Schematic of a typical diesel engine aftertreatment system. Credit: ACT.

Schematic of a typical, bulky and costly three-part diesel engine aftertreatment system. Credit: Pyzik, Ziebarth, Han and Yang; ACT.

Mostly through sheer coincidence, I have been running across several articles (see below) regarding approaches to NOx emissions control. The latest comes from a new paper in the International Journal of Applied Ceramic Technology (DOI: 10.1111/j.1744-7402.2010.02539.x) from a group of Dow Chemical researchers who are demonstrating how an experimental form ultra-high porosity acicular mullite can be used for combined NOx reduction, soot filtration and control of other emissions from diesel engines. They say their strong, porous ceramic scaffold, while serving as a carrier several functional materials that perform while on creating a back-pressure comparable to existing exhaust filtration systems.

Aleksander Pyzik, Rob Ziebarth, Chan Han, Kwanho Yang write in ACT that much of the work on this topic is being driven by environmental regulations in different parts of the world that are mandating reductions in NOx, CO2, SOx, HC and particulate matter (PM). Currently, the most sophisticated approaches have been to use a three-part system that is comprised of a diesel oxidation catalyst, a ceramic diesel particulate filter and a separate NOx catalyst on a ceramic substrate. They note that while this three-part system works, it is “complex, bulky and expensive.”

Although technically feasible, they note that while just dumping different catalytic washes on a typical ceramic substrate (such as silicon carbide, cordierite, and aluminum titanate)  might work, the back pressure on the engine (a measure of how much power must be wasted pushing the exhaust through the filter/catalyst) would be substantial and, practically speaking, make this approach unworkable. Further, any attempts to increase porosity in the typical substrate materials would be offset by a reduction in strength.

So, the researchers looks around for another candidate for a new substrate and found one to be acicular mullite, an advanced ceramic material. Dow has a proprietary method for making the acicular mullite, and this group experimented with extruded catalyzed acicular mullite honeycombs created with a variety of porosities (between 64% and 80%) showed excellent NOx reduction and pressure drop that is comparable with state-of-art commercial filters without any catalyst coating.

They found that the 64% porosity sample given a catalytic wash still cause a back pressure problem, but when porosity was raised to 75% and 80%, back pressure declined significantly. They report that the 75% porosity sample was “robust, easy to process and easy to handle,” while the 80% sample was more fragile, but still could be handled without damage. More importantly, when the 75% and 80% samples were given catalyst washcoats of Pt and Ba compounds dispersed in Al2O3, they achieved NOx reductions of 93% and 98%, respectively. Even heavier catalyst loading caused only modest increases in back pressure.

Not surprisingly, their conclusion is that the acicular mullite isn’t just a good candidate for a combined NOx control and soot filtration system, but is is also a prime candidate for a multifunctional system that simultaneously and successfully tackles NOx, soot, hydrocarbons, CO and CO2.

For other recent stories on NOx-reduction materials, see:

Titanium dioxide-coated anti-NOx roof tiles now being marketed in U.S.

Titania–concrete combination in roadways reduces NOx levels 25-45%

Strontium-doped perovskite-based catalytic converters offer cheaper alternative to platinum for diesel, gas exhaust

Brick, stained glass that cleans the air