Effect of TiO2 acidic pre-treatment on the photocatalytic properties for phenol degradation

Photocatalytic oxidation of phenol was performed over acid pre-treated TiO2 prepared by a sol–gel method. Several oxoacids were used in the acid pre-treatment (nitric, sulfuric and phosphoric acids). Wide structural and surface characterisation of catalysts was carried out in order to establish a correlation between the oxoanion stability during calcination and the further photocatalytic behaviour of TiO2. Pre-treatment with sulfuric and phosphoric acids clearly stabilises TiO2 catalyst against sintering, maintaining anatase phase and relatively high surface area values with respect to untreated TiO2. On the other hand, nitric pre-treatment seems to favour sintering process. Acid treatment leads to an excess of adsorbed protons (Brönsted acid sites) incorporated to the TiO2 surface. During calcination, the elimination of these hydroxyl groups would be the responsible for the generation of a highly defective material that readily losses the oxygen ions from their surface. Different stability of adsorbed oxoanions would stabilise the anatase crystal phase as well these oxygen vacancies formed during calcination, preventing at the same time the sintering process. The occurrence of oxygen vacancies could favour the appearance of small rutile phase dispersed onto the surface of an anatase matrix performing the photoinduced electronic process. Best photocatalytic behaviour is found for pre-sulfated TiO2 samples calcined at such high temperature for which sulfates have been eliminated.