Semiconductor-semiconductor catalyst system

The Matrix TiOa photocatalytic treatment system is a technology that destroys dissolved organic contaminants in water in a continuous-flow process at ambient temperature. The technology uses ultraviolet (UV) light and a titanium dioxide (TiOa) semiconductor catalyst to break hydroxide ions (OH ) and water (H2O) into hydroxyl radicals (OH ). The radicals oxidize the organic contaminants to form carbon dioxide, water, and halide ions (if the contaminant was halogenated). 

The low reproducibility of certain of the systems involving heterogeneous catalysts, described above, as well as the potential for charge separation using semiconductor catalysts has led to the use of heterogeneous catalysts supported on semiconducting materials, e.g. Ti02. 

Colloidal Semiconductors in Systems for the Sacrificial Photolysis of Water. 1. Preparation of a Pt/Ti02 Catalyst by Heterocoagulation and its Physical Characterization 254... 

Photocatalytic reduction of C02 to organic compounds was carried out [327] in a semiconductor suspension system under simulated solar power using a Ti02 catalyst. Experimental results show that the photocatalytic activity can be... 

On the basis of the above discussion, in its most simple description photocatalysis implies a catalysed process preceded by absorption of a photon by a material acting as the catalyst. Where the photocatalyst is a semiconductor nanoparticulate system, e.g. TiOi, which is the material treated in most detail in this chapter, absorption of photons of energy greater than 3.2 eV (for anatase 3.0 eV for the rutile polymorph) leads to formation of conduction-band electrons and valence-band holes, which subsequently diffnse to the particle surface in competition with bnlk recombination. 

Many speciahzed laboratory reactors and operating conditions have been used. Sinfelt has alternately passed reactants and inert materials through a tubular-flow reactor. This mode of operation is advantageous when the activity of the fixed bed of catalyst pellets changes with time. A system in which the reactants flow through a porous semiconductor catalyst, heated inductively, has been proposed for studying the kinetics of high-temperature (500 to 2000°C) reactions. An automated microreactor... 

T., A semiconductor gas sensor system for high throughput screening of heterogeneous catalysts for the production of benzene derivatives, Afeas. Sci. Technol., 16(1), 229-234, 2005. 

Semiconductor oxides are also important support materials. Even if a support is inactive in the reaction imder consideration, it can considerably change the reactivity of the catalyst that it supports. As an example, metals such as Ni and Ag are often applied to doped AI2O3 by vapor-phase deposition. The resulting catalyst system behaves like a rectifier in that electrons flow from the support through the catalyst metal to the reac-tauts (Eq. 5-61). Hence in this case acceptor reactions are favored. 

In water photooxidation by semiconductor photocatalysis, a sacrificial electron acceptor A, such as Fe or Ag" ions, is usually added to the system to prevent accumulation of any photogenerated electrons. Transition metal oxides, such as RUO2 or Ir02, which are recognised O2 evolution catalysts, are often deposited on the surface of the semiconductor catalyst to improve the efficiency of water oxidation. 

The semiconductor catalyst is generally used as powder suspended in a hquid medium. The inconvenience of this approach at large scale is the catalyst-recovering step from the solution at the end of operation. The sohd-liquid separation is an extremely important issue for the development of the photocatalytic technology indeed, the best possible recovery of particles must be ensured, in order to prevent their wash out and consequent decrease of their amount in the reactor system. 

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