Photocatalysis
Encyclopedia
In chemistry
, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysed photolysis, light is absorbed by an adsorbed
substrate. In photogenerated catalysis, the photocatalytic activity (PCA) depends on the ability of the catalyst to create electron–hole pairs
, which generate free radicals (hydroxyl radical
s: ·OH) able to undergo secondary reactions. Its comprehension has been made possible ever since the discovery of water
electrolysis
by means of the titanium dioxide
.
Commercial application of the process is called advanced oxidation process
(AOP). There are several methods of achieving AOP's, that can but do not necessarily involve TiO2 or even the use of UV light. Generally the defining factor is the production and use of the hydroxyl
radical
.
reactions through the formation of adsorbed radicals on TiO2 surfaces. The photocatalytic activity of TiO2 depends on the relative rates of generation and recombination of electron-hole pairs as well as the levels of adsorbed radical-forming species on TiO2 surfaces.
The two most commonly used phases of TiO2 are anatase
and rutile
. While rutile exhibits a lower band gap (~3.0 eV) in comparison to anatase (~3.2 eV) and can thus be excited by irradiation at longer wavelengths, anatase is generally exhibits superior photocatalytic activity to rutile as a result of a significantly higher surface area and thus higher levels of adsorbed radicals. It is likely that mixed phase anatase-rutile materials exhibit enhanced photocatalytic activity through an improvement in electron-hole separation, as conduction band elections become trapped in the rutile phase.
By using titanium dioxide photocatalysts and UV-A radiation from the sun, the hydrocarbons found in crude oil can be turned into H2O and CO2. Higher amounts of oxygen and UV radiation increased the degradation of the model organics. These particles can be placed on floating substrates, making it easier to recover and catalyze the reaction. This is relevant since oil slicks float on top of the ocean and photons from the sun target the surface more than the inner depth of the ocean. By covering floating substrates like woodchips with epoxy adhesives, water logging can be prevented and TiO2 particles can stick to the substrates. With more research, this method should be applicable to other organics.
The removal and destruction of organic contaminants in groundwater can be addressed through the impregnation of adsorbents with photoactive catalysts. These adsorbents attract contaminating organic atoms/molecules like tetrachloroethylene to them. The photoactive catalysts impregnated inside speed up the degradation of the organics. Adsorbents are placed in packed beds for 18 hours, which would attract and degrade the organic compounds. The spent adsorbents would then be placed in regeneration fluid, essentially taking away all organics still attached by passing hot water counter-current to the flow of water during the adsorption process to speed up the reaction. The regeneration fluid then gets passed through the fixed beds of silica gel photocatalysts to remove and decompose the rest of the organics left. Through the use of fixed bed reactors, the regeneration of adsorbents can help increase the efficiency.
Triethylamine
(TEA) was utilized to solvate and extract the polyaromatic hydrocarbons (PAHs) found in crude oil. By solvating these PAHs, TEA can attract the PAHs to itself. Once removed, TiO2 slurries and UV light can photocatalytically degrade the PAHs. The figure shows the high success rate of this experiment. With high yielding of recoveries of 93–99% of these contaminants, this process has become an innovative idea that can be finalized for actual environmental usage. This procedure demonstrates the ability to develop photocatalysts that would be performed at ambient pressure, ambient temperature, and at a cheaper cost.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysed photolysis, light is absorbed by an adsorbed
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
substrate. In photogenerated catalysis, the photocatalytic activity (PCA) depends on the ability of the catalyst to create electron–hole pairs
Carrier generation and recombination
In the solid state physics of semiconductors, carrier generation and recombination are processes by which mobile charge carriers are created and eliminated. Carrier generation and recombination processes are fundamental to the operation of many optoelectronic semiconductor devices, such as...
, which generate free radicals (hydroxyl radical
Hydroxyl radical
The hydroxyl radical, •OH, is the neutral form of the hydroxide ion . Hydroxyl radicals are highly reactive and consequently short-lived; however, they form an important part of radical chemistry. Most notably hydroxyl radicals are produced from the decomposition of hydroperoxides or, in...
s: ·OH) able to undergo secondary reactions. Its comprehension has been made possible ever since the discovery of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
electrolysis
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
by means of the titanium dioxide
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
.
