This work centered on the optimization and scale-up of photocatalytic reactors employing silica-titania composites (STC) for two applications involving the abatement of hazardous air pollutants (HAPs) from industrial facilities: (1) degradation of HAPs, particularly methanol, emitted from pulp and paper mills; and (2) recovery of mercury emitted from chlor-alkali plants.

STC were synthesized with varying pore sizes (50 Å, 120 Å, and 260 Å) and TiO₂ loadings (0-60%) and were tested for the removal of methanol from a humid air stream. The efficiency of methanol oxidation was dependent on the surface area of the STC and the space time of the gas in the reactor. For 120 Å 12% and 260 Å 12% STC irradiated with UVA light, a lag time of 1.0 s and 1.2 s, respectively, was observed before mineralization began. After this lag time, which was zero for the 50 Å 12% STC, the data followed pseudo-first order reaction kinetics and the rate constant, k, was 0.40 s^-1 for all pore sizes. Using the 50 Å STC, the efficiency was further improved by using a 4% TiO2 loading and UVC lamp, which generated a higher photon flux compared to a UVA lamp. The presence of H₂S in the gas stream decreased methanol removal efficiency and resulted in SO₂ and SO₄ ^2- oxidation byproducts. When compared to other catalyst supports, the STC was more efficient in a low-humidity gas stream with a relative humidity (RH) of less than 0.22% at 23°C. In a high humidity gas stream (RH = 95% at 23°C), the efficiency of the STC was inhibited by water vapor due to its surface chemistry and performed similarly to TiO₂-coated activated carbon. When compared to TiO₂-glass spheres, the use of an adsorbent catalyst support resulted in higher degradation efficiencies. Based on the promising bench-scale results, a 40 ACFM pilot reactor was fabricated employing a packed bed of STC and a 4.3 s space time through the packed bed. The pilot reactor achieved methanol removal rates up to 66 ± 7% with less than 1 ppm_v formaldehyde production at steady state.

A pilot-scale photocatalytic reactor packed with STC was tested at a chlor-alkali facility over a three-month period. This pilot reactor treated up to 10 ACFM of end-box exhaust and achieved 95% mercury removal. The pilot reactor was able to maintain excellent removal efficiency even with large fluctuations in influent mercury concentration (400-1600 ug/ft³). The STC pellets were regenerated ex-situ with hydrochloric acid and performed similarly to virgin STC pellets when returned to service. Based on these promising results, two full-scale reactors with in-situ regeneration capabilities were installed and operated. After optimization, these reactors performed similarly to the pilot reactor. A cost analysis was performed comparing the treatment costs (i.e., cost per pound of mercury removed) for sulfur-impregnated activated carbon and the STC system. The STC proved to be both technologically and economically feasible for this installation.