Photoelectrochemical oxidation

Photoelectrochemical oxidation (PECO) is the process by which incident light enables a semiconductor material to promote a catalytic oxidation reaction. While a photoelectrochemical cell typically involves both a semiconductor (electrode) and a metal (counter-electrode), at sufficiently small scales, pure semiconductor particles can behave as microscopic photoelectrochemical cells. PECO has been used for the detoxification of air and water, hydrogen production, and other applications.

Reaction Mechanism

The process by which a photon initiates a chemical reaction directly is known as photolysis; if this process is aided by a catalyst, it is called photocatalysis.^[1] If a photon has more energy than a material's characteristic band gap, it can free an electron upon absorption by the material. The remaining, positively charged hole and the free electron may recombine, generating heat, or they can take part in photoreactions with nearby species. If the photoreactions with these species result in regeneration of the electron-donating material—i.e., if the material acts as a catalyst for the reactions—then the reactions are deemed photocatalytic. PECO represents a type of photocatalysis whereby semiconductor-based electrochemistry catalyzes an oxidation reaction—for example, the oxidative degradation of an airborne contaminant in air purification systems.

The principal objective of photoelectrocatalysis is to provide low-energy activation pathways for the passage of electronic charge carriers through the electrode electrolyte interface and, in particular, for the photoelectrochemical generation of chemical products.^[2] With regard to photoelectrochemical oxidation, we may consider, for example, the following system of reactions, which constitute TiO₂-catalyzed oxidation.^[3]

TiO₂ (hv) → TiO₂ (e⁻ + h⁺)

TiO₂(h⁺) +RX → TiO₂ + RX^.+

TiO₂(h⁺) + H₂O → TiO₂ + HO^. + H⁺

TiO₂(h⁺) + OH⁻ → TiO₂ + HO^.

TiO₂(e⁻) + O₂ → TiO₂ + O₂^.−

This system shows a number of pathways for the production of oxidative species that facilitate the oxidation of the species, RX, in addition to its direct oxidation by the excited TiO₂ itself. PECO concerns such a process where the electronic charge carriers are able to readily move through the reaction medium, thereby to some extent mitigating recombination reactions that would limit the oxidative process. The “photoelectrochemical cell” in this case could be as simple as a very small particle of the semiconductor catalyst. Here, on the “light” side a species is oxidized, while on the “dark” side a separate species is reduced.^[4]

Photochemical oxidation (PCO) versus PECO

The classical macroscopic photoelectrochemical system consists of a semiconductor in electric contact with a counter-electrode. For n-type semiconductor particles of sufficiently small dimension, the particles polarize into anodic and cathodic regions, effectively forming microscopic photoelectrochemical cells.^[2] The illuminated surface of a particle catalyzes a photooxidation reaction, while the “dark” site of the particle facilitates a concomitant reduction.^[5]

Photoelectrochemical oxidation may be thought of as a special case of photochemical oxidation (PCO). Photochemical oxidation entails the generation of radical species that enable oxidation reactions, with or without the electrochemical interactions involved in semiconductor-catalyzed systems, which occur in photoelectrochemical oxidation. An example of a photochemical oxidation system that is not strictly speaking photoelectrochemical in nature would be the oxidative degradation of organic contaminants by the H₂O₂/UV process.^[3] Here, direct photolysis of H₂O₂ generates the hydroxyl radicals needed for oxidative degradation without the use of a semiconductor catalyst.

Applications

The primary applications for photochemical / photoelectrochemical oxidation include air disinfection / purification, water disinfection / purification, hydrogen production (i.e., through water splitting), and environmental remediation, to name a few. Significant work has been done in the realm of air and water treatment, which is described in more detail below.

Air purification

PECO has shown promise for the disinfection (i.e., removal of biological aerosols) and purification (i.e., more generally, removing contaminants) of air in HVAC and other applications. Photocatalytic oxidation has been used to successfully oxidize various organic contaminants, including alcohols, chlorocarbons, and BTEX compounds.^[1] With an effective catalyst, and light source of sufficient intensity and appropriate wavelength, virtually any volatile organic compound can be removed, at least in part, from an air source.

