William S. Epling talks with
ScienceWatch.com and answers a few questions about
this month's Emerging Research Front Paper in the field of
Article: Overview of the fundamental reactions and
degradation mechanisms of NOx storage/reduction
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
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
Why do you think this paper is so highly
This paper was the first comprehensive review of the science behind and
application of, NOx storage and reduction catalysis. At the
time, NOx storage and reduction catalysis was being studied to
address the challenge of NOx 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-NOx 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."
NOx 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 NOx storage and reduction catalysis for NOx
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 NOx storage and
reduction catalysts for natural gas turbine NOx control.
Under the guidance of Dr. Larry Campbell, we were exploring "outside the
box" ideas in further developing NOx 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
Where do you see your research leading in the
There are two areas where we are focusing our efforts. First, although
application of NOx 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 NOx in an oxidizing environment. We are
currently working on these topics.
Secondly, one of the interesting results in studying the NOx
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
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
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