PhotoCatalytic Air Purifier
The photocatalytic air purifier is based on photocatalytic oxidation (PCO), an emerging air purifier technology that converts fine particles and toxic gasses into safer compounds.
A photocatalytic air cleaner uses broad spectrum ultraviolet light, which reacts with a thin-film titanium dioxide-based chemical catalyst, in the presence of water, to create hydroxyl radicals and super-oxide ions which oxidize volatile organic compounds (VOCs), and eliminate microorganisms adsorbed on the catalyst surface.
Oh, that makes everything perfectly clear.
Let’s analyze this paragraph one concept at a time: a real bright light shines on a wet metal thing which makes tiny pac-man people that get rid of bad stuff stuck on the surface and turn it into carbon dioxide and water. When the pac-people are done, they turn back into to oxygen and hydrogen.
Yeah, I knew that.
A high-intensity broad spectrum ultraviolet lamp, with UVC (254 nm) germicidal light wavelengths included, is the first part of photocatalytic air purification technology. This light is similar to those covered in the preceding article, “UV Air Purifier: The Twilight Zone,” but includes additional wavelengths. As we saw there, ultraviolet light can reduce microbial populations in our air.
A wider set of ultraviolet wavelengths below 385nm are employed for catalyst activation. Although UV light alone will degrade toxic organic compounds, reaction rates are much faster with photocatalytic assistance.
A cataylst is a substance that can speed up a chemical reaction without being used up itself. The catalyst material in the photocatalytic purifier is titanium dioxide, TiO2, which is applied as a thin film on aluminum or ceramic substrate.
Titanium dioxide is a relatively inexpensive pigment and is used to add whiteness to products such as paints and coatings, plastics, paper, inks, fibers, food, toothpaste and cosmetics.
Titanium dioxide is also a semiconductor. When a semiconductor is bombarded with light of certain wavelengths, electrons in the material’s valence band are excited into the conduction band. This means they are free to move and their energy ends up spliting nearby water molecules into two parts, hydroxyl radicals and super-oxide ions.
Free radicals are uncharged atoms or molecules with unpaired electrons. Unpaired electrons are highly reactive, so free radicals are likely to quickly engage in chemical reactions. Hydroxyl radicals are among the most powerful oxidizers in the world, stronger than chlorine, ozone, and peroxide, and consequently very short lived.
Superoxide is created by the addition of one electron to oxygen. This free radical has a relatively long half-life: less than one second.
Here is what photocatalytic machine vendors claim their air purifiers can do. Organic substances, like germs and volatile organic compounds, are held together by carbon-carbon, carbon-oxygen or carbon-hydrogen bonds. Oxidizers chop these bonds and cut the molecule into smaller compounds which are broken over and over until only carbon dioxide and water are left.
A photocatalytic purifier can eliminate particles down to 0.001 microns from air, including the very tiny lung penetrating particles I keep hollering about. Remember most HEPA air filters balk at particles smaller than .3 microns.
Pollens, dust mite allergens, pet dander, mold, bacteria, and viruses in the air, are on the list of items removed. Larger particles, such as pet hair, require a prefilter. Photocatalytic air purifiers can eliminate toxic gas phase VOC pollutants including formaldehyde, exhaust fumes, benzene, toluene, and odors like ammonia and hydrogen sulphide from our air. Even gases like carbon monoxide and nitrous oxide, untouched by every other technology on this site, are destroyed.
Could you say that again?
Air contaminants in every category, particles, gasses, and biologicals, are not accumulated in filter media, not collected on metal plates, they don’t fall on the floor, they are gone, dissapeared, transmuted into harmless substances, really gone!
Awesome air cleaning technology, ideal for multiple chemical sensitivity, allergy, asthma, and just about everything else. Sounds like a sci-fi movie script.
Two to beam up, Scotty.
Or maybe it's just air purifier marketing.
But in the fiction-like atmosphere of air purifier marketing some vendors gotta ‘zaggerate. They claim their photocatalytic air purifier works over impossible distances:
“Air-borne hydro peroxides, super oxide ions and hydroxide ions travel through the house”
The marketing department has these air purifiers targeted at that big money flowing to Ionic Breeze air purifiers, whose main weakness is lack of circulation. The ozone generator, which actually does fill the whole house, is looking vulnerable on safety issues. Hey, it’s only a little exaggeration.
