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Researchers Discover How Certain NOx Reduction Catalysts Form; Potential Boost for Wider Use of Lean-Burn Engines
22 February 2008
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| Researchers found that, in the presence of water, aluminum ions (gray) on the surface of alumina bond to six oxygen ions (red). Heating removes the water and leaves some aluminum ions with only five oxygen ions. This creates a bonding site for the NOx-removing catalyst barium oxide. Click to enlarge. |
Researchers at the Department of Energy’s Pacific Northwest National Laboratory have recorded the first observations of how certain catalyst materials used in emission control devices are constructed. The discovery could help remove a barrier to widespread use of diesel and other fuel-efficient lean burn vehicle engines.
The PNNL team observed how barium oxide attaches itself to the surface of gamma-alumina. Barium oxide is a compound that absorbs NOx from tail-pipe emissions. Gamma alumina is a form of aluminum oxide that is used as a support for catalyst materials, such as barium oxide, that are the active ingredients in exhaust systems.
The manner in which barium oxide anchors onto alumina suggests the exact site where catalytic materials begin to form—and where they can be available to absorb NOx emissions.
Lean burn engines deliver up to 35% better fuel economy because they mix more air with gasoline than standard internal combustion engines. But the more efficient engines have difficulty in meeting stricter emissions standards because current aftertreatment devices don’t effectively reduce NOx emissions. New catalysts are essential before the economic and environmental benefits of lean burn engines can be realized.
Alumina is a common and relatively inexpensive catalyst support material. Its surface structure, formation and thermal stability have been the subjects of much research, but the alumina particles are too small and poorly crystalline for traditional surface analysis. Researchers used the world’s first 900-MHz nuclear magnetic resonance spectrometer to reveal the anchoring behavior. The instrument is located at the William R. Wiley Environmental Molecular Sciences Laboratory, a DOE national scientific user facility at PNNL.
Scientists know that the aluminum ions in alumina coordinate, or bond, to either four or six oxygen ions. When water is present, 10 to 15% of the aluminum ions on the surface bond to six oxygen ions: one underneath to the bulk of the alumina, four in a square on the surface and one on top to an oxygen ion in the water molecule.
Removing the water by heating leaves the aluminum ion with only five oxygen bonds. In this penta-coordinated state, the aluminum is open for bonding to the barium oxide. Results from the NMR spectrometer showed that the catalyst filled every available penta-coordinated site, atom-for-atom.
The team is now examining the interaction of gamma-alumina with other metal and metal oxide particles to determine if penta-coordinated aluminum ions are suitable bonding locations for other catalytic materials.
DOE’s Office of Basic Energy Sciences, Division of Chemical Sciences funded the research, which was facilitated by the laboratory’s Institute for Interfacial Catalysis.
Resources
Ja Hun Kwak, Jain Zhi Hu, Do Heui Kim, Janos Szanyi and Charles Peden. Penta-coordinated Al3+ Ions as Preferential Nucleation Sites for BaO on γ-Al2O3. Journal of Catalysis 251(1):189-194. July 2007
February 22, 2008 in Catalysts, Engines | Permalink | Comments (4) | TrackBack (0)
Comments
Posted by: sjc | February 22, 2008 at 08:22 AM
There are four reasons for the great expense of lean NOx traps:
1) fuel savings only occur if the engine actually spends most of its time in lean burn mode. During purge events, the engine is operated in slightly rich mode so all of the accumulated NOx can be desorbed and neutralized using CO and HC. Keeping these fuel-guzzling events brief requires a large surface area and large amounts of precious metals (platinum, palladium, rhodium).
2) only atoms exposed to the gas flow can be catalytically active. Presently, washcoat manufacturing techniques still lead to extremely small grains of catalyst material rather than a coating with individual atoms. The grains are also partially or even completely buried in the matrix. Worse, exposure to heat makes the grains mobile - over the lifetime of the device, they tend to agglomerate, which reduces the catalytically active surface area. To compensate, an excess of precious metals must be used.
3) the NOx adsorption sites are subject to poisoning by sulfur compounds. Purging the sulfur requires exposing the catalyst to temperatures above 550 degC for several minutes, which guzzles gas and causes some permanent degradation in conversion efficiency. Meeting the 150,000 mile life expectancy target therefore means even greater amounts of precious metals must be applied.
By 2010, the US and EU will have completed the transition to ultra low sulfur diesel (15 and 10 ppm limit, resp.) This eases the problem sufficiently to make LNTs viable at all, but only naturally sulfur-free fuels such a biodiesel and xTL eliminate it altogether.
4) the engine control unit must support transparent mode shifts for all points of the engine map. The application effort in development is therefore very high.
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IFF the discovery described in the article also applies to the precious metals, new manufacturing techniques based on it could reduce precious metal loading by addressing points 1 and 2. Even so, device cost will remain high because of rapid growth in the demand for the raw materials. Recycling alone will not be nearly enough to meet it. Diesel engines are expected to win double-digit market share in the US by 2015. Tightening emissions regs in Europe will make NOx aftertreatment unavoidable by 2014. China, India and Russia all more or less track European standards with several years' delay.
Of course, SCR systems represent an alternative to LNTs that requires far lower amounts of precious metals. However, their complexity makes them very expensive as well.
Posted by: Rafael Seidl | February 22, 2008 at 09:17 AM
Hi five for the the IFF Rafael! Ya have the best comments meng. :)
Posted by: yesplease | February 24, 2008 at 01:31 AM
Rafael-
I was surprised by your statement "China, India and Russia all more or less track European standards with several years delay."
Do you mean with respect to fuel quality or emissions? I was under the impression that those places were somewhat of a free-for-all. I could look it up myself, but like yesplease said....
JRod.
Posted by: JRod | February 25, 2008 at 07:57 PM
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Now this seems like something to go after. 35% is a significant gain and the material is low cost. Maybe with all these techniques converging we can end up with a workable solution.