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The advantage of coupling a diffuse-reflectance infrared cell with a continuous-feed recycle reactor for in situ kinetic and mechanistic studies is illustrated in the study of CO2 methanation over Ru/TiO2. From transient and steady-state experiments, metal-adsorbed CO is identified as a major reaction intermediate and at 120°C occupies approximately 40% of the available ruthenium surface, while methane formation proceeds at a turnover frequency of 2.8 × 10-3 s-1. In contrast, CO methanation is totally hindered under the same conditions. A simple kinetic model is proposed to account for the main trends observed. For CO2 methanation, the temperature insensitivity of adsorbed CO coverage is interpreted on the basis of a generalized (two-stage) intermediate supply/consumption mechanism involving the reverse water-gas shift reaction.