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 160oC to 320oC) have been investigated at 1 bar and H2/CO2 = 4 over a 10 % Ru/γ-Al2O3 catalyst. Thermodynamic equilibrium was reached at T ≈ 280oC 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 217oC 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 260oC 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 320oC 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 320oC. 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 = 240oC). 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. http://www.sciencedirect.com/science/article/pii/S0926337314000228