TransForum Vol. 4, No. 1

NEW BIFUNCTIONAL CATALYSTS PROMISE DRAMATIC NOx REDUCTIONS FOR HEAVY-DUTY DIESEL VEHICLES

Truck manufacturers will need new technologies to help them meet EPA regulations that require a 95% cut in allowed NOx emissions by 2007.

The U.S. Environmental Protection Agency (EPA) typically aims the start date of a new standard toward a point in the future when it projects that the technologies designed to meet the standard will become commercialized. So it was in December 2000, when the agency ruled that a 95% cut in allowed nitrogen oxide (NOx) emissions would take effect in 2007. The exhaust aftertreatment technology to meet that standard does not currently exist. But there is a growing consensus that a technology called "selective catalytic NOx reduction" will make an ideal aftertreatment approach if it can be developed and implemented in time.

A number of catalysts have shown promise over the years in systems used to treat diesel exhaust, including the metal-exchanged zeolite Cu-ZSM-5. But the catalysts often lose activity when exposed to the amounts of water vapor typically present in diesel exhaust. They also tend to require temperatures that are too high and produce relatively large amounts of undesired side products, including nitrous oxide (N2O) and carbon monoxide (CO).

Argonne researchers recently developed a series of bifunctional catalysts that overcome all of the show-stopping drawbacks of other catalysts. The bifunctional catalysts can operate at diesel exhaust temperatures and are more effective at NOx removal when water vapor is present. In addition, NOx reduction selectivities under lean-burn conditions range from 95% to nearly 100% and are accompanied by few or no NOx side products and virtually no hydrocarbon slippage. Carbon monoxide levels remain higher than the researchers would like, but they have been lowered to about 25% of those obtained using Cu-ZSM-5 alone, and work continues to reduce the amount even more. The researchers' goal is to develop an efficient NOx reduction system that operates passively, using waste heat and a minimal slipstream of unburned hydrocarbons.

The new catalysts are bimodal, meaning that they consist of two different components: a base metal-exchanged zeolite, such as Cu-ZSM-5, and a special oxide additive. The term "bifunctional" indicates that different types of reactions occur at two different sites. The metal zeolite phase provides the catalytic sites that are responsible for NOx reduction, while the additive phase improves oxidation performance and possibly contributes to oxidizing NO to form N2O, which is more easily reduced. As a bonus, the additive improves the water stability of the catalyst.

Researchers are using x-ray absorption spectroscopy at Argonne's Advanced Photon Source to trace the state of copper on Cu-ZSM-5 during H2 reduction as copper moves from Cu2+ to a metallic-like state that can be undesirable for NOx aftertreatment. Tools such as these are bringing new understanding of the catalyst's structure and interactions.

Argonne conducts basic research on the catalysts by using a variety of in situanalytical techniques at the Advanced Photon Source, including infrared spectroscopy, extended x-ray absorption fine structure, and x-ray absorption near-edge spectroscopy.

The researchers plan to test the long-term stability of the materials under reaction conditions and evaluate their resistance to the presence of sulfur oxides, which interact with various metals and, together with water vapor, yield sulfuric acid. Argonne's bifunctional catalysts have attracted the interest of major diesel engine manufacturers and have led to a Cooperative Research and Development Agreement with BP.