Every scientific and engineering source I have found asserts that oxidation occurs on the surface of the catalyst, not throughout the house.
These aggressive oxidizers, especially hydroxyl radicals, have a very short half-life in air (billionths of a second): they defy measurement by precision laboratory techniques. They do not go wafting from room to room like ozone can, with its half-life in air measured in days.
Engineers are working with additional metals, like silver, added to the catalyst film, trying to extend the short time window of the photoelectric charge.
But that little marketer's exaggeration casts a shadow of doubt on all the wonderful claims of salvation above.
I wonder if all this sci-fi stuff really works.
As we have seen, free radical oxidizers do not persist long enough to travel through air away from the catalyst surface. Consumers will not be exposed to superoxide ion and hydroxyl radical.
They cannot build up in closed rooms.
These oxidizers are much safer than ozone.
But people, pets, and plants should not be close to the air purifier.
To me a question arises: is there complete elimination of contaminants inside the unit?
Free radical oxidation of organic molecules is not a one step process: as each bond is broken smaller molecules are created in a cascade.
Eventually the free radical cascade terminates, but intermediate products from the oxidation of organic compounds might escape into the room.
I don’t think we can know the answer here, just the question.
One vendor claims: ”plants thrive on the carbon dioxide those pollutants are converted to.”
I don’t consider carbon dioxide harmless.
I notice the lack of oxygen after a while in any closed room, and absolutely will not sleep without venting. Not a big hazard, but just how much CO2 is produced by the PCO air purifier?
Probably not much, just enough to make plants “thrive.”
Happy air purifier marketing.
During the catalytic free radical generation process ozone is temporarily created. Some escapes, with varying claims from vendors. Some assert zero ozone while others publish ozone tests ranging from .01-.04 ppm. Most people can smell ozone at .01 ppm, and I consider .05 ppm, the federal limit for medical devices, way too high for routine use.
If I can smell it I won’t use it.
Fluorescent lamps produce UV radiation by emissions from low-pressure mercury gas. Some photocatalytic air purifier systems incorporate a glass UV mercury bulb without sufficient protection from breakage. It is important to shield the bulb from breakage which could release mercury.
High benefit-to-cost ratios make photocatalytic technology air purifiers even more attractive. With no air filters to replace or collecting plates to clean, the photocatalytic device requires little consumer attention. Yearly or longer replacement of lamps, and/or photocatalytic reaction modules, is the norm.
I have seen units advertised from under $300 to $1095.00.
The photocatalytic purifier operates with air at room temperature so there are no burn or fire hazards.
Recent improvements to PCO technology include adding other element's oxides to the catalyst; vanadium, tungsten, copper, zinc, rhodium, silver and nickel. New hydration compounds help attract more moisture to the surface. The most advanced models will have multiple metals in the catalyst film.
Added chromium, vanadium and manganese have produced visible light responsive TiO2 photocatalysts that can harness the full power of sunlight. Ultraviolet represents less than 2% of this energy.
Future potential of PCO air purification technology is enormous. The HVAC engineering community has taken notice, with a number of global conferences. Photocatalysis has also gained tremendous interest in booming manufacturing areas like South and East Asia, including Japan, China, Korea, and Taiwan.
Japan leads the world in photocatalysis research and development, with 2,000 companies. The Daikin air purifier is one product gaining market share.
Many products are emerging, such as the clean air lamp, a standard sized fluorescent tube like so many we see every day, but with the purifying photocatalyst on its surface! Visible light PCO is in paint on walls and ceilings, and on building exteriors to prevent dirt buildup and detoxify the air.
I even found a toilet with PCO technology to oxidize odors.
Anyone still thinking of buying ozone generator "air purifiers" should stop now. For them, the photocatalytic air cleaner is a go right now.
Many air cleaners now (2011) incorporate PCO - it is very inexpensive to add a sheet of TiO2 coated aluminum to an existing UV device - but no mainstream manufacturer has adopted the PCO technology.
Recent innovations, including a 3 dimensional substrate which increases the surface area of the catalyst, have gone begging for major OEM support in the shrinking air cleaner market.
The safety record of the similar plasma-ion oxidizer technology (Sharp Plasmacluster has sold over 25 million units without any personal injury claim) suggests that photocats pose no major health hazards, but I'd still avoid very close contact with PCO, like installation near the bed.