In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalyzed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity depends on the ability of the catalyst to create electron–hole pairs, which generate free radicals (e.g. hydroxyl radicals: •OH) able to undergo secondary reactions. Its practical application was made possible by the discovery of water electrolysis by means of titanium dioxide (TiO
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 (ZnO) 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 TiO
2 , 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 TiO
2 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.
Research in photocatalysis again paused until1964, when V.N. Filimonov investigated isopropanol photooxidation from ZnO and TiO
2 ; while in 1965 Kato and Mashio, Doerffler and Hauffe, and Ikekawa et al. (1965) explored oxidation/photooxidation of CO
2 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 TiO
2 electrode irradiated with ultraviolet light was electrically connected to a platinum electrode. As the ultraviolet light was absorbed by the TiO
2 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 TiO
2 and PtO2 in ethanol, respectively.
Photocatalysis has not been developed for commercial purposes. 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".
Types of photocatalysis
In homogeneous photocatalysis, the reactants and the photocatalysts exist in the same phase. The most commonly used homogeneous photocatalysts include ozone and photo-Fenton systems (Fe+ and Fe+/H2O2). The reactive species is the •OH radical, which is used for various purposes. The mechanism of hydroxyl radical production by ozone can follow two paths:
- O3 + hν → O2 + O(1D)
- O(1D) + H2O → •OH + •OH
- O(1D) + H2O → H2O2
- H2O2 + hν → •OH + •OH
Similarly, the Fenton system produces hydroxyl radicals by the following mechanism:
- Fe2+ + H2O2→ HO• + Fe3+ + OH−
- Fe3+ + H2O2→ Fe2+ + HO•2 + H+
- Fe2+ + HO• → Fe3+ + OH−
In photo-Fenton type processes, additional sources of OH radicals should be considered, such as photolysis of H2O2 and reduction of Fe3+ ions under UV light:
- H2O2 + hν → HO• + HO•
- Fe3+ + H2O + hν → Fe2+ + HO• + H+
The efficiency of Fenton type processes is influenced by several operating parameters like the concentration of hydrogen peroxide, pH and intensity of UV. The main advantage of this process is the ability of using sunlight with light sensitivity up to 450 nm, thus avoiding the high costs of UV lamps and electrical energy. These reactions have been proven more efficient than other examples of photocatalysis but the disadvantages of the process are the low pH values, which are required since iron precipitates at higher pH values and the fact that iron has to be removed after treatment.
Homogeneous photocatalysis can also be conducted by Cu(II)/Cu(I) complexes.The photoredox behavior of Cu(II) complexes, similar to Fe(III) complexes, is derived mostly from the reactive decay of their LMCT states. Excitation to LMCT states can be achieved by direct sunlight when the ionization energy of the ligands coordinated to Cu(II) is not very high. In consequence of the reactive decay of the LMCT excited state by inner-sphere electron transfer, the Cu(II) central atom is reduced to Cu(I), whereas the ligand is oxidized to its radical and leaves the coordination sphere:
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 void region of energy, which extends from the top of the filled valence band to the bottom of the vacant conduction band, is called the band gap. When a photon with energy equal to or greater than the material's band gap is absorbed by the semiconductor, an electron is excited from the valence band to the conduction band, generating a hole in the valence band. Such a photogenerated electron-hole pair is termed 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 efficieny. 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 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 (MO) surface:
- 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− + H2O + H+ → H2O2 + O2
- H2O2 → 2 •OH
Ultimately, hydroxyl radicals are generated in both reactions. These radicals are oxidative in nature and nonselective with a redox potential of E0 = +3.06 V.
2 is a common choice for heterogeneous catalysis. Inertness to chemical environment and long-term photostability has made TiO
2 an important material in many practical applications. TiO
2 is a wide band-gap semiconductor. It is commonly investigated in the rutile (bandgap 3.0 eV) and anatase (bandgap 3.2 eV) phases. Photocatalytic reactions are initiated by the absorption of illumination with energy equal to or greater than the band gap of the semiconductor. 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 TiO
2 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.
Plasmonic antenna-reactor photocatalysis
2 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 800–1,000 °C (1,470–1,830 °F).
A Fe catalyst combined with a Cu light absorber can produce hydrogen from ammonia (NH
3) at ambient temperature using visible light. Conventional Cu-Ru production operates at 650–1,000 °C (1,202–1,832 °F).
Photoactive catalysts have been introduced over the last decade, such as TiO
2 and ZnO nano rodes. Most suffer from the fact that they can only perform under UV irradiation due to their band structure. Other photocatalysts, including a graphene-ZnO nanocompound counter this problem.
