Development and characterizations of MWCNT@TiO2 core-shell nanocomposites for photocatalytic reduction of carbon dioxide under visible light irradiation

Carbon dioxide (CO2) emission is one of the most well-known causes of global warming. Conversion of CO2 into useful chemical products in a process such as photocatalytic reduction is deemed as an attractive approach in permanently sequestrating CO2 from its emission source. Photocatalytic reaction applies the concept of radical-chain reactions in which proton and anion radicals are formed from electron (e-) and proton (h+) transfer between metal oxide photocatalysts and the reactants. This work demonstrated the synthesis of multi-walled carbon nanotubes (MWCNT)@titanium dioxide (TiO2) core-shell nanocomposites using a newly developed simple coating approach. UV-vis analysis revealed that the photoactivity of the nanocomposites in visible light range was significantly enhanced by the addition of visible light inducible MWCNTs. The MWCNT@TiO2 core-shell nanocomposites exhibited excellent activities for converting CO2 into methane, ethylene and ethane in a continuous process under a low power visible light irradiation at atmospheric pressure. Photoluminescence analysis revealed that the inhibition of electron-hole pair recombination was greater with the increase in MWCNT loading, indicating that the presence of MWCNTs in the nanocomposites enhanced the electron transfer and reduced the electron-hole pair recombination rates. The core-shell structures with mixed-phase of anatase-rutile shell layer were obtained from thermal treatment in the temperature range of 300-700oC for the durations of 2-6h. The various anatase-rutile crystal compositions greatly influenced the visible light absorption band-edge. The MWCNT@TiO2 was further enhanced with the addition of transition metal oxide dopants which include, iron oxides (Fe2O3), copper oxides (CuO), nickel oxides (NiO), cobalt oxides (CoO) and zinc oxides (ZnO). Doping with visible-light-responsive metal oxides i.e. CuO, Fe2O3 significantly enhanced the photoreactivity of the MWCNT@TiO2 core-shell. The metal oxides could function as ‘charge-carrier traps’ that transport electrons from TiO2 through the heterojunction of the TiO2-metal oxides. Doping of MWCNT@TiO2 with plasmonic noble metal, i.e. silver (Ag) dopant, was found to greatly improve the CO2 photoreduction performance. 2wt% Ag-doped MWCNT@TiO2 was found to possess the best performance in CO2 reduction. The optimum reaction conditions that gave the maximum formation of methane, ethylene and ethane were determined to be at CO2 flowrate of 7.5mL/min, H2O:CO2 molar ratio of 0.11mol H2O/mol CO2, and under the irradiation with light intensity of 600μW/cm2. The maximum total formation of methane, ethylene and ethane formation under the optimum conditions were found to be ca. 4.50, 0.82, 5.75μmol/g-catalyst, respectively for an 8 h photoreduction reaction. Upon obtained the optimum conditions, the work continued on a simple kinetic study based on one-site Langmuir-Hinshelwood model