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Hydrogen from Catalytic Reforming of Biomass-Derived Hydrocarbons in Liquid Water
Nature 418, 964 (2002), with R. D. Cortright and R. R. Davda
In the search for a nonpolluting source of energy, hydrogen is often cited as a potential source of unlimited clean power. Currently, most hydrogen is made from fossil fuels like natural gas using multi-step and high-temperature processes. Dumesic and his group have developed a new process that produces hydrogen fuel from plants. This source of hydrogen is non-toxic, non-flammable and safely transportable in the form of sugars.
Writing in the journal Nature, Dumesic’s team describes a process by which glucose, the same energy source used by most plants and animals, is converted to 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.
The process is greenhouse-gas neutral. Carbon dioxide is produced as a byproduct, but the plant biomass grown for hydrogen production will fix and store the carbon dioxide released the previous year.
Glucose is manufactured in vast quantities — for example, in the form of corn syrup — from corn starch, 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.
Because the Wisconsin process occurs in a liquid phase at low reaction temperatures (e.g., 225oC), the hydrogen is made without the need to vaporize water. That represents a major energy savings compared to ethanol production or other conventional methods for producing hydrogen from fossil fuels based on vapor-phase, steam-reforming processes.
In addition, the low reaction temperatures result in very low carbon monoxide (CO) concentrations making it possible to generate fuel-cell-grade hydrogen in a single-step process. The lack of CO in the hydrogen fuel clears a major obstacle to reliable fuel cell operation. CO poisons the electrode surfaces of low-temperature hydrogen fuel cells.
Dumesic and Randy Cortright have formed Virent Energy Systems to move the technology from the engineering laboratory to the marketplace (see Virent.com). The Wisconsin Alumni Research Foundation is pursuing two patents based on the technology and has granted Virent Energy Systems an exclusive license. In lieu of licensing fees, WARF has taken an equity stake in Virent.
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Raney Ni-Sn catalyst for H2 Production from biomass-derived hydrocarbons
Science 300, 2075 (2003), with G. W. Huber and J. W. Shabaker.
Writing in the journal Science, Dumesic and co-workers 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 offers new opportunities toward the transition of a world economy based on fossil fuels to one based on hydrogen produced from renewable resources.
Dumesic and his team 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. These researchers had shown earlier that nickel was very active, but it allowed reaction to continue beyond hydrogen producing methane. They found that adding tin to a Raney-Nickel catalyst decreased the rate of methane formation without compromising the rate of hydrogen production.
Catalytic Reforming of Oxygenated Hydrocarbons for Hydrogen with Low Levels of Carbon Monoxide
Angewandte Chemie International Edition 42 4068 (2003), with R. R. Davda.
Dumesic and Davda published in the German journal Angewandte Chemie, International Edition a new process to produce a higher quality of hydrogen using a supported platinum catalyst. Among the key accomplishments first cited in the 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. 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.
Powering Fuel Cells with CO via Aqueous Polyoxometalates and Gold Catalysts
Science 305, 1280 (2004), with W. B. Kim, T. Voitl, and G. J. Rodriguez-Rivera.
Hydrocarbons such as gasoline, natural gas or ethanol must be reformed into a hydrogen-rich gas to be useful in a power-generating fuel cell. A large, costly and critical step to this process requires generating steam and reacting it with CO in the water-gas shift reaction to produce hydrogen and carbon dioxide (CO2). Additional steps reduce the CO levels further before the hydrogen enters a fuel cell.
Reporting in Science, Dumesic and co-workers eliminated the water-gas shift reaction from the process, removing the need to transport and vaporize liquid water in the production of energy for portable applications. The team uses an environmentally benign polyoxometalate (POM) compound to oxidize CO in liquid water at room temperature. The compound not only removes CO from gas streams for fuel cells, but also converts the energy content of CO into a liquid that subsequently can be used to power a fuel cell.
Partial oxidation of hydrocarbons leads primarily to CO and hydrogen. Conventional systems follow with a series of these water-gas shift steps. The discovery by Dumesic has the potential of eliminating those steps. Instead, the CO can be passed through their process at room temperature to selectively remove the CO out of the CO:H2 gas mixture to make energy.
