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.