The Ohio State University
Lean burn engines are
well known for their high power output achieved with higher engine efficiencies
and significantly cleaner engine out emissions.
However failure to meet ever stringent emission regulations for NOx, through conventional aftertreatment
technologies, such as three way catalysts, has limited the use of lean burn
engines. Significant research efforts
are being focused on the development of exhaust gas aftertreatment
which makes use of unburned hydrocarbons in the exhaust stream for the
reduction of NOx species under lean burn
conditions. Methane, in particular, has
attracted much attention for hydrocarbon SCR due to its low cost and
availability. Despite being effective as
a reducing agent, its use is somewhat limited since methane combustion is more
favoured than NOx reduction under lean
We previously developed
an integrated oxidation and selective reduction catalytic system where NO was
first oxidized to NO2, then reduced selectively with hydrocarbons
[1-6]. This method combines a physical mixture of oxidation and reduction
catalyst components, such as a supported cobalt oxide catalyst and a Pd
catalyst supported on sulphated zirconia. The dual catalyst system performs three
distinct catalytic functions, specifically NOx
reduction, CO oxidation and hydrocarbon combustion. The dual-catalyst approach for lean-burn
exhaust after-treatment takes advantage of the stronger oxidizing potential of
NO2 compared to NO, which in turn helps to utilize the reducing
capability of unburned hydrocarbons in the exhaust. Furthermore the cobalt-based oxidation
catalyst can re-oxidize any gaseous NO present in the system, a consequence of
partially reduced NO2, through a competitive reaction during direct
NO2 reduction. NO oxidation to NO2 is an exothermic and
reversible reaction which is thermodynamically limited at high temperatures;
however when this reaction occurs in close proximity to a reduction catalyst,
NO2 is removed from the system by NO2-SCR thus pushing
the equilibrium in the forward direction.
The oxidation catalyst assumes a multi-purpose function in the
dual-catalyst scheme; in addition to oxidizing NO or re-oxidizing partially
reduced NO