58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Most transition metal oxides are reducible. Many of them exist in different phases and exhibit multiple oxidation state. Some of them, such as VO_x, Fe₃O₄, and CeO₂ are catalysts or active components in heterogeneous catalysis. These reducible oxides play important roles in many catalytic processes. Generally, they exist in three categories: nanoparticles of a doped oxide, a thin film of a doped oxide on an inert support, and nanoparticles of an oxide MO_y with immobilized metal or oxide nanoclusters. Our hypothesis was that the surface chemistry (oxidation state, density of surface oxygen vacancies, composition, electronic state, etc) and structure of the doped bulk oxide catalysts during catalysis are quite different from those of doped surfaceoxide. Based on this hypothesis, the surface chemistry and structure of oxide could experience significant change under reaction condition or during catalysis. To understand catalysis on oxide thin film, in-situ studies of surface chemistry is necessary. In the last year (September 2012-August 2013) we have performed operando studies of surface chemistry of that of oxide catalysts and simultaneously measured their catalytic performances for CO₂ conversion with H₂ to CH₄. We prepared Ru-Co mixed oxide and catalytic performance of this catalyst for CO₂ reduction with H₂. Figure 1 shows the TEM images of the catalysts for CO₂ conversion with H₂ to CH₄. Activity and selectivity in CO₂ reduction was measured toward mechanistic understanding the difference of catalytic behaviors between bulk and surface doped oxides. Figures 2a and 2b present the measured catalytic activity and selectivity for conversion of CO₂ to CH₄. We put emphasis on mechanistic understanding of the difference in catalytic performance between doped bulk oxide and doped surface oxide using the new approach of operando studies. In detailed we have performed operando studies of CO₂ reduction with H₂ on (Co_1-xRu_x)₃O_4+y nanorods and thin film by using in-house ambient pressure XPS. Figure 3 is the photoemission features of Co 2p and Ru 3p as a function of catalytic temperatures in the mixture of reactants of CO₂ and H₂. A correlation between catalytic performances of the two catalysts and their surface chemistry under reaction conditions was built. Active phases of the two catalysts are metallic cobalt and bimetallic Co−Ru, respectively. Light-off temperature of Co−Ru catalyst is lower than that of a cobalt catalyst. Selectivity to production of CH₄and activity on the Ru-doped cobalt oxide are obviously enhanced by formation of bimetallic Co−Ru ultrathin film in its surface region in contrast to that of cobalt catalyst in the temperature range of 200−340 °C.

Figure 1. TEM images of synthesized Co₃O₄nanorods. (a) Largescale image; (b) high-resolution image of a nanorod.

Figure 2. Catalytic conversion (a) and selectivity (b) of CO₂ conversion on Co₃O₄(black line) and (Co_0.95Ru_0.05)₃O₄(red line).

Figure 3.Photoemission feature of Co 2p (a) and Ru 3p (b) of (Co_0.95Ru_0.05)₃O₄under reaction conditions in the temperature of 180−420 °C. The ambient pressure XPS measurements were performed in the temperature range of 30−420 °C. Data at temperature below 180 °C is the same as those of 180 °C.