EMERGING RESEARCH FRONTS - 2010
February 2010

William S. Epling talks with ScienceWatch.com and answers a few questions about this month's Emerging Research Front Paper in the field of Physics.
	Article: Overview of the fundamental reactions and degradation mechanisms of NOx storage/reduction catalysts Authors: Epling, WS;Campbell, LE;Yezerets, A;Currier, NW;Parks, JE Journal: CATAL REV-SCI ENG, 46 (2): 163-245 SEP 2004 Addresses: EmeraChem, 2375 Cherahala Blvd, Knoxville, TN 37932 USA. EmeraChem, Maryville, TN USA. Adv Catalyst Syst, Maryville, TN USA. Cummins Inc, Columbus, IN USA. Oak Ridge Natl Lab, Natl Transportat Res Ctr, Knoxville, TN USA.

  Why do you think this paper is so highly cited?

This paper was the first comprehensive review of the science behind and application of, NO_x storage and reduction catalysis. At the time, NO_x storage and reduction catalysis was being studied to address the challenge of NO_x to N₂ reduction in an oxidizing environment, such as that found in diesel engine exhaust.

Previous experience with this system for stationary power exhaust suggested that it was indeed well-suited for diesel engine application. We correctly anticipated that it would be a practical lean-NO_x technology solution for companies looking to be at the forefront of emissions reduction, especially before other technologies, such as urea selective catalyst reduction (urea-SCR), became available.

For over a decade prior to this paper's release, there were published studies on this catalyst topic, by engine companies and catalyst manufacturers primarily, and also a few by academics, mostly out of Europe. The success of the paper stems from the analysis of previous work by those authors as well as work performed by us to understand not only the catalyst chemistry but also the numerous discrepancies apparent in the literature.

"...huge fuel economy gains could be realized, decreasing the standard emissions as well as CO2, plus decreasing our use of, and therefore dependence on, petroleum."

NO_x storage and reduction catalysis is arguably the most complex of the commercial emission control catalysts. However, answers became clear, through the review and subsequent experimentation, that this catalyst system has a strong "memory" which partially led to the different findings being reported.

Furthermore, due to the integral nature of the catalyst and the completely transient nature of the system, there were not just the challenges in understanding the chemistry, but also the aspects of reaction engineering, which also led to the need for new tools to evaluate the catalyst.

Based on our analysis, we were able to draw a simple picture of the steps involved in the process. Although each individual step is admittedly complex, by establishing that each step affects the others, this clarified that sample history was of critical importance in evaluating the relevant chemistry and understanding what was being observed in actual practice.

  Does it describe a new discovery, methodology, or synthesis of knowledge?

It is a synthesis of knowledge, while at the same time highlighting methodology's importance in experiments directed at understanding these catalysts. By providing a comprehensive summary of the available knowledge, it reconciled a number of apparent mechanistic contradictions discussed in the literature. Furthermore, it clarified glaring omissions in our current understanding of how the catalyst worked, and therefore provided direction for future work in the area.

  Would you summarize the significance of your paper in layman's terms?

The paper describes/summarizes the existing knowledge, to that time, regarding NO_x storage and reduction catalysis for NO_x emissions control in diesel engine exhaust. The paper provided a background of the existing knowledge pertaining to the chemistry of the multiple steps involved in the reduction process, demonstrated why discrepancies existed in the literature, contained results describing the fundamental chemistry involved, and provided a set of questions concerning the chemistry of the catalysis that were then still unanswered.

  How did you become involved in this research and were any particular problems encountered along the way?

My research history has focused on environmental catalysis and on discovering the mechanisms of catalytic reactions. This specific topic was the focus of my first industrial job at GoalLine Environmental Technologies, a company which had developed NO_x storage and reduction catalysts for natural gas turbine NO_x control.

Under the guidance of Dr. Larry Campbell, we were exploring "outside the box" ideas in further developing NO_x storage and reduction technology, specifically to expand the technology into the diesel engine exhaust market. Through this work, I was introduced to Cummins Inc., and, while working there, learned from my colleagues, Dr. Alex Yezerets and Neal Currier, the importance of methodology in performing experiments and, therefore, logical experiment design.

Also, I was exposed to the seemingly incongruous literature evidence that led me to tying the existing literature knowledge together and designing experiments to answer questions regarding the apparent discrepancies that existed.

  Where do you see your research leading in the future?

There are two areas where we are focusing our efforts. First, although application of NO_x storage and reduction catalysts has been successful for diesel engine applications, there are ever increasingly stringent emissions regulations, which will soon include those for off-road vehicles. This is a significant challenge due to the even more transient operating conditions—full load and idle are dominant. Keeping the catalyst active at the two extremes is the challenge.

Furthermore, there are the new passenger vehicle technologies being implemented, including lean-burn gasoline, hybrid and mixed-mode diesel combustion engines. Low-temperature exhaust gas is associated with the latter two technologies, making catalytic activity a challenge, whereas the lean-burn gasoline engine exhaust is still at a relatively high temperature, leading to catalyst deactivation concerns, with the same challenge of reducing NO_x in an oxidizing environment. We are currently working on these topics.

Secondly, one of the interesting results in studying the NO_x storage and reduction catalysts was the realization of how important understanding the integral nature of the catalyst was. Although more fundamental studies provided very relevant information regarding the catalytic chemistry, measurements within the reactor/catalyst itself provided mechanistic insight that was critical for understanding the phenomena observed.

I am therefore focusing on developing (and using existing) methodology to study reactions where there are evolving gradients within the reactor, along the catalyst bed. Such results have been essential in understanding emissions, reforming, and partial oxidation catalysis, but will also provide new and exciting results in other systems where temperature, surface, and gas species concentrations exist.

  Do you foresee any social or political implications for your research?

There are definitely social implications derived from this research. Diesel engine exhaust is now significantly cleaner than just a few years ago. And, since they are inherently more fuel efficient, thereby resulting in a coincident decrease in CO₂ emissions relative to today’s gasoline vehicles, we could see a shift in consumer interest back to diesel vehicles. This of course is predicated on society's continued interest in decreasing our impact on the environment, as well as saving money on fuel costs.

Also, imagine a catalyst that can reduce these emissions to zero, thereby freeing the engineers working on the engine itself to focus solely on fuel economy. Right now, there are limitations on what they can do, as they need to focus on both emissions and fuel economy simultaneously. If such a catalyst is developed, huge fuel economy gains could be realized, decreasing the standard emissions as well as CO₂, plus decreasing our use of, and therefore dependence on, petroleum.

Bill Epling, Ph.D.
Assistant Professor of Chemical Engineering
University of Waterloo
Waterloo, Ontario, Canada
Web

KEYWORDS: EMISSIONS; NOX STORAGE/REDUCTION CATALYSTS; NO OXIDATION; NOX RELEASE; NOX STORAGE; CATALYST; LEAN-BURN CONDITIONS; STORAGE-REDUCTION CATALYSTS; GAS-SHIFT REACTION; EMISSION CONTROL CATALYSIS; SILICA-SUPPORTED PLATINUM; GROUP METAL-CATALYSTS; NITROGEN-OXIDES; SELECTIVE REDUCTION; NO(X) STORAGE; EXCESS OXYGEN.