(460e) Methane Oxidation over Iridium Dioxide Catalysts

The (110) surface facet of rutile IrO₂ can activate methane below room temperature, but the active sites on this surface are blocked in the presence of very low concentrations of gas phase oxygen. Selective transformations of methane to higher value chemicals over IrO₂-based catalysts are therefore challenging despite the ability to activate methane at low temperatures. To gain a better understanding of IrO₂-based catalysts our group has investigated IrO₂ supported on TiO₂ with different structures and particle sizes. Under complete oxidation conditions (CH₄:O₂ of 1:10), both the structure and the particle size of the TiO₂ nanoparticle support influence the catalytic activity. Catalyst characterizations reveal that on anatase TiO₂ small IrO₂ nanoparticles are visible, while a thin layer of IrO₂ is observed on both brookite and rutile TiO₂ nanoparticles. The IrO₂-TiO₂ interactions result in more electron-rich IrO_x species on anatase TiO₂ compared with the other TiO₂ supports. This appears to be beneficial in the complete oxidation of methane, as the IrO₂ supported on anatase TiO₂ is more active compared with IrO₂ supported on brookite TiO₂, and the least active catalyst is IrO₂ supported in rutile TiO₂ (Figure 1a). However, the activity of these catalysts is also influenced by the particle size of the support. The best performing catalyst is IrO₂ supported on anatase TiO₂ with an average particle size of 10 nm, and IrO₂ on 5-nm or 15-nm anatase TiO₂ exhibit inferior activity. In fact, the worst performing catalyst of all IrO₂/TiO₂ tested is the IrO₂ supported on 5-nm anatase TiO₂, likely due to an unstable support. Under partial oxidation conditions (CH₄:O₂ of 2:1), the influence of TiO₂ structure on the activity is less significant (Figure 1b). This is due to the reaction pathway, which involves complete combustion while oxygen is present, followed by dry and steam reforming of methane.