Catalytic and adsorption studies for the hydrogenation of CO₂ to methane

CO₂ methanation has been evaluated as a means of storing intermittent renewable energy in the form of synthetic natural gas. A range of process parameters suitable for the target application (4720 h⁻¹ to 84,000 h⁻¹ and from 160 °C to 320 °C) have been investigated at 1 bar and H₂/CO₂ = 4 over a 10% Ru/γ-Al₂O₃ catalyst. Thermodynamic equilibrium was reached at T ≈ 280 °C at a GHSV of 4720 h⁻¹. Cyclic and thermal stability tests specific to a renewable energy storage application have also been conducted. The catalyst showed no sign of deactivation after 8 start-up/shut-down cycles (from 217 °C to RT) and for total time on stream of 72 h, respectively. In addition, TGA-DSC was employed to investigate adsorption of reactants and suggest implications on the mechanism of reaction. Cyclic TGA-DSC studies at 265 °C in CO₂ and H₂, being introduced consecutively, suggest a high degree of short term stability of the Ru catalyst, although it was found that CO₂ chemisorption and hydrogenation activity was lowered by a magnitude of 40% after the first cycle. Stable performance was achieved for the following 19 cycles. The CO₂ uptake after the first cycle was mostly restored when using a H₂-pre-treatment at 320 °C between each cycle, which indicated that the previous drop in performance was not linked to an irreversible form of deactivation (sintering, permanent poisoning, etc.). CO chemisorption on powder Ru/γ-Al₂O₃ was used to identify metal sintering as a mechanism of deactivation at temperatures higher than 320 °C. A 10% Ru/γ-Al₂O₃//monolith has been investigated as a model for the design of a catalytic heat exchanger. Excellent selectivity to methane and CO₂ conversions under low space-velocity conditions were achieved at low hydrogenation temperatures (T = 240 °C). The use of monoliths demonstrates the possibility for new reactor designs using wash-coated heat exchangers to manage the exotherm and prevent deactivation due to high temperatures