Structure Sensitivity of TiO2-Supported IrO2 Catalysts in the Complete Oxidation of Methane

TiO₂-supported catalysts exhibit high catalytic performance in numerous reactions, and the catalytic activities are often dependent on the TiO₂ structure. Yet, detailed knowledge of how the TiO₂ structure affects active metals or metal oxides, and thus the catalytic performance, is still lacking. Therefore, rational catalyst design of TiO₂-supported catalysts remains a challenge. To determine the effects of TiO₂ structure on IrO₂-based catalysts in the oxidation of methane, three TiO₂ phases were selected, anatase, rutile, and brookite TiO₂, and used as catalyst supports. Different loadings of IrO₂ were also used to vary the IrO₂ particle size and tune IrO₂–TiO₂ interactions, and three different particle sizes of anatase TiO₂ were evaluated. The results reveal that the IrO₂–TiO₂ interactions are structure sensitive and the catalytic activity is not only dependent on the titania structure but also the TiO₂ particle size and the influence of iridium loading on the reaction is dependent on the TiO₂ structure. The best-performing catalyst is IrO₂ supported on anatase TiO₂ with an average particle size of 10 nm, due to its high catalytic activity and better stability during 50 h on stream. This catalyst consists of small and well-dispersed IrO₂ nanoparticles on the TiO₂ support. In contrast, strong metal–support interactions result in a thin IrO₂ film on the surfaces of both the rutile and brookite TiO₂ supports. These interactions result in IrO₂ species that are more difficult to reduce compared with the IrO₂ on the anatase TiO₂ supports. This is further evidenced in the X-ray photoelectron spectroscopy (XPS) measurements, which reveal the formation of reduced IrO₂ on the anatase TiO₂ support after reaction. This indicates that surface oxygen vacancies in IrO₂ play a critical role in the reaction. This study also reveals that catalyst deactivation over IrO₂ supported on rutile TiO₂ is likely due to loss in active IrO₂ sites, while the observed loss in activity with time on stream over the IrO₂ supported on brookite TiO₂ is due to degradation of the brookite TiO₂ support. These results underscore the importance of support structure and particle size in TiO₂-supported IrO₂ catalysts for methane oxidation, effects that are likely to influence also other catalyst systems and reactions, highlighting their importance in rational catalyst design.