Catalytic activity of γ-AlOOH (001) surface in syngas conversion: Probing into the mechanism of carbon chain growth

Identifying the exact mechanism of C-C chain formation in syngas conversion to C-2 oxygenate is a significant yet challenging issue. Based on a detailed density functional theory study, we find for the first time that gamma-AlOOH (0 0 1) surface not only has specific activity for the carbon chain growth but also prefers an unconventional route for syngas conversion to ethanol. Our results reveal that the optimal route starts with CO hydrogenation to CHO, followed by reacting with CO to form key intermediate CH and release CO2, then the initial C-C chain is formed via CO insertion into CH along with conquering a 0.76 eV barrier and exothermic by 1.90 eV, finally CHCO undergoes successive hydrogenation to produce ethanol in which the step of CH formation is identified as the rate-determining step of the overall process. In addition, as determined from the Bader charge analysis, carbon chain growth proceeding by CO insertion into CH is more facile. Moreover, the arising of CO2 during the process of syngas conversion to ethanol is inevitable, which is well consistent with our previous experiments. Notably, compared to our previously reported gamma-AlOOH (1 0 0) surface, the gamma-AlOOH (0 0 1) surface is inclined to trigger syngas conversion because the more saturated adsorption sites on the gamma-AlOOH (0 0 1) surface lower the barriers associated with corresponding steps by weakening the absorption of key intermediates with suitable binding energy. The insights might be valuable for rational tailoring and designing of better, stable catalysts with superior reactivity involved in syngas conversion.