The process is especially promising for producing electrical energy from renewable biomass-derived oxygenated hydrocarbons, such as ethylene glycol derived from corn, because these fuels generate H2 and CO in nearly equal amounts during catalytic decomposition. The hydrogen could be used directly in a proton-exchange-membrane fuel cell operating at 50-percent efficiency and the remaining CO could be converted to electricity via the researchers' new process. The overall efficiency of such a system is equal to 40-percent and does not require water as would be needed in traditional ethylene glycol reforming. The overall efficiency is equivalent to 60 percent of the energy content of octane.
The advance will make possible a new generation of inexpensive fuel cells operating with solutions of reduced POM compounds. While higher current densities can be achieved in fuel cells using electrodes containing precious metals, the researchers found that good current densities can be generated using a simple carbon anode.
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Hydrogen CTS discovery
Text from NSF web-page under construction
One of the hottest topics in a globally warming world is the prospect of a “hydrogen economy” in which the clean-burning gas would save civilization from its century-long addiction to diminishing stores of petroleum.
Hydrogen can be used in internal-combustion engines or in fuel cells that generate electricity from chemical reactions. Either way, there is no less-polluting fuel. When hydrogen burns in air, its only byproduct is water vapor. That makes it enormously attractive as an alternative to fossil fuels. And there’s certainly no shortage of the stuff: Hydrogen is far and away the most common element in the universe.
But here on Earth, hydrogen is hard to find in the pure gaseous state, H2. Its atoms are almost always bound into compounds such as hydrocarbons, carbohydrates or water. And getting it out in quantity can be a very messy business.
Industry produces large quantities of hydrogen chiefly by extracting it from fossil fuels such as natural gas or coal. In particular, about 95% of all H2 produced today comes from the processing of petroleum by a process called stream-reforming. Steam-reforming requires lots of energy and high temperatures, and generates substantial volumes of carbon dioxide (CO2), a notorious greenhouse gas, as well as poisonous carbon monoxide (CO). Furthermore, while this process is well-developed and highly efficient, it is still based on the utilization of non-renewable, petroleum-based resources that are in diminishing supply, especially in the United States, and are the main cause of global warming. Thus, according to Dumesic, the full environmental benefits of using H2 as an energy carrier comes when H2 is produced from renewable resources, such as biomass.
The main alternative to steam-reforming is electrolysis – running a powerful electrical current through water to separate the hydrogen and oxygen. (A fuel cell does the mirror opposite: combining hydrogen and oxygen to produce electricity and water.) But the current has to come from somewhere; and most of America’s electrical power is generated by burning coal, which produces CO2 as well as other polluting gases. These polluting gases produced by coal combustion would largely offset the gain from using hydrogen.
That situation has been a discouraging obstacle for planners who hope to see hydrogen emerge as the environmentally friendly fuel of the future.
But over the past few years, chemical engineers at the University of Wisconsin-Madison have been developing catalytic techniques that may make hydrogen production a much cleaner and greener endeavor.
Their raw materials are carbohydrates, which form the major component of plants or biomass, including ordinary wood bits or the unused portions of agricultural crops instead of nonrenewable fossil fuels, and their methods operate at around 440 degrees Fahrenheit – no hotter than a batch of cookies in a home oven. When hydrogen is produced from carbohydrates the net CO2 production is zero, because any CO2 produced in the hydrogen production step is recycled and consumed in the biomass regrowth step. Obtaining transportation fuels from domestically available biomass resources would also allow the U.S. to become more energy self-sufficient. Currently, the U.S. uses only 80 % of its farmland, and using carbohydrates as a hydrogen source would provide another large market, significantly improving the U.S. economy.
That’s why their novel experimental methods, developed with support from the National Science Foundation’s Division of Chemical & Transport Systems and other NSF programs, have attracted national attention.
Chemical engineer Dumesic and colleagues devised catalysts – substances that prompt chemical reactions without themselves being permanently changed in the process – that make it possible to extract hydrogen directly from biomass.
