Tracing the conversion of aurichalcite to a copper catalyst by combined X-ray absorption and diffraction

Abstract

EVER since X-ray sources first became available, the merit of deploying diffraction and absorption spectroscopic studies simultaneously has been acknowledged¹. Information on oxidation states and local (~6-Å radius) atomic environments is now obtained routinely from X-ray absorption measurements using synchrotron sources^2–4. Synchrotron radiation is also used commonly for high-resolution powder diffraction crystallography. We report here an instrumental arrangement that has allowed us to extract quantitative short- and long-range structural information on samples undergoing chemical change by measuring X-ray absorption spectra and X-ray diffraction patterns in situ and within a few seconds of one another, using a synchrotron X-ray source. To illustrate the combination of these techniques, we have followed the structural and chemical changes that occur within the layered mineral aurichalchite (Cu_5−xZn_x(OH)₆(CO₃)₂) when heated in dry air to ~ 450 °C. Despite marked changes in crystallinity, the local environment and electronic state of the Cu²⁺ ions remain unchanged, even when at ~ 450 °C the material is converted to a mixture of CuO and ZnO. Heating this mixture in H₂/N₂ produces an active catalyst for the water-gas shift reaction (CO₂ + H₂ → CO + H₂O), which our studies show to consist of small particles of copper metal (with some zinc incorporated) supported on ZnO.