Active Phase of IrO₂ in the Catalytic CH₄ Combustion Reaction: Operando Infrared Spectroscopy and Online Gas Analysis

With operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and online gas analysis, we studied the catalytic methane combustion over 2 mol % IrO₂ supported on rutile TiO₂ under reducing (CH₄:O₂ = 1:1) and oxidizing (CH₄:O₂ = 1:4) reaction conditions. Two types of catalysts are studied: the samples are pretreated at 320 °C either with O₂ (ox-IrO₂@TiO₂) to ensure full oxidation or with CH₄ (red-IrO₂@TiO₂) to partially reduce IrO₂. Ox-IrO₂@TiO₂ and red-IrO₂@TiO₂ are stable under oxidizing and reducing methane oxidation conditions, respectively. DRIFTS indicates, however, that under reducing conditions CO is formed on red-IrO₂@TiO₂, while no CO formation is observed for ox-IrO₂@TiO₂ under oxidizing conditions. Methane oxidation under reducing conditions transforms ox-IrO₂@TiO₂ into red-IrO₂@TiO₂ showing strong CO bands in DRIFTS and higher activity than oxidized IrO₂. This latter observation is in stark contrast to methane oxidation studies over single crystalline IrO₂(110), thus manifesting a material gap in terms of chemical reducibility. Highest methane conversion is achieved with red-IrO₂@TiO₂ under the oxidizing reaction feed. The formation of adsorbed CO on the catalyst evidences that the reaction mechanism proceeds via the formaldehyde intermediate.