Issue Date: August 10, 2009
Hydrogen From Sun And Water
Sunlight can readily liberate hydrogen from water as a result of a novel solid catalyst that mediates that reaction with unprecedented efficiency, according to researchers in China who developed the catalyst. The study advances the decades-old search for an inexpensive way to produce hydrogen, a versatile fuel, from water, an abundantly available resource.
A key challenge to tapping into solar energy on a broad scale is developing an effective way to store that energy. One strategy calls for using sunlight to produce fuels such as hydrogen, which in many ways is considered an ideal energy carrier. Using sunlight to evolve hydrogen from water photolytically is one direct route to converting solar energy into fuels. But most photocatalysts suffer from significant shortcomings.
For example, many photocatalysts facilitate water splitting only under ultraviolet light, which constitutes just a few percent of the energy in the broad solar spectrum. Other catalysts have been designed to exploit the visible wavelengths of sunlight. But they do so only with limited effectiveness. A standard measure of that effectiveness is known as quantum efficiency, which can be expressed as the ratio of the number of product molecules to incident photons.
Among synthetic catalysts activated by light in the visible range, the highest quantum efficiency for hydrogen production from water reported until now is about 60%. In contrast, the quantum efficiency of natural catalytic systems that drive photosynthesis can reach 95%.
Now, researchers at Dalian Institute of Chemical Physics, in China, have developed a three-component semiconductor-based catalyst that can produce hydrogen from water when irradiated with light in the visible-wavelength region (420 nm) with a quantum efficiency as high as 93% (J. Catal., DOI: 10.1016/j.jcat.2009.06.024).
The catalyst, prepared from cadmium sulfide doped with low concentrations of palladium sulfide and platinum, does not convert water into hydrogen and oxygen. It evolves hydrogen alone and does so only from water solutions containing sulfur-based "sacrificial" reagents that consume oxygen, according to Can Li, who led the study and directs the institute's catalysis laboratory.
While searching for the composition that yields the most active photocatalyst, Li, Hongjian Yan, Jinhui Yang, and coworkers observed that pure CdS evolves hydrogen from water very slowly. The team found that doping CdS with platinum or palladium greatly increases the hydrogen yield. But doping CdS with both metals offers little additional benefit, they found. So the team took another approach: They doped CdS with PdS, which boosted CdS's activity by more than a factor of 260. Then they co-doped CdS with PdS and platinum and found the three-component material to be 380 times more active than pure CdS.
The group proposes that PdS serves as an oxidation cocatalyst (co-dopant), and platinum's role is to facilitate reduction. Li notes that further work is already under way to firmly establish the roles of each of the components.
"The reported quantum efficiency of hydrogen evolution is remarkably high," says Kazunari Domen, a photocatalysis expert at the University of Tokyo. He adds that the requirement for sacrificial sulfur compounds might be exploited, for example, to produce hydrogen from sulfur-laden petroleum products. But in Domen's view, the most significant aspect of the work is the novel combination of cocatalysts, which will likely motivate other researchers to explore that strategy in the search for better performing catalysts.
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