A Fourier Transform Infrared Spectroscopic Study of CO2 Methanationon Supported Ruthenium
Diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy has been used to study in situ, the low-temperature (T < 200°C) methanation of CO2 over Ru on TiO2 supports and on Al2O3. For 3.8% Ru/TiO2, the reaction exhibits an activation energy (Ea) of 19 kcal/mol, is 0.43 ± 0.05 (approximately one-halt) order in H2 concentration, and essentially independent of C02 concentration. At 110°C, 40% of the available metal sites are occupied by CO (Qco = 0.4), a known methanation intermediate. In contrast to Ru/TiO2, Ru/Al2O3, despite having the same Ea and Qco = 0.2, is 15 times less active. Batch catalyst screening experiments showed no dependence of methanation activity on adsorbed CO (COa) formation rate (as modeled by HCOOH dehydration) or on Qco. In view of this, and the fact that CO dissociation is known to be structure-sensitive, heterogeneity in the active sites is invoked to reconcile the data. The high Ru dispersion on TiO2 is believed to contribute to the enhanced activity over this support. Adsorbed CO2 and H2 react, possibly at the metal-support interface, to form COa via rapid equilibration of the reverse water-gas shift reaction, in which HCOOH (and/or HCOO- ion) play a major role. According to this view, the COa and HCOO-a intermediates seen by FTIR represent accumulated reservoirs en route to CH4, in which the COa hydrogenation step is rate-controlling. An interesting synergy occurs for mixtures of Ru/anatase and Ru/rutile, the former being a better catalyst for CO. supply while the latter is more effective in COa hydrogenation.