Commercial application of the process is called advanced oxidation process
Advanced Oxidation Process
Advanced oxidation processes refers to a set of chemical treatment procedures designed to remove organic and inorganic materials in waste water by oxidation...
(AOP). There are several methods of achieving AOP's, that can but do not necessarily involve TiO2 or even the use of UV light. Generally the defining factor is the production and use of the hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
radical
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
.
Basic principle
When TiO2 is subjected to radiation exceeding the material's band gap, electron-hole pairs, known as excitons, are generated so that additional electrons enter the conduction band, while holes remain in the valence band. These photo-generated electron-hole pairs facilitate redoxRedox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
reactions through the formation of adsorbed radicals on TiO2 surfaces. The photocatalytic activity of TiO2 depends on the relative rates of generation and recombination of electron-hole pairs as well as the levels of adsorbed radical-forming species on TiO2 surfaces.
The two most commonly used phases of TiO2 are anatase
Anatase
Anatase is one of the three mineral forms of titanium dioxide, the other two being brookite and rutile. It is always found as small, isolated and sharply developed crystals, and like rutile, a more commonly occurring modification of titanium dioxide, it crystallizes in the tetragonal system; but,...
and rutile
Rutile
Rutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:...
. While rutile exhibits a lower band gap (~3.0 eV) in comparison to anatase (~3.2 eV) and can thus be excited by irradiation at longer wavelengths, anatase is generally exhibits superior photocatalytic activity to rutile as a result of a significantly higher surface area and thus higher levels of adsorbed radicals. It is likely that mixed phase anatase-rutile materials exhibit enhanced photocatalytic activity through an improvement in electron-hole separation, as conduction band elections become trapped in the rutile phase.
Applications
- Conversion of water to hydrogen gas by photocatalytic water splittingPhotocatalytic water splittingPhotocatalytic water splitting is the term for the production of hydrogen and oxygen from water by directly utilizing the energy from light. Hydrogen fuel production has gained increasing attention as oil and other nonrenewable fuels become increasingly depleted and expensive...
. An efficient photocatalyst in the UVUltravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
range is based on a sodium tantaliteTantaliteTantalite, [2O6], is a mineral group that is close to columbite. The two are often grouped together as a semi-singular mineral called coltan or "columbite-tantalite" in many mineral guides. However, tantalite has a much greater specific gravity than columbite...
(NaTaO3) doped with La and loaded with a cocatalyst nickel oxideNickel oxideNickel oxide may refer to:* Nickel oxide, NiO, green, well-characterised oxide* Nickel oxide, Ni2O3, black, not well-characterised oxide...
. The surface of the sodium tantalite crystals is grooved with so called nanosteps that is a result of doping with lanthanumLanthanumLanthanum is a chemical element with the symbol La and atomic number 57.Lanthanum is a silvery white metallic element that belongs to group 3 of the periodic table and is the first element of the lanthanide series. It is found in some rare-earth minerals, usually in combination with cerium and...
(3–15 nm range, see nanotechnologyNanotechnologyNanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...
). The NiO particles which facilitate hydrogen gas evolution are present on the edges, the oxygen gas evolves from the grooves. - Use of titanium dioxideTitanium dioxideTitanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
in self-cleaning glassSelf-cleaning glassSelf-cleaning glass is a specific type of glass with a surface which keeps itself free of dirt and grime.-Introduction:The field of self-cleaning coatings on glass is divided into two categories: hydrophobic and hydrophilic....
