TiO2-SiO2 Photocatalysts Research Articles

Photocatalytic Reduction of Greenhouse Gas CO2 to Fuel

Sun is the Earth’s ultimate and inexhaustible energy source. One of the best routes to remedy the CO2 problem is to convert it to valuable hydrocarbons using solar energy. In this study, CO2 was photocatalytically reduced to produce methanol, methane and ethylene in a steady-state optical-fiber reactor under artificial light and real sunlight irradiation. The photocatalyst was dip-coated on the optical fibers that enable the light to transmit and spread uniformly inside the reactor. The optical-fiber photoreactor, comprised of nearly 120 photocatalyst-coated fibers, was designed and assembled. The XRD spectra indicated the anatase phase for all photocatalysts. It is found that the methanol yield increased with UV light intensity. A maximum methanol yield of 4.12 μmole/g-cat h is obtained when 1.0 wt% Ag/TiO2 photocatalyst was used under a light intensity of 10 W/cm2. When mixed oxide, TiO2–SiO2, is doped with Cu and Fe metals, the resulting photocatalysts show substantial difference in hydrocarbon production as well as product selectivity. Methane and ethylene were produced on Cu–Fe loaded TiO2–SiO2 photocatalyst. Since dye-sensitized Cu–Fe/P25 photocatalyst can fully harvest the light energy of 400–800 nm from sunlight, its photoactivity was significantly enhanced. Finally, CO2 photoreduction was studied by in situ IR spectroscopy and possible mechanism for the photoreaction was proposed.

Photooxidation of a Mustard Gas Simulant over TiO2−SiO2 Mixed-Oxide Photocatalyst: Site Poisoning by Oxidation Products and Reactivation

The photooxidation of 2-chloroethyl ethyl sulfide (2-CEES), a simulant for mustard gas, was studied using transmission IR spectroscopy on a mixed-oxide TiO2-SiO2 photocatalyst. Ultraviolet irradiation in the photon energy range from 2.1 to 5 eV was employed at a catalyst temperature of 200 K. Rapid photooxidation was observed by the loss of infrared intensity in the v(CHx) stretching region, and concomitant infrared features of adsorbed oxidation products were observed to develop. The oxidation products, captured on the photocatalyst at 200 K, were found to block 2-CEES readsorption. Upon heating the poisoned photocatalyst to about 300 K, infrared measurements indicate that the adsorbed CO2 oxidation product was desorbed. The capability for full readsorption of 2-CEES was achieved upon heating the poisoned photocatalyst to 397 K, and continued rapid photooxidation of the 2-CEES was then possible at about 1/3 the rate found for the fresh catalyst. Thus thermal treatment at 397 K of oxidation-product-poisoned TiO2-SiO2 material is able to partially restore the TiO2-SiO2 photooxidation activity.

Electron exchange on TiO2–SiO2 photocatalysts during O2 and organic molecule adsorption – the role of adsorbate electrophilicity

The mixed oxide photocatalyst, TiO2–SiO2 has been observed to contain trapped conduction band electrons after annealing in vacuum. These electrons are consumed by adsorption of particular electrophilic molecules. It is found that O2 and an adsorbed organic molecule containing a Cl moiety can remove trapped electrons, whereas a similar organic molecule, not containing Cl, is ineffective in charge removal. The measurement of trapped electronic charge is made by observation of the intensity of the background IR absorption in the 4000 to 1400 cm−1 region, which originates from trapped electron excitation into a continuum of excited electronic states by IR photons.