Thermal evolution of the adsorbed methoxy species on Ce xZr 1− xO 2 solid solution samples: a FT-IR study

Abstract

Thermal reduction of a series of Ce x Zr 1− x O 2 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 Zr 4+ or Ce 4+ ions, are well differentiated. Upon the thermal treatment, Ce 4+ sites appear to be exclusively the reactive ones through their reduction into Ce 3+. First, on-top methoxy species adsorbed on Ce 4+ 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, CO 2, H 2, H 2O 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 not reduced by H 2 uptake, but by framework oxygen consumption, due to methoxys oxidation into formate species.