Exploiting Heat Transfer in CO2 Hydrogenation

We are investigating tandem catalysts for hydrogenation of CO₂ into fuels in a single reactor. During the CO-mediated pathway, endothermic reverse water-gas shift (RWGS) is coupled with exothermic Fischer-Tropsch Synthesis (FTS). The CO intermediate diffuses to a secondary active site that catalyzes the carbon coupling reaction, with the diffusion seen as a kinetically integral step in the reaction pathway. However, the effect of heat transfer of the coupled exo- and endothermic reactions is neglected. We hypothesize that exploiting localized heat transfer will decrease the quantity of external thermal energy needed to run the tandem reaction, increasing energy efficiency and raising economic viability of the approach. While we are ultimately interested in exploiting heat transfer limitations to enhance CO₂ hydrogenation kinetics, FTS results in a wide distribution of hydrocarbon products, adding undesired complexity into the system. To focus on tuning reaction kinetics, we substitute FTS with CO methanation as the heat source. We first attempt to identify appropriate catalysts for the exothermic and endothermic reactions: the RWGS catalyst needs to be nearly 100% selective toward CO without CH4 formation, and be inert for CO activation; the CO methanation catalyst should be almost 100% selective towards CH₄, and inert for CO₂ activation. The catalysts are synthesized using incipient wetness impregnation, and screened using temperature programmed reactions at atmospheric pressure to evaluate their effectiveness at producing either CO or CH₄. The candidates we are evaluating are Pt/Al₂O₃ and Ni/ZrO₂ for RWGS and CO methanation, respectively. The former appears to be a promising RWGS candidate, active and selective for RWGS and inactive for CO methanation. On the other hand, Ni/ZrO₂ performs well for CO methanation, but remains active for the undesired CO₂ methanation side-reaction.