Recent advances in CO₂ reduction with renewable reductants under hydrothermal conditions: towards efficient and net...

The ever-growing atmospheric CO₂ concentration threatening the environmental sustainability of humankind makes the reduction of CO₂ to chemicals or fuels an ideal solution. Two priorities are anticipated for the conversion technology, high efficiency and net carbon benefit, to ensure the mitigation of the CO₂ problem both promptly and sustainably. Until now, catalytic hydrogenation or solar/electro-chemical CO₂ conversion have achieved CO₂ reduction promisingly while, to some extent, compromising to fulfill the two rules, and thus alternative approaches for CO₂ reduction are necessary. Natural geochemical processes as abiotic CO₂ reductions give hints for efficient CO₂ reduction by building hydrothermal reaction systems, and this type of reaction atmosphere provides room for introducing renewable substances as reductants, which offers the possibility to achieve CO₂ reduction with net carbon benefit. While the progress in CO₂ reduction has been abundantly summarized, reviews on hydrothermal CO₂ reduction are relatively scarce and, more importantly, few have focused on CO₂ reduction with renewable reductants with the consideration of both scale of efficiency and sustainability. This review provides a fundamental and critical review of metal, biomass and polymer waste as reducing agents for hydrothermal CO₂ reduction. Various products including formic acid, methanol, methane and multi-carbon chemicals can be formed, and effects of operational parameters such as temperature, batch holding time, pH value and water filing as well as detailed reaction mechanisms are illustrated. Particularly, the critical roles of high temperature and pressure water as reaction promotor and catalyst in hydrothermal CO₂ conversion are discussed at the mechanistic level. More importantly, this review compares hydrothermal CO₂ reduction with other methods such as catalytic hydrogenation and photo/electrocatalysis, evaluating their efficiency and potential for net carbon benefit. The aim of this review is to promote the understanding of CO₂ activation under a hydrothermal environment and provide insights into the efficient and sustainable strategy of hydrothermal CO₂ conversion for future fundamental research and industrial applications.