What is Photocatalytic Oxidation?

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Written By Jamila W.

Photocatalytic Oxidation (PCO) is an innovative technology that can transform indoor air quality. It destroys diverse air pollutants, turning them into carbon dioxide and water. In this article, we explain:

  • What photocatalytic oxidation is
  • How PCO works
  • The use of photocatalytic oxidation in air cleaning
  • Potential hazards of using PCO

What is Photocatalytic Oxidation?

close up, water

Photocatalytic oxidation (PCO), also known as Ultra-Violet Photocatalytic Oxidation (UVPCO) is an innovative air-cleaning, cleaning, and disinfection technology capable of destroying air pollutants with a 1-nanometer (0.001 microns) diameter. For comparison, the smallest molecules filtered by HEPA filters are 300 times larger at 0.3 microns.

The powerful air purification of photocatalytic oxidation is due to its unique ability to degrade particles at a molecular level. This makes photocatalytic air cleaners effective at cleaning and disinfecting indoor air, removing pollutants that include: 

  • Biological agents including viruses, bacteria, fungi, parasites, mites, pollen 
  • Vola¬≠tile organic compounds (VOCs)[1]
  • Nicotine
  • Methanol

PCO uses UV light to complete the chemical process of photocatalysis. A photocatalytic unit uses an installed photocatalyst to generate reactive oxygen species (ROS, free radicals, superoxide ions, free hydroxyl radicals) that oxidize and destroy airborne particulates and chemicals generated within indoor environments. 

The history of photocatalytic oxidation

The concept of photocatalysis was first proposed in the early 20th century by the German chemist Dr. Alexander Eibner. He described the bleaching of Prussian blue using an illuminated zinc oxide catalyst. 

In 1938, scientists Doodeve and Kitchener discovered the photocatalyst titanium dioxide (TiO2). Later in 1972, Akira Fujishima and Kenichi Honda discovered that in the presence of UV light, TiO2 could be used for the electrochemical photolysis of water.

During the 1970s and 80s, scientists and engineers began to develop a range of applications for photocatalysis including water treatment, the photocatalytic oxidation air filter, and the generation of gaseous hydrocarbons from CO2.

After many years of research and development, the photocatalytic air purifier has become commercially available. Widespread concern regarding poor indoor air quality has led to indoor air cleaning becoming a primary application of photocatalytic oxidation. This is because it can reduce concentrations of volatile organic compounds (VOCs) and other gas-phase pollutants that cannot be eliminated by air filters.

How does photocatalytic oxidation work?

Photocatalytic oxidation involves the irradiation of titanium dioxide with UV light. The irradiated catalyst undergoes a chemical conversion, enabling it to degrade contaminants, reducing them to carbon dioxide, water, and harmless detritus. 

This photochemical reaction is built into air-cleaning systems that draw indoor air across a honeycomb series of UV/titanium dioxide chambers called monolith reactors. The reactors are coated with titanium dioxide and irritated with UV-C light (wavelength 254 nm) or UV-A light (365nm) from a series of fluorescent bulbs. 

Within the chambers, contaminants in the air are adsorbed on the TiO2 catalyst. Once the catalyst is UV activated, the adsorbed pollutants are attacked and broken down by free radicals produced by the photocatalyst. Biological agents have their cellular structure and DNA completely broken down, leaving clean, safe exhaust air.

Other applications of photocatalytic conversion

In addition to uv air purifiers, photocatalytic oxidation has numerous applications that harness the effectiveness of a UV-activated catalyst to degrade molecular bonds. Related uses include

  • Solar water disinfection (SODIS)
  • Self-sterilizing coatings for use in food preparation and healthcare settings
  • Surgical instrument sterilization 
  • Removal of fingerprints and other contaminants from precision electronic and optical hardware
  • ePaint, a zinc-oxide-rich, eco-friendly alternative to marine antifouling paint [3]

Photocatalytic oxidation for indoor air purification

building, outside, aircon

Photocatalytic oxidation shows outstanding potential for air cleaning because of its ability to consistently degrade gas-phase organic contaminants in room air at room temperature. It is an ideal solution for elimination of gaseous and biological pollutants from indoor air in large buildings. PCO has the following benefits:

UVPCO is efficient and cost-saving

By breaking down VOCs, and particulates, UVPCO air cleaners can purify indoor air without increasing ventilation, saving the energy and expense of upgrading HVAC to increase the supply of outdoor air. The honeycomb photocatalytic monoliths maximize surface area for high conversion rates even if air pressure is low.

