Structure and reactivity of zinc–chromium mixed oxides. Part 2.—Study of the surface reactivity by temperature-programmed desorption of methanol

Abstract

The interaction of methanol with zinc–chromium mixed oxides has been studied by temperature-programmed desorption (t.p.d.) in the range 430–770 K. Both the amount of methanol adsorbed and the amount of desorbed species in t.p.d. experiments, which can be taken as an indicator of the number of active sites responsible for methanol adsorption, are maximum for the sample with a Zn/Cr ratio of 1.000. The number of active sites increases for some catalysts after pretreatment in hydrogen. The stoichiometric spinel, ZnCr₂O₄, shows oxidizing properties only for the adsorbed methanol. Non-stoichiometric spinels have some properties typical of ZnO and others typical of the stoichiometric spinel, depending on the Zn/Cr ratio and the thermal treatment. For a spinel with Zn/Cr between 1.500 and 4.000 (the range of composition of industrial catalysts) the reducing treatment increases the adsorption power for methanol. The F.t.i.r. experiments showed that in the case of the stoichiometric spinel after adsorption of methanol, methoxy, formate and carbonate groups are present at the surface, but on the non-stoichiometric spinels only methoxy and formate groups are present. The desorption of CO₂ and H₂ at 590 K are attributed to the decomposition of surface formates and the desorption of CO and H₂ at the same temperature either to formates or to methoxy species. The desorption of CO₂ at 720 K is attributed to surface oxidation of formates to carbonates. Very likely the excess of zinc in the spinel, localized essentially in octahedral positions, is responsible for methanol adsorption on the surface of zinc–chromium mixed oxides.