Synthesis of TiO2−x visible-light photocatalyst using N2/Ar/He thermal plasma for low-concentration elemental mercury removal

Oxygen-vacant titanium dioxide (TiO2−x) nanoparticles possessing a narrower band gap were synthesized with a single-step process using N2/Ar/He thermal plasma as a heating source. Sample characterization showed that the formed TiO2−x nanoparticles had a size within 5–40 nm and were in a mixing form of anatase and rutile. An evident absorption red shift to 470 nm wavelength was observed. The deconvolution results of XPS spectra supported the formation of TiO2−x due to the presence of Ti3+2p peaks. The Hg removal efficiency of TiO2−x nanoparticles increased with increasing O2 concentration and in the presence of light irradiation, suggesting an enhancement in Hg0 oxidation. H2O molecules, however, greatly reduced the Hg removal of TiO2−x under visible-light (VL) irradiation. The photoinduced hydrophilicity of TiO2−x under VL irradiation was suspected to amplify the competitive adsorption of H2O that decreased Hg capture. Hg removal performances at 50 °C were better than those at 25 and 100 °C, suggesting that both adsorption and catalytic oxidation limit the Hg removal. These observations also implicate that the bonding between TiO2−x and adsorbed Hg was weak and adsorption reaction may be reversible.

TiO2−x nanoparticles were synthesized using N2/He/Ar thermal plasma. Adding N2 not only increased the plasma temperature but also enhanced the oxygen-lean environment. Consequently, TiO2−x instead of TiO2−xNx, was the major product.Figure optionsDownload as PowerPoint slideHighlights
► TiO2−x nanoparticle was developed with thermal plasma via a single-step process.
► An insight into the characteristics of TiO2−x photocatalyst was provided.
► Hg removal using TiO2−x was demonstrated under UV and visible-light irradiation.
► Enhancement on Hg0 removal by O2 with TiO2−x was shown.
► Competitive adsorption between H2O and Hg0 on active sites of TiO2−x was proved