Green-energy fans have long dreamed of using the sun's rays to make clean-burning fuel. For decades, researchers tinkered with light-triggered catalysts that encourage water molecules to release hydrogen gas--but none of the catalysts were sufficiently cheap and stable. Now a team at Duquesne University in Pittsburgh, Pennsylvania, has modified a catalyst in a way that might bring the long-sought goal within reach.
Experiments with titanium dioxide (TiO2) date back to the early 1970s. TiO2 absorbs photons, which excite electrical charges in the catalyst. These charges can then split water molecules to produce hydrogen gas. TiO2's big advantage is that it is stable under prolonged sunlight, and the material, which is added to everything from paint to sunscreen, is cheap. But TiO2 only absorbs ultraviolet light, a small fraction of the spectrum of sunlight that reaches Earth.
Chemist Shahed Khan thought that part of the problem was the high-temperature process of turning titanium metal into TiO2: It creates other compounds that do a poor job of absorbing light. So along with his graduate students Mofareh Al-Shahry and William Ingler Jr., he designed a furnace that roasts a sheet of titanium metal in a flame of natural gas. Methane in the gas breaks down into carbon dioxide and water vapor when it burns; the vapor helps turn titanium metal to TiO2. Crucially, it also added some carbon to the mix. When the group burned the metal at 850°C, the resulting catalyst converted 8.5% of the energy in sunlight to hydrogen gas, more than eightfold the usual amount, Khan reports in the 26 September Science . The reason: In addition to absorbing UV light, the carbon-enriched TiO2 catalyst also absorbed longer wavelength photons in the violet, blue, and green regions of the spectrum.
"That's an excellent result," says T. Nejat Veziroglu, a hydrogen energy specialist at the University of Miami, Florida. The efficiency still falls just below the U.S. Department of Energy's 10% benchmark for a commercially viable catalyst, notes Eric Miller, an electrical engineer at the University of Hawaii, Manoa. But he predicts that Khan's team has a real chance to clear the hurdle. "It's a good lead in a good direction," Miller says.
Shahed Khan's home page