Application of Optical-fiber Photoreactor for CO2 Photocatalytic Reduction
An optical-fiber photoreactor, comprised of 216 catalyst-coated fibers, was designed and assembled to transmit and spread light uniformly inside the reactor. The power loss of light transmission inside an optical fiber was calculated using beam propagation method. The optimum length of optical fiber was estimated to be near 11 cm long in order to entirely spread out light energy over surface catalyst. Vapor-phase CO2 was photocatalytically reduced to methanol using the photoreactor under UV irradiation in a steady-state flow system. The solutions of metal-loaded titania were prepared by thermal hydrolysis method. Metal-loaded TiO2 film was coated on optical fibers by dip-coating method. TiO2, Cu/TiO2 and Ag/TiO2 films were uniformly on the fibers and their thicknesses ranged from 27 to 33 nm. The films consisted of very fine spherical particles with diameters of 10–20 nm. The XRD spectra indicated anatase phase for all films. Methanol yield increased with UV irradiative intensity. Maximum methanol rate was 4.12 μmole/g-cat h using 1.0 wt%-Ag/TiO2 catalyst at 1.13 bar of CO2, 0.03 bar of H2O pressures, and 5,000 s mean residence time under 10 W/cm2 UV irradiation.
KeywordsPhotocatalysis Optical-fiber photoreactor CO2 conversion Renewable energy
Almost industrial chemical reactions are thermal-driven processes currently. The required thermal energy is mostly supplied by fossil fuel which, in fact, stored the solar energy in prehistoric times. Photocatalysis provides an alternative route to carry out reactions using photo energy, which may be directly supplied from the sun. The photonitration and photonitrosation of phenol were reported using TiO2 catalyst under UV irradiation . Photocycloadditions provided elegant routes to synthesize aromatic compounds from non-aromatics. Compared to the normal intramolecular arene–alkene, photocyclization improved the efficiency of such reactions . Hydrogen will be a major energy source once the fuel cell is operated commercially. Photo-driven water splitting can generate hydrogen from solar energy without producing CO2 [3, 4, 5, 6].
In the last 20 years, photocatalysis has been shown to be effective for removing trace levels of persistent and toxic organic pollutants from both water and air. The photocatalyst and photo kinetics have been studied for years. However, most applications are still in the environmental remediation. Now, it is time to develop a photoreactor to extend more application in chemical processes. The major barrier for commercial applications is its high cost as well as its low overall rate and energy efficiency. This is due, among other factors, to this low-order dependency of reaction rates on radiation intensity and a limited capacity to deliver photons and reactants to catalyst surfaces. How to evenly distribute just enough photons to the catalyst surface is an crucial design factor in a photoreactor.
Our motivation is to seek photo-driven or photo-assisted chemical reactions using solar energy directly. The ideal is to use highly focused light to accelerate a photo reaction, so that mass production of chemicals is viable. The light can be collected from the sun using focus reflection dish. TiO2-coated fiber-optical cable reactors have demonstrated some inherent advantages over packed-bed reactors in photo reactions [7, 8]. Optical fibers are used to spread the light uniformly inside the module of the photoreactor. This kind of design provides an economical way to deliver photons uniformly in a large volume.
Theoretical Light Transmission in an Optical-fiber
The TiO2 solutions for coating on optical fibers were prepared using a thermal hydrolysis method. Titanium butoxide (97%, Aldrich, USA) and polyethylene glycol (PEG, molecular weight of 20,000, Merck, Darmstadt, Germany) were added to a 0.1 M nitric acid solution. An appropriate amount of metal precursor, i.e. CuCl2 or AgNO3, was added to obtain near 1 wt% metal loading of TiO2. The mixed solution was stirred and heated to 80 °C for 8 h. PEG was added to prevent cracking during the drying and calcination of the film. Detailed preparation procedure was reported in literature .
