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Abstract: The analysis of FTIR absorption spectra of CO and of12CO-13CO isotopic mixtures is reported for gold catalysts supported on a number of oxides. Relationships between the nanostructure and the CO adsorbing properties of the different gold catalysts are discussed. In particular, on titania supported gold, where particles of metallic nature are present, two kinds of surface sites do adsorb CO, the 6-coordinated corners and the 7- coordinated edges, while the terrace sites, 8 coordinated, are not able to adsorb even at 90 K. Moreover, the adsorbing sites are not isolated, but mutually interacting. On zirconia supported gold, the adsorbing sites, mutually interacting, are exposed at the surface of non metallic gold nanoclusters. Almost isolated and negatively charged gold adsorbing nanoclusters have been highlighted on the sample supported on reduced ceria, and, finally, isolated cationic sites on oxidized gold supported on a mixed ceria-titania oxide have been detected.

Gold‐loaded zirconia and sulfated zirconia catalysts were tested in the low‐temperature water gas shift reaction. The samples were characterized by N2adsorption analysis, temperature‐programmed reduction, X‐ray diffraction, pulse‐flow CO chemisorption, FTIR spectroscopy, and high‐resolution transmission electron microscopy. A reference catalyst, Au/TiO2, provided by the World Gold Council was investigated for comparison. CO chemisorption and FTIR data indicate the presence of only highly dispersed gold clusters on the sulfated sample and both small clusters and small particles on the non‐sulfated sample. Both gold–zirconia catalysts are much more active than the Au/TiO2reference sample over all the temperature range investigated. The sample prepared on sulfated zirconia exhibits higher stability than the catalyst on unmodified zirconia. The prominent role in the water gas shift reaction of gold clusters in close contact with the support was deduced.

Three Au/TiO2catalysts, with the same Au loading and with different particle sizes, were prepared by the deposition–precipitation method followed by calcination at three different temperatures, 473, 573, and 873 K. The mean diameters of Au particles were 2.4, 2.5, and 10.6 nm, respectively. On all the samples the CO adsorption and different CO–O2interactions were examined by FTIR at 90 K and at room temperature. The higher catalytic activity on CO oxidation found for the samples calcined at 473 and 573 K is related to the higher concentration of step sites over the Au surfaces and to a higher concentration of step sites at the borderline with the support. At 90 K, CO and molecular oxygen are competitively adsorbed on step sites. By CO preadsorption on hydrated catalysts, the reaction with O2gives CO2already at 90 K, while by oxygen preadsorption the reaction is completely inhibited, unless moisture is present in the gas phase. An effect of CO coadsorption has been evidenced on water dissociation on gold sites or at the interface with the support, producing atomic hydrogen. The hydrogen reacts with the oxygen, producing a reactive species, quickly dissociated in nascent oxygen and OH groups. Moreover, the reaction with 18O2at 90 K in the presence of moisture produces only C16O18O, giving evidence that there is no direct participation of oxygens of the support and of the water in the reaction. At room temperature, other reaction channels become operative, involving oxygen species activated on the support, as shown by the extensive exchange reactions occurring with the support oxygen.

The interaction of NO at room temperature (RT) with pure and Cr-doped SnO2self-supporting pellets, pretreated in dry oxygen at 673-873 K, has been studied by FT-IR spectroscopy. With both pure and doped pellets treated at 673 K, NO at pressures ⩽ 10 N m−2causes a reaction (NO⇌NO++ e−) that is responsible for the increase of a broad electronic absorption, which is irreversible to evacuation at RT, but no peaks due to surface species appear. For pressures 10 N m−2, peaks due to nitrites, nitrosyls and N2O appear, without further increase of the electronic absorption, which actually becomes completely reversible at RT, while surface nitrites and nitrosyls are irreversible under evacuation. The formation of these species is responsible for the shift of the equilibrium involving electrons. The electronic absorption is due to the overlap of two bands: one at 0.1–0.18 eV, assigned to the electron transition from monoionized oxygen vacancies to the conduction band; the other, at 0.45–0.6 eV, assigned to a similar electronic transition involving oxygen divacancies. Thermal treatment at 873 K affects the electronic response of both the pellets. On Cr-doped pellets the electronic response is completely killed. SnO2pellets show the behaviour described for pretreatment at 673 K, except for the shape of the electronic absorption: the relative intensities of the two overalapping bands at 0.1–0.18 and 0.45–0.6 eV bands change in favour of the latter.