Solvent-free synthesis of nanostructured TiO2 in a continuous flow spinning disc reactor for application to photocatalytic reduction of CO2

Nanostructured TiO₂ catalysts were synthesised via the sol-gel method using solvent-free titanium (IV) n-butoxide and acidified water at pH 1 in a continuous flow spinning disc reactor (SDR). The influence of disc rotational speed, total flow rate of the reagents, and molar hydrolysis ratio (molar ratio of the acidified water to the titanium (IV) n-butoxide) on the particle size, phase distribution, band gap energy and photocatalytic activity for CO₂ reduction was studied. Increasing the disc rotational speed from 400 ​rpm to 1400 ​rpm results in highly sheared, uniformly mixed thin films where small particles (up to ca. 40 ​nm mean diameter) with narrow particle size distribution (polydispersity index of up to 0.5) are formed even at the lower molar hydrolysis ratio of 113. Increasing the molar hydrolysis ratio from 113 to 301 favours anatase phase transformation to rutile phase, thus improving photocatalytic activity. Larger TiO₂ particles from the SDR are associated with an increase in their band gap energy whilst doping with copper narrows the band gap energy from 3.00 ​eV down to 2.53 ​eV.

The photocatalytic performance of the TiO₂ nanoparticles was evaluated for CO₂ reduction in the form of bicarbonate ions using a meso-structured photocatalytic reactor at a TiO₂ loading of 0.5 ​g L^-1 and flow rate of 4 ​mL min^-1. A formate production rate of 500 ​μmol ​g^-1 ​h^-1 is achieved after 2 ​h of irradiation (λ ​= ​254 ​nm) on a bare TiO₂ catalyst, with no apparent trend observed with SDR operating conditions used in the production of the nanoparticles. However, for copper-doped TiO₂, there is a clear correlation between the anatase to rutile ratio and formate production rate.