Kinetic study of photocatalytic oxidation of adsorbed carboxylic acids at TiO2 porous films by photoelectrolysis

Development of chemical oxygen demand on-line monitoring system based on a photoelectrochemical degradation principle

A simple, rapid, and sensitive on-line chemical oxygen demand (COD) determination method has been proposed and experimentally validated. The method is based on a photoelectrochemical oxidative degradation principle and operates under a continuous flow mode. The method employs a specially designed thin-layer photoelectrochemical cell that incorporates a highly effective nanoparticulate TiO2 photoanode. This approach overcomes many problems associated with the conventional COD determination techniques such as long analysis time, consumption of expensive and toxic reagents, production of secondary toxic waste, and poor reproducibility. The effect of important experimental parameters on the analytical signal generation was systematically investigated, and the optimum conditions were obtained. The method was successfully applied to determine the COD of real samples from various industrial wastewaters. The COD value of real samples determined by this method agreed well with the standard dichromate method. The assay time of 1-5 min/sample can be readily achieved. A practical detection limit of 1 mg L(-1) COD with a linear range of 1-100 mg L(-1) was achieved under the optimum conditions.

Effects of Polyoxometalate and Fluoride on Adsorption and Photocatalytic Degradation of Organic Dye X3B on TiO2: The Difference in the Production of Reactive Species

It has been reported that addition of polyoxometalates (POM) or fluoride anions into the TiO(2) dispersions can significantly enhance the photocatalytic degradation (PCD) of weakly adsorptive organic pollutants in water such as chlorophenol. In this work, however, contradictory effects of POM and fluoride were observed on the PCD of highly adsorptive substrate X3B, an anionic organic dye, under similar conditions. The total rate of X3B PCD, determined by total loss of X3B both in solution and on the catalyst surface, was increased in the presence of fluoride, but the rate was decreased in the presence of POM. In both cases, the dark adsorption of X3B on TiO(2) was greatly decreased, ascribed to competitive adsorption of POM or fluoride that reduces the positive charges on the catalyst surface. The spectral analysis and the kinetic study using tert-butyl alcohol as hydroxyl radical scavenger revealed that the PCD of X3B on naked TiO(2) was predominately initiated by direct hole transfer, whereas addition of POM or fluoride into the TiO(2) dispersions enhanced the degradation of X3B via hydroxyl radical pathway. It is proposed that the surface occupation of POM on TiO(2) accelerates the production of surface-bound hydroxyl radicals, due to enhanced charge separation, whereas the fluoride replacement of surface hydroxyl groups of TiO(2) increases the production of free hydroxyl radicals in solution, due to enhanced hole availability for water oxidation. Assume that the relative reactivity among various active follows the order of free hydroxyl radicals > subsurface holes > surface-bound hydroxyl radicals, the proposal could account for the observed effects of POM and fluoride on the PCD of both weakly and highly adsorptive organic substrates over TiO(2) such as chlorophenol and X3B.

Ab initio nonadiabatic molecular dynamics of the ultrafast electron injection across the alizarin-TiO2 interface

The observed 6-fs photoinduced electron transfer (ET) from the alizarin chromophore into the TiO2 surface is investigated by ab initio nonadiabatic (NA) molecular dynamics in real time and at the atomistic level of detail. The system derives from the dye-sensitized semiconductor Grätzel cell and addresses the problems of an organic/inorganic interface that are commonly encountered in photovoltaics, photochemistry, and molecular electronics. In contrast to the typical Grätzel cell systems, where molecular donors are in resonance with a high density of semiconductor acceptor states, TiO2 sensitized with alizarin presents a novel case in which the molecular photoexcited state is at the edge of the conduction band (CB). The high level ab initio analysis of the optical absorption spectrum supports this observation. Thermal fluctuations of atomic coordinates are particularly important both in generating a nonuniform distribution of photoexcited states and in driving the ET process. The NA simulation resolves the controversy regarding the origin of the ultrafast ET by showing that although ultrafast transfer is possible with the NA mechanism, it proceeds mostly adiabatically in the alizarin-TiO2 system. The simulation indicates that the electron is injected into a localized surface state within 8 fs and spreads into the bulk on a 100-fs or longer time scale. The molecular architecture seen in the alizarin-TiO2 system permits efficient electron injection into the edge of the CB by an adiabatic mechanism without the energy loss associated with injection high into the CB by a NA process.

Photoelectrochemical measurements of a heterosupramolecular system under visible light irradiation

A photochemical system utilising a modular approach characterised through interpretation of photoelectrochemical measurements is discussed. A photoanode was prepared by the chemisorption of a photosensitiser, cis-bis-(2,2'-bipyridine)-(4,4'-bis-(methyl)phosphonato-2,2'-bipyridine)ruthenium(II) dichloride (RuL2L'2+), to a mixed nanoporous nanocrystalline RuO2:TiO2 thin film, calcined on a fluorine doped SnO2 conducting glass substrate. Similarly, an electron relay molecule, 1-ethyl-1'-(2-phosphonoethyl)-4,4'-bipyridinium dichloride (EVP), was covalently bound to a platinum electroplated nanoporous nanocrystalline TiO2 thin film, and the electrodes connected in a photoelectrocatalytic cell (PCC). Irradiation with lamda > 420 nm gave a measurable photocurrent. Interpretation of the photocurrents obtained from this assembly provides a means for understanding photochemical reactions under low light intensities. Optimised conditions of the electrolyte solution were determined to be pH = 5 and illumination yielded eta = 0.0036% with an apparent quantum yield (AQY)= 1.6%.