Modification of Cu/ZnO/SiO2 catalysts by high temperature reduction

In this overview results obtained in three laboratories on Cu/ZnO/SiO2 catalysts prepared by homogeneous deposition-precipitation are summarised. It is concluded that the system adopts a layered 'zincsilite' structure. In this copper-zinc hydrosilicate the Cu and/or Zn cations are located in octahedral sites and in the interlayer. When copper or zinc is present in excess of the zincsilite ratio (Cu-y + Zn1-y :Si = 0.75) or at relatively high copper content (y greater than or equal to 0.40), copper-zinc and copper hydroxocompounds are formed, the latter yielding CuO after calcination. Cu/ZnO/SiO2 shows increased activity in ester hydrogenolysis and methanol synthesis reactions as a function of reduction temperature, in contrast to the unpromoted catalyst. Inert treatment at 700 K significantly decreases activity, whereas subsequent reductive treatment restores the high activity mode. The copper metal surface area of the promoted catalyst increases modestly with reduction temperature when the catalyst is carefully evacuated preceding a N2O chemisorption measurement. Significantly lower surface areas result after prolonged evacuation at higher temperatures. It was shown using infrared spectroscopy that upon high temperature (700 K) reduction cationic copper sites are created. Investigation of Cu-63/(ZnO)-Zn-68/SiO2 by low energy ion scattering demonstrates that the surface of the catalyst reduced at 700 K is enriched in zinc. In the 600 K reduced catalyst the zinc enrichment was less prominent. It is proposed that the Cu-ZnO interface plays a crucial role in catalysing the reactions studied. It is not yet possible to decide between two models arriving at a situation where this interface is enlarged: (1) Reversible formation of flat epitaxial particles upon high temperature reduction from the fraction of the copper dissolved in the mixed oxide catalyst precursor phase. (2) Migration of partly reduced ZnO on top of copper. (C) 2000 Elsevier Science B.V. All rights reserved.