Showing papers by "Jeffrey C.S. Wu published in 2010"

Artificial photosynthesis over crystalline TiO2-based catalysts: fact or fiction?

Abstract: The mechanism of photocatalytic conversion of CO2 and H2O over copper oxide promoted titania, Cu(I)/TiO2, was investigated by means of in situ DRIFT spectroscopy in combination with isotopically labeled 13CO2. In addition to small amounts of 13CO, 12CO was demonstrated to be the primary product of the reaction by the 2115 cm−1 Cu(I)−CO signature, indicating that carbon residues on the catalyst surface are involved in reactions with predominantly photocatalytically activated surface adsorbed water. This was confirmed by prolonged exposure of the catalyst to light and water vapor, which significantly reduced the amount of CO formed in a subsequent experiment in the DRIFT cell. In addition, formation of carboxylates and (bi)carbonates was observed by exposure of the Cu(I)/TiO2 surface to CO2 in the dark. These carboxylates and (bi)carbonates decompose upon light irradiation, yielding predominantly CO2. At the same time a novel carbonate species is produced (having a main absorption at ∼1395 cm−1) by adsorption of photocatalytically produced CO on the Cu(I)/TiO2 surface, most likely through a reverse Boudouard reaction of photocatalytically activated CO2 with carbon residues. The finding that carbon residues are involved in photocatalytic water activation and CO2 reduction might have important implications for the rates of artificial photosynthesis reported in many studies in the literature, in particular those using photoactive materials synthesized with carbon containing precursors

CO2 photoreduction using NiO/InTaO4 in optical-fiber reactor for renewable energy

Abstract: The photocatalytic reduction of CO2 into fuels provides a direct route to produce renewable energy from sunlight. NiO loaded InTaO4 photocatalyst was prepared by a sol–gel method. Aqueous-phase CO2 photoreduction was performed in a quartz reactor to search for the highest photoactivity in a series of NiO/InTaO4 photocatalysts. Thereafter, the best NiO/InTaO4 was dip coated on optical fibers and calcined at 1100 °C. A uniform NiO/InTaO4 layer of 0.14 μm in thickness was observed on the optical fiber. An optical-fiber photoreactor, comprised of ∼216 NiO/InTaO4-coated fibers, was designed to transmit and spread light uniformly inside the reactor. The UV–vis spectra of powder InTaO4 as well as NiO loaded InTaO4 prepared via the same procedure indicated that both photocatalysts could absorb visible light. XRD confirmed that InTaO4 was in single phase. Vapor-phase CO2 was photocatalytically reduced to methanol using the optical-fiber reactor under visible light and real sunlight irradiation in a steady-state flow system. The rate of methanol production was 11.1 μmol/g h with light intensity of 327 mW/cm2 at 25 °C. Increasing the reaction temperature to 75 °C increased the production rate to 21.0 μmol/g h. Methanol production rate was 11.30 μmol/g h by utilizing concentrated sunlight which was comparable to the result of using artificial visible light. The quantum efficiencies were estimated to be 0.0045% and 0.063% in aqueous-phase and optical-fiber reactors, respectively, per gram NiO/InTaO4 photocatalyst. The quantum efficiency increased due to the superior light-energy utilization of NiO/InTaO4 thin film in the optical-fiber reactor

In situ DRIFTS study of photocatalytic CO 2 reduction under UV irradiation

Abstract: Photocatalytic reduction of CO2 on TiO2 and Cu/TiO2 photocatalysts was studied by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) under UV irradiation. The photocatalysts were prepared by sol-gel method via controlled hydrolysis of titanium (IV) butoxide. Copper precursor was loaded onto TiO2 during sol-gel procedure. A large amount of adsorbed H2O and surface OH groups was detected at 25°C on the TiO2 photocatalyst after being treated at 500°C under air stream. Carbonate and bicarbonate were formed rapidly due to the reaction of CO2 with oxygen-vacancy and OH groups, respectively, on TiO2 surface upon CO2 adsorption. The IR spectra indicated that, under UV irradiation, gas-phase CO2 further combined with oxygen-vacancy and OH groups to produce more carbonate or bicarbonate. The weak signals of reaction intermediates were found on the IR spectra, which were due to the slow photocatalytic CO2 reduction on photocatalysts. Photogenerated electrons merge with H+ ions to form H atoms, which progressively reduce CO2 to form formic acid, dioxymethylene, formaldehyde and methoxy as observed in the IR spectra. The well-dispersed Cu, acting as the active site significantly increases the amount of formaldehyde and dioxymethylene, thus promotes the photoactivity of CO2 reduction on Cu/TiO2. A possible mechanism of the photocatalytic CO2 reduction is proposed based on these intermediates and products on the photocatalysts.

