Study Of Cu/CeO₂ Catalyst For Water-Gas Shift Reaction

The purpose of this work is trying to get an excellent catalytic material for monolithic WGS catalyst.Cu/CeO2 catalysts are prepared by co-precipitation .The effects of preparation conditions including reduction pretreatment, copper content, calcinations temperature on the catalytic activity are investigated. Catalysts are characterized by means of XRD, BET, TPR, TPD, XPS, EPR technique, and the WGS reaction mechanism over Cu/CeO2 is discussed. Firstly, the low-temperature activity of catalyst is affected by reduction temperature in pretreatment process. Catalyst reduced at 200℃, similar to the one without reduction pretreatment shows the highest activity. As reduction temperature is increased, the activity decreases. Therefore, prior reduction is not necessary for Cu/CeO2 catalysts.Secondly, copper content has significant effect on the low-temperature activity, but has little on the high-temperature activity. The low-temperature activity increases linearly with copper content from 10at% to 50at%, reaching the highest value at 50at% copper content, and then decreases gradually with the farther increase of copper content. Amorphous and nanocrystalline CuO are responsible for WGS activity. Bulk CuO particles contribute little to the catalytic activity.Thirdly, the low temperature activity is related to crystallinity of catalysts. Catalyst calcined at 400℃ has suitable CeO2, CuO crystallite size, and shows the highest activity. Poor crystallinity of CeO2 and grown CuO particle size both lead to decreases of catalytic activities, which correspondingly attribute to lower and higher calcinations temperature.Fourthly, the XRD and TPD results obtained from reduced catalysts show that, after in situ reduction at 200℃, CuO is reduced to metallic Cu, on which CO adsorb and generate Cu-CO formation. Additionally, part of CeO2 are reduced to Ce3+ under conditions of WGS, which indicates that ceria may play a role of transporting oxygen in reaction procedure.Finally, the redox mechanism over Cu/CeO2 water-gas shift catalyst is proposed: CO adsorbed on the copper reacts with oxygen transported from CeO2 , releasing CO2 and generating oxygen vacancy on the surface of CeO2, and then water dissociates on the oxygen vacancy site of CeO2 to H2 and atomic oxygen which re-oxidizes the CeO2.In this work, Cu/CeO2 catalysts show excellent catalytic performance for WGS reaction. Compared to iron-based HTS catalyst (FBD), Cu/CeO2 catalysts are characterized by higher catalytic activity at low-temperature and a wider activity temperature range (200℃-500℃). Cu/CeO2 catalyst is expected to be the activity species for monolithic WGS catalyst.