Effect of sodium promoters on Ni/Al₂O₃ catalyst for CO₂ hydrogenation: The carbon fixation as carbon nanofiber and reverse-water gas reactions

Reducing CO₂ emissions has become a critical priority in addressing the environmental impact of carbon dioxide. Technologies for CO₂ fixation primarily involve carbon capture storage and utilization (CCSU) and chemical conversion, typically pursued along independent tracks. In our research, we have focused on developing a catalytic reaction for CO₂ hydrogenation that integrates the benefits of both CCSU and chemical conversion. The use of sodium-promoted Ni/Al₂O₃ catalyst (NiNa_x/Al₂O₃) in the CO₂ hydrogenation process exhibits a synergistic effect, leading to the simultaneous production of CO, CH₄, and carbon nanofibers (CNFs). Notably, the conversion of CO₂ into CNFs represents a novel approach for carbon fixation, in addition to the formation of CO and CH₄. Importantly, the formation of CNFs on the NiNa_x/Al₂O₃ catalyst does not compromise its catalytic activity for CO₂ hydrogenation to CO and CH₄, which is a noteworthy finding. This bifunctional catalyst has the potential to offer significant advantages, including high CO production and the generation of valuable carbon materials. The CNFs are predominantly formed at the interfacial sites between NiNa_x and Al₂O₃. The formation process involves the reaction between H₂ bound to Ni and adsorbed CO₂, leading to the formation of hydrogen carbonate species, which subsequently give rise to CNFs. The reduction of CO₂ to CNFs is facilitated by the transfer of electrons from Al³⁺ species to the interfacial sites between NiNa_x and Al₂O₃.