CO2 Conversion to Alcohols over Cu/ZnO Catalysts: Prospective Synergies between Electrocatalytic and Thermocatalytic Routes

Abstract

The development of efficient catalysts is one of the main challenges in CO₂ conversion to valuable chemicals and fuels. Herein, inspired by the knowledge of the thermocatalytic (TC) processes, Cu/ZnO and bare Cu catalysts enriched with Cu⁺¹ were studied to convert CO₂ via the electrocatalytic (EC) pathway. Integrating Cu with ZnO (a CO-generation catalyst) is a strategy explored in the EC CO₂ reduction to reduce the kinetic barrier and enhance C-C coupling to obtain C₂₊ chemicals and energy carriers. Herein, ethanol was produced with the Cu/ZnO catalyst, reaching a productivity of about 5.27 mmol·g_cat^-1·h^-1 in a liquid-phase configuration at ambient conditions. In contrast, bare copper preferentially produced C₁ products like formate and methanol. During CO₂ hydrogenation, a methanol selectivity close to 100% was achieved with the Cu/ZnO catalysts at 200 °C, a value that decreased at higher temperatures (i.e., 23% at 300 °C) because of thermodynamic limitations. The methanol productivity increased to approximately 1.4 mmol·g_cat^-1·h^-1 at 300 °C. Ex situ characterizations after testing confirmed the potential of adding ZnO in Cu-based materials to stabilize the Cu¹⁺/Cu⁰ interface at the electrocatalyst surface because of Zn and O enrichment by an amorphous zinc oxide matrix; while in the TC process, Cu⁰ and crystalline ZnO prevailed under CO₂ hydrogenation conditions. It is envisioned that the lower *CO binding energy at the Cu⁰ catalyst surface in the TC process than in the Cu¹⁺ present in the EC one leads to preferential CO and methanol production in the TC system. Instead, our EC results revealed that an optimum local CO production at the ZnO surface in tandem with a high amount of superficial Cu¹⁺ + Cu⁰ species induces ethanol formation by ensuring an appropriate local amount of *CO intermediates and their further dimerization to generate C₂₊ products. Optimizing the ZnO loading on Cu is proposed to tune the catalyst surface properties and the formation of more reduced CO₂ conversion products.

Keywords: C2+ products; CO dimerization; CO2 conversion; Cu/ZnO; alcohols.