Investigation of the role of oxygen vacancies in CuZn catalysts for the formation of higher alcohols from syngas

Abstract

The catalytic conversion of syngas to higher alcohols (C2+OH) has drawn widespread attention because it can alleviate the oil crisis and realize the clean and efficient use of coal. Nevertheless, the seesaw problem between C2+OH and total alcohol selectivity has not been solved. In recent years, catalytic processes involving oxygen vacancies have been widely investigated. However, the existing literature on this topic is fragmentary and unsystematic. Herein, we report a significant improvement in C2+OH synthesis by varying the oxygen vacancies on CuZn catalysts and systematically elaborate the structural and catalytic effects of the oxygen vacancies. Oxygen vacancy content shows almost linear positive correlations with many of the physical and chemical properties of the catalysts, such as SBET, Vp, Dp, Cu0 size, H2 consumption, and Cu0/(Cu0 + Cu+) ratio. The presence of oxygen vacancies drives catalysis in two ways. One, the dissociation of water adsorbed on oxygen vacancies causes the formation of hydroxyl groups, which interact with activated CO* to form surface formate (HCOO*) groups. This is followed by successive hydrogenation to form surface CHxO* intermediates. The CHxO* or CO* species are then coupled with surface CHx* to form ethanol on Znδ+ defect sites, realizing the growth of carbon chains. Two, oxygen vacancies drive electron transfer from ZnO to Cu, generating more Znδ+ defects and regulating the Cu0/(Cu0 + Cu+) ratio. The presence of electron-rich Cu is conducive to CO dissociation and adsorption, which promotes the formation of the critical surface intermediate CHx*. Consequently, the optimized CZ-0.80 catalyst exhibits exceptional catalytic performance, achieving 9.98 % CO conversion as well as ethanol and C2+OH proportions of up to 54.27 % and 59.95 %, respectively. Thus, by developing a new catalyst system that avoids the use of F-T elements such as Co or Fe for C2+OH synthesis from syngas, this work provides a new understanding of the growth of carbon chains.