User:TChem987/Photocatalysis

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In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a photocatalyst, the excited state of which "repeatedly interacts with the reaction partners forming reaction intermediates and regenerates itself after each cycle of such interactions." In many cases, the catalyst is a solid that upon irradiation with UV- or visible light generates electron–hole pairs that generate free radicals. Photocatalysts belong to three main groups; heterogeneous, homogeneous, and plasmonic antenna-reactor catalysts. The use of each catalysts depends on the preferred application and required catalysis reaction.

Types of photocatalysis[edit]

Heterogeneous photocatalysis

In heterogeneous catalysis the catalyst is in a different phase from the reactants. Heterogeneous photocatalysis is a discipline which includes a large variety of reactions: mild or total oxidations, dehydrogenation, hydrogen transfer, ¹⁸O₂–¹⁶O₂ and deuterium-alkane isotopic exchange, metal deposition, water detoxification, and gaseous pollutant removal.

Most heterogeneous photocatalysts are transition metal oxides and semiconductors. Unlike metals, which have a continuum of electronic states, semiconductors possess a void energy region where no energy levels are available to promote recombination of an electron and hole produced by photoactivation in the solid. The difference in energy between the filled valence band and the empty conduction band in the MO diagram of a semiconductor is the band gap. When the semiconductor absorbs a photon with energy equal to or greater than the material's band gap, an electron excites from the valence band to the conduction band, generating a electron hole in the valence band. This electron-hole pair is an exciton. The excited electron and hole can recombine and release the energy gained from the excitation of the electron as heat. Such exciton recombination is undesirable and higher levels cost efficiency. Efforts to develop functional photocatalysts often emphasize extending exciton lifetime by improving electron-hole separation using diverse approaches that may rely on structural features such as phase hetero-junctions (e.g. anatase-rutile interfaces), noble-metal nanoparticles, silicon nanowires and substitutional cation doping. The ultimate goal of photocatalyst design is to facilitate reactions of the excited electrons with oxidants to produce reduced products, and/or reactions of the generated holes with reductants to produce oxidized products. Due to the generation of positive holes (h⁺) and excited electrons (e^-), oxidation-reduction reactions take place at the surface of semiconductors irradiated with light.

In one mechanism of the oxidative reaction, holes (h⁺) react with the moisture present on the surface and produce a hydroxyl radical. The reaction starts by photo-induced exciton generation in the metal oxide (MO) surface by photon (hv) absorption:

MO + hν → MO (h⁺ + e⁻)

Oxidative reactions due to photocatalytic effect:

h⁺ + H₂O → H⁺ + •OH

2 h⁺ + 2 H₂O → 2 H⁺ + H₂O₂

H₂O₂→ 2 •OH

Reductive reactions due to photocatalytic effect:

e⁻ + O₂ → •O₂⁻

•O₂⁻ + HO₂• + H⁺ → H₂O₂ + O₂

H₂O₂ → 2 •OH

Ultimately, both reactions generate hydroxyl radicals. These radicals are oxidative in nature and nonselective with a redox potential of E₀ = +3.06 V. This is significantly greater than many common organic compounds, which typically are not greater than E₀ = +2.00 V. This results in the non-selective oxidative behavior of these radicals.

TiO₂, a wide band-gap semiconductor, is a common choice for heterogeneous catalysis. Inertness to chemical environment and long-term photostability has made TiO₂ an important material in many practical applications. Investigation in the rutile (bandgap 3.0 eV) and anatase (bandgap 3.2 eV) phases is common for TIO₂. The absorption of photons with energy equal to or greater than the band gap of the semiconductor initiates photocatalytic reactions. This produces electron-hole (e⁻ /h⁺) pairs:

${\ce {TiO2 ->[{hv}] e-(TiO2) + h+(TiO2)}}$

Where the electron is in the conduction band and the hole is in the valence band. The irradiated TiO₂ particle can behave as an electron donor or acceptor for molecules in contact with the semiconductor. It can participate in redox reactions with adsorbed species, as the valence band hole is strongly oxidizing while the conduction band electron is strongly reducing.

Homogeneous photocatalysis[edit]

In homogeneous photocatalysis, the reactants and the photocatalysts exist in the same phase. The process by which the atmosphere self-cleans and removes large organic compounds is a gas phase homogenous photocatalysis reaction. The ozone process is often referenced when developing many photocatalysts:

${\ce {2 O3(g) + H2O(g) ->[{hv}] O2(g) + 2OH.}}$

Most homogeneous photocatalytic reactions are aqueous phase, with a transition-metal complex photocatalyst. The wide use of transition-metal complexes as photocatalysts is in large part due to the large band gap and high stability of the species. Homogeneous photocatalysts are common in the production of clean hydrogen fuel production, with the notable use of cobalt and iron complexes.

Iron complex hydroxy-radical formation using the ozone process is common in the production of hydrogen fuel (similar to Fenton's reagent process done in low pH conditions without photoexcitation):

${\ce {Fe^2+ +H2O2 ->[{hv}] Fe^3+ +OH- + HO.}}$

${\ce {Fe^3+ +H2O2 ->[{hv}] Fe^2+ H+ + HO2.}}$

${\ce {Fe^2+ +HO. -> Fe^3+ +OH-}}$

Complex-based photocatalysts are semiconductors, and operate under the same electronic properties as heterogeneous catalysts.

Applications[edit]

Filtration membranes

Photocatalyst radical generation species allow for the degradation of organic pollutants into non-toxic compounds at a high efficiency. Use of CuO nanosheets to breakdown azo bonds in food dyes is one such example, with 96.99% degradation after only 6 minutes. Degradation of organic matter is a highly applicable property, particularly in waste processing.

The use of photocatalyst TiO2 as a support system for filtration membranes shows promise in improving membrane bioreactors in the treatment of wastewater. Polymer-based membranes have shown reduced fouling and self-cleaning properties in both blended and coated TiO₂ membranes. Photocatalyst-coated membranes show the most promise, as the increased surface exposure of the photocatalyst increases its organic degradation activity.

Photocatalysts are also highly effective reducers of toxic heavy metals like hexavalent chromium from water systems. Under visible light the reduction of Cr(VI) by a Ce-ZrO₂ sol-gel on a silicon carbide was 97% effective at reducing the heavy metal to trivalent chromium.

Air Filtration

Light2CAT was a project funded by the European Commission from 2012 to 2015. It aimed to develop a modified TiO₂ that can absorb visible light and include this modified TiO₂ in construction concrete. The TiO₂ degrades harmful pollutants such as NOx into NO₃⁻. The modified TiO₂ is in use in Copenhagen and Holbæk, Denmark, and Valencia, Spain. This “self-cleaning” concrete led to a 5-20% reduction in NOx over the course of a year.