Mechanism of Photocatalysis for Air Purification

With the discovery of photoinduced water cleavage on titanium dioxide (TiO₂) semiconductor electrodes by Fujishima and Honda in the early 1970 ’s, it was soon realised that this phenomenon could be applied for environmental remediation.Photocatalytic oxidation (PCO) first saw use as a technique for water purification, following from Frank and Bard’s investigation into the decomposition of cyanide using an aqueous TiO₂ suspension in 1977. Aqueous suspensions of catalysts, such as TiO₂, were found to be effective at breaking down organic pollutants. However, due to the inherent inefficiency of the process (the need to filter out the TiO₂ after purification), techniques had to be developed to immobilise TiO₂ onto support surfaces. This has lead to a technology that lends itself to air purification.

Photocatalytic Oxidation (PCO) can be defined as a chemical reaction influenced or initiated by light that removes electrons from a catalyst and adds those electrons to a compound. This definition highlights the main ingredients that make photocatalytic air purification possible: a light source, a catalyst, and reactants.

Crucially PCO requires the formation of an interface between, in general, a solid photocatalyst and a liquid or gas phase containing the reactants and/or products of the photoreaction.

The series of events following the illumination of a photocatalyst-gas interface may be initiated by either (A) light absorption by the catalyst, which leads to the activation of the reactant, or by (B) direct excitation of the reactant, which is then quenched by the catalyst. Mechanisms (A) and (B) may operate simultaneously at a semiconductor-gas interface, though (A) is generally considered to be the primary step for photocatalytic oxidation.

An important step of the photoreaction is the formation of electron-hole pairs. These are created when energy is provided to overcome the atomic band gap, between valence band (VB) and conduction band (CB). When a photon, with excitation energy (hu) greater than the band gap energy (EBG), is absorbed an electron hole pair is created.

The charge is transferred between the electron-hole pairs and an adsorbed, ground state, reactant on the photocatalyst surface. Resulting in the photo-oxidation and photo-reduction of reactants, see Fig. 1. To what extent electron-hole pairs play a part in the destruction of pollutants is still debated. They may be too short lived to be able to react directly, unless the concentration of the pollutant is high or is strongly adsorbed.

Figure 1. Electron-hole pair generation.

With the presence of water, as vapour form in air, the oxidizing agents known as reactive oxygen species (ROS) can be formed. These include oxygen (O₂), superoxide (O₂^- ), peroxide (O₂^–2), and hydroxide (OH). These species can participate in a host of oxidation-reduction (redox) reactions, which are highly effective at the chemical destruction of VOC’s, particulate matter, microbes, ozone, NO_x, and SO_x. The creation of hydroxyl radicals from water and subsequent destruction of an organic compound are shown in equations 2 and 3. It is shown that, as with e-h pair oxidation, the final products of organic compound oxidation are water and carbon dioxide.

Semiconducting materials (photocatalysts) are key to the photocatalytic process. Many have been studied in either pure or doped form. The most common semiconductors researched for PCO applications have been: TiO₂, ZnO, CdS, with Fe (III) and precious metals being the most common dopants. TiO₂ has proved to be the most suitable candidate, and is the most widely used.It is considered almost ideal for PCO applications. Firstly TiO₂ is relatively inexpensive.It is easy to produce, in large supply and is used throughout the world in a wide range of applications (e. g. as a colorant for paint, paper, and plastics, even food; and for UV protection). TiO₂ is highly stable chemically, so is unlikely to participate in unwanted reactions. Importantly, the photogenerated holes are highly oxidizing (+2.53V vs SHE), and the photogenerated electrons are reducing enough (-0.52V vs SHE) to produce reactive oxygen species. The down side to TiO₂ is that it cannot be activated by visible light.

TiO₂ has a large band gap, E_BG = 3.2–3.0 eV.It is therefore limited to activation by radiation wavelengths equal to or below UV light. UV light makes up only 5% of the solar spectrum. There are three crystalline forms of TiO₂: anatase, rutile, and brookite. The anatase form has been found to have the most favourable characteristics for PCO, as it appears to be the most active and easiest to produce of the three. Irradiation with light of 385 nm or less will generate electron-hole pairs in anatase TiO₂. The anatase form is predominantly used in most commercial PCO processes.

Source:

http://www.manchesteruniversitypress.co.uk/uploads/docs/47 to58. pdf