Photocatalytic oxidation eliminates odors

Because photocatalytic oxidation is effective at degrading volatile organic matter, it can be used for odor control as well as air cleaning. UVPCO can break down the following foul odor-causing chemicals into odor-free nitrogen, carbon dioxide, and water:

  • Ammonia (urine)
  • Skatole (feces)
  • Isovaleric acid (sweaty feet)
  • Amines (fish odors)

PCO air cleaners are ideal for large buildings

UVPCO air cleaning is used in large public and commercial buildings to safeguard occupant health and comfort. It is particularly useful for protecting indoor environments from the accumulation of bioaerosols. However, it is important to note that nitrogen dioxide, radon, and other inorganic gasses that can build up in smaller residential buildings, are not removed by UVPCO. 

Photocatalytic oxidation does have disadvantages

A variety of research studies that have examined the performance of PCO air cleaners have highlighted important concerns about their use:

Airflow determines UVPCO air cleaning performance 

The efficiency of ultraviolet photocatalytic oxidation air cleaners depends on the duration of contact between contaminated room air and the UV light-activated titanium dioxide catalyst. A 2007 investigation of the performance of UVPCO for indoor air purification found that the efficiency of the breakdown of common indoor VOCs by UVPCO air cleaners varied between 20% and 80%. 

This large disparity in performance was directly related to the rate of airflow through the machine, with prolonged exposure of contaminants to the catalytic chambers producing the best results. 

Are photocatalytic oxidation air purifiers safe?

Accelerated airflow rate not only reduced the VOC conversion efficiency but also led to incomplete conversion of VOCs, generating toxic intermediates like formaldehyde, acetone, carbon monoxide, nitrogen dioxide, and acetaldehyde. [3]

In addition to airflow, the generation of toxic intermediates may be affected by:

  • Pollutant concentration, where high levels of indoor air pollution saturate the catalysts and cause incomplete degradation or bypassing of the oxidative process.
  • Light wavelength with suboptimal UV light wavelengths failing to activate the titanium oxide catalyst.
  • Generation and release of ozone by certain UVC light sources.

This is an important disadvantage of photocatalytic oxidation, necessitating air quality monitoring and timely servicing of UVPCO air cleaners to ensure their safe function. Industrial and consumer photocatalytic oxidation air cleaners are also expensive, as this is still an emerging technology. 

Environmental Protection Agency PCO air cleaner guidance 

In 2018, the Environmental Protection Agency (EPA) reviewed UVPCO air cleaners as part of a comprehensive technical summary of residential air cleaners. Their review of this technology noted academic studies that found that these air cleaners often fail to fully degrade pollutants.

The EPA found manufacturers of PCO air cleaners who prevent the release of intermediate chemicals by installing additional adsorbent media air filters to adsorb any hazardous byproducts. It is also important to note that the catalysts within PCO air purifiers have a finite lifespan. 

PCO has few real-world studies, no effectiveness standards, and no ANSI/ASHRAE rating metric, it is not possible to determine whether consumer PCO devices will work as advertised.

In conclusion

sky

Photocatalytic oxidation is a technology that is capable of eliminating some of the most hazardous pollutants in indoor air. It is already being used in large buildings and consumer PCO air cleaners are already available. 

However, the efficiency of the catalytic conversion of pollutants is key to the efficiency and safety of PCO air purifiers. There is still some way to go before the performance of this promising solution for indoor air pollution is fully rated and standardized.