Mixed RuxIr1-xO2 Supported on Rutile TiO2: Catalytic Methane Combustion, a Model Study

With a modified Pechini synthesis, mixed RuxIr1-xO2 is grown on rutile-TiO2 with full control of the composition x, where the preformed TiO2 particles serve as nucleation sites for the active component. Catalytic and kinetic data of the methane combustion over RuxIr1-xO2@TiO2 and unsupported RuxIr1-xO2 catalysts reveal that the least active catalyst is RuO2@TiO2 (onset temperature: 270 degrees C), while the most active catalyst is Ru0.25Ir0.75O2 with an onset temperature below 220 degrees C. Surprisingly, even Ru0.75Ir0.25O2@TiO2 is remarkably active in methane combustion (onset temperature: 230 degrees C), indicating that little iridium in the mixed RuxIr1-xO2 oxide component already improves the activity of the methane combustion considerably. We conclude that iridium in the mixed RuxIr1-xO2 oxide enables efficient methane activation, while ruthenium promotes the subsequent oxidation steps of the methyl group to produce CO2. Kinetic data provide a reaction order in O-2 of zero, while that of methane is close to one, indicating that the methane activation is rate limiting. The apparent activation energy varies among RuxIr1-xO2 from 110 (x=0) to 80 kJ mol(-1) (x=1). This variation in the apparent activation energy may be explained by the variation in adsorption energy of oxygen. Under the given reaction conditions the catalyst's surface is saturated with adsorbed oxygen and only if oxygen desorbs, methane can be activated and the methyl group can be accommodated at the liberated surface metal sites.

Automated summary

By growing mixed RuxIr1-xO2 oxide on rutile-TiO2, the composition can be controlled to improve the activity of methane combustion. Even small amounts of iridium in the mixed oxide significantly enhance the activity of methane combustion, while ruthenium plays a role in promoting oxidation steps. Adsorption energy of oxygen may contribute to the variation in apparent activation energy among RuxIr1-xO2.