Structure and reactivity of zinc–chromium mixed oxides. Part 3.—The surface interaction with carbon monoxide

Abstract

The reactivity towards carbon monoxide of a series of Zn–Cr non-stoichiometric 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^–1 and the C—O stretching frequency in the range 2192–2205 cm^–1. The strongest active site for CO coordination are surface CR³⁺ ions acting as σ-acceptors, but also capable of d–π backdonation. The number of active sites for CO coordination is very low in the stoichiometric spinel ZnCr₂O₄(inactive in methanol synthesis), whereas it grows in the zinc-excess non-stoichiometric spinels owing to modifications of the ‘collective’ properties of the solid introduced by the presence of the excess zinc into the spinel lattice.