The Nature of Loading-Dependent Reaction Barriers over Mixed RuO2/TiO2 Catalysts

Mixed-metal oxides are one of the most frequently used catalysts in chemical industry, because the superior catalytic reactivity can be achieved by taking advantage of the synergetic effects of their parent oxides. However, the interfacial electronic interactions between metal oxides remain unclear, because of their structural complexity. This paper describes the modulation of catalytic performance of mixed RuO2RuO2/TiO2 catalysts via adjusting the loading amount of RuO2. We show that, at very low loadings, the majority of of RuO2 catalysts can be anchored at the defective sites of TiO2 substrates. Spectroscopic studies, combined with density functional calculations, indicate that electrons transfer from defective sites of substrates to RuO2, causing an increase in the apparent reaction barrier of CO oxidation. As the loading of Ru increases, RuO2 starts to appear on the terrace of TiO2, and the apparent reaction barrier of CO oxidation decreases. At the medium loading of similar to 0.75 wt %, the lowest apparent reaction barrier is achieved. The phenomenon can be attributed to the electron transfer from RuO2 to the terrace of TiO2. By further increasing the loading of Ru, the apparent reaction barrier rises again. When the loading of Ru is more than 2 wt %, the apparent reaction barrier is found to be very comparable to that over RuO2 catalysts supported on an inert substrate, indicating that the electronic effect of TiO2 is isolated underneath thick RuO2 overlayers. The demonstrated loading-electron transfer-reaction barrier relationship at RuO2/TiO2 catalysts provides an insight into the interfacial interaction between mixed oxides and can be readily extended to many other catalytic systems.