The transmission infra-red technique has been used to investigate the surface of tin(1v) oxide as a function of evacuation temperature in the range 293-773 K. Molecular water is largely removed by evacuation at 320 K and is fully removed at 473 K. Hydrogen-bonded hydroxyl groups are present after evacuation at 773 K, and isolated hydroxyl groups exhibit a stretching frequency at 3640 cm-'. Room temperature carbon dioxide adsorption has been studied as a function of thermal pretreatment in the range 320-618 K and the spectra obtained indicate a surface carbonate and a surface bicarbonate. The adsorption of carbon monoxide has been investigated in a similar 1IIsu1lzeT. A carbonate species is proposed from the infra-red data, and the partial reduction of the tin(w) oxide to a tin(n) species has been confirmed by tin-119 m Mossbauer spectroscopy. No evidence was obtained for the physical adsorption of either carbon dioxide or carbon monoxide.

The tin@) oxide surface is strongly oxidising towards smaI1, reactive organic molecules. Methanol is chemisorbed to give methoxy groups but these are readily oxidised to a surface formate at temperatures >320 K. Acetone and acetaldehyde are adsorbed predominantly as acetates but some evidence is found for an en01 form which may be responsible for the rapid deuterium exchange between hexadeuteroacetone and surface hydroxyl groups. The spectra of the formate and acetate structures are confirmed by adsorption of formic and acetic acids, where evidence is also found for undissociated acid molecules bonded to the surface. As expected the acetate is thermally more stable than

Infrared transmission spectroscopy has been used to study the adsorption of CO, NO and CO + NO mixtures on tin(Iv) oxide gels containing CrlIr, Mn", FelI1, CoII, Nil1 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 2200-2180cm-I for all the oxidised gels (except the FelIrexchanged sample). The increase in absorption frequency above that of the gas phase value (2143 cm-l) 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 CulI 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", FelI1 and Coll samples. In contrast, exposure of CO-reduced Mn", FerI1, CoI1 and NiI1 samples to CO+Oz mixtures resulted in the formation of unidentate carbonate complexes bound to transition metal ion sites. Nitric oxide is chemisorbed on all gels except the Mnrl-exchanged sample, but the nature of the chemisorbed species varies. All the samples catalysed the CO-NO reaction, and physisorbed C 0 2 was present in the Mn", FelI1, ColI and NilI samples as well as physisorbed N20 in the case of Mn". 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 FerlI) show a marked selectivity for reduction of NO to N20, whereas the Cu"exchanged gel, which preferentially adsorbs CO, exhibits a similar selectivity for reduction to Nz.

The carbonyl infrared spectra of catalysts prepared by adsorption of Rhs(C0)16 onto ligandmodified silica have been assigned with the assistance of 13C0 substitution experiments. I n the fully carbonylated state the catalysts contain Rh(CO)2 groups and partial decarbonylation leads to the formation of Rh(C0) groups. On the basis of infrared spectroscopic and other data it is deduced that the rhodium is not present as hexarhodium clusters but as mononuclear surface complexes, L,Rh(C0)2 and LmRh(CO), where L are surface attached ligands and n and m are unknown.

For carbon monoxide adsorbed in the coordination spheres of five transition metal cations (Cr3+, Mn3+, Co2+, Ni2+, Cu+) in the tin(1v) oxide surface, correlations are found between the stretching frequency and the (calculated) electric field strength at the adsorption site and between the stretching frequency and the integrated band intensity. The bonding between carbon monoxide and the adsorption sites present on metal oxide surfaces is discussed in view of these correlations.