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In chemistry, photocatalysis is the acceleration of a Photochemistry 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 . 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.
In 1938 Doodeve and Kitchener discovered that , a highly-stable and non-toxic oxide, in the presence of oxygen could act as a photosensitizer for bleaching dyes, as ultraviolet light absorbed by led to the production of active oxygen species on its surface, resulting in the blotching of organic chemicals via photooxidation. This was the first observation of the fundamental characteristics of heterogeneous photocatalysis.
A breakthrough occurred in 1972, when Akira Fujishima and Kenichi Honda discovered that electrochemical photolysis of water occurred when a electrode irradiated with ultraviolet light was electrically connected to a platinum electrode. As the ultraviolet light was absorbed by the electrode, electrons flowed from the anode to the platinum cathode where hydrogen gas was produced. This was one of the first instances of hydrogen production from a clean and cost-effective source, as the majority of hydrogen production comes from natural gas reforming and gasification. Fujishima's and Honda's findings led to other advances. In 1977, Nozik discovered that the incorporation of a noble metal in the electrochemical photolysis process, such as platinum and gold, among others, could increase photoactivity, and that an external potential was not required. Wagner and Somorjai (1980) and Sakata and Kawai (1981) delineated hydrogen production on the surface of strontium titanate (SrTiO3) via photogeneration, and the generation of hydrogen and methane from the illumination of and PtO2 in ethanol, respectively.
For many decades photocatalysis had not been developed for commercial purposes. However, in 2023 multiple patents were granted to a U.S. company, (Pure-Light Technologies, Inc.) that has developed various formulas and processes that allow for widespread commercialization for VOC reduction and germicidal action.R Young. US Patents 11,906,157; 11,680,506;17/393,065 Chu et al. (2017) assessed the future of electrochemical photolysis of water, discussing its major challenge of developing a cost-effective, energy-efficient photoelectrochemical (PEC) tandem cell, which would, “mimic natural photosynthesis".
Most heterogeneous photocatalysts are transition metal oxides and . 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 Electron 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 an 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, 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 , 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−), Redox reactions take place at the surface of semiconductors irradiated with light.
In one mechanism of the oxidative reaction, holes 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:
Oxidative reactions due to photocatalytic effect:
Reductive reactions due to photocatalytic effect:
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.
Titanium dioxide, a wide band-gap semiconductor, is a common choice for heterogeneous catalysis. Inertness to chemical environment and long-term photostability has made an important material in many practical applications. Investigation of TiO2 in the rutile (bandgap 3.0 eV) and anatase (bandgap 3.2 eV) phases is common. 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:
Where the electron is in the conduction band and the hole is in the valence band. The irradiated 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.
Most homogeneous photocatalytic reactions are Aqueous solution 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. (2025). 9780128211922 ISBN 9780128211922 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):
Complex-based photocatalysts are semiconductors, and operate under the same electronic properties as heterogeneous catalysts. (2025). 9780128230077 ISBN 9780128230077
A Silicon dioxide catalyst combined with an Au light absorber accelerated hydrogen sulfide-to-hydrogen reactions. The process is an alternative to the conventional Claus process that operates at .
A Fe catalyst combined with a Cu light absorber can produce hydrogen from ammonia () at ambient temperature using visible light. Conventional Cu-Ru production operates at .
conversion of [[|carbon dioxide]] into gaseous hydrocarbons. The proposed reaction mechanisms involve the creation of a highly reactive carbon radical from carbon monoxide and carbon dioxide which then reacts with photogenerated protons to ultimately form [[methane]]. Efficiencies of -based photocatalysts are low, although nanostructures such as [[carbon nanotube]]s and [[metal]]lic nanoparticles help.
Photocatalysis of organic reactions by polypyridyl complexes, (2025). 9783527335602 ISBN 9783527335602 porphyrins, or other dyes can produce materials inaccessible by classical approaches. Most photocatalytic dye degradation studies have employed . The anatase form of has higher photon absorption characteristics.
The use of photocatalyst TiO2 as a support system for filtration shows promise in improving membrane 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.
Specific FTIR systems are used to characterize photocatalytic activity or passivity, especially with respect to volatile organic compounds, and representative binder matrices.
Mass spectrometry allows measurement of photocatalytic activity by tracking the decomposition of gaseous pollutants such as nitrogen NOx or .
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