Perhaps more important for HVAC applications in general is the disinfection of air. In many homes and workplaces, allergens can hamper comfort and productivity in sensitive individuals; these materials can be oxidized via a PECO system, provided the residence time in the treatment media is sufficient.^[4] More troubling are virus and / or bacteria, which can cause infectious diseases if they are allowed to accumulate in indoor air. UV treatment for air disinfection has been proven effective; however, use of an effective photocatalyst (e.g., TiO₂ in a PECO air cleaning system) with UV radiation leads to significantly higher reductions in populations of bacteria.^[6]

Air treatment in HVAC applications can lead to unseen benefits. In a process referred to HVAC load reduction (HLR), air treatment may minimize the number of air changes required to maintain sufficient indoor air quality, thereby reducing energy use.^[7]

Water purification

As with air, PECO treatment can be applied to aqueous systems for disinfection / purification purposes. Some of the early work in investigating semiconductor-catalyzed detoxification processes was done on water systems, including that of Kinney and Ivanuski.^[8] A good example of photocatalytic water purification methods as they pertain to environmental remediation can be seen in the field pilot test performed at the Tyndall Air Force Base, near Panama City, Florida, in 1992.^[9] Results showed approximately 90 – 100 percent reduction in contaminant concentration (predominately BTEX) with treatment time of approximately 45 minutes.

History

An important early effort in the field is that of Goodeve and Kitchener, who were among the first to demonstrate the “photosensitization” of TiO₂—e.g., as evidenced by the fading of paints incorporating it as a pigment.^[10] Another important work is that of Markham and Laidler.^[11] They demonstrated through several measurements and a process of deduction that the generation of hydrogen peroxide by aqueous suspensions of ZnO under UV irradiation must occur by transferring a freed electron from the semiconductor, forming the perhydroxyl radical. They further indicated that when organic compounds were present, these compounds were oxidized at the semiconductor surface by oxidizing species with increased production of hydrogen peroxide.

Early forays into the realm of macro-scale photoelectrochemical cells began at Bell Laboratories, where researchers measured electrochemical responses of various semiconductors, including TiO₂, to light and dark.^[12] Boddy demonstrated the evolution of oxygen using rutile-phase TiO₂ as an anode in a cell containing various types of electrolytic solutions under UV irradiation.^[13] In this case, n-type TiO₂ is shown to liberate the electrons (as free radicals) necessary to split water and generate gaseous oxygen. Subsequent researchers continued to elucidate the potential for semiconductor-based photoelectrochemical water splitting, beginning with the work of Fujishima and Honda.^[14]

An important early study in the photoelectrochemical oxidation of organic contaminants in water was undertaken by Kinney and Ivanuski.^[8] They showed the potential for a variety of metal oxides, including TiO₂, to catalyze the oxidation of dissolved organic materials (phenol, benzoic acid, acetic acid, sodium stearate, and sucrose) under illumination by sunlamps. Additional work by Carey et al showed similar oxidative degradation in the photodechlorination of PCBs in the presence of TiO₂.^[15]

Significant work has been done designing photo reactors that make use of the basic principle behind PECO for detoxification. Many of these reactors use solar radiation as the photon source, although in some cases other light sources may be employed. Pacheco and Tyner developed a simple solar photocatalytic reactor modeled on the well-known parabolic trough concentrator design.^[16] They distributed the catalyst (TiO₂) in TCE-contaminated water for treatment. Mehos and Turchi applied a similar reactor to groundwater remediation.^[17] An alternate strategy for water treatment is the falling-film reactor, which may use sunlight as the irradiation source in a central receiver-type arrangement.^[18]

Goswami was the first to propose a system for photocatalytic air disinfection through the incorporation of photocatalyst material, light source, and filter in existing HVAC systems.^[19] This strategy was specifically intended to address bio-aerosols in air streams that, if left unchecked, could lead to “sick building syndrome.” Later work by Goswami incorporated PECO processes to treat bio-aerosols and other contaminants (e.g., VOCs) in a similar system for air and water purification.^[4]

See also

• Photocatalysis
• Photolysis
• Photochemistry
• Photocatalytic water splitting
• Photoelectrochemical cell
• List of photochemists

Further reading

• I. U. I. A. Gurevich, I. U. V. Pleskov, and Z. A. Rotenberg, Photoelectrochemistry. New York: Consultants Bureau, 1980.
• M. Schiavello, Photoelectrochemistry, photocatalysis, and photoreactors: Fundamentals and developments. Dordrecht: Reidel, 1985.
• A. J. Bard, M. Stratmann, and S. Licht, Encyclopedia of Electrochemistry, Volume 6, Semiconductor Electrodes and Photoelectrochemistry: Wiley, 2002.

References