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Adsorption behaviour and CO oxidation activity of and gold catalysts, prepared by TiO 2 -ZrO 2 -supported adsorption of gold colloids on the supports, have been investigated by pulse thermal analysis and di †use reÑectance FTIR spectroscopy. Structural investigations using transmission electron microscopy, X-ray di †raction and X-ray photoelectron spectroscopy showed that Au particles, mainly in the metallic state and with similar particle size distribution (2.0È3.0 nm), could be prepared on both supports, but the as-prepared samples contained carbonaceous impurities from the preparation procedure. Upon calcinations in 20 vol.% oxygen/He only slight sintering occurred up to 600 ¡C. Above 600 ¡C, the gold particles sintered on both supports. CO oxidation activity, studied by injection of CO pulses into an oxygen stream in the thermoanalyzer, depended strongly on the calcination temperature of the catalysts. Maximal activity was obtained for the catalysts calcined at 500 ¡C and 560 ¡C respectively. Catalysts calcined (Au/TiO 2 ) (Au/ZrO 2 ), at higher temperatures exhibited lower activity due to sintering of the gold particles. CO adsorption was found to be weak on both catalyst systems. No signiÐcant adsorption of and could be detected by pulse O 2 CO 2 thermal analysis. CO adsorption increased for catalysts calcined up to 200È400 ¡C. Calcination temperatures leading to maximal CO adsorption did not coincide with calcination temperatures a †ording maximal CO oxidation activity. Moreover, the removal of carbonaceous residues during calcination was found to be correlated to the activity. The results of the adsorption studies do not support a MarsÈvan Krevelen mechanism, where lattice oxygen would be involved. CO oxidation by the reaction of adsorbed CO with molecularly adsorbed oxygen seems more likely.

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Vanadia−silica mixed oxides were prepared via the sol−gel method involving acid catalysis together with prehydrolysis in order to achieve matching of the reactivities of vanadium(V) oxide triisopropoxide and tetraethoxysilicon(IV) precursors. Gelation was forced by the addition of basic solution. The as-received gels were supercritically dried by semicontinuous extraction with supercritical CO2 at 313 K (low-temperature aerogels). The effects of composition, aging, and calcination temperature on the chemical, structural, and textural properties of the solids were investigated. The oxides were characterized by N2 physisorption, XRD, vibrational spectroscopy, thermal analysis, UV−vis, and 51V NMR. The low-temperature vanadia−silica aerogels were mesoporous and highly disperse. The increasing V content from 5 to 20 wt % nominal V2O5 caused a gradual decline in V dispersion. For 30 wt % “V2O5” the continuous formation of V−O−V connectivity resulted in crystallization of V2O5. The effect of aging in basic medium confined to the textural properties, significantly increasing BET surface area and especially pore volume. The prepared aerogels revealed a marked lack of stability against both apolar solvents in the presence of peroxides and polar solvents. The marked thermal stability in air at ≤873 K, however, combined with mesoporosity and high V dispersion, render these solids promising catalysts for gas-phase reactions.