Increasing the photoefficiency in heterogeneous photocatalysis through controlled periodic illumination

Heterogeneous photocatalysis is an advanced oxidation process for water purification. Ultraviolet illumination of titanium dioxide powders in water catalyzes reactions that completely mineralize many dissolved organic compounds. Low photoefficiencies limit the possibilities for implementing this technology on a large scale. This research presented, developed, tested and applied a new theory to increase the photoefficiency. The theory proposes that controlled periodic illumination (intermittent light) will increase the photoefficiency in photocatalysis. The theory was tested in a novel open channel reactor by determining the optimal illumination and dark recovery times for the oxidation of the formate anion. These experiments showed that when the illumination time was less than 70 ms and the dark time was greater than 2 seconds, the photoefficiency increased by a factor of three. Further experiments in this reactor showed that the photoefficiency decreases as the light intensity increases and that the dark time remains constant for various illumination times.; Based on these results, a unique Taylor-Couette flow device was constructed as a pilot scale reactor that incorporated controlled periodic illumination via fluid mixing. The Taylor vortex reactor consisted of coaxial Pyrex cylinders. The outer cylinder was fixed while the inner cylinder rotated from 5 to 300 rpm. The vortices moved the catalyst particles into and out of the illuminated region of the reactor. With a catalyst weight loading of 10 g/L and a rotation rate of 300 rpm, the photoefficiency increased by a factor of three. This reactor behaved as ideal plug flow reactor because of the intense radial mixing of the vortices with little axial exchange. This reactor represents one of many possible configurations for implementing controlled periodic illumination into large scale photocatalytic water purification systems.