Abstract:
Infrared transmission spectroscopy has been used to study the adsorption of CO, NO and CO + NO mixtures on tin(IV) oxide gels containing CrIII, MnII, FeIII, CoII, NiII and CuII 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 FeIII 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 CuII exchanged gel which was bonded via oxygen. Bands due to bidentate carbonate complexes associated with transition metal sites were also observed for the oxidised MnII, FeIII and CoII samples. In contrast, exposure of CO-reduced MnII, FeIII, CoII and NiII 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 MnII-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 MnII, FeIII, CoII and NiII samples as well as physisorbed N2O in the case of MnII. 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 (CoII, NiII and FeIII) show a marked selectivity for reduction of NO to N2O, whereas the CuII- exchanged gel, which preferentially adsorbs CO, exhibits a similar selectivity for reduction to N2.
