In Water High Yield and Selectivity of CH₄ and H₂ Production Using UVC Light and a SiO₂-surface-modified TiO₂ Photocatalysts

Abstract

To improve CH₄ and H₂ formation from CO₂ photoreduction using non-organic, non-laborious, and inexpensive photocatalysts, we have prepared two surface-silicate-modified TiO₂ catalysts: P25-SiO₂ and AmTiO₂-SiO₂ (amorphous TiO₂) to be tested in water and using UV light. The last catalyst produces more CH₄ and H₂ in water than P25 (3:1 TiO₂ anatase: rutile) under UV light irradiation of HCO₃^- and CO₂; am-TiO₂-SiO₂ at pH = 7, produces 8 times more CH₄ and H₂ than P25 with selectivity at the reactor headspace of 30% and 53%, respectively. Using CO₂ (pH = 3), 80 times more CH₄ than P25 under the same conditions is obtained with a yield of 71%. This corresponds to a production of 8.9 μmol g_cat^-1·h^-1, one of the highest reported rates of CH₄ production from CO₂ using carbon-free semiconductors. H₂ is also produced by water splitting using Am-TiO₂-SiO₂ and water at low pH. The enhanced reactivity compared to P25 is attributed to three main factors: a) Low catalyst PZC (4.1) that facilitates CO₂ adsorption and proton availability at the active site to catalyze the e transfer from Ti at the TiO₂-SiO₂-carbonate adduct b) SiO₂ acts as electron trap reducing carriers recombination (External intramolecular trapping (EIT) mechanism) and c) am-TiO₂-SiO₂, light collection efficiency, surface area and irregular atoms distribution. Catalysts were also tested for Methylene blue (MB) photooxidation. P25 is quite a better catalyst in oxidizing MB via OH radicals, probably due to the more positive valence band potentials in the SiO₂-modified catalysts that avoid the OH radical formation from water; however, when bicarbonate is added to MB solution, am-TiO₂-SiO₂ catalysts reactivity increases as a consequence of its valence band down-bending.