A TWIST ON WATER SPLITTING PHOTOCATALYSTS: Nanosized crystal
patchwork generates hydrogen, raising alternative-energy hopes
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F WATER COULD be separated economically into ox-
ygen and hydrogen, a clean-burning fuel, the world’s oceans would represent a free and virtually limitless feedstock for producing energy. A new strategy for designing light-activated catalysts that split water may help bring that alternative-energy goal a step closer to reality. Researchers in China have found that crystals of the semiconductor Ga2O3 that are composed of a patchwork of structurally distinct nanosized domains can split water photocatalytically (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201207554).The study demonstrates that the interface between polymorphic crystal phases can play a key role in light-stimulated water splitting. The work may lead to photocatalysts that are more active than the relatively inefficient ones available today. Photocatalysts split water by directing energy absorbed from light—often sunlight—to break water’s chemical bonds. The heart of the process is the light absorption event, which generates pairs of negatively charged electrons and positively charged holes (electron vacancies). The key to capitalizing on the energy absorbed from light is keeping the charges separated. Charge recombination can dissipate the absorbed energy before bonds are broken. The standard strategy for maximizing charge separation in semiconductors calls for selectively doping the material to juxtapose positively (p-type) and negatively (n-type) charged regions. Researchers also make these
types of interfaces, known as p-n junctions, by depositing two types of semiconductors side by side. The new study, which was conducted by Xiang Wang, Can Li, and coworkers of the Dalian Institute of Chemical Physics, demonstrates an alternative way to keep charges separated. Rather than relying on p-n junctions to do the job, the Dalian team exploits the interfaces between structurally dissimilar nanosized domains of Ga2O3, which is known to crystallize in five polymorphic phases. The researchers show that a simple heat treatment can be used to tailor the distribution of nanosized domains of Ga2O3’s so-called α and β phases. They further show that mixed α-β samples prepared by heating the starting material to roughly 600 °C are up to seven times more catalytically active in splitting water than samples composed of TINY JUNCTION The interfaces either phase alone. between nanosized domains of α- (brown) The University of Toand β- (blue) Ga2O3 mediate photocatalytic kyo’s Kazunari Domen charge separation and water splitting. comments that although in this proof-of-concept study the overall activity of the Ga2O3 photocatalysts is not especially high, the strategy described here represents a new approach to boosting catalytic water-splitting efficiency. He adds that the method’s applicability may be limited, however, as a result of the relatively small number of materials known to exhibit similar kinds of polymorphs.—MITCH JACOBY
SHALE Sasol moves ahead with massive Louisiana chemicals and fuels projects The South African fuels and chemicals maker Sasol will proceed with engineering and design for building gas-to-liquids and ethylene plants at its Lake Charles, La., complex. The project, which may cost as much $21 billion to complete over a five- to seven-year period, is being billed as the largest manufacturing investment ever for the state of Louisiana and one of the largest-ever direct foreign investments in the U.S. The gas-to-liquids facility will reform natural gas extracted from shale into a mixture of hydrogen and carbon monoxide. It will then use a Fischer-Tropsch process, which converts the mixture into
hydrocarbons, to make fuels and chemicals such as diesel, jet fuel, naphtha, paraffin, and base oils. Expected to cost between $11 billion and $14 billion, the gas-to-liquids plant will open in phases. The first phase will start up in 2018 with 48,000 barrels per day of capacity. The second phase will double the plant’s size the following year. The ethylene cracker will use shalebased ethane as its feedstock. Downstream from the 1.5 million-metric-ton cracker, Sasol plans to build polyethylene, ethylene oxide, synthetic alcohols, 1-octene, and ethoxylation facilities. The complex will cost between $5 billion and $7
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billion and is slated for completion in 2017. The project is predicated on North American natural gas remaining a much cheaper raw material than oil, Sasol Senior Group Executive André M. de Ruyter told reporters on a conference call. Recent price ratios of a barrel of oil to a million Btu of natural gas have been about 30 to 1, he noted, and the project would still be viable at a ratio of 15 to 1. “Gas-to-liquids would make an enormous amount of sense if we could have the plant up and running today,” he said. Sasol has delayed plans for a gas-to-liquids project in Canada until it completes the Lake Charles plant.—ALEX TULLO
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