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

Abstract

To improve CH<sub>4</sub> and H<sub>2</sub> formation from CO<sub>2</sub> photoreduction using non-organic, non-laborious, and inexpensive photocatalysts, we have prepared two surface-silicate-modified TiO<sub>2</sub> catalysts: P25-SiO<sub>2</sub> and AmTiO<sub>2</sub>-SiO<sub>2</sub> (amorphous TiO<sub>2</sub>) to be tested in water and using UV light. The last catalyst produces more CH<sub>4</sub> and H<sub>2</sub> in water than P25 (3:1 TiO<sub>2</sub> anatase: rutile) under UV light irradiation of HCO<sub>3</sub><sup>-</sup> and CO<sub>2</sub>; am-TiO<sub>2</sub>-SiO<sub>2</sub> at pH = 7, produces 8 times more CH<sub>4</sub> and H<sub>2</sub> than P25 with selectivity at the reactor headspace of 30% and 53%, respectively. Using CO<sub>2</sub> (pH = 3), 80 times more CH<sub>4</sub> than P25 under the same conditions is obtained with a yield of 71%. This corresponds to a production of 8.9 μmol g<sub>cat</sub><sup>-1</sup>·h<sup>-1</sup>, one of the highest reported rates of CH<sub>4</sub> production from CO<sub>2</sub> using carbon-free semiconductors. H<sub>2</sub> is also produced by water splitting using Am-TiO<sub>2</sub>-SiO<sub>2</sub> 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 CO<sub>2</sub> adsorption and proton availability at the active site to catalyze the e transfer from Ti at the TiO<sub>2</sub>-SiO<sub>2</sub>-carbonate adduct b) SiO<sub>2</sub> acts as electron trap reducing carriers recombination (External intramolecular trapping (EIT) mechanism) and c) am-TiO<sub>2</sub>-SiO<sub>2</sub>, 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 SiO<sub>2</sub>-modified catalysts that avoid the OH radical formation from water; however, when bicarbonate is added to MB solution, am-TiO<sub>2</sub>-SiO<sub>2</sub> catalysts reactivity increases as a consequence of its valence band down-bending.