NiGa Unsupported Catalyst for CO₂ Hydrogenation at Atmospheric Pressure. Tentative Reaction Pathways

The hydrogenation of CO₂ is presented as a catalytic strategy for CO₂ utilization and an approach to the development of sustainable processes to obtain products with high added value, such as CO, CH₄, and CH₃OH. An unsupported Ni_xGa_y model catalyst with high surface area was synthesized by an ultrasonic-assisted coprecipitation method. The in situ H₂-treatment and the activation temperature (600, 700, and 800 °C) are key in the phases present in the catalyst and, therefore, in the reactivity in the CO₂ hydrogenation reaction at atmospheric pressure. Operating conditions have been selected that maximize the selectivity to be able to correlate phases, surface, and reactivity. Independently of the H₂/CO₂ feed ratio, after treatment in hydrogen at 600 °C, CO selectivity values are higher than 97%, via a reverse water gas shift reaction, with Ni₁₃Ga₉ and Ni metallic being the main phases detected. The H₂-treatment at 700 °C increases the proportion of Ni⁰ and Ni₁₃Ga₉ phases, and even with the use of a H₂/CO₂ feeding ratio of 3, methanation is the only reaction that occurs. At 800 °C, Ni₅Ga₃ is the main phase detected, and methanol is formed at 150 °C, through the intervention of a necessary formate species; when the reaction temperature is higher than 400 °C, the rWGS is the only observable reaction, with complete CO formation.