Comparative surface studies of high-Zn-level and commercial Cu/ZnO/Al2O3 catalysts

Abstract

By altering component formulation and modifying a conventional coprecipitation procedure, we have synthesized a high-Zn-, low-Cu-level Cu/ZnO/Al2O3 catalyst which showed a 54.4 mmol/(mL catalyst h) of methanol yield and 66% CO conversion measured at 230°C, 8 MPa, and 10000 h-1. Comparative surface studies of this catalyst and the catalyst prepared according to the commercial standard (low-Zn-, high-Cu-level) were performed using microreactor study, gas (N2 and N2O) adsorption and X-ray photoelectron and Fourier transfer infrared spectroscopies and temperature-programmed desorption. It was found that the high-Zn-level catalyst had better performance, including a substantial (15-25%) increase in methanol yield and some other advantages under identical conditions imitating industrial process. The high catalytic activity is ascribed to the high concentrations of Cu, Zn, and oxygen vacancies detected on the surface of the high-Zn catalyst. It is proposed that the active center is Cu□ZnO and that the catalytic process may follow a carbonate-formate-methoxy-methanol mechanism. The high Zn level of the catalyst facilitates the hydrogen heterolysis with the presence of Cu, resulting in the high concentration of oxygen vacancies, as well as the existence of more Cu1+ ions on the surface, and thus leading to the increased CO/CO2 adsorption, activation, and conversion.