New insights into methanol synthesis catalysts from X-ray absorption spectroscopy

X-ray absorption spectroscopic studies from several different groups provide a consistent structural picture of methanol synthesis catalysts. Copper metal is the principal Cu species detected in all in-situ XAS studies. A small amount of Cu(II) persisted in many samples reduced below 600 K, but it appears to be catalytically irrelevant. Cu(I) was not detected in any of the in-situ XAS studies. The Zn structure did not change in response to chemical treatments in any of the ZnO-containing methanol synthesis catalysts. We conclude that the slow approach to steady-state rate of methanol synthesis from CO/H-2 mixtures cannot result from changes in the Cu metal component of Cu/SiO2 catalysts. By eliminating this possibility, we provide indirect evidence for the proposal that initial induction periods reflect slow changes in the surface of the SiO2 or ZnO component in the catalysts. A bifunctional mechanism involving the formation of formate from CO on support hydroxyl groups would increase methanol synthesis rates as OH groups are formed by hydrolysis of Si-O bonds using the H2O formed in slow methanation side reactions. The bifunctional mechanism is not required for the synthesis of methanol from CO2-containing mixtures, because formate can form on Cu directly from CO2 and H-2. (C) 1999 Elsevier Science B.V. All rights reserved.