Mxene modified layered double hydroxide nanocomposite for photocatalytic carbon dioxide reduction to renewable fuels

Photocatalytic conversion of carbon dioxide (CO2) to solar fuels is a promising solution to resolve the energy crisis and global warming issues. The overall efficiency of photoreduction of CO2 to fuels can be improved through the development of highly efficient catalyst and suitable photoreactor configuration. Hence, the main objective of this research work was to design a photocatalytic reactor system and synthesize cobalt-aluminium-lanthanum-layered double hydroxide (CoAlLa-LDH) modified with graphitic carbon nitride (g-C3N4) and titanium carbon (Ti3C2TA/R) MXene for enhanced photocatalytic reduction of CO2 to renewable fuels. Initially, a novel CoAlLa-LDH was synthesized by co-precipitation method that has hexagonal nanosheet structure. The Ti3C2TA/R was synthesized through controlled etching with hydrogen fluoride acid that resulted in the formation of layered structured Ti3C2 MXene embedded with anatase and rutile phases of titanium dioxide (TiO2). The Ti3C2TA/R having layered structure and embedded TiO2 effectively acted as electrons reservoir and electrons mediator, respectively. Graphitic carbon nitride (g-C3N4) nanosheets were obtained through thermal heating and subsequent sonication. g-C3N4, CoAlLa-LDH and Ti3C2TA/R were hybridized to obtain g-C3N4/CoAlLa-LDH, g-C3N4/Ti3C2TA/R, Ti3C2TA/R/CoAlLa-LDH and g-C3N4/Ti3C2TA/R/CoAlLa-LDH composites with layer-by-layer assemblies. The performance of photocatalysts was investigated through photocatalytic reduction of CO2 with water (H2O), dryreforming (DRM) and bireforming (BRM). Among LDHs the Co2Al0.95La0.05-LDH showed maximum photocatalytic reduction of CO2 with H2O resulting in production rate of 21.80 and 25.5 µmolegcat-1h-1 for CO and CH4, respectively. The g-C3N4/Ti3C2TA/R/Co2Al0.95La0.05-LDH sample resulted in maximum CO and CH4 production rate of 106 and 49.8 µmole gcat-1h-1 through photocatalytic reduction of CO2 with H2O. The g-C3N4/Ti3C2TA/R sample showed very good performance in photocatalytic DRM with production of 73.31 and 51.24 µmole gcat-1h-1 for CO and H2, respectively. The g-C3N4/Ti3C2TA/R/Co2Al0.95La0.05-LDH sample, through photocatalytic BRM showed maximum CO and H2 production of 47.81 and 73.31 µmole gcat-1h-1 with higher selectivity towards H2 production that is a high-quality syngas. The best performing g-C3N4/Ti3C2TA/R/Co2Al0.95La0.05-LDH catalyst in the fixed bed photoreactor for photocatalytic BRM was compared with multistage mesh photoreactor (MSM). The MSM showed syngas production of 1.81 and 1.22 folds higher as compared to fixed bed photoreactor for CO and H2 respectively. The effects of various parameters such as amount of catalyst, feed ratio and illumination time for the photocatalytic reduction of CO2 was studied to optimize yield and selectivity of fuel products through response surface methodology. It was found that the optimum CO production was obtained at 0.143 g, 4.48 h and 1.67 while the optimum H2 production obtained was at 0.143 g, 4.93 h, 1.41 of catalyst loading, time and feed ratio, respectively. Finally, Langmuir-Hinshelwood model was developed to investigate adsorption behaviours and photocatalytic oxidation and reduction process, fitted well with the experimental data. It was determined that CO and H2 production were dependent on quantity of CO2 and CH4 in the feed, respectively. In conclusion the Ti3C2TA/R and g-C3N4 modified CoAlLa-LDH catalyst can produce high quality renewable syngas fuel with high selectivity towards H2 production