4. Nanomaterials for CO₂ Hydrogenation

The use of fossil fuels such as coal, oil, and natural gas has allowed a fast and unprecedented development of human society. However, this has led to a continuous increase in anthropogenic CO₂ emissions, which affect human life and the ecological environment through global warming and climate changes. There are various strategies to mitigate the atmospheric concentration of CO₂, such as capture, separation, and utilization. Among them, CO₂ hydrogenation to obtain different products through catalytic processes is a strategy of great interest. Thus, the catalytic combination of CO₂ and hydrogen not only mitigates anthropogenic emissions into Earth’s atmosphere, but it also produces carbon compounds that can be used as fuel or precursors for the production of different chemicals.

This chapter reviews the use of different nanomaterials for CO₂ hydrogenation. Three different processes are distinguished, depending on the final product: (i) CO₂ hydrogenation to carbon monoxide, (ii) methanol production by CO₂ hydrogenation, and (iii) CO₂ hydrogenation to methane. It has been included both nanomaterials that act as support and those that can replace the active metal phase. Concerning CO₂ hydrogenation to CO, one-dimensional transition metal carbides have received increasing attention because their unique electronic structure allows similar catalytic properties to the expensive noble metals. Attending the high thermal requirements of CO synthesis, emerging metal oxides nanocatalysts are focused to prevent the metal sintering by increasing the metal-support interactions. Controlling the support’s morphology at nanoscale can enhance both catalytic activity and stability at high temperatures up to twice with respect to those conventional micro-sized catalysts. Regarding to methanol production, the nanomaterials most commonly used as supports are those based on carbon, e.g., carbon nanotubes, carbon nanofibers, and graphene oxide. The main advantage of using these materials is their high surface area, which improves metallic phase dispersion, higher thermal and electrical conductivities, and greater mechanical resistance. In addition, the use of intermetallic nanoparticles as an active phase is very promising. The combination of two metals in the same nanoparticle greatly increases the interface between components, which clearly leads to a synergistic effect between them. The use of these nanomaterials improves the activity and selectivity to methanol between 2 and ~50%, compared with classical catalysts. Moreover, similar strategies are equally valid in methane production. Catalysts based on nanoparticles, such as Ni or NiO, supported on traditional metal oxides have been recently reported to improve catalytic activity in CO₂ methanation with high resistance to coke deposition. Other supports, such as carbon nanofibers and carbon nanotubes previously mentioned, have shown excellent results, with CO₂ conversions higher than 90% and complete selectivity to methane. Finally, TiO₂-based catalysts are a promising solution for methane production by the still undeveloped photocatalytic reduction. This reaction can be performed under mild temperatures and pressure conditions, which is a clear advantage for methane synthesis.