Titanium dioxide catalysts break down pollutants Pacifichem '95 I Honolulu Ï Titanium dioxide, that stolid industrial mainstay, has an alter ego. While serving ably as an ingredient in paint pigments and scratch-resistant optical coatings, this unassuming semiconductor also displays a talent for photocatalysis that chemists and chemical engineers are beginning to exploit. Ti0 2 , it appears, has seemingly limitless potential—under even indoor room light—to break down organics in not only trivially annoying things like bad breath, but more dangerous contaminants, such as cigarette smoke. In Japan, consortiums of companies are inventing ingenious ways to harness its reactivity. Ti0 2 -impregnated paints, papers, glasses, and sheets can prevent buildup of dusty grease on walls of tunnels and roofs of buildings, oxidize nitrogen oxides (NOx) into nitric acid, and even kill bacteria. At Pacifichem '95 last month, Ti0 2 was the focus of a session during a symposium on photocatalysis of semiconductor particles, coorganized by Nick Serpone, chemistry professor at Concordia University in Montreal. Chemists and chemical engineers at the session from Japan and the U.S. highlighted some of the material's more promising uses. In the eyes of chemical engineers, Ti0 2 is an ideal substance for mass use. It is nontoxic, extremely stable, works at room temperature, and is cheap and plentiful. Ti0 2 's propensity for oxidation when exposed to light has been known for some time. To show full catalytic potential, Ti0 2 requires both oxygen and water, both of which are abundant in air. Photons impinging on Ti0 2 dislodge electrons, creating holes and free electrons. The holes and free electrons oxidize water and oxygen, respectively, to form hydroxyl and superoxide radicals, both ruthless bond cleavers. "Once those [radicals] are formed, they are completely indiscriminating. They will destroy anything that is organic," Serpone said. "If there's one drawback, perhaps if s
that it's not selective enough/' added David R Ollis, chemical engineering professor at North Carolina State University, Raleigh. "It oxidizes everything." More than a decade ago, researchers in Ollis' lab realized Ti0 2 's ability to photocatalytically decompose organics like trichloromethane and trichloroethylene might be used to treat contaminated environments such as groundwater. However, after an initial flurry of research, the consensus was that Ti0 2 probably isn't efficient enough to tackle large-volume problems such as polluted lakes, rivers, or industrial effluents. But Ti0 2 can be quite effective against a host of dilute, airborne pollutants that cause innumerable headaches, literally and figuratively, for humans and the environment. Like kitchen grease, for instance. "In Japan, people cook tempura almost every day," Akira Fujishima, chemistry professor at the University of Tokyo, told the Pacifichem symposium. And Americans' love of deep-fried food is no secret. Consequently, kitchen fans the world over clog with oil, grease, and dirt. But a fan belt infused with some Ti0 2 will break down the grease under the kitchen fluorescent light, minimizing clean-up, he said. Although strong light sources, such as the sun or ultraviolet light, were generally thought to be necessary for efficient photocatalysis, Fujishima presented a host of experimental evidence showing that room light is sufficient to activate the process. In one experiment, cigarette smoke adsorbed on a Ti0 2 -coated glass or Ti0 2 -containing paper disappeared within two hours under black fluorescent light and within one week under a less intense regular fluorescent light, Fujishima said. Ti0 2 also breaks down foul-smelling compounds such as methyl mercaptan, hydrogen sulfide, and acetaldehyde. As a result of these and many other experiments, scores of companies are now jumping on the Ti0 2 product bandwagon, including Ishihara Sangyo Kaisha, the largest supplier of Ti0 2 products in Japan; Fuji Electric Co.; the paint company Okitsumo; and the cement company Chichibu Onoda. Fujishima's group has also studied the ability of Ti0 2 to kill bacteria such
as Escherichia coli, Staphylococcus aureus, and Pseudomonas aerguinosa. The bactericidal mechanism is still not clear, Fujishima said, but his group suspects the hydroxyl or superoxide radicals may attack cell membranes. The Japanese firm TOTO is now manufacturing Ti0 2 -coated ceramic tiles for hospitals, to help maintain a bacteria-free environment. Ti0 2 can even ameliorate the housekeeping bane of the well-to-do: cleaning the chandelier. "If you coat it with Ti0 2 , it is not necessary to clean it," Fujishima said, because dust particles will not cling to the surface. Photocatalytic paint is a more tricky problem, Fujishima said. Ti0 2 , which is commonly used as white pigment in paints, must be coated with silica; oth-
Fujishima: room light can activate process
Ibusuki: Tt02 catalyzes NOx oxidation JANUARY 15,1996 C&EN
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SCIENCE/TECHNOLOGY erwise its catalytic action degrades the paint base within a couple of days. Researchers are in the process of developing paint bases impervious to oxidation, which will pave the way for selfcleaning houses, tunnels, and roofs. NO x emissions are a serious air pollution problem in Japan despite governmental efforts to reduce them, Takashi Ibusuki, chief senior researcher at Japan's National Institute for Resources & Environment, told the symposium. He and his colleagues, including Koji Takeuchi, chief of NIRE's Photoenergy Application Laboratory, are developing sheet catalysts made with a mixture of fluororesins and catalyst powders—Ti02 and activated carbon— that oxidize NO x into nitric acid. A square-meter section of the sheet removes about 3 mmol of NO x per day, lasting about a week before requiring regeneration. About 50 to 60% of the nitric acid can be recovered with a simple water wash that also regenerates the catalyst, Takeuchi said. Chemical engineering professor Adam Heller at the University of Texas, Austin, and colleagues have developed a Ti0 2 coating for car windows that oxidizes the organic film that builds up on the surface. Heller's colleague, John G. Ekerdt, also a chemical engineer, described at the symposium another technique developed by the Texas researchers—using buoyant nanocrystalline Ti02-coated glass microbubbles to break down organic contaminants like oil on water surfaces. The list of possible applications will continue to expand, Ollis said, but he warned of wrinkles in Ti0 2 photocatalyst technology not yet ironed out. While many of the material's potential uses seem promising, not all will prove successful, he predicted. Some may not be economically feasible. Additionally, he said, the possible degradation of Ti0 2 's catalytic ability needs to be considered more thoroughly. Ideally, the catalysts should work for months or years without deteriorating to be commercially viable. Ollis predicted catalyst deactivation will become the next focus for chemists and engineers in this area. "It doesn't mean there's not an easy solution. It's just the research has not yet begun," he said. "It's a wonderfully exciting area with tons of potential work remaining." Elizabeth Wilson
