Use of an optofluidic microreactor and Cu nanoparticles synthesized in ionic liquid and embedded in TiO2 for an efficient photoreduction of CO2 to methanol

The slow kinetics in the photocatalytic reduction of CO2, as well as the low quantum efficiencies achieved, directly related to the photocatalyst and reactor configuration applied, limit the widespread use of this technology. In light of this, the main objective of this work is to evaluate the continuous photocatalytic conversion of CO2 into methanol in an optofluidic microreactor (with enhanced mass transport, large volume/active area ratio and uniform light distribution) using Cu nanoparticles synthesized in the hydrophilic 3-methyl-n-butylimidazolium tetrafluoroborate (BMIm.BF4) ionic liquid and embedded in TiO2 (P25). The ionic liquid not only acts as a template to control the size of the nanoparticles but also as a stabilizing agent. The analysis includes the effect of structural parameters of the photoactive layer such as Cu content (from 0.8 to 6.8 wt%) and photocatalyst loading (0.5–3 mg·cm−2), as well as operating variables such as UV and visible light intensities (2.5–10 mW·cm−2) and cell configuration (i.e. one or two compartments). The maximum methanol yield reached from the continuous transformation of CO2 is r = 230.3 µmol∙g−1∙h−1 at 2 wt% Cu content, photocatalyst loading of 2 mg·cm−2, UV light intensity of 10 mW·cm−2 and a two-compartment microreactor configuration. This result outperforms the values previously reported for Cu/TiO2-based systems using optofluidic microreactors, as well as most of those in common CO2 photoreactors.