TiO_2−x nanoparticles synthesized using He/Ar thermal plasma and their effectiveness on low-concentration mercury vapor removal

Oxygen-vacant titanium dioxide (TiO_2−x ) nanoparticles were synthesized using thermal plasma as a heating source at various applied plasma currents and He/Ar ratios. Samples with diverse characteristics were developed and the mercury removal effectiveness was subsequently evaluated. TiO₂ nanoparticles possessing high purity and uniform particle sizes were successfully synthesized using metal titanium and O₂ as precursors and Ar as plasma gas. TiO_2−x in anatase phase with a particle size at 5–10 nm was formed at the He/Ar volume ratio of 25/75. Further increasing the He/Ar ratio elevated the plasma temperature, causing the tungsten to melt, vaporize from the cathode, and then dope into the formed TiO₂ nanoparticles. The doped W appeared to inhibit the growth of nanoparticles and decrease the crystallinity of formed anatase. The effectiveness of oxygen-vacant sites on Hg⁰ removal under the visible light circumstance was confirmed. Hg⁰ removal by the TiO_2−x nanoparticles was enhanced by increasing the O₂ concentration. However, moisture reduced Hg⁰ capture, especially when light irradiation was applied. The reduction in Hg⁰ capture may be resulted from the competitive adsorption of H₂O on the active sites of TiO_2−x with Hg⁰ and transformed Hg²⁺.