Micro-sized ZnO tetrapodal particles added to pilot paper production. The most common are one-dimensional nanostructures, such as nanorods, nanotubes, nanofibers, nanowires, but also nanoplates, nanosheets, nanospheres, tetrapods. ZnO is strongly oxidative, chemically stabile, with enhanced photocatalytic activity, and has a large free-exciton binding energy. It is non-toxic, abundant, biocompatible, biodegradable, environmentally friendly, low cost, and compatible with simple chemical synthesis. ZnO faces limits to its widespread use in photocatalysis under solar radiation. Several approaches have been suggested to overcome this limitation, including doping for reducing the band gap and improving charge carrier separation.
Photocatalytic water splitting separates water into hydrogen and oxygen. The most prevalently investigated material, TiO
2 , has limited production efficiency, was mixed with nickel oxide (NiO). NiO allows a significant explоitation of the visible spectrum. One efficient photocatalyst in the UV range is based on sodium tantalite (NaTaO3) doped with lanthanum and loaded with a nickel oxide cocatalyst. The surface is grooved with nanosteps from doping with lanthanum (3–15 nm range, see nanotechnology). The NiO particles are present on the edges, with the oxygen evolving from the grooves.
Titanium dioxide takes part in self-cleaning glass. Free radicals generated from TiO
2 oxidize organic matter. The rough wedge-like TiO
2 surface can be modified with a hydrophobic monolayer of octadecylphosphonic acid (ODP). TiO
2 surfaces that were plasma etched for 10 seconds and subsequent surface modifications with ODP showed a water contact angle greater than 150◦. The surface was converted into a superhydrophilic surface (water contact angle = 0◦) upon UV illumination, due to rapid decomposition of octadecylphosphonic acid coating resulting from TiO
2 photocatalysis. Due to TiO
2 's wide band gap, light absorption by the semiconductor material and resulting superhydrophilic conversion of undoped TiO
2 requires ultraviolet radiation (wavelength <390 nm) and thereby restricts self-cleaning to outdoor applications.
Disinfection and cleaning
- Water disinfection/decontamination, a form of solar water disinfection (SODIS). Adsorbents attract organics such as tetrachloroethylene. Adsorbents are placed in packed beds for 18 hours. Spent adsorbents are placed in regeneration fluid, essentially removing organics still attached by passing hot water opposite to the flow of water during adsorption. The regeneration fluid passes through fixed beds of silica gel photocatalysts to remove and decompose remaining organics.
2 self-sterilizing coatings (for application to food contact surfaces and in other environments where microbial pathogens spread by indirect contact).
- Magnetic TiO
2 nanoparticle oxidation of organic contaminants agitated using a magnetic field.
- Sterilization of surgical instruments and removal of fingerprints from electrical and optical components.
Hydrocarbon production from CO
2 conversion of CO
2 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 TiO
2 -based photocatalysts are low, although nanostructures such as carbon nanotubes and metallic nanoparticles help.
ePaint is a less-toxic alternative to conventional antifouling marine paints that generates hydrogen peroxide.
Photocatalysis of organic reactions by polypyridyl complexes, porphyrins, or other dyes can produce materials inaccessible by classical approaches. Most photocatalytic dye degradation studies have employed TiO
2. The anatase form of TiO
2 has higher photon absorption characteristics.
2 nanoparticle decomposition of crude oil can turn hydrocarbons into H2O and CO2. The particles can be placed on floating substrates, which are easier to recover and catalyze the reaction. This is relevant since oil slicks float on the ocean suface and photons from the sun reach the surface. Covering floating substrates with epoxy adhesives prevents waterlogging and TiO
2 particles can stick to them.
Decomposition of polyaromatic hydrocarbons (PAHs). Triethylamine (TEA) solvates and extracts PAHs in crude oil. TEA attracts the PAHs to itself. TiO
2 slurries can degrade the PAHs. Recoveries of 93–99% of these contaminants have been demonstrated at ambient pressure and temperature, and at lower cost.
Light2CAT was a project funded by the European Commission from 2012 to 2015. It aimed to develop a modified TiO
2 that can absorb visible light and include this modified TiO
2 into construction concrete. The TiO
2 degrades harmful pollutants such as NOx into NO3−. The modified TiO
2 was utilized 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.
ISO 22197-1:2007 specifies a test method for the measurement of NO
2 removal for materials that contain a photocatalyst or have superficial photocatalytic films.
Mass spectrometry allows measurement of photocatalytic activity by tracking the decomposition of gaseous pollutants such as nitrogen NOx or CO
- Light harvesting materials
- Photoelectrochemical cell
- Photocatalytic water splitting
- Photoredox catalysis
- Photoelectrochemical oxidation
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