Photo reduction of CO 2 to methanol using optical-fiber photoreactor

Abstract

Greenhouse gases such as CO 2 are the primary cause of global warming. One of the best routes to remedy CO 2 is to transform it to hydrocarbons using photo reduction. CO 2 was photocatalytically reduced to produce methanol using a Hg lamp with wavelength 365 nm in a steady-state optical-fiber photoreactor. The optical-fiber photoreactor, comprised of nearly 120 Cu/TiO 2-coated fibers, was designed and assembled to transmit and spread light uniformly inside the reactor. TiO 2 film was coated on optical fiber using a dip-coating method. Cu-loaded titania solutions were prepared by a thermal hydrolysis method. The thickness of Cu/TiO 2 film was 53 nm. The coating film consisted of very fine spherical particles with diameters of near 14 nm. The XRD spectra indicated the anatase phase for all TiO 2 and Cu/TiO 2 films. The wavelength of absorption edge on Cu/TiO 2 was near 367 nm, equivalent to a bandgap of 3.3 eV. The most active Cu species on TiO 2 surface were Cu 2O clusters, and they played an important role for the formation of methanol. The methanol yield increased with UV irradiative intensity. Maximum methanol rate was 0.45 μmole/g cat h using 1.2 wt.%-Cu/TiO 2 catalyst at 1.29 bar of CO 2, 0.026 bar of H 2O, and 5000 s mean residence time under 16 W/cm 2 UV irradiation. Higher than 1.2 wt.% Cu loading gave a lower rate of methanol yield because of the masking effect of Cu 2O clusters on the TiO 2 surface. The Langmuir–Hinshelwood model was established by correlating experimental data to describe the kinetic behavior. An optimum pressure ratio of H 2O/CO 2 was found in the photo reduction of CO 2 for maximum methanol yield.