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Coating To reduce Smog Could Be Causing It!

New evidence suggests that coatings designed for smog reduction and self-cleaning might be producing the very chemicals they’re supposed to get rid of.

Graduate students in the Raff laboratory are recording conditions of an experiment to investigate the amount of NOx produced when TiO2 in the presence of ammonia is irradiated with simulated sunlight.

Titanium dioxide (TiO2) is in everything these days, from toothpaste to latex paint. Valued for opacity, it’s also a powerful anti­mic­ro­bial and the key ingredient in “smog-eating” building products. That’s because TiO2 acts as a catalyst in the presence of UV light, helping break down virtually any organic compound—­whether it’s a bacterium or a formaldehyde molecule.

But TiO2’s reactivity may be harder to control than previously thought. New findings suggest the chemical could actually produce more smog-forming chemicals than it breaks down.

Studies of smog-eating coatings typically look at the direct conversion of the smog-forming compounds nitrogen oxide and nitric oxide (collec­tively called NOx) into nitrate on the building’s surface. In theory, the TiO2 coating neutralizes the NOx before it can turn into smog, and the nitrate trickles harmlessly away the next time it rains.

The problem with this scenario is that there are many other organic compounds in ambient air. These include ammonia—and NH3 spells trouble for smog-eating coatings, according to Jonathan Raff, Ph.D., assistant professor of chemistry at Indiana University–­Bloomington. Based on Raffs findings published recently in the Journal of the American Chemistry Society, TiO2 converts small concen­tra­tions of ammonia into NOx with water as a catalyst (the reaction picks up speed at about 30% relative humidity and slows down at around 50%). So with widespread use of smog-eating technology, “you could potentially have fluxes of NOx coming off these surfaces that are 13% of urban sources,” meaning the coatings themselves would be producing a significant amount of the city’s overall smog, Raff told EBN.

Ammonia in the air isn’t the only thing to worry about. Raff argues that “a big-picture study” should also look at ozone, ammonia, and nitrate concen­tra­tions on the surfaces of buildings, not just in the air. Researchers in France have already shown that nitrate deposited on smog-eating surfaces can “renoxify” if it isn’t washed away, but Raff says there are other problems as well, like the possibility that nitrate could turn into nitrous acid. “Nitrous acid is really important for kick-starting smog, especially in the morning hours,” he said.

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