Relationship concerning the nature and concentration of Fe(III) species on the surface of TiO2 particles and photocatalytic activity of the catalyst

There have been contradictory results concerning the effect of doping TiO2 with Fe(III) upon its photocatalytic activity. We believe that this is due to the method used in adding the Fe(III) to TiO2. This paper addresses this issue by using a precise adsorption process for loading stable aqueous suspensions (sols) of TiO2 doped with Fe(III). In this manner, we control the speciation of Fe(III) on the surface of TiO2 and subsequently use these loaded sols to produce stable thin-film photocatalysts. Adsorption isotherms for these systems at different pH (2.0 and 2.5) values showed that both pH and the concentration of Fe(III) in solution influence the adsorption density of Fe(III), the stability of the resulting sols, and the speciation (degree of polymerization) of Fe(III) ions on the surface of the TiO2 nano-particles. UV-Vis spectroscopy on selected systems was the technique employed to confirm these results. The pore structure of the resulting gels (xerogels) prepared from these sols, as well as thermal stability of porous materials prepared by firing the xerogels have been determined by measuring N2 adsorption as a function of firing temperature. Thermal analysis (TG–DTA) studies on xerogels obtained from some of these systems showed that the presence of Fe(III) species on the TiO2 retards the conversion of anatase into rutile and therefore increases the thermal stability of the gel microstructure. Lastly, porous films, resulting from the casting of the sols on glass rings and subsequent firing, were used in photocatalytic studies. In the presence of UV light, these supported films were capable of degrading ethanol in the gas phase. While photocatalytic activity for Fe(III) loaded TiO2 films is less than for TiO2 films without Fe(III), the speciation of Fe(III) on the surface directly influences the behavior of the intermediate—acetaldehyde—formed in this reaction. Since we have only evaluated the effect of iron speciation on the degradation of ethanol, other target species may be influenced either advantageously or deleteriously depending upon the state of Fe(III) on the surface of TiO2 photocatalysts.