Catalytic enhancement of production of solar thermochemical fuels: opportunities and limitations

Abstract

Solar thermochemical fuels are a promising low-carbon alternative to conventional fossil fuels, which must be swiftly phased out to mitigate the consequences of climate change. Thermochemical cycles powered by concentrating solar energy at high temperatures have demonstrated efficiency in the conversion of solar to chemical energy exceeding 5% and have been assayed in pilot scale facilities of up to 50 kW. This conversion route implies the use of a solid oxygen carrier that enables CO₂ and H₂O splitting, generally operating in two consecutive stages. The primary product of the combined thermochemical conversion of CO₂ and H₂O is syngas (CO + H₂), which for practical applications must be catalytically transformed into hydrocarbons or other chemicals such as methanol. This link between thermochemical cycles, involving the transformation of the whole solid used as an oxygen carrier, and catalysis, occurring only on the material surface, calls for exploitation of the synergies between these two unlike but interconnected gas–solid processes. Accordingly, in this perspective we discuss the differences and similitudes between these two transformation routes, consider the practical impact of kinetics in thermochemical solar fuel generation and explore the limits and opportunities of the catalytic promotion. With this aim, first, the potential benefits and hurdles of direct catalytic enhancement of CO₂ and H₂O dissociation in thermochemical cycles are discussed and then, the possibilities of improving the catalytic production of hydrocarbon fuels, basically methane, are also assessed. Finally, an outlook of the future opportunities of the catalytic promotion of thermochemical solar fuel productions is also provided.