Formation of Formaldehyde and Other Byproducts by TiO₂ Photocatalyst Materials

Photocatalysts promised to control pollution in an environmentally benign manner, inexpensively, and with a low or cheap energy input. However, the limited chemical activity of photocatalysts has prevented their widespread use. This limitation has two important consequences; in addition to limited removal efficiency for pollution, photocatalysts may also generate unwanted byproducts due to incomplete reaction. This study focuses on the byproducts formed in the photocatalytic degradation of dimethyl sulfide (DMS) on titanium dioxide (TiO₂), using a continuous flow reactor and detection via proton transfer reaction mass spectrometry. TiO₂, activated carbon (AC), TiO₂/AC (1:1) and TiO₂/AC (1:5) were tested using either a laser-driven light source or LED lamps at 365 nm. The samples were characterized using a N₂-BET surface area and pore size distributions, Scanning Electron Microscopy, X-ray Diffraction, and X-ray Photoelectron Spectroscopy, which confirmed that TiO₂ was successfully coated on activated carbon without unexpected phases. TiO₂ and activated carbon showed different removal mechanisms for DMS. The maximum yield of formaldehyde, 11.4%, was observed for DMS reacting on a TiO₂/AC (1:5) composite operating at a DMS removal efficiency of 31.7% at 50 ${}^{\circ }$C. In addition to formaldehdye, significant products included acetone and dimethyl disulfide. In all, observed byproducts accounted for over half of the DMS material removed from the airstream. The TiO₂/AC (1:5) and TiO₂/AC (1:1) composites have a lower removal efficiency than TiO₂, but a higher yield of byproducts. Experiments conducted from 20 ${}^{\circ }$C to 70 ${}^{\circ }$C showed that as temperature increases, the removal efficiency decreases and the production of byproducts increases even more. This is attributed both to decreased surface activity at high temperatures due to increased recombination of reactive species, and to the decreased residence time of volatile compounds on a hot surface. This study shows that potentially dangerous byproducts are formed by photocatalytic reactors because the reaction is incomplete under the conditions generally employed.