. Free radicals generated from TiO2 oxidize organic matterOrganic compoundAn organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
. - Disinfection of water by titanium dioxideTitanium dioxideTitanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
photocatalysis. - Oxidation of organic contaminants using magneticMagnetismMagnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
particles that are coated with titanium dioxideTitanium dioxideTitanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
nanoparticles and agitated using a magnetic fieldMagnetic fieldA magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
while being exposed to UV light. - Conversion of carbon dioxideCarbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
into gaseous hydrocarbonHydrocarbonIn organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s using titanium dioxideTitanium dioxideTitanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
in the presence of water. As an efficient absorber in the UVUltravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
range, titanium dioxideTitanium dioxideTitanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
nanoparticles in the anataseAnataseAnatase is one of the three mineral forms of titanium dioxide, the other two being brookite and rutile. It is always found as small, isolated and sharply developed crystals, and like rutile, a more commonly occurring modification of titanium dioxide, it crystallizes in the tetragonal system; but,...
and rutileRutileRutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:...
phases are able to generate excitonExcitonAn exciton is a bound state of an electron and hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids...
s by promoting electrons across the band gapBand gapIn solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...
. The electrons and holes react with the surrounding water vapor to produce hydroxyl radicalHydroxyl radicalThe hydroxyl radical, •OH, is the neutral form of the hydroxide ion . Hydroxyl radicals are highly reactive and consequently short-lived; however, they form an important part of radical chemistry. Most notably hydroxyl radicals are produced from the decomposition of hydroperoxides or, in...
s and protonProtonThe proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s. At present, proposed reaction mechanisms usually suggest the creation of a highly reactive carbon radical from carbon monoxide and carbon dioxide which then reacts with the photogenerated protons to ultimately form methaneMethaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
. Although the efficiencies of present titanium dioxide based photocatalysts are low, the incorporation of carbon based nanostructures such as carbon nanotubeCarbon nanotubeCarbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...
s and metalMetalA metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
lic nanoparticles have been shown to enhance the efficiency of these photocatalysts. - Sterilise surgical instruments and remove unwanted fingerprints from sensitive electrical and optical components.
- A less-toxic alternative to tin and copper-based antifouling marine paints, ePaint, generates hydrogen peroxide by photocatalysis.
- Decomposition of crude oil with TiO2 nanoparticles
By using titanium dioxide photocatalysts and UV-A radiation from the sun, the hydrocarbons found in crude oil can be turned into H2O and CO2. Higher amounts of oxygen and UV radiation increased the degradation of the model organics. These particles can be placed on floating substrates, making it easier to recover and catalyze the reaction. This is relevant since oil slicks float on top of the ocean and photons from the sun target the surface more than the inner depth of the ocean. By covering floating substrates like woodchips with epoxy adhesives, water logging can be prevented and TiO2 particles can stick to the substrates. With more research, this method should be applicable to other organics.
- Decontamination of water with photocatalysis and adsorption
The removal and destruction of organic contaminants in groundwater can be addressed through the impregnation of adsorbents with photoactive catalysts. These adsorbents attract contaminating organic atoms/molecules like tetrachloroethylene to them. The photoactive catalysts impregnated inside speed up the degradation of the organics. Adsorbents are placed in packed beds for 18 hours, which would attract and degrade the organic compounds. The spent adsorbents would then be placed in regeneration fluid, essentially taking away all organics still attached by passing hot water counter-current to the flow of water during the adsorption process to speed up the reaction. The regeneration fluid then gets passed through the fixed beds of silica gel photocatalysts to remove and decompose the rest of the organics left. Through the use of fixed bed reactors, the regeneration of adsorbents can help increase the efficiency.
- Decomposition of polyaromatic hydrocarbons (PAHs)
Triethylamine
Triethylamine
Triethylamine is the chemical compound with the formula N3, commonly abbreviated Et3N. It is also abbreviated TEA, yet this abbreviation must be used carefully to avoid confusion with triethanolamine, for which TEA is also a common abbreviation....
(TEA) was utilized to solvate and extract the polyaromatic hydrocarbons (PAHs) found in crude oil. By solvating these PAHs, TEA can attract the PAHs to itself. Once removed, TiO2 slurries and UV light can photocatalytically degrade the PAHs. The figure shows the high success rate of this experiment. With high yielding of recoveries of 93–99% of these contaminants, this process has become an innovative idea that can be finalized for actual environmental usage. This procedure demonstrates the ability to develop photocatalysts that would be performed at ambient pressure, ambient temperature, and at a cheaper cost.