Review of CO2 Reduction on Supported Metals (Alloys) and Single-Atom Catalysts (SACs) for the Use of Green Hydrogen in Power-to-Gas Concepts

The valorization of carbon dioxide by diverting it into useful chemicals through reduction has recently attracted much interest due to the pertinent need to curb increasing global warming, which is mainly due to the huge increase of CO2 emissions from domestic and industrial activities. This approach would have a double benefit when using the green hydrogen generated from the electrolysis of water with renewable electricity (solar and wind energy). Strategies for the chemical storage of green hydrogen involve the reduction of carbon dioxide to value-added products such as methane, syngas, methanol, and their derivatives. The reduction of CO2 at ambient pressure to methane or carbon monoxide are rather facile processes that can be easily used to store renewable energy or generate an important starting material for chemical industry. While the methanation pathway can benefit from existing infrastructure of natural gas grids, the production of syngas could be also very essential to produce liquid fuels and olefins, which will also be in great demand in the future. In this review, we focus on the recent advances in the thermocatalytic reduction of CO2 at ambient pressure to basically methane and syngas on the surface of supported metal nanoparticles, single-atom catalyst (SACs), and supported bimetallic alloys. Basically, we will concentrate on activity, selectivity, stability during reaction, support effects, metal-support interactions (MSIs), and on some recent approaches to control and switch the CO2 reduction selectivity between methane and syngas. Finally, we will discuss challenges and requirements for the successful introduction of these processes in the cycle of renewable energies. All these aspects are discussed in the frame of sustainable use of renewable energies.