Photoreduction of carbon dioxide to methanol using copper modified carbon nitride and titanium dioxide nanocomposites

Abstract

Photoreduction of CO2 to useful chemicals have shown promising results from the research on CO2 conversion and utilization. The objective of this study is to synthesize copper and carbon nitride based titanium dioxide nanocomposites for selective photoreduction of carbon dioxide to methanol under visible light irradiations. The nanocomposites were synthesized by a chemical precipitation method and characterized using XRD, FT-IR, FESEM, TEM, DRS, BET and XPS. The XRD results confirmed the presence of TiO2, g-C3N4 and Cu in the nanocomposite by their characteristic peaks. The doping of Cu metal reduced the intensity of the PL emission and the rate of recombination. The most effective catalysts was g−C3N4/(3% Cu/TiO2) which gave a maximum methanol yield of 948.14μmol/g.cat after 2 h. Cu doped TiO2 enhanced its photoactivity by fostering carrier separation. The position of Cu in the composite affected the distribution of electrons and hence the photo-activity. Parameters investigated were weight percent ratio, effect of time and stability. The position of Cu in the composite affected the distribution of electrons and hence the photo-activity. After 8 h of photoreaction, a maximum CH3OH yield of 2574 μmol/g. cat was obtained using visible light. The ratio of g-C3N4 to Cu/TiO2 dictated the efficiency of the composite and the visible light was seen to demonstrate higher efficiency compared to the ultraviolet light. The higher emitting power UV light provided more photons for photoexcitation of more electrons, but photo-oxidation of CH3OH to HCOOH affected the product yield while using UV light. The low band gap, electronic structure and light absorption capacity of g-C3N4 assisted in the transfer of photogenerated electrons to Cu/TiO2 in the composite thereby aiding maximal usage of the irradiated light. Cu/TiO2 demonstrated a high selectivity for photoreduction of CO2 to CH3OH in the nanocomposite. The photostability of the composite was maintained even after three cycles. Possible reaction mechanisms were proposed to understand the type of catalysts and light irradiations on yield and selectivity.