Synergetic effect of Cu active sites and oxygen vacancies in Cu/CeO2–ZrO2 for the water–gas shift reaction

Abstract

A series of Cu/CeO₂–ZrO₂ catalysts with different Ce/(Ce + Zr) molar ratios and Cu loadings were prepared by a simple citrate sol–gel method, characterized by various techniques, and evaluated for water–gas shift (WGS) under H₂-rich and low H₂O/CO molar ratio conditions of the sorption enhanced WGS process. It was shown that the oxygen vacancy concentration and the Cu dispersion were important structural factors affecting the catalytic activity of Cu/CeO₂–ZrO₂, but neither of them alone was capable of correlating with the turnover frequency (TOF) for WGS based on kinetic analysis. By contrast, a positive linear correlation was established between the TOF and the ratio of oxygen vacancy concentration to Cu dispersion, demonstrating the synergetic effect of Cu active sites and oxygen vacancies for WGS. The stability of Cu/CeO₂–ZrO₂ was found to be dependent on the crystal structure of CeO₂–ZrO₂ solid solution, and the cubic to tetragonal phase transformation of CeO₂–ZrO₂ led to a decreased stability. Among all Cu/CeO₂–ZrO₂ investigated, the catalyst with a Ce/(Ce + Zr) molar ratio of 0.6 and a Cu loading of 50 wt% showed the highest activity and stability under 20% H₂, 40% CO and 40% steam in a gas hourly space velocity of 20 000 h⁻¹ at 350 °C, with the CO conversion slightly varying from 77% to 75% over 60 h on stream and meanwhile 100% selectivities to H₂ and CO₂.