Kinetic modelling of the methanol synthesis from CO₂ and H₂ over a CuO/CeO₂/ZrO₂ catalyst: The role of CO₂ and CO hydrogenation

This work addresses the kinetics of the CO₂ hydrogenation to methanol over a Cu/CeO₂/ZrO₂ catalyst studied using single-site, dual-sites and three adsorption sites kinetic models. Physicochemical constraints and statistical indicators are used as tool for model discrimination. The best performing model is used to elucidate the reaction mechanism and the relative roles of the Cu-sites and oxygen vacancies. The results show that the dissociative adsorption of H₂ occurs on the Cu⁰ sites, while CO₂ is attracted to the oxygen vacancies created by the CeO₂-ZrO₂ solid solution. Then, the adsorbed H interacts preferentially with the carbon atom, favouring the so-called “formate” route. The CO formed via the r-WGS reaction could either desorb to the gas phase or react via hydrogenation to methanol. Analysis of the relative contributions of the CO₂ and CO hydrogenation (i.e. direct and indirect pathways, respectively) to the methanol synthesis reveals that the latter is in fact preferential at high temperatures (i.e. about 100% of methanol is produced from CO at 260 ⁰C and 30 bar), and it shows an optimum vs the H₂:CO₂ ratio (c.a. 7 at 200 ⁰C and 30 bar), which corresponds to the saturation of the Cu⁰ sites with H₂. Thus, this work provides an essential tool (i.e., kinetic model) for the design of reactors and processes based on novel catalysts, and importantly, it offers a deeper understanding of the reaction mechanism as basis for further catalyst development.