The raw materials could be anything from corn stalks and leaves to wheat straw or sawdust, mixed with water, placed in a reactor vessel, and heated. Another unexpected source of H2 for their catalytic process is glycerol, which is currently an unwanted by-product in the formation of bio-diesel from animal fats and vegetable oils. As for the catalyst, platinum is famously effective in petroleum processing, automobile catalytic converters and other uses. So for the first stage of tests, the researchers used a specially engineered combination of platinum and aluminum oxide for the catalyst, and a selection of natural compounds found in, or easily derived from, biomass, such as glucose and wood alcohol.
When the sugar or alcohol molecules touch the surface of the catalyst, chemical reactions break and rearrange many of the carbon bonds, causing the atoms to be “reformed” into new configurations and liberating hydrogen in the process. In fact, about half the product is hydrogen gas. The researchers estimate that, if the system is fully developed, it will be able to turn a liter of biomass into about 1,000 watts of power.
The remaining byproducts consist of carbon dioxide and methane (or other readily combustible hydrocarbons). In a commercial application, the hydrocarbons could be separated and burned to provide all the heat needed to run the reactor, making the whole process energy-neutral.
And although the process does generate carbon dioxide, growing the plants to feed the reactor would absorb approximately as much CO2 as the reforming process produces.
That discovery, published in Nature in 2002, was only the beginning. In order to devise a practical, cost-effective system, new catalysts would be needed.
The research team at the University of Wisconsin realized at the outset that platinum is expensive (price ~ $8000/lb) and it is already being used extensively for other catalytic processes, such as for emissions control from automobiles and for production of gasoline from petroleum oil. In addition, platinum is employed at both the anode and the cathode for H2 fuel cells. Therefore, the researchers began a search for less-expensive catalysts that would allow the difficult production of H2 from biomass.
The group eventually tested more than 300 catalysts, and determined that a nickel-aluminum-tin combination actually outperformed the costly platinum version, with hydrogen making up to 70 percent of the reaction products. They reported their results in Science in 2003. Meanwhile, they were already at work on yet another vexing impediment to a “hydrogen economy.”
Carbon monoxide, an unwelcome byproduct of H2 generation, has a deadly effect on the platinum electrodes used in a widely favored type of fuel cell. So it is important to remove as much CO as possible from hydrogen gas before use. The standard process is called “water-gas shift,” in which CO and H2O are combined as liquid water turns to steam. The oxygen atom in each water molecule binds to the CO, producing CO2. The leftover hydrogen atoms are released as molecular hydrogen gas.
Water-gas shift must be carried out at moderate temperatures (for example, 480 degrees Fahrenheit) to achieve low levels of CO, requiring large amounts of catalyst because of low reaction rates. The process also requires large amounts of liquid water, and thus is unsuitable for small-scale or portable uses – for example, in automobiles.
Dumesic and coworkers found a room-temperature substitute for the entire water-gas procedure: letting CO react with a liquid “oxidizing” compound made from tiny clusters of molybdenum oxide, containing 12 molybdenum atoms, 40 oxygen atoms, as well as a central phosphorous atom. When CO mixed with this agent in the presence of nano-gold particles about 1,000 times thinner than a human hair, nearly every carbon monoxide molecule was converted to CO2.
Better yet, the group noted in a 2004 Science paper, the process turns the oxidizing compound into a mix of highly charged molecules, which makes it ideal for powering a fuel cell on its own.
To be sure, the Wisconsin researchers’ work has not removed all the scientific and practical barriers to a hydrogen-fueled future. But it has identified that renewable biomass resources may well play an important role in the hydrogen economy. In fact, these researchers have recently started a new company (Virent Energy Systems) to explore the practical aspects of their work, and helped by funding from the NSF-STTR program as well as other federal agencies, they are beginning to build prototype units for the production of energy by their aqueous-phase reforming process. Dumesic says: “The production of energy and valuable chemicals from renewable biomass resources is a rapidly growing area of research, with the aim of identifying new catalytic processes that may be integrated to form environmentally-benign, cost-effective biorefineries for the future. In addition, an essential role of academic research in this area is to educate future generations of scientist and engineers who will be able to discover new catalysts and invent new processes that will allow society to utilize in the most efficient manner our precious biomass resources.”
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