Researchers at the University of Wisconsin in Madison have discovered a much lower cost catalyst for producing hydrogen from organic matter.
MADISON – It is thousands of times less expensive than platinum and works nearly as well.
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.
The precious metal platinum (Pt) is well known to be an excellent catalyst in a number of chemical reactions. It is one component in a car's catalytic converter, for example, that helps remove toxins from automobile exhaust. Yet, platinum is rare and very expensive, costing more than $17 per gram (about $8,000 per pound).
Catalytic platinum (Pt) and nickel (Ni) stand out from other metals (such as copper or iron) because they process reaction molecules much faster. But pure nickel, unlike platinum, recombines the hydrogen product with carbon atoms to make methane, a common greenhouse gas. Dumesic and his colleagues tested over 300 catalysts to find one that could compete with platinum and perform in the APR process. Using a specially designed reactor that can test 48 samples at one time, the team finally found a match in a modified version of what researchers call a Raneynickel catalyst, named after Murray Raney, who first patented the alloy in 1927.
Raney-nickel is a porous catalyst made of about 90 percent nickel (Ni) and 10 percent aluminum (Al). While Raney-nickel proved somewhat effective at separating hydrogen from biomass-derived molecules, the researchers improved the material's effectiveness by adding more tin (Sn), which stops the production of methane and instead generates more hydrogen. Relative to other catalysts, the Raney-NiSn can perform for long time periods (at least 48 hours) and at lower temperatures (roughly 225 degrees Celsius).
According to Dumesic, a substitute for platinum catalysts is essential for the success of hydrogen technology. "We had to find a substitute for platinum in our APR process for production of hydrogen, since platinum is rare and also employed in the anode and cathode materials of hydrogen fuel cells to be used in products such as cars or portable computers," he said.
While this is an important advance by itself it does not make biomass a viable major energy source. The problem with growing crops for biomass is that it takes energy to make and transport the fertilizer, run tractors, run irrigation equipment, harvest, transport, and so on. It remains to be seen whether there is a crop that will yield enough biomass energy to make it worthwhile.
This catalyst may be useful on smaller scales in places where there is already a great amount of biomass waste being produced. For instance, the processing of existing crops produces biomass waste. Equipment to convert that biomass waste into useful hydrogen energy could be installed next to agricultural product processing facilities if this new catalyst turns out to work well in industrial use. Still, all the existing biomass waste is not sufficient as an energy source to replace much of the currently consumed fossil fuels.
Other enabling technologies such as fuel cells need to mature ot make hydrogen a more useful energy source once it has been produced. Those advances will come with time. What strikes me as less certain is whether biomass will ever become a major energy source for producing hydrogen. Plants have to be planted, tended, harvested, and processed. They are vulnerable to insects and droughts. They do not convert most of the light that hits them into stored chemical energy.
There are competing approaches that may be cheaper in the longer run. Advances in nanotechnology will eventually yield photovoltaic materials that will be cheap to produce. Then the electricity from the photovoltaics will could be used to run hydrolysis reactions to produce hydrogen from water. Also, some materials may be found that can absorb light to drive a direct catalysis reaction to produce hydrogen from water without first producing electricity. Such materials would probably be more efficient than plants at converting sunlight to energy and would even be able to do so all year around (albeit at lower rates during the shorter days of the year).
Update: Some Tufts researchers have also recently discovered a way to reduce the amount of precious metals used as catalysts to make hydrogen.
"A lot of people have spent a lot of time studying the properties of gold and platinum nanoparticles that are used to catalyze the reaction of carbon monoxide with water to make hydrogen and carbon dioxide," said Maria Flytzani-Stephanopoulos, professor of chemical and biological engineering at Tufts and the lead researcher of the project. "We find that for this reaction over a cerium oxide catalyst carrying the gold or platinum, metal nanoparticles are not important. Only a tiny amount of the precious metal in non metallic form is needed to create the active catalyst. Our finding will help researchers find a cost-effective way to produce clean energy from fuel cells in the near future"
She and her two colleagues, doctoral student Qi Fu and research professor Howard Saltsburg, were funded by a $300,000 three-year grant from the National Science Foundation, and have filed a provisional patent for their research. Their cutting-edge work in catalytic fuel processing to generate hydrogen for fuel cell applications is one of the major undertakings at Tufts' Science and Technology Center at the University's Medford campus.
The Tufts researchers' article is based on the "water-gas shift" reaction they use to make hydrogen from water and carbon monoxide over cerium oxide loaded with gold or platinum. Typically, a loading of 1-10 weight percent of gold or other precious metals is used to make an effective catalyst. But the Tufts team discovered that, after stripping the gold with a cyanide solution, the catalyst was just as active with a slight amount of the gold remaining – one-tenth the normal amount used.
According to Flytzani-Stephanopoulos, "This finding is significant because it shows that metallic nanoparticles are mere 'spectator species' for some reactions, such as the water-gas shift. The phenomenon may be more general, since we show that it also holds for platinum and may also hold true for other metals and metal oxide supports, such as titanium and iron oxide."
She adds, "It opens the way for new catalyst designs so more hydrogen can be produced with less precious metal. This can pave the way for cost-effective clean energy production from fuel cells in the near future."
|Share |||Randall Parker, 2003 June 26 05:52 PM Energy Biomass|