Development of crystallinity and photocatalytic properties in porous TiO2 by mild acid treatment

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Abstract
Crystallization of amorphous mesoporous TiO2 synthesized by a surfactant-templated sol–gel route has been investigated by different methods in order to generate materials with photocatalytic properties. Crystallization by hydrothermal treatment is not a convenient alternative, as it leads to a strong reduction of the textural properties due to excessive coalescence of the inorganic framework. A new crystallization route, based on the treatment of m-TiO2 xerogels with acid–ethanol mixtures under reflux, has been investigated. Sulfuric and phosphoric acids cause severe structural and porous damage, whereas nitric and hydrochloric acids lead to the development of TiO2 photocatalysts with convenient textural properties. These last two acids produce the selective crystallization into anatase, the most active phase of titania in photocatalysis, with small nanocrystals being present within the pore walls. The materials so obtained are characterized by exhibiting high surface areas (210–260 m2 g−1), with an important contribution of microporosity and good photocatalytic activity for trichloroethylene degradation in the aqueous phase. In particular, the best balance between the textural and photocatalytic properties was achieved when the crystallization was carried out by treatment with a 0.5 wt% HCl–ethanol mixture.