29 August 2003—Topping the list of favorite alternatives to fossil fuels and the climate-changing chemical compounds produced when they are burned is hydrogen. While hydrogen can be either combusted directly, producing only water and heat as byproducts, or used in fuel cells to create electricity, a big obstacle to a hydrogen economy is producing enough of it in pure form to replace all the coal, oil, and natural gas the world now depends on.
Current methods of isolating hydrogen are costly, use more energy than can be gained from the hydrogen, or are harmful to the environment. But researchers at the University of Wisconsin, Madison, say they have improved one process enough to make it both cheap and environmentally benign.
Two common processes used to isolate hydrogen are electrolysis, which requires electricity to separate the hydrogen from the oxygen in water—making for a chicken-and-egg problem, and steam reforming of hydrocarbons, a two-step process that eventually yields hydrogen, carbon dioxide, and waste products that must be disposed of.
Instead the researchers focused on the chemical conversion of biomass, such as wood pulp, cheese whey, or alcohol made from sugar. Converting biomass—perhaps the least environmentally problematic method because its feedstock is renewable and the conversion process is highly efficient—makes better economic sense now that they have found a way to do without expensive metal catalysts made, for example, from US $17-a-gram platinum.
In the 27 June issue of Science, the University of Wisconsin’s James Dumesic, George Huber, and John Shabaker reported on the development of a catalyst that separates hydrogen from other compounds in biomass just as well as precious metals do, but costs one-thousandth as much.
Although a cheap catalyst that can strip away hydrogen is commercially available, its main byproduct is methane, a greenhouse gas that is much more potent than carbon dioxide. Raney-nickel, a porous metal alloy made up of 90 percent nickel and 10 percent aluminum, is very good at manipulating the position of hydrogen atoms, either for stripping them from simple carbohydrates and sugars or strategically adding them to other molecules. It is used in several situations, including the food processing industry, where extra hydrogen atoms are forced into the molecular branch chains that make up vegetable oils, turning them into semi-solid fats such as margarine and shortening.
When stripping hydrogen atoms away from a molecule, Raney-nickel is just as active a catalyst as platinum, but unlike its more expensive counterpart, it allows a chemical reaction known as a water-gas shift to carry on too long. Instead of sugar (carbon, hydrogen, and oxygen, or CHO) reacting with water (H 2 O) to form carbon dioxide (CO 2 ) plus hydrogen (H 2 ), Raney-nickel allows the CO 2 to dissociate into molecular oxygen and free carbon, which recombines with the hydrogen. The net result is methane (CH 4 ).
If hydrogen is to ever become the panacea some think it could be, the researchers reasoned, the processes that break its chemical bonds with other elements must not aggravate the greenhouse effect. After testing more than 300 metal combinations, the Wisconsin group hit upon a variant of Raney-nickel that suppresses the formation of methane without affecting the production of hydrogen. According to Huber, the addition of a small amount of tin made all the difference. He told IEEE Spectrum that he and his colleagues still are unsure how the chemistry works on a molecular level, but they think the tin fills in the places on the surface of the catalyst’s crystal lattice where there are faults. These fault areas, so-called nickel-defect sites, are more likely to catalyze the recombination of carbon with hydrogen to produce methane.
To take full advantage of the catalyzing properties of this new alloy, the researchers knew they had to put it in contact with the biomass feedstock— in this case, sorbitol, a cheap, natural sweetener in toothpaste and other products—under the right temperature and pressure. They settled on a process called aqueous phase reforming, a highly technical way to say that they threw some sorbitol and water into a special pressure cooker along with some pellets of the catalyst. When the temperature and pressure reached 225 ° C and 2500 kPa, respectively, hydrogen and carbon dioxide bubbled up and was piped away to be cooled.
Sorbitol is used because the reaction gives you more than you pay for. Adding a single hydrogen molecule (H 2 ) to a molecule of glucose (C 6 H 12 O 6 ), creates a molecule of sorbitol, which, in turn, yields 13 hydrogen molecules in the catalytic reforming process. Ethylene glycol yields even more hydrogen during the process, but the researchers dismissed it as a potential feedstock because it is much more expensive to produce than sorbitol.
When asked why the release of carbon dioxide into the atmosphere didn’t concern the team, Huber, a Ph.D. candidate working in Dumesic’s lab, told Spectrum that it was because the net addition of CO 2 to the atmosphere is zero. The carbon dioxide emitted during the reforming process in a given year will be reabsorbed by the crop grown as feedstock the following year.
Though he would not give specifics for patent reasons, Huber said the team is currently at work improving the catalyst so it will produce more hydrogen per unit of its volume. They are also reworking the design of the reactor so that it will require less energy to operate and will improve the contact between the catalyst and the feedstock. Advances in both will someday yield an on-board reformer that can be used in applications such as cars and laptop computers. ”That’s the dream, anyway,” said Huber.