Biodiesel Synthesis by Simultaneous Esterification and Transesterification Using Oleophilic Acid Catalyst

Abstract: Solid-acid catalysts can perform transesterification and esterification simultaneously so that free fatty acids (FFAs) in oil can be converted into biodiesel to avoid the disposal of biomaterial. A carbon catalyst was prepared by pyrolyzing glucose at 400 °C under a N2 stream. The catalyst was further sulfated using concentrated sulfuric acid. Transesterification of soybean oil and methanol was carried out at 150 °C and 1.7 MPa in a pressurized autoclave. More than 90% biodiesel yield was achieved within 2 h with a molar ratio of methanol to soybean oil of 30:1. The total biodiesel yield for the mixture of soybean oil and palmitic acid decreased to 85% when 20 wt % palmitic acid was used. A rate equation based on the Langmuir−Hishelwood mechanism was established to describe the kinetic behavior of transesterification. The adsorption equilibrium constant of soybean oil was higher than those of the other species, implying an oleophilic surface of the sulfated carbon catalyst.

Platinum nanoparticles embedded in pyrolyzed nitrogen-containing cobalt complexes for high methanol-tolerant oxygen reduction activity

Abstract: High oxygen reduction activity of methanol-tolerant catalysts was successfully reported using platinum nanoparticles embedded in cobalt-based nitrogen-containing complexes supported on carbon blacks (Pt–N-complex/C). The oxygen reduction reaction (ORR) of the Pt–N-complex/C was attributed to four-electron transfer pathway in which oxygen was directly reduced to water, yielding four electrons. In a methanol-containing solution, the platinum intrinsically favors the methanol oxidation reaction over the ORR, which is a major drawback for direct methanol fuel cells (DMFCs). In comparison, when the Pt–N-complex/C is introduced in a methanol-containing solution, not only is the methanol oxidation suppressed but also the four-electron-transfer in the ORR is maintained up to the diffusion-limiting region. Physicochemical characterization of the Pt–N-complex/C indicates that pyrrolic N-type poly-aromatic hydrocarbons were formed in a network structure around the catalysts and prevented them from the methanol oxidation reaction. In a DMFC test at elevated methanol concentrations, the one with the Pt–N-complex/C cathode showed superior stability over the one with the Pt-based cathode, which may offer a solution to the methanol crossover problem in DMFCs.

Photocatalytic reduction of NO pollutant using an optical‐fibre photoreactor at room temperature

Abstract: Photo‐assisted catalytic reduction of nitric oxide (NO) was studied over different metal‐loaded TiO2 catalysts at room temperature. The activities of metal‐loaded (Pt, Ag, Cu) TiO2 photocatalysts, prepared by the sol–gel method, were compared in a batch system using CH4 as the reducing agent. The Pt/TiO2 catalyst showed the highest activity for NO reduction. Thus, Pt/TiO2 was coated on optical fibres and used in a continuous‐flow optical‐fibre photoreactor. The optical‐fibre photoreactor provides light irradiation on the photocatalyst through the optical fibre, thus improving the efficiency of photoreactions. Ten per cent conversion of NO was found using CH4 as the reducing agent. The NO conversions increased to 90% in the presence of water vapour and oxygen. However, most NO was oxidized to NO2. Hydrogen had superior reducing capabilities over CH4 on Pt/TiO2 photocatalyst, and the conversion of NO reached 85%. But the conversion of NO was substantially decreased to less than 10% in the presence of water ...

Renewable Energy from the Photocatalytic Reduction of CO2 with H2O

Abstract: Sun is the Earth’s ultimate and inexhaustible energy source. With the advantage of generating renewable energy, one of the best routes to remedy CO2 is to transform it to hydrocarbons using photoreduction. CO2 was photocatalytically reduced to produce methanol in an aqueous batch reactor and a steady-state optical–fiber reactor under UV irradiation. Titania and Cu-loaded titania were synthesized using titanium (IV) butoxide by sol-gel method. The catalyst was dip-coated on optical fiber. The optical–fiber photoreactor, comprised of nearly 120 photocatalyst-coated fibers, was designed and assembled to transmit and spread light uniformly inside the reactor. The coating film consisted of very fine spherical particles with diameters of near 14 nm. The XRD spectra indicated the anatase phase for all TiO2 and Cu/TiO2 catalysts. XPS analysis revealed primary Cu2O species on the TiO2 supports. The most active Cu species on TiO2 surface were Cu2O clusters and they played an important role for the formation of methanol. The methanol yield rates increased with UV irradiative intensity. Maximum methanol rate was 0.45 μmol/g cat h using 1.2 wt% Cu/TiO2 catalyst at 129 kPa of CO2, 2.6 kPa of H2O, and 5,000 s mean residence time under 16 W/cm2 UV irradiation. Higher than 2 wt% Cu loading gave a lower rate of methanol yield rate because of the masking effect of Cu2O clusters on the TiO2 surface. A Langmuir–Hinshelwood model was established by correlating experimental data to describe the kinetic behavior. An optimum pressure ratio of H2O/CO2 was found in the photoreduction of CO2 for maximum methanol yield rate.