Titanium dioxide (TiO2) is a semiconductor photocatalyst with a band gap energy of 3.2 eV. When this material is irradiated with photons of less than 385 nm, the band gap energy is exceeded and an electron is promoted from the valence band to the conduction band. The resultant electron-hole pair has a lifetime in the space-charge region that enables its participation in chemical reactions. The most widely postulated reactions are shown here.

OH- + h+ ____________>.OH
O2 + e- _____________> 02-

Hydroxyl radicals and super-oxide ion are highly reactive species that will oxidize volatile organic compounds (VOCs) absorbed on the catalyst surface. They will also kill and decompose absorbed bioaerosols. The process is referred to as heterogeneous photocatalysis or, more specifically, photocatalytic oxidation (PCO).

Several attributes of PCO make it a strong candidate for indoor air quality (IAQ) applications. Pollutants, particularly VOCs, are preferentially absorbed on the surface and oxidized to (primarily) carbon dioxide (CO2). Thus, rather than simply changing the phase and concentrating the contaminant, the absolute toxicity of the treated air stream is reduced, allowing the photocatalytic reactor to operate as a self-cleaning filter relative to organic material on the catalyst surface.

Photocatalytic reactors may be integrated into new and existing heating, ventilation, and air conditioning (HVAC) systems due to their modular design, room temperature operation, and negligible pressure drop. PCO reactors also feature low power consumption, potentially long service life, and low maintenance requirements. These attributes contribute to the potential of PCO technology to be an effective process for removing and destroying low level pollutants in indoor air, including bacteria, viruses and fungi.

(The above information was provided courtesy of Dr. Bill Jacoby)

Photochemical Treatment of Pollutants
Photochemistry for a Cleaner Environment

Researchers from NREL are shedding light on one of our most pressing societal problems: environmental pollution. Recognized nationally as a leader in environmental photochemistry, the NREL team has developed a pollution control technique – photocatalytic oxidation, or PCO – that uses the energy in light to destroy environmental contaminants. Applicable as both a waste clean-up and a pollution control technique, PCO could help thousands of businesses comply with today's – and tomorrow's – environmental regulations.

Photocatalytic Oxidation: Cleansing Air and Water with Light Energy

Photons – the packets of energy that make up light – are abundant and can be precisely controlled. The photocatalytic oxidation (PCO) process harnesses photon energy to destroy many toxic organic compounds that are hazardous to human health and the environment. The key to PCO is the photocatalyst, a chemical compound that becomes highly reactive when exposed to various wavelengths of light. In the presence of organic pollutants – such as solvents, alcohols, dyes, and fuel oils – the activated photocatalyst attacks the pollutants' chemical bonds, converting the toxic compounds into benign constituents such as water and carbon dioxide. The PCO technique destroys pollutants in both air and water. Although system designs differ depending on the pollution stream being treated, the basic operating principles are the same. The photocatalyst – typically titanium dioxide (TiO2) – is coated on the inside of a reactor vessel. The reactor is exposed to ultraviolet light, either from electric lamps or from the sun. As the polluted stream circulates through the reactor, the pollutants are destroyed. Cleansed air or water exits the system. more