Writing this week in the journal Science (June 27) University of Wisconsin-Madison chemical and biological engineers report the discovery of a nickel-tin catalyst that can replace the precious metal platinum in a new, environmentally sustainable, greenhouse-gas-neutral, low-temperature process for making hydrogen fuel from plants.
The new catalyst, together with a second innovation that purifies hydrogen for use in hydrogen fuel cells, offers new opportunities toward the transition of a world economy based on fossil fuels to one based on hydrogen produced from renewable resources.
James Dumesic, a professor of chemical and biological engineering, and graduate students George Huber and John Shabaker describe testing more than 300 materials to find a nickel-tin-aluminum combination that reacts with biomass-derived oxygenated hydrocarbons to produce hydrogen and carbon dioxide without producing large amounts of unwanted methane.
"Platinum is very effective but it's also very expensive," says Dumesic. "It's also problematic for large-scale power production because platinum is already in demand for use as anode and cathode materials in hydrogen fuel cells. We knew nickel was very active, but it allowed reaction to continue beyond hydrogen producing methane. We found that adding tin to what's known as a Raney-Nickel catalyst decreased the rate of methane formation without compromising the rate of hydrogen production."
Dumesic, research scientist Randy Cortright (now at Virent Energy Systems) and graduate student Rupali Davda first reported the catalytic reforming process for hydrogen production in the Aug. 29, 2002 issue of the journal Nature.
The simple, single-step process employs temperature, pressure and a catalyst to convert hydrocarbons such as glucose, the same energy source used by most plants and animals, into hydrogen, carbon dioxide, and gaseous alkanes with hydrogen constituting 50 percent of the products. More refined molecules such as ethylene glycol and methanol are almost completely converted to hydrogen and carbon dioxide. Because plants grown as fuel crops absorb the carbon dioxide released by the system, the process is greenhouse-gas neutral.
Glucose is manufactured in vast quantities -- for example, in the form of corn syrup -- from cornstarch, but can also be made from sugar beets, or low-cost biomass waste streams like paper mill sludge, cheese whey, corn stover or wood waste. While hydrogen yields are higher for more refined molecules, Dumesic says glucose derived from waste biomass is likely to be the more practical candidate for cost-effectively generating power. In addition, hydrogenation of glucose to the sugar-alcohol sorbitol allows the hydrogen from glucose to be extracted more efficiently.
Because the Wisconsin process occurs in a liquid phase at low reaction temperatures (225 degrees Celsius, 440 degrees Fahrenheit) the hydrogen is made without vaporizing water. That represents a major energy savings compared to ethanol production or conventional fossil fuel-based hydrogen-generation methods that require water to be boiled away.
In addition to finding the new catalyst, Dumesic and Davda will soon publish in the German journal Angewandte Chemie, International Edition refinements to the system that produce a higher quality of hydrogen using the platinum catalyst. Among the key accomplishments first cited in the August Nature article was the production of hydrogen with very low CO (carbon monoxide) concentrations on the order of 300 parts per million (PPM). Now, Dumesic and Davda report enhancements to the process that achieve CO concentrations of 60 PPM. By comparison, more common steam-reforming processes for hydrogen production require a complex and costly combination of techniques to achieve hydrogen fuel with CO levels between 100 and 500 PPM.
The dramatic reduction in CO contamination achieved by the team's new "ultra-shift" process confronts a major obstacle in the efficient operation of hydrogen fuel cells. Carbon monoxide poisons the electrode surfaces of the devices hampering their reliability.
Ultimately, the researchers will create a combined process whereby the nickel-tin catalyst reforms oxygenated hydrocarbons to produce relatively clean hydrogen which is then passed to a second-stage ultra-shift catalyst where carbon monoxide is removed and the hydrogen made even more pure for use in fuel cells. Using strategies similar to those that revealed the nickel-tin catalyst, Dumesic is confident his team can find a similar low-cost replacement for the platinum catalyst in the second stage.
Jim Beal, (608) 263-0611, email@example.com