With a modified Pechini synthesis, mixed Ru x Ir 1À x O 2 is grown on rutile-TiO 2 with full control of the composition x, where the preformed TiO 2 particles serve as nucleation sites for the active component. Catalytic and kinetic data of the methane combustion over Ru x Ir 1À x O 2 @TiO 2 and unsupported Ru x Ir 1À x O 2 catalysts reveal that the least active catalyst is RuO 2 @TiO 2 (onset temperature: 270°C), while the most active catalyst is Ru 0.25 Ir 0.75 O 2 with an onset temperature below 220°C. Surprisingly, even Ru 0.75 Ir 0.25 O 2 @TiO 2 is remarkably active in methane combustion (onset temperature: 230°C), indicating that little iridium in the mixed Ru x Ir 1À x O 2 oxide component already improves the activity of the methane combustion considerably. We conclude that iridium in the mixed Ru x Ir 1À x O 2 oxide enables efficient methane activation, while ruthenium promotes the subsequent oxidation steps of the methyl group to produce CO 2 . 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 Ru x Ir 1À x O 2 from 110 (x = 0) to 80 kJ • mol À 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.

The effect of the carrier material on the intrinsic activity of three catalytic total oxidation reactions, namely CO oxidation as well as methane and propane combustion over highly dispersed RuO2...

The solid solution of a reducible oxide with a (non or) less reducible oxide may open the way to incorporate substantial amounts of hydrogen by the simple exposure to H2...

Mixed oxide catalysts Ru x Ir 1-x O 2 with varying composition x (x = 0, 0.25, 0.5, 0.75, 1.0) supported on CeO 2 , γ-Al 2 O 3 or ZrO 2 are successfully prepared and tested in the catalytic propane combustion in terms of activity and stability. Pure IrO 2 reveals a significantly lower activity than Ru x Ir 1-x O 2 with x � 0.25. For low conversion, pure RuO 2 on CeO 2 turns out to be the most active catalyst, while at higher conversion, Ru 0.75 Ir 0.25 O 2 on ZrO 2 is found to be more active than RuO 2 , pointing towards synergism of Ru and Ir sites. Long-term stability and also the resistance against water poisoning are highest for ZrO 2 -supported catalysts. The higher the Ir concentration in the active component Ru x Ir 1-x O 2 the more susceptible is the catalyst to water poisoning. Water poisoning is shown to be reversible, consistent with a blocking of catalytically active sites by water adsorption.