Design and operational guidelines of solar-driven catalytic conversion of CO₂ and H₂ to fuels

Sunlight-powered catalytic conversion of CO₂ and (green) H₂ into fuels and chemicals via Sabatier and reverse water gas shift (RWGS) processes offers a promising solution to reduce greenhouse gas emissions and increase renewable energy utilization. The success of this approach relies on the development of efficient catalysts and reactors. Prior research on reactor design is based on fixed-bed concepts using conventional transition metal thermocatalysts that typically require high-temperature activation. The utilization of photothermal catalysts yields fast reaction kinetics and enhanced product selectivity at relatively low temperatures, and, therefore, requires new design and operational guidelines. A comprehensive steady-state model is described to assess the performance of solar-driven photothermal catalytic Sabatier and RWGS processes, with an emphasis on the development of a 1D heat and mass transfer model for a plate-shaped transparent flow reactor. The model allows for the prediction of the temperature profile, pressure drop and reaction extent along the reactor channel, CO₂ and H₂ conversion, total fuel yield, as well as system efficiency for a variety of design and operational choices. The effects of these parameters are strongly coupled, and a low packed bed porosity of 40% combined with high gas inlet pressure at 18–20 bar leads to both, high fuel yield and high system efficiency, for both the Sabatier and RWGS processes. The maximum system efficiency is predicted via simultaneous optimization of relevant variables within meaningful ranges while also respecting the practical temperature constraints of both the glass and catalysts. Compared with the baseline case, the optimized scenario achieves higher efficiencies of 26.3% (vs 6.7%) and 10.1% (vs 5.4%) for the Sabatier and RWGS processes, respectively, at 20 kW/m² irradiance. The model also identifies optimal reactor conditions under different concentrated solar irradiance, thus offering design and operational guidelines for solar-driven catalytic conversion of CO₂ and H₂ processes.