Photocatalytic air purifiers

by Chris Woodford. Last updated: August 11, 2011.

Light is an amazing source of energy—the power behind virtually everything that happens on Earth. Light from the Sun brightens the dark depths of space, makes plants leap to life, and (indirectly) powers our bodies. But did you know it can help to clean things as well? In air purifiers that work using a method called photocatalysis, light energy kick-starts a process that zaps all kinds of nasty air pollutants and turns them into harmless substances instead. For people who suffer from asthma and allergies, light-powered air purifiers like these are another weapon in the fight for cleaner air and better health. Now photocatalysis might sound horribly complicated, but it works in a relatively simple way. Let's take a closer look!

Photo: Designed by US DOE/NREL (National Renewable Energy Laboratory), this photocatalyser unit uses ultraviolet light to help purify the air inside cars, turning harmful volatile compounds (from such things as petroleum and exhaust fumes), bacteria, and viruses into less harmful substances instead. Domestic air purifiers that use photocatalysers in a similar way are made by companies such as Daikin and Zander. Photo by Warren Gretz courtesy of US DOE/NREL.

What is a catalyst?

Crystalline structure of a zeolite catalyst.

You've probably heard of catalysts already—in things like catalytic converters (the exhaust-cleaning systems fitted to cars) and zeolites (rock-like crystals used in all kinds of products and industrial processes). Catalysts are hugely important in industry; there are many types, they work in many different ways, and they're used in the manufacture of almost every chemical product you can think of.

Simply speaking, a catalyst is a substance that makes a chemical reaction more likely to happen by reducing the energy needed to kick start it ("activation energy," as it's known). A catalyst can speed up a chemical reaction or make it happen at a lower temperature. Once the reaction has finished, the catalyst isn't used up, though it may be physically changed in some way.

What about photocatalysts? When you see "photo" attached to a word (as in photocopier, photograph, photomultiplier, photoelectric...), you can be pretty sure light is involved: phōtos is the Greek word for "light." Photocatalysis means light is involved in making a catalyst do its job. In other words, light provides the energy that allows the catalyst to work.

Photo: The structure of a zeolite catalyst. Photo by courtesy of NASA Marshall Space Flight Center (NASA-MSFC).

How does a photocatalytic air purifier work?

In air purifiers, the catalyst that cleans the air is typically titanium dioxide (sometimes called titania) and it's energized by ultraviolet (UV) light. UV is the short-wavelength light just beyond the blue/violet part of the electromagnetic spectrum that our eyes can detect. The bad thing about it is that it gives you sunburn. The good thing is that it has much more energy than ordinary, visible light—and exactly the right amount of energy to get titanium dioxide excited.

Titanium dioxide is a semiconductor (a bit like materials such as silicon, used in integrated circuits). You don't actually need much titanium dioxide: just a thin film covering the surface of a backing material called a substrate, which is usually made from a ceramic or a piece of metal (such as aluminum).

Here's how the titanium dioxide catalyst in an air purifier breaks apart molecules of air pollution:

Animation showing how photocatalysis works.

When UV light (the big yellow arrow shown here) shines on the titanium dioxide, electrons (the tiny, negatively charged particles inside atoms) are released at its surface. It's the electrons that do the useful work for us.
The electrons interact with water molecules (H2O) in the air, breaking them up into hydroxyl radicals (OH·), which are highly reactive, short-lived, uncharged forms of hydroxide ions (OH−).
These small, agile hydroxyl radicals then attack bigger organic (carbon-based) pollutant molecules, breaking apart their chemical bonds and turning them into harmless substances such as carbon dioxide and water. This is an example of oxidation—and that's why air purifiers that work this way are sometimes also described as PCO (photocatalytic oxidation) air cleaners.

Here, then, is the big advantage that photocatalytic air purifiers have over other air-cleaning technologies, such as filters: instead of simply trapping pollutants (which still have to be disposed of somehow), they completely transform the harmful chemicals and effectively destroy them. The disadvantage is that photocatalytic purifiers also produce tiny amounts of ozone (O3), a chemical variant of the oxygen in the air that is, in itself, a toxic air pollutant! Purifier makers claim the amounts of ozone produced are well within the guideline limit (0.05 parts per million) suggested by the US FDA but, even so, this is something to bear in mind.

Another thing worth noting is that all the interesting stuff happens on the surface of the titanium dioxide catalyst. That's why air purifiers need fans to suck polluted air in at one end and blow clean out out of the other. It's also why air purifiers take some time (typically up to 30 minutes) to clean a large room properly.

Photocatalysis in practice

A photocatalyser tube designed to purify air inside cars.

Photo: Another view of NREL's photocatalytic air purifier. Photo by Warren Gretz courtesy of US DOE/NREL.

Photocatalysis only tackles certain, chemical forms of air pollution and doesn't solve the problem of particulates (soot and dirt). That's why photocatalytic air purifiers combine UV-activated, titanium-based catalysers with other cleaning and filtering technologies to form a comprehensive system that can tackle a whole range of dirt and pollutants. A typical purifier draws incoming air past a series of different cleaning stages, each of which tackles a different kind of airborne pollutant:

A relatively coarse prefilter captures large particles of household dust, hair, and pet hair. This filter is made of polypropylene netting coated with catechin (a bitter-tasting natural substance, found in green tea, that works as an antibacterial agent and deodorant).
A fine HEPA filter removes airborne viruses, bacteria, spores, and mold.
A plasma ionizer gives a positive electrical charge to any remaining dust and pollen particles so they stick to a negatively charged metal grid further along the machine. (This is much like the system used in an electrostatic smoke precipitator that scrubs soot from smokestacks.)
A photocatalyst made from titanium apatite (similar to but more effective than titanium dioxide) chemically destroys remaining organic pollutants such as exhaust fumes, volatile organic compounds, and so on.