Tin oxide surfaces. Part 9.—Infrared study of the adsorption of CO, NO and CO + NO mixtures on tin(IV) oxide gels containing ion-exchanged CrIII, MnII, FeIII, CoII, NiII and CuII

Abstract

Infrared transmission spectroscopy has been used to study the adsorption of CO, NO and CO + NO mixtures on tin(IV) oxide gels containing Cr^III, Mn^II, Fe^III, Co^II, Ni^II and Cu^II as ion-exchanged cations. Exposure to CO results in the formation of linear physisorbed CO species exhibiting a single absorption band in the range 2200–2180 cm^–1 for all the oxidised gels (except the Fe^III exchanged sample). The increase in absorption frequency above that of the gas phase value (2143 cm^–1) is rationalised by considering the strong electric field due to the transition metal ion, and it was concluded that the carbon monoxide is adsorbed perpendicular to the surface, probably via carbon, at a cationic transition metal site, except for Cu^II exchanged gel which was bonded via oxygen. Bands due to bidentate carbonate complexes associated with transition metal sites were also observed for the oxidised Mn^II, Fe^III and Co^II samples. In contrast, exposure of CO-reduced Mn^II, Fe^III, Co^II and Ni^II samples to CO + O₂ mixtures resulted in the formation of unidentate carbonate complexes bound to transition metal ion sites. Nitric oxide is chemisorbed on all gels except the Mn^II-exchanged sample, but the nature of the chemisorbed species varies. All the samples catalysed the CO–NO reaction, and physisorbed CO₂ was present in the Mn^II, Fe^III, Co^II and Ni^II samples as well as physisorbed N₂O in the case of Mn^II. All samples showed spectra due to carbonate species consistent with a redox mechanism for the CO—NO reaction catalysed by these oxides. Additionally, those oxides with exchanged cations which adsorb NO from CO + NO mixtures (Co^II, Ni^II and Fe^III) show a marked selectivity for reduction of NO to N₂O, whereas the Cu^II- exchanged gel, which preferentially adsorbs CO, exhibits a similar selectivity for reduction to N₂.