An EXAFS study of Cu/ZnO and Cu/ZnO–Al₂O₃: methanol synthesis catalysts

Cu K-absorption edge and EXAFS measurements on binary Cu/ZnO and ternary Cu/ZnO–Al₂O₃ catalysts of varying compositions on reduction with hydrogen at 523 K, show the presence of Cu microclusters and a species of Cu¹⁺ dissolved in ZnO apart from metallic Cu and Cu₂O. The proportions of different phases critically depend on the heating rate especially for catalysts of higher Cu content. Accordingly, hydrogen reduction with a heating rate of 10 K/min predominantly yields the metal species (> 50%), while a slower heating rate of 0.8 K/min enhances the proportion of the Cu¹+ species (∼ 60%). Reduced Cu/ZnO–Al₂O₃ catalysts show the presence of metallic Cu (upto 20 %) mostly in the form of microclusters and Cu¹⁺ in ZnO as the major phase (∼ 60%). The addition of alumina to the Cu/ZnO catalyst seems to favour the formation of Cu¹⁺/ZnO species.