Tailoring performance of La-modified TiO2 nanocatalyst for continuous photocatalytic CO2 reforming of CH4 to fuels in the presence of H2O

Abstract

Photo-induced CO2 reforming of CH4 in the presence of H2O over La-modified TiO2 nanoparticles in a continuous flow photoreactor has been investigated. The structure and properties of the catalyst samples, synthesized by a sol-gel method, were systematically characterized by XRD, Raman, SEM, TEM, CO2-TPD, TGA, N2-sorption, XPS, UV–Vis DRS and PL spectroscopy. The crystallite size was reduced while, BET surface area and basicity were increased due to the presence of La2O3. The La-modified TiO2 nanocatalysts were tested under different catalyst loading, irradiation time, reaction temperature and type of reductants. The main products detected over La/TiO2 catalysts during photo-induced CO2-CH4 reaction system were CO, H2 and C2H6. The amount of CO produced over 5 wt.% La/TiO2 was 9.6 fold more the amount of CO produced by pure TiO2. When H2O was added to the CO2-CH4 reaction system, the yield of CO increased 37 fold higher over 5 wt.% La/TiO2 compared to TiO2. The enhanced photocatalytic performances can be attributed to the synergistic effect of La2O3 for CO2 adsorption with hindered charge recombination rate by La3+ and appropriate redox potentials. The photocatalytic turnover productivity (PTOP), calculated for the first time, presented amounts of products evolved with the photon energy consumption. The highest PTOP number achieved for CO production using the CO2-CH4-H2O reaction system was 3.83 fold higher than PTOP achieved in CO2-CH4 reaction system. However, PTOP for the production of H2 and C2H6 in CO2-CH4 system was 1.2 and 2.1 fold higher than the CO2-CH4-H2O reaction system, respectively. The stability test revealed prolonged life time of La/TiO2 in cyclic runs for dynamic CO2-CH4 conversion to fuels in the presence of H2O than using only CO2-CH4 reaction system. Therefore, CO2-CH4 could efficiently be converted to fuels over a La/TiO2 catalyst while the addition of H2O could promote both photoactivity and stability.