Adsorption and photocatalytic oxidation of formaldehyde on a clay-TiO2 composite

We investigated the adsorption capacity and photocatalytic removal efficiency of formaldehyde using a hectorite-TiO₂ composite in a bench flow reactor. The same experimental conditions were applied to pure TiO₂ (Degussa P25) as a reference. The catalysts were irradiated with either a UVA lamp (365 nm) or with one of two UVC lamps of 254 nm and 254 + 185 nm, respectively. Formaldehyde was introduced upstream at concentrations of 100–500 ppb, with relative humidity (RH) in the range 0–66% and residence times between 50 and 500 ms. Under dry air and without illumination, saturation of catalyst surfaces was achieved after ∼200 min for P25 and ∼1000 min for hectorite-TiO₂. The formaldehyde uptake capacity by hectorite-TiO₂ was 4.1 times higher than that of P25, almost twice the BET surface area ratio. In the presence of humidity, the difference in uptake efficiency between both materials disappeared, and saturation was achieved faster (after ∼200 min at 10% RH and ∼60 min at 65% RH). Under irradiation with each of the three UV sources, removal efficiencies were proportional to the Ti content and increased with contact time. The removal efficiency decreased at high RH. A more complete elimination of formaldehyde was observed with the 254 + 185 nm UV source.