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US researchers have used surface-enhanced Raman spectroscopy (SERS) to reveal the structure and makeup of molecules sitting on the surface of palladium-covered gold nanoshell particles. The work could lead to an improved method for cleaning up the stubborn pollutant TCE (trichloroethene). Michael Wong of Rice University and colleagues hoped to find a better way to clean up the pollutant TCE and have now developed an approach that lets them watch the molecules break down on the surface of a catalyst as individual chemical bonds are formed and broken. Trichloroethene is a common volatile organic solvent but is one of the most pervasive and troublesome groundwater pollutants. It is a carcinogen and is unfortunately found in almost two thirds of the contaminated waste sites on the US Environmental Protection Agency's Superfund National Priorities List. The US government has estimated the cost of cleaning up TCE contamination at US military bases alone will cost billions of dollars. Finding an inexpensive but effect method of eradicating this substance could reduce costs significantly. As part of the effort to find a catalytic cleanup agent, the Rice team has used SERS to monitor bond making and breaking on the surface of experimental nanoparticles. "With SERS, we can see the vibrations of the bonds between the atoms of our molecules," explains Wong, "By watching the way these vibrations change frequency and intensity with time, we can watch how molecules transform into other molecules step-by-step." Their approach utilises nanoparticles consisting of gold and silica called nanoshells, invented a decade ago at Rice by nanophotonics pioneer Naomi Halas. The nanoshells are about a twentieth of the size of a red blood corpuscle, and can amplify light waves and focus them so sharply that the researchers can use them to detect just a few molecules of almost any given target chemical. By building the catalyst directly on to the surface of these nanoshells they can then directly follow chemical reactions that take place on the surface. Wong's team have had a palladium-gold catalyst to break down TCE into non-toxic components for several years. Their early experiments showed it worked remarkably quickly. In fact, it was more efficient than predicted, based on the best available theories. "The gold was definitely playing a role that we didn't fully understand," Wong said. To learn more, Wong approached Halas and Rice theoretical chemist Gustavo Scuseria. Halas' spectroscopic skills used gold nanoshells for chemical detection and analysis. Wong's four-nanometre particles have a gold centre covered with palladium atoms and he and graduate student Kimberly Heck mused on whether they could coat Halas' much larger gold nanoshells with palladium atoms and then use the nanoshells to detect the elusive TCE chemical reaction. The experiments took about eighteen months, but the results were worth waiting for and post-doctoral researcher Ben Janesko provided sophisticated theoretical calculations to support the findings. "We think we parsed it out pretty well," Wong said of the hydrodechlorination reaction. "Millions of surface-bound molecules are reacting simultaneously, but with a lot of work we've uncovered at least seven chemical steps." Wong says that the study is not only helping them understand better the catalytic degradation of TCE, but the researchers also think that their new method will prove useful in obtaining details about surface catalysis in general. "Nanoshells are among the world's most effective chemical sensors," says Halas, "and this study reveals another area where they are uniquely valuable." She emphasises that there is no other known method that provides this level of detail about metal-catalysed chemical reactions that run in water. Given the overwhelming interest in biofuels processing and other water-based reactions, the same technology used to find a catalyst for breaking down TCE could also become a very useful tool in understanding the chemistry of biofuels too. "There was a question of whether we could do Raman spectroscopy and catalysis at the same time," Wong says, "There's no other method that lets you 'see' these catalyzed reactions in water while the reaction is happening, and some of the most interesting of these - like the reactions needed to upgrade glycerol and cellulose into chemicals and fuels - occur in water." Related links:
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Wong, using SERS to reveal catalytic rationale
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