behaviors of oxygen species during H₂ oxidation in a temperature range of 25-400 °C at β-MnO₂ surfaces. It is revealed that O₂ is dissociated simultaneously into terminal-type oxygen (M²⁺-O^2-) and bridge-type oxygen (M⁺-O^2--M⁺) via adsorption at the Mn cation with an oxygen vacancy. O₂ adsorption is inhibited if the Mn cation is covered with terminal-adsorbed species (O, OH, or H₂O). In a temperature range of 110-150 °C, OH at Mn cation becomes reactive and its reaction product (H₂O) can desorb from the Mn cation, resulting in the formation of bare Mn cation for O₂ adsorption and dissociation. At a temperature above 150 °C, OH is reactive enough to leave bare Mn cation for O₂ adsorption and dissociation. These results suggest that bare metal cations with oxygen vacancies are important to improve the performance of reducible metal oxide catalysts.