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In the sol-gel Cu/TiO 2 system, nanocrystalline brookite, which was stable even after annealing the sample at 400 °C, was generated in high concentrations. This was achieved with copper chloride as copper precursor. The respective samples had a light absorption band at 562 nm that corresponded to an energy bandgap of 2.30 eV, which is smaller than the values reported for titania. When copper sulfate was the precursor, brookite was absent, but nanocrystalline anatase, the main phase, was stabilized even at 800 °C by sulfate ions. After annealing the samples at low temperatures, copper was well dispersed for 1 wt % copper, but part of it segregated forming copper compounds when the copper concentration was 10 wt %. By annealing the samples at 800 °C, copper oxidized to CuO; except in the 10 wt %-copper sample prepared with copper sulfate, where it formed antlerite.

Adsorption isotherms of aromatic BTX (benzene, toluene, and p-xylene) hydrocarbons on pure and Ag-doped SiO 2 microporous substrates synthesized by the sol-gel procedure have been measured by the gas chromatographic technique at several temperatures in the range between 423 and 573 K. Both Langmuir and Dubinin adsorption models can reasonably fit every set of BTX adsorption data at all temperatures. The uptake degree of these hydrocarbon adsorptives on both SiO 2 and Ag/SiO 2 microporous substrates is temperature-dependent. Additionally, the degrees of interaction of BTX molecules with SiO 2 and Ag/SiO 2 microporous walls have been investigated by evaluation of the corresponding isosteric heats of adsorption (q st ) by means of a Clausius-Clapeyron-type equation. The isosteric heats of adsorption of BTX hydrocarbons at different adsorbate loadings have been inferred from the adsorption isotherms and found to follow the decreasing sequence q st (benzene) < q st (toluene) < q st (p-xylene). Isosteric heats of adsorption of BTX compounds on SiO 2 microporous substrates have been found, in general, to increase with an increase in the adsorbate loading as a consequence of the cohesive interactions between adsorbate molecules. Addition of Ag atoms to the SiO 2 structure helps to homogenize the magnitudes of the isosteric heats of adsorption of the BTX compounds.

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