Thermal evolution of the adsorbed methoxy species on CexZr1-xO2 solid solution samples:: a FT-IR study

Thermal reduction of a series of CexZr1-xO2 solid solution samples by methanol adsorbed at room temperature was investigated using infrared spectroscopy (IR), Methoxy species resulting from methanol dissociation and adsorbed as on-top or bridging species, either on Zr4+ or Ce4+ ions, are well differentiated. Upon the thermal treatment, Ce4+ sites appear to be exclusively the reactive ones through their reduction into Ce3+. First, on-top methoxy species adsorbed on Ce4+ sites are oxidised to mobile formate species in the 423-473 K temperature range, with the subsequent partial cerium reduction. Second, bridging methoxy species and formate ones decomposed in the 473-523 K temperature range through processes involving CO, CO2, H-2, H2O gaseous evolution. The reduction of surface cerium ions is then complete, The production of H-2 molecules was established by a complementary temperature programmed desorption study. The different temperature range at which mixed oxide reduction and hydrogen evolution take place demonstrates that the catalysts are nor reduced by H-2 uptake, but by framework oxygen consumption, due to methoxys oxidation into formate species, (C) 1999 Elsevier Science B.V. All rights reserved.