Abstract
This study elucidates the factors reducing the CO2 methanation reaction temperature of TiO2-supported Ru catalysts prepared via the polygonal barrel-sputtering method (Ru/TiO2(BS)) to investigate the structure-sensitivity mechanism. The smaller nanoparticles deposited in Ru/TiO2(BS) (<4 nm) were amorphous RuO2 because of air exposure after the preparation, and their surfaces were changed to island-shaped structures consisting of amorphous RuO2 and amorphous Ru metal by H2 exposure. In this case, dissociative hydrogen was also adsorbed in abundance on the amorphous Ru metals. Such hydrogen atoms were not observed in conventional Ru/TiO2 catalysts. Under the supplied CO2 + H2 at a stoichiometric ratio of 1:4, these hydrogen atoms not only contributed to the generation of a unique CO intermediate (Ru–CO–Ru–H) from room temperature, but also reduced this CO adsorbate to methane even in low-temperature ranges (≤120 °C). These reaction steps were completely different from the reported mechanisms. Accordingly, the formation of amorphous Ru metals and the adsorption of hydrogen atoms on them are essential for reducing the CO2 methanation temperature. These are key factors of structure-sensitivity, which would also be useful for improving activities of various catalysts.
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