Photocatalysis
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.
History
Early mentions (1911–1938)
The earliest mention came in 1911, when German chemist Dr. Alexander Eibner integrated the concept in his research of the illumination of zinc oxide on the bleaching of the dark blue pigment, Prussian blue. Around this time, Bruner and Kozak published an article discussing the deterioration of oxalic acid in the presence of uranyl salts under illumination, while in 1913, Landau published an article explaining the phenomenon of photocatalysis. Their contributions led to the development of actinometric measurements, measurements that provide the basis of determining photon flux in photochemical reactions. After a hiatus, in 1921, Baly et al. used ferric hydroxides and colloidal uranium salts as catalysts for the creation of formaldehyde under visible light.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.
1964–2024
Research in photocatalysis again paused until 1964, when V.N. Filimonov investigated isopropanol photooxidation from ZnO and ; while in 1965 Kato and Mashio, Doerffler and Hauffe, and Ikekawa et al. explored oxidation/photooxidation of and organic solvents from ZnO radiance. In 1970, Formenti et al. and Tanaka and Blyholde observed the oxidation of various alkenes and the photocatalytic decay of N2O, respectively.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 and Sakata and Kawai delineated hydrogen production on the surface of strontium titanate 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. Chu et al. assessed the future of electrochemical photolysis of water, discussing its major challenge of developing a cost-effective, energy-efficient photoelectrochemical tandem cell, which would, "mimic natural photosynthesis".
Types of photocatalysis
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 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, 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 and excited electrons, oxidation-reduction 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 surface by photon 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 and anatase 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 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.
Homogeneous photocatalysis
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: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 :
Complex-based photocatalysts are semiconductors, and operate under the same electronic properties as heterogeneous catalysts.
Plasmonic antenna-reactor photocatalysis
A plasmonic antenna-reactor photocatalyst is a photocatalyst that combines a catalyst with attached antenna that increases the catalyst's ability to absorb light, thereby increasing its efficiency.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.