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In situ X-ray absorption spectroscopy XAFS at the Cu and Zn K-edge has been used to unravel the Cu/Zn interaction and identify the possible active site of Cu-based methanol synthesis catalysts in the Cu/ZnO/SiO 2 ternary system. These highly dispersed silica supported catalysts, whose activity increases sharply as a function of the reduction temperature, were studied calcined, reduced at 200, 300, and 400°C, and for each reduction temperature under passivation/rereduction and methanol synthesis conditions. Results showed that the calcined form consists mainly of a mixed Cu/Zn hydrosilicate that is progressively transformed as the reduction temperature increases into (i) Cu metal particles, (ii) increasingly dispersed ZnO species on SiO 2 , and (iii) finally a Zn metallic phase forming segregated bimetallic CuÀZn α-brass alloy particles. These different structures and Cu/Zn interfaces may correspond to different active phases and activities in methanol synthesis. After reduction at 200 and 300°C, Cu 0 is likely composing most of the active phase, whereas above 300°C, the sharp increase in the number Zn 0 -based sites formed as a function of the reduction temperature could explain the major role played by this parameter in controlling the activity of these catalysts. The dynamic Cu/Zn interaction as a function of the temperature and gas environment pointed out in this ternary system may be at the origin of the existence of different and sometimes contradictory models to account for the mechanisms of the methanol synthesis.

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The evolution of Co-Al (1 : 1) hydroxycarbonate with a hydrotalcite-like structure during its consequent calcination in an inert gas flow and reduction in H 2 has been studied by means of X-ray diffraction, infrared spectroscopy, UV-VIS diffusive reflectance spectroscopy and magnetic susceptibility measurements. The oxidation of Co 2+ species to Co 3+ state during the calcination in the inert gas medium occurs simultaneously with the formation of a highly inverted anion-modified spinel-like structure. The samples reduced at 480 C and 620 C contain metallic cobalt particles. Ferromagnetic and paramagnetic contributions are distinguished and evaluated. The spontaneous magnetization of the sample reduced at 480 C increases monotonously with the temperature in the range of 78-300 K. The correlation of the results with earlier data on physico-chemical and catalytic properties has been discussed.