Redox Behavior of Copper Oxide /Zinc Oxide Catalysts in the Steam Reforming of Methanol studied by in situ X-ray Diffraction and Absorption Spectroscopy

The bulk structure of copper in various binary Cu/ZnO catalysts for steam reforming of methanol under activation and working conditions is studied by in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). The evolution of bulk phases from CuO/ZnO precursors during activation with hydrogen was studied using temperature programmed reduction (TPR) (448 – 523 K, 2 vol-% H2 with and without water vapor). With decreasing copper content the onset of reduction is shifted from 473 K (pure CuO) to 443 K (40 mol-% Cu) accompanied by a decrease in Cu crystallite sizes (from 210 Å to 40 Å). Using time -resolved in situ XANES measurements at the Cu K edge during TPR experiments the degree of reduction was monitored. It is shown that Cu(I) oxide forms prior to Cu. Adding oxygen to the feed gas leads to the formation of a mixture of Cu(II) and Cu(I) oxide accompanied by a complete loss of activity. After switching back to steam reforming conditions a higher activity is attained while the catalyst shows an increased Cu crystallite size (up to 40%). EXAFS measurements at the Cu K and the Zn K edge indicate a structural disorder of the Cu particles in the medium range order based on increasing Debye-Waller factors for higher Cu-Cu shells . Furthermore, the dissolution of Zn atoms (up to ~ 4 mol-%) in the copper lattice is detected. Upon oxidation/reduction cycles activity is increased, the disorder in the copper particles increases, and Zn segregates out of the copper bulk. A structural model is proposed which ascribes the enhanced activity to structurally disordered (strained) copper particles due to an improved interface interaction with ZnO.