‘Self-Cleaning’ Coatings Raise Concerns

FRIDAY, JUNE 14, 2013

A group of environmental chemists say widespread use of technology used in “self-cleaning” paints and building surfaces may do more harm than good.

The researchers from Indiana University in Bloomington, IN, found that titanium dioxide (TiO2)-containing coatings—seen as promising for their role in breaking down airborne pollutants on contact—are likely, in real-world conditions, to convert abundant ammonia to nitrogen oxide, according to a report on the research.

Nitrogen oxides are the "key precursor of ozone pollution," the researchers add.

"As air quality standards become more stringent, people are going to be thinking about other technologies that can reduce pollution," said Jonathan D. Raff, assistant professor in the School of Public and Environmental Affairs at the university and an author of the study.

"Our research suggests that this may not be one of them."

"Photooxidation of Ammonia on TiO2 as a Source of NO and NO2 under Atmospheric Conditions" was published online by the Journal of the American Chemical Society. Other authors include doctoral students Mulu Kebede and Nicole Scharko; Mychel Varner of the University of California-Irvine; and R. Benny Gerber of UC-Irvine and the Hebrew University in Jerusalem.

Ozone Formation Concern

The researchers calculate that in areas where the TiO2 technology is used, ammonia degradation could account for up to 13 percent of the nitrogen oxides in the immediate vicinity.

“This suggests that widespread use of the technology could contribute significantly to ozone formation,” according to the researchers.

Jonathan D. Raff
Indiana University

Researcher Jonathan D. Raff from Indiana University says that while titanium dioxide coatings can break down chemical grime on building surfaces, the mineral catalyzes the incomplete breakdown of ammonia. 

The team noted that their research is timely as the Environmental Protection Agency is developing stricter regulations for ground-level ozone, a primary component in photochemical smog. The pollution is linked to serious health problems, including breathing difficulties and heart and lung disease.

Ozone is produced by reactions involving nitrogen oxides (NOx), which come primarily from motor vehicle emissions, and volatile organic compounds resulting from industrial processes. Equipping cars with catalytic converters has been effective at reducing ozone in urban areas.

However, different technologies may be needed to meet stricter air-quality standards of the future, the researchers said.

‘Not a Permanent Removal Process’ for Smog

The need has sparked interest in TiO2, a prime pigment that is used as a whitening agent in paints. When it is treated through a special process to do so, TiO2 acts as a photocatalyst, breaking down nitrogen oxides, ammonia and other pollutants in the presence of sunlight.

"Self-cleaning" surfaces coated with TiO2 can break down chemical grime that will otherwise adhere to urban buildings, the scientists said.

However, Raff and his colleagues found that, in normal environmental conditions, TiO2 also catalyzes the incomplete breakdown of ammonia into nitrogen oxides. Ammonia is an abundant constituent in motor vehicle emissions, and its conversion to nitrogen oxides could result in increases in harmful ozone concentrations.

Lotus leaf
Wikimedia Commons

The hydrophobic surface structure of lotus leaves has inspired many self-cleaning paints. The study, however, focuses specifically on TiO2-containing coatings.

"We show that uptake of atmospheric NH3 (ammonia) onto surfaces containing TiO2 is not a permanent removal process, as previously thought, but rather a photochemical route for generating reactive oxides of nitrogen that play a role in air pollution and are associated with significant health effects," the scientists added.

Raff said other studies have missed the effect on ammonia because they investigated reactions that occur with high levels of emissions under industrial conditions, not the low levels and actual humidity levels typically present in urban environments.

Other TiO2 Uses Questioned

The findings also call into question other suggestions for using TiO2 for environmental remediation—for example, to remove odor-causing organic compounds from emissions produced by confined livestock feeding operations. The mineral has also been suggested as a geo-engineering substance that could be injected into the upper atmosphere to reflect sunlight away from the Earth and combat global warming.

Further studies in Raff's lab are aimed at producing better understanding of the molecular processes involved when TiO2 catalyzes the breakdown of ammonia.

The results could suggest approaches for developing more effective pollution-control equipment as well as improvements in industrial processes involving ammonia, according to the scientists.

Testing in Germany

Related research is under way in Germany. There, a team of researchers from Fraunhofer Institute for Molecular Biology and Applied Ecology IME in Schmallenberg are examining the effectiveness of photo-catalytic coatings at removing nitrogen oxides.

The two-year project is also measuring how coated building test panels perform over time. See: “Scientists to Test Smog-Eating Coatings” for more information on that research.


Tagged categories: Air pollution control; Air quality; Coating chemistry; Coatings Technology; Coatings technology; Environmental Protection; Pigments; Self-cleaning coatings; Titanium dioxide

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