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Stirred and shaken

发布时间:2019-03-08 08:03:06来源:未知点击:

By Lila Guterman THE words “mysterious” and “bizarre” don’t often come up in conversations among chemists. But that’s how they are describing a way to split water into hydrogen and oxygen at room temperature using a simple catalyst. Japanese researchers now say that the energy needed to break the bonds that hold water molecules together seems to come from stirring the liquid. When Kazunari Domen and his colleagues at Tokyo Institute of Technology first reported that a powdered cuprous oxide catalyst could split water at room temperature, chemists sat up and took notice (This Week, 28 February, p 10). This reaction usually takes place at 3000 °C and is driven by light. If its efficiency could be improved, hydrogen would become much more attractive as a cheap, clean fuel. Domen initially thought that the energy driving the low-temperature reaction came from light—a mechanism chemists could just about live with. But now he reports that the reaction continues in the dark for hundreds of hours. The quicker the container is stirred, says Domen, the more hydrogen and oxygen are produced (Chemical Communications, p 2185). Domen believes that the mechanical energy is converted to chemical energy without first being converted to thermal energy. “This is the first example of mechanical energy being converted into chemical energy by catalysis,” he says. But Domen cannot explain how this works. To most chemists, the idea is heresy. And the prospect of producing a source of fuel by a mechanism no one understands evokes memories of the “cold fusion” saga of 1989. But Domen’s results were carefully reviewed before they were published. He has also avoided making any bold claims about practical applications. “A lot of colleagues were incredulous but I think Domen is a very serious scientist,” says Michael Grätzel of the Swiss Federal Institute of Technology in Lausanne. Grätzel suspects that something more mundane is occurring, however, such as friction between the catalyst and the glass container causing a build-up of electrical charge. The resulting tiny sparks could break down small amounts of water. “There’s an easy way to rule that out,” he says. Dissolving a salt in the water would change its conductivity and the amount of gases produced. Arthur Nozik, a chemist at the National Renewable Energy Laboratory in Golden, Colorado, doubts if the effect is really caused by catalysis. “I wouldn’t be surprised if they are actually using up some of the metal oxide,” he says. Nozik suspects that the cuprous oxide might react with water, liberating hydrogen. The oxygen could come from small quantities of the gas dissolved in the water. Domen says that less than 5 per cent of the energy from stirring appears to end up splitting water, producing a small amount of hydrogen and oxygen gas. But he thinks he can improve the yield by learning more about how the reaction works. “If we can increase the efficiency by an order of magnitude, then it may be possible to think about practical applications,” he says. “It sounds odd, but maybe it only sounds odd because it’s new,” says Sean McGrady of King’s College London. He likens the situation to the discovery in the 1980s that sound can drive chemical reactions, which has led to the burgeoning field of sonochemistry. “When these things show up they always challenge people’s conceptions of how energy can get into a chemical reaction,