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Catalysts for change - Could some cunning chemistry make hydrogen the fuel of the 21st century?

点击量:   时间:2017-12-26 05:02:11

By Peter Hadfield and Rebecca Warden Tokyo & Barcelona THE prospect of a cheap supply of hydrogen has moved closer to reality now that researchers working independently in Japan and Spain have succeeded in splitting the elements of water at room temperature with a catalyst. Hydrogen is widely touted as the fuel of the future and water is an abundant source of the element. Researchers at the Tokyo Institute of Technology have succeeded in extracting hydrogen and oxygen from water using visible light and a cuprous oxide catalyst. Meanwhile, researchers at the University of Valencia have produced hydrogen and oxygen from water with a molybdenum catalyst. Hydrogen is well-suited to motor vehicles. It is virtually pollution-free, but making the gas from water by electrolysis is quite expensive. Although researchers have tried to use cuprous oxide to split water before, the process did not work in visible light. Sunlight reduces cuprous oxide to metallic copper, which halts the process. The Japanese researchers managed to avoid this problem by using powdered cuprous oxide. The researchers added 0.5 grams of cuprous oxide powder to 200 cubic centimetres of distilled water. Visible light with wavelengths of between 460 and 650 nanometres was shone from a lamp on one side of the glass vessel. As the catalyst broke down the water, the researchers were able to extract oxygen and hydrogen. Thirty test runs were carried out, producing varying ratios of hydrogen to oxygen. During these tests, the researchers discovered that the process of splitting water slowed down if the oxygen gas built up to a pressure above 500 pascals. To double-check that the oxygen was coming from the water—and not from the cuprous oxide—the researchers tested the catalyst with water containing a higher proportion of the oxygen-18 isotope as well as the more common oxygen-16. The mixture of isotopes in the gas corresponded to that in the water, proving that the gas was not coming from another source (Chemical Communications, vol 70, p 287). The experiment showed that cuprous oxide in powder form can act as a stable catalyst for at least 1900 hours. “We believe this is a breakthrough in our research,” says Kazunari Domen of the Tokyo Institute of Technology, which is working on the project in conjunction with Nikon Corporation. However, he warned that “this is still basic research”. Domen says that the researchers need to improve the efficiency of the process. The efficiency of the experiment, measured as the number of hydrogen gas molecules produced, divided by the light absorbed by the catalyst, was around 0.3 per cent at wavelengths between 550 and 600 nanometres. Domen wants to increase that figure by making better use of the cuprous oxide catalyst and finding new catalysts that are active with light at wavelengths above 600 nanometres. The team is currently experimenting with a copper iron oxide catalyst. The researchers also need to overcome the effects of the build-up of oxygen. The Spanish team, headed by Antonio Cervilla and Elisa Llopis at the Institute of Materials Science in Valencia, has produced hydrogen from water using a molybdenum catalyst. The discovery means “a revolution in the way of obtaining hydrogen and generating energy in a clean way”, says Cervilla. However, he also says that a lot of work is needed before the method can be used to produce hydrogen on an industrial scale. The catalyst is a new compound of molybdenum, which has been developed by the Spanish team. The scientists will not disclose more details about the compound until they have been granted a patent for it. However, Cervilla says that the catalyst splits water in hydrogen and oxygen at room temperature. He believes that purification and storage of hydrogen will be relatively simple. The Spanish scientists are convinced this new method has great potential for producing cheap hydrogen. “You do not need an energy source to carry out this reaction,” says Cervilla. However, chemical engineers warn that many questions still need to be answered, including how much catalyst is consumed in the process and how much hydrogen can be produced. The discovery is significant, says Cervilla,