On the role of BmimPF6 and P/F- containing additives in the sol-gel synthesis of TiO2 photocatalysts with enhanced activity in the gas phase degradation of methyl ethyl ketone

Abstract

The role of phosphate and fluoride ions brought by the BmimPF6 ionic liquid via hydrolysis during the sol gel synthesis of TiO2 has been investigated by replacing BmimPF6 with phosphorous- and fluorine-containing additives, phosphoric acid (PA) and sodium fluoride (NaF), respectively. Correlation between the physico-chemical properties and the photocatalytic behavior of the synthesized TiO2 materials was established. High crystallinity anatase TiO2 photocatalysts with controlled crystallite size and shape resulting from P/F-induced modifications displayed strongly improved photocatalytic activity and mineralization yield under UV-A illumination in the degradation of methyl ethyl ketone (MEK) taken as model Volatile Organic Compound (VOC), compared to the TiO2 Aeroxide P25 reference and sol-gel TiO2 counterpart synthesized in absence of any BmimPF6. We showed that the BmimPF6 ionic liquid could be efficiently substituted by cheap PA and NaF additives in an adequate ratio in the sol-gel synthesis for synthesizing TiO2 photocatalysts without altering the main physico-chemical properties and the photocatalytic activity. We proposed a similar synthesis mechanism in both ionic liquid- and P/F-assisted sol-gel syntheses of TiO2, involving a combined role of phosphate and fluoride ions. The reaction of phosphates with titanium hydroxide network in the early stage of the sol-gel synthesis resulted in a size control of TiO2 crystallites and thus in higher specific surface area, in favor of a higher MEK conversion rate, while the fluoride ions were hypothesized to cause an anisotropic TiO2 crystal growth during the aging step of the sol gel synthesis, in favor of a higher selectivity to CO2 through a favored adsorption of intermediate products of MEK degradation on the exposed TiO2 {001} facets.