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

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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.

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