New Catalyst More Efficiently Removes CO from Hydrogen; Benefit for Fuel Cells
20 March 2008
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| The new catalyst is a core of ruthenium surrounded by one to two layers of platinum atoms. Click to enlarge. |
Researchers at the University of Wisconsin-Madison and University of Maryland (UM) have designed from first principles a new type of chemical catalyst that efficiently oxidizes carbon monoxide (CO). CO is a contaminant in hydrogen produced via the reformation of hydrocarbons that poison the expensive platinum catalyst that runs the fuel cell reaction, thereby reducing the efficiency of fuel cells.
Writing in this week’s Advance Online Publication of Nature Materials, UW-Madison chemical and biological engineering Professor Manos Mavrikakis and UM chemistry and biochemistry Professor Bryan Eichhorn describe a new type of catalyst created by surrounding a nanoparticle of ruthenium (Ru) with one to two layers of platinum (Pt) atoms. The result is a robust room-temperature catalyst that improves the preferential oxidation (PROX) of CO in the presence of hydrogen.
A conventionally constructed catalyst combining ruthenium and platinum must be heated to 70° C (158° F) to drive the PROX reaction, but the same elements combined as core-shell nanoparticles operate at room temperature. The lower the temperature at which catalyst activates the reactants and makes the products, the more energy is saved.
The distinct catalytic properties of these well-characterized core–shell nanoparticles were demonstrated for preferential CO oxidation in hydrogen feeds and subsequent hydrogen light-off. For H2 streams containing 1,000 p.p.m. CO, H2 light-off is complete by 30° C, which is significantly better than for traditional PtRu nano-alloys (85° C), monometallic mixtures of nanoparticles (93° C) and pure Pt particles (170° C).
The new core-shell catalyst works so well for two primary reasons, according to Mavrikakis. First is the core-cell nanostructure and composition, which can sustain significantly less CO on its surface than pure Pt would. Because the binding is weaker, Mavrikakis says fewer sites on the core-cell nanostructure are available to bind with CO than would occur with Pt alone. That leaves empty sites for oxygen to come in and react.
The second reason is that there is a completely new reaction mechanism that makes this work so well. We call it hydrogen-assisted CO oxidation. It uses atomic hydrogen to attack molecular oxygen and make a hydroperoxy intermediate, which in turn, easily produces atomic oxygen. Then, atomic oxygen selectively attacks CO to produce CO2, leaving much more molecular hydrogen free to be fed to the fuel cell than pure Pt does.
—Manos Mavrikakis
While the breakthrough is important to the development of fuel-cell technology, the researchers say it’s even more significant to catalysis in general because of the combination of precise nanoscale fabrication (as opposed to combination in bulk) and the use of design from theory.
For the field of catalysis, the pairing of these approaches could bridge the gap between surface science and catalysis, opening new paths to novel and more energy-efficient materials discovery for a variety of industrially important chemical processes.
Resources
Selim Alayoglu, Anand U. Nilekar, Manos Mavrikakis & Bryan Eichhorn, Ru–Pt core–shell nanoparticles for preferential oxidation of carbon monoxide in hydrogen, Nature Materials 7, 333 - 338 (2008) Published online: 16 March 2008 | doi:10.1038/nmat2156
CO-related performance degradation is especially problematic for direct fuel cells that consume methanol or even ethanol rather than hydrogen, which is much harder to produce, distribute and store on board. If this new catalyst can be produced at reasonable cost and proves effective in direct fuel cells, it could represent a big advance.
However, even more useful would be raising the operating temperature of mobile fuel cells. This would speed up reaction kinetics and hence, specific power. It would also eliminate the CO problem thermally, without any need for special nanocatalysts. Finally, it would reduce the cross-section of the radiator required to shed the waste heat. The main problem appears to be that the most efficient separator proton exchange membrane material is a polymer that becomes unstable at elevated temperatures, destroying the stack. Also, platinum is more easily oxidized at higher temperatures in an acid medium, which is what you need for H+ (proton) exchange.
Last year, the University of Offenburg (Germany) demonstrated an alkaline direct ethanol fuel cell that uses an anionic membrane (permeable by OH- ions) and doesn't need any noble metals. Unfortunately, it's still very far from production-ready:
http://www.acta-nanotech.com/index.php?option=com_content&task=view&id=130&Itemid=66
Posted by: Rafael Seidl | 20 March 2008 at 10:10 AM
This might be a convenient and lower cost way of reforming SNG to H2 for PEMs. If this can make a reformer cheaper, more efficient with less CO poisoning, then it would be useful, but then you could just put the CO into an SOFC and use it as a fuel while converting it to CO2.
Posted by: sjc | 20 March 2008 at 08:01 PM