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The nature and amount of active sites on the surface of nano-sized SiO 2 -TiO 2 oxides were studied by FTIR spectroscopy and back-titration methods. Increasing the TiO 2 content in the SiO 2 -TiO 2 composites increased the amount of activated surface H 2 O and adsorbed CO 2 . This increased the amount of active basic centers on the oxide surface and caused the fi rst of two observed mechanisms for benzaldehyde adsorption (with and without its activation) to begin to prevail.Introduction. Catalysis in organic chemistry is currently focused on the potential of using nano-sized materials. Surface effects in nano-sized systems can be especially prominent because the amount of coordinatively unsaturated cations in them reaches tens of percent; on the microparticle surface, units and fractions of percent. Surface acidic and basic sites of various strengths are responsible for the scientifi c and practical interest in using them as catalysts of organic reactions because the catalytic activity of a heterogeneous catalyst (increase of the reaction rate) is known to depend on the amount and nature of active sites involved in the catalytic process [1].It was shown earlier [2] that active sites on the surface of nano-sized oxides (nano-oxides) are metal atoms surrounded by O atoms, free OH groups, coordinated H 2 O, and also O atoms of carboxylic groups of metal hydroxycarbonates formed by adsorption of atmospheric CO 2 .Research on the adsorption (coordination) of organic molecules on the surface of nano-oxides can provide information on intermolecular and chemical interactions occurring on the solid surface and on the amount and nature of active sites on the nano-oxide surface and also suggest the mechanism of their catalytic activity. This would enable the optimum catalyst for a given reaction to be selected advantageously [3]. We studied the Biginelli reaction, which is the condensation of urea, acetoacetic ester, and a substituted aldehyde and produces 4-arylsubstituted dihydropyrimidines, analogs of nifedipine-type cardiotropic drugs. As shown earlier, metal nano-oxides catalyze the Biginelli reaction. This was explained by activation of the reagents after adsorption on the nano-oxide surface [4].The goal of the present work was to develop approaches for investigating the surface of metal nano-oxides that enabled the nature and amount of surface active sites to be evaluated and a prediction about the effectiveness of using them as catalysts to be made.Experimental. The sorbents were mixed Ti-Si oxides of various compositions and morphologies (core-shell, coprecipitated, hybrid xerogels produced by a sol-gel method) in addition to Si nano-oxide [5,6]. The primary particles in these samples were 10-50 nm in size and were clumped into loose aggregates up to 10 μm in size.Given amounts of tetraethoxysilane and titanium tetraisopropoxide were mixed in i-PrOH, stirred vigorously, and treated dropwise with a H 2 O:EtOH mixture in order to prepare the coprecipitated SiO 2 -TiO 2 composites. The order of mixing the reagents was...

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