Photocatalysis

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According to Plotnikow, the phenomenon of photocatalysis ( photocatalysis in standard language ) describes every chemical reaction triggered by light . Balzani describes kinetically inhibited exergonic reactions between two partners A and B that are induced by light excitation as photocatalysis.

Wilhelm Ostwald, on the other hand, provides the following classic definition of catalysis that is still valid today :

“Finally there are numerous substances whose addition, even in very small quantities, changes the speed of a chemical reaction. In the majority of cases these substances do not take part in the chemical process insofar as they are found again in practically unchanged quantities during and after it. However, this does not preclude them from participating in the response, but only requires the assumption that the participation is temporary; H. that these substances (...) are released from the reaction products in an unchanged state. "

This reveals a problem: photons themselves cannot be regarded as catalysts , since they are consumed during the reaction and are not available again in an “unchanged state”. However, a large number of reactions are known that take place only moderately or not at all at room temperature , but are considerably accelerated when exposed to light. An example of this is photosynthesis in plants , in which the photonic energy absorbed by sunlight is transferred to the catalytic reaction of carbon dioxide and water to form carbohydrates and oxygen .

In modern literature, photocatalytic reactions are divided into two categories:

  • the catalytic photoreaction and the
  • photosensitized catalysis , the latter being differentiated into one
    • photoinduced catalytic reaction and a
    • photo-assisted catalytic reaction .

Catalytic photoreaction

Outline sketch of a catalytic photoreaction

A photoactivated molecule interacts with a catalyst that is in the electronic ground state. Without a photochemical excitation of the substrate (A), no reaction takes place despite the presence of the catalyst (left side). The reaction is catalyzed by the catalyst (K) only after the photochemical excitation of the substrate (A ') has taken place.

Photosensitized Catalysis

The catalyst is converted directly into an electronically excited state. The catalyst is said to have the same effect as classic catalysis according to Wilhelm Ostwald , but the special properties of electronically excited species must be taken into account when describing the course of the reaction.

Photo-induced catalytic reaction

Outline sketch of a catalytic photoreaction

One-time photochemically excited catalyst (K *) causes catalytic substrate conversion (S) to a transition state (P *) with the help of which a catalyst molecule is excited again.

Example of a photographic process: the catalyst, metallic silver, is generated during exposure according to the above principle and then catalyzes the reduction of further silver bromide with simultaneous oxidation of the developer .

Photo-assisted catalytic reaction

Outline sketch of a catalytic photoreaction

The photo-assisted catalytic reaction requires permanent exposure to light of a suitable wavelength to generate the excited catalyst state (K *). If the light radiation is stopped, the reaction also stops.

Anatase , a modification of titanium dioxide , is often used as a catalyst . A photo-assisted catalytic reaction is made up of a number of physical and chemical processes. It all starts with the absorption of light. Electrons are excited in states with higher energy, with a semiconductor like titanium dioxide from states of the valence band to states of the conduction band (see also band model ). This is equivalent to the formation of an electron-hole pair. The next step is the spatial separation of electron and hole and their diffusion on the surface of the semiconductor. There the electron triggers a reduction and the hole triggers oxidation (e.g. a very reactive radical (OH . ) Can be formed from a hydroxide ion (OH - ) ). Often, further chemical reactions follow.

Applications

literature

Web links

supporting documents

  1. ^ Wilhelm Ostwald: Principles of Chemistry. An introduction to all chemistry textbooks . BoD - Books on Demand, 2011, ISBN 3-86195-689-6 , pp. 461 ( limited preview in Google Book search).