Understanding methanol synthesis from CO/H2 feeds over Cu/CeO2 catalysts

Abstract

Understanding copper-based methanol synthesis catalysts as a function of catalyst type and applied reaction conditions is an area of active industrial and academic research. In this work, results of a methanol synthesis study over a Cu/CeO2 catalyst using CO/H2 feeds are presented, using catalyst performance data combined with catalyst characterisation information (DRIFTS, XPS, TPR, XRD) obtained during start-up and under steady state methanol synthesis conditions. The results indicate that the active site and reaction mechanism for methanol synthesis over Cu/CeO2 are different from the conventional Cu/ZnO/Al2O3 catalyst, with CO, rather than CO2, being the carbon source for methanol. Fixed-bed micro reactors were employed to obtain catalyst performance data in discrete bed sectors using experimental spatial discretisation methods. Methanol synthesis activity over Cu/CeO2 from CO/H2 is preceded by transient CO2 formation, with the onset of methanol synthesis activity observed when the CO2 formation reaches its peak. Considerable differences between reactor bed sectors are observed during start-up, with CO2 formation, CO2 re-adsorption (as surface carbonates and formates), methanol formation and transient methanol decomposition occurring to different degrees and at different time scales.The interfaces of defective CeO2−x in contact with highly dispersed copper particles form the active site of the catalyst under steady-state methanol synthesis conditions, with the copper oxidation state being a combination of 0 and +1 states. The CeO2−x defect sites are formed during start-up upon reaction of CeO2 with CO. The CO2 thus formed leads to catalyst deactivation due to build-up of carbonate and formate species on the catalyst surface. A gradient of progressive poisoning/deactivation is observed going further down the catalyst bed, where the level of deactivation can be controlled by modifying the CO content in the feed. CO2 thus acts as a poison for the Cu/CeO2 catalyst, and addition of even low levels of CO2 to the feed leads to an evenly deactivated catalyst. During start-up in CO/H2, transient CO2 and H2O formation is also observed in the case of Cu/ZnO/Al2O3, ascribed to partial reduction of ZnO, similar to the partial reduction of CeO2 as seen in Cu/CeO2. The key difference between the two systems is the absence of CO2 and H2O re-adsorption on Cu/ZnO/Al2O3, resulting in stable methanol production distributed equally over the catalyst bed. Introduction of CO2 to the CO/H2 feed in case of Cu/ZnO/Al2O3 leads to reduction of Cu+ to Cu0 and a change in methanol synthesis mechanism from CO to CO2 hydrogenation.