High-pressure CO2 photoreduction, FSP and Z-scheme: a promising synergy

The first successful attempts use light to catalyze a process were dated around the 1970s. Since then, photocatalysis was deepened and in the last years CO2 photoreduction has gained attention as a possible way to valorize carbon dioxide. The main problem of this process is the low conversion and direct Z-scheme heterojunction seems to be a way to increase the catalyst efficiency 1. The latter are created joining two semiconductors together, with the effect to reduce the electron-hole recombination rate by quenching the photogenerated holes. High purity, thermally stable mixed metal oxides semiconductors can be obtained through Flame Spray Pyrolysis (FSP) synthesis. The FSP apparatus is reported in Figure 1. Both pure semiconductors, namely WO3 and TiO2, and Z-scheme composed of WO3/TiO2 in a 40/60 mass ratio have been synthetized through FSP synthesis. Ammonium metatungstate hydrate (AMT) and titanium tetra-isopropoxide (TTIP) were used as precursors. Dimetilformamide (DMF) was used as solvent. Catalysts have been tested in a three-phase high pressure photoreactor 2. 18 bar have been used as operating pressure in presence of Na2SO3 as hole scavenger. The performances of the two bare oxides were comparable, although they showed a showed a slightly lower productivity (ca. 14 mol/kgcat h) compared to the P25 benchmark result (ca. 16,8 mol/kgcat h). The Z-scheme WO3/TiO2 40/60 showed an astonishing 2.5-times increase compared to FSP titania and about 2-times increase with respect to the benchmark P25 catalyst under the same conditions. XRD, BET, DRS, SEM/TEM analysis have been conducted to show the physical difference and to explain the improvement due to this promising synergy.