Structure and reactivity of Zn-Cr mixed oxides. III: The surface interaction with carbon monoxide

The reactivity towards carbon monoxide of a series of Zn-Cr nonstoichiometric spinels (NSS) with an excess of zinc, active in the synthesis of methanol and higher alcohols, has been investigated by infrared spectroscopy and adsorption microcalorimetry. A fraction of the total CO taken up is irreversibly oxidized to surface carbonates by the chromate groups present. A second fraction is strongly but reversibly coordinated in the form of a 'carbon-down' CO complex, the heat of adsorption being between 80 and 50 kJ mol-' and the C-0 stretching frequency in the range 2192-2205 cm-'. The strongest active sites for CO coordination are surface Cr3+ ions acting as a-acceptors, but also capable of d-n backdonation. The number of active sites for CO coordination is very low in the stoichiometric spinel ZnCr,O, (inactive in methanol synthesis), whereas it grows in the zinc-excess nonstoichiometric spinels owing to modifications of the ' collective ' properties of the solid introduced by the presence of the excess zinc into the spinel lattice