A surface science perspective on TiO2 photocatalysis

Electronic structure intrinsically controls the light absorbance, redox potential, charge-carrier mobility, and consequently, photoreactivity of semiconductor photocatalysts. The conventional approach of modifying the electronic structure of a semiconductor photocatalyst for a wider absorption range by anion doping operates at the cost of reduced redox potentials and/or charge-carrier mobility, so that its photoreactivity is usually limited and some important reactions may not occur at all. Here, we report sulfur-doped graphitic C(3)N(4) (C(3)N(4-x)S(x)) with a unique electronic structure that displays an increased valence bandwidth in combination with an elevated conduction band minimum and a slightly reduced absorbance. The C(3)N(4-x)S(x) shows a photoreactivity of H(2) evolution 7.2 and 8.0 times higher than C(3)N(4) under lambda > 300 and 420 nm, respectively. More strikingly, the complete oxidation process of phenol under lambda > 400 nm can occur for sulfur-doped C(3)N(4), which is impossible for C(3)N(4) even under lambda > 300 nm. The homogeneous substitution of sulfur for lattice nitrogen and a concomitant quantum confinement effect are identified as the cause of this unique electronic structure and, consequently, the excellent photoreactivity of C(3)N(4-x)S(x). The results acquired may shed light on general doping strategies for designing potentially efficient photocatalysts.

Unique Electronic Structure Induced High Photoreactivity of Sulfur-Doped Graphitic C3N4

Photocatalysis (by semiconductors, molecules and ions) is used in such diverse applications as water hydrolysis for producing hydrogen as fuel, organic synthesis and the recovery of polluted effluents. This tutorial review discusses the common principles of such applications and their role in green chemistry.

Photocatalysis. A multi-faceted concept for green chemistry

This paper reviews recent studies on the semiconductor photocatalysis based on surface-modified TiO2 of which application is mainly focused on environmental remediation. TiO2 photocatalysis that is based on the photoinduced interfacial charge transfer has been extensively studied over the past four decades. A great number of modification methods of semiconductor photocatalysts have been developed and investigated to accelerate the photoconversion, to enable the absorption of visible light, or to alter the reaction mechanism to control the products and intermediates. In this regard, various modification methods of TiO2 are classified according to the kind of surface modifiers (metal-loading, impurity doping, inorganic adsorbates, polymer coating, dye-sensitization, charge transfer complexation) and their effects on photocatalytic reaction mechanism and kinetics are discussed in detail. Modifying TiO2 in various ways not only changes the mechanism and kinetics under UV irradiation but also introduces visible light activity that is absent with pure TiO2. Each modification method influences the photocatalytic activity and mechanism in a way different from others and the observed modification effects are often different depending on the test substrates and conditions even for the same modification method. Better understanding of the modification effects on TiO2 photocatalysis is necessary to obtain reliable results, to assess the photoconversion efficiency more quantitatively, and to further improve the modification methods.

Surface modification of TiO2 photocatalyst for environmental applications

https://core.ac.uk/download/pdf/10903854.pdf

Heterogeneous photocatalytic degradation of phenols in wastewater: A review on current status and developments

Six hydroxylated aromatic compounds, p-benzoquinone and four aliphatic compounds were detected as intermediates of the reaction of phenol photocatalytic oxidation in the presence of illuminated TiO2. The concentration of catechol, hydroquinone and p-benzoquinone was determined quantitatively along with that of phenol in the course of the photoreaction. The reactions of TiO2-photooxidation of phenol and its primary intermediates—dihydroxybenzenes show 1st order behaviour. The use of initial reaction rates for kinetic studies is recommended because of many competitive reactions in illuminated suspensions of titanium dioxide. A possible mechanism of phenol full mineralisation is presented.

Phenol destruction by photocatalysis on TiO2: an attempt to solve the reaction mechanism

This paper describes the basics of photocatalysis on semiconductors, mainly on TiO2 and the application of photocatalytic processes to water purification from organic matter. The second chapter is devoted to metal cocatalysts introduced in order to improve the photocatalytic action of titania. Finally, a short review of more interesting practical applications of the processes is presented.

Water purification by photocatalysis on semiconductors

The influence of silver additives on titania photoactivity in the photooxidation of phenol.

Photodeposition of gold on TiO2 was conducted in situ in the photoreactor used for the reaction of phenol photodestruction. The Au/TiO2 system shows better photocatalytic properties in the destruction of phenol contaminants in water than pure TiO2. The gold cocatalyst prevents also the decrease of the rate of the photocatalytic reaction at higher phenol concentration observed for pure TiO2. Hydroquinone, p-benzoquinone, and catechol are main intermediates of the photoreaction, which finally leads to total phenol mineralization.

Water Detoxification: Photocatalytic Decomposition of Phenol on Au/TiO2

Photocatalytic oxidation of hydroquinone and p-benzoquinone, the main intermediates which are formed in the process of phenol oxidation on irradiated titania, was studied. The reaction proceeds via several steps. Some of the intermediates were detected by HPLC and GC-MS: ethanedial, glycerol, and 1,2,4-trihydroxybenzene. Under illumination in the presence of TiO2, p-benzoquinone initially transforms partly to hydroquinone, partly oxidizes to unidentified intermediates. The residue, about 4% of the concentration of hydroquinone, undergoes slow oxidation together with the hydroquinone. The rate of hydroquinone photooxidation shows 1st order behaviour.

Andrzej Sobczyński

Papers

Phenol destruction by photocatalysis on TiO2: an attempt to solve the reaction mechanism

Water purification by photocatalysis on semiconductors

The influence of silver additives on titania photoactivity in the photooxidation of phenol.

Water Detoxification: Photocatalytic Decomposition of Phenol on Au/TiO2

Photocatalysis by Illuminated Titania:Oxidation of Hydroquinone and p-Benzoquinone