Optical fibers were obtained from the E-Tone Technology Company of Taiwan. The polymeric shield on the optical fiber was burned off in a furnace at 400 °C. The remaining quartz fiber had a diameter of 112 μm. Each quartz fiber was cleaned by a 5 M NaOH solution in an ultrasonic cleaner, then rinsed in de-ionized water and dried before applying dip-coating procedure. The bare fiber was immersed into the solution vertically, then pulled up at a rate of 30 mm/min. The TiO2 film was dried in air at 150 °C by a rate of 1 °C/min from the ambient temperature, and maintained at 150 °C for 3 h. Then it was calcined at 500 °C for another 5 h. The same procedure of TiO2 coatings was also applied to glass plates for characterization.
The TiO2 phase of the film was determined using the X-ray diffractometer, MAC Science M03XHF. The UV–visible absorption was measured by transmission mode using a Varian Cary spectrophotometer 100. The microstructure of the TiO2 film on the optical fiber was inspected using a scanning electro microscope (SEM) LEO 1530 (Germany).
Reaction products were analyzed via a sampling loop of 2.5 mL on-line by a GC equipped with FID using a 2 m long Porapak Q column. Methanol, formic acid and methyl formate were detected. The Quantitative analysis indicated that methanol was the dominant hydrocarbon. The amounts of formic acid and methyl formate were much smaller than that of methanol. Methane was reported to be one of the products in the photoreduction of CO2 on the titanium oxide prepared within the zeolite cavities . But we did not detect methane due to the technique difficult of the GC analysis. Blank reactions were carried out to ensure that hydrocarbon production was solely from the photo reduction of CO2. One blank was UV-irradiated with bare optical fibers, and another was in the dark with TiO2-loaded optical fibers under the same experimental conditions. No hydrocarbon was detected in the above blank tests.
Results and Discussion
Characteristics of TiO2 Film
Characteristics of TiO2, Cu/TiO2, Ag/TiO2 films
Crystal sizea (nm)
Band gapb (eV)
Specific areac (m2/g)
Photo Reduction of CO2
Apparent quantum efficiency (%) of methanol production on TiO2, Cu/TiO2, Ag/TiO2 catalysts under 365 nm UV irradiation
The heterogeneous photo reduction of CO2 involves two procedures, photo activation and catalytic reaction on the catalyst. The factors of photo activation include (1) the excitation of electron-hole pair by photon may not be efficient. Most of photon convert to thermal energy and is dissipated; (2) the recombination of electron and hole could be substantial; and (3) the effective electrons for the catalytic reduction of CO2 may be only a small portion of total electrons due to migration loss. The factors of catalytic include (1) the adsorption of CO2 may be limited on catalyst surface; (2) one of the elementary reaction steps is rate-limiting; and (3) reverse reaction may be occurred. Considering the above factors together, therefore, the overall apparent quantum efficiency can be very low in the CO2 photo reduction.
An optical-fiber photoreactor is designed and applied to the photo reduction of CO2 with H2O using TiO2, Cu/TiO2 and Ag/TiO2-coated optical fibers. The photo reduction of CO2 is one of the best routes to renewable energy similar to photosynthesis. So far, the maximum methanol yield was 4.12 μmole/g-cat h under 365 nm UV irradiation. Compared with a traditional packed-bed reactor, an optical-fiber provides a medium to transmit light uniformly throughout a reactor. In addition, a higher processing capacity is possible because the catalyst can disperse on a large external area of optical fibers in a given reactor volume. The advantage of photo-driven reaction is clearly benefited from the un-limited solar energy. A high-efficient photoreactor is the first step toward a commercial-scale application to produce chemicals.
Financial supports of Institute of Nuclear Energy Research, Atomic Energy Council, Taiwan under contract no. 95-2001-INER-035, and of Ministry of Economic Affairs, Taiwan, under grant 94-EC-17-A-09-S1-019, and of National Science Council under grant NSC95-EPA-Z-002-007 are gratefully acknowledged.