Hydrothermal synthesis of ZnZrOₓ catalysts for CO₂ hydrogenation to methanol: the effect of pH on structure and activity

Abstract

With the growing necessity of achieving carbon neutrality in the industrial sector, the catalytic hydrogenation of carbon dioxide into methanol has been widely considered one of the key strategies for the utilization of captured CO₂. For this reason, the development of alternative catalysts such as ZnZrOₓ has attracted considerable interest, given its superior stability and versatility in comparison to the conventional Cu-based materials. In this work, ZnZrOₓ has been produced by a hydrothermal synthesis method at varied synthesis pH between 7 and 10 and a positive association between pH and catalytic CO₂ conversion is observed. At 2.0 MPa and 250 °C, ZnZrOₓ produced at pH 10 shows a methanol selectivity of 95% at a CO₂ conversion of 3.4%. According to characterization, basic pH conditions enable the formation of abundant t-ZrO₂ and the subsequent incorporation of Zn²⁺ into this phase, although the content of surface Zn does not increase between pH 8 and 10. Nevertheless, synthesis pH values can be correlated with surface oxygen content and CO₂ adsorption capacity, which could be important contributors to the higher catalytic activity observed as a result of higher synthesis pH values. However, upon synthesis at pH 10, an inferior selectivity to methanol is observed above 250 °C, as a possible result of the excessive formation of ZnO. Interestingly, this secondary phase can be prevented and the selectivity can be slightly improved by utilizing NH₄OH instead of NaOH in the hydrothermal method.