Photocatalysis

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 radicals: ·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.

Contents 1 Basic principle 2 Applications 3 See also 4 References 5 External links

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 redox 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.^[1][2]

Applications

• Conversion of water to hydrogen gas by photocatalytic water splitting.^[3] An efficient photocatalyst in the UV range is based on a sodium tantalite (NaTaO₃) doped with La and loaded with a cocatalyst nickel oxide. The surface of the sodium tantalite crystals is grooved with so called nanosteps that is a result of doping with lanthanum (3–15 nm range, see nanotechnology). The NiO particles which facilitate hydrogen gas evolution are present on the edges, the oxygen gas evolves from the grooves.
• Use of titanium dioxide in self-cleaning glass. Free radicals^[4] generated from TiO₂ oxidize organic matter.^[5]
• Disinfection of water by titanium dioxide photocatalysis.^[6]
• Oxidation of organic contaminants using magnetic particles that are coated with titanium dioxide nanoparticles and agitated using a magnetic field while being exposed to UV light.^[7]
• Conversion of carbon dioxide into gaseous hydrocarbons using titanium dioxide in the presence of water.^[8] As an efficient absorber in the UV range, titanium dioxide nanoparticles in the anatase and rutile phases are able to generate excitons by promoting electrons across the band gap. The electrons and holes react with the surrounding water vapor to produce hydroxyl radicals and protons. 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 methane. Although the efficiencies of present titanium dioxide based photocatalysts are low, the incorporation of carbon based nanostructures such as carbon nanotubes^[9] and metallic nanoparticles^[10] have been shown to enhance the efficiency of these photocatalysts.
• Sterilization of surgical instruments and removal of unwanted fingerprints from sensitive electrical and optical components.^[11]
• 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 (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.

See also

• Photoelectrochemical cell
• Photolysis
• Photocatalytic water splitting

References

External links

• Fraunhofer Photocatalysis Alliance (in German)