(42f) Synthesis of Titania Photocatalyst in Supercritical CO2: Effect of Metallic Precursor and Hydrolysis Reactant

Titanium dioxide is a well known photocatalyst and it has been extensively tested in environmental applications dealing with photodegradation of organic pollutants in water and air [1-3]. A numerous of organic pollutants in water have been degraded by photocatalytic processes with TiO2 including herbicides, aniline, oil and dyes [4]. More than water and air depollution applications, photocatalysis using TiO2 is widely used in the self-cleaning of TiO2 covered surfaces [1] and in photochemical solar cells [5].

The Supercritical Fluid (SCF) method is a suitable method for the synthesis of titania nanoparticles. In these proceses, crystallization step is very fast and particle growth can be avoided and/or controlled. TiO2 (rutile or anatase) nanoparticles have been prepared by hydrothermal synthesis from stabilized TiCl4 solutions [6]. Also, some researchers have used the combination of hydrolysis and polycondensation of titanium tetra-isopropoxide, (TTIP), in supercritical alcohol or mixtures alcohol/CO2 [7]. The conventional hydrothermal or sol-gel methods yield larger particles and hence the SCF technology is the most viable one for producing nanoparticles with the shortest residence time.

In the present work, TiO2 nanoparticles have been synthesized in supercritical carbon dioxide (SC-CO2) by thermohydrolysis of two different precursors diisopropoxititanium bis(acetylacetonate) (DIPBAT) and titanium tetraisopropoxide, (TTIP). Three different hydrolysis reactants have been tested (ethanol, isopropyl alcohol and water), and experimental results are presented for different molar ratios. The reaction takes place at 300 ºC and 20 MPa, both selected from a previous work, already published [8]. The photocatalytic activity of the obtained powders has been determined for the oxidation of methylorange, as a prototype molecule and compared with that of commercial TiO2. Experimental results show the influence of the acetylacetonate ligand in the precursor molecule on the structural properties of the obtained powder, and therefore on its photocatalytic activity. The photocatalytic activity of TiO2 is influenced by the crystal structure (anatase and/or rutile), surface area, size distribution, porosity, surface hydroxyl group density, etc.

Moreover the results obtained with the different hydrolysis reactants suggest significant differences on reaction mechanism for TTIP and DIPBAT, related with the chemical stability of the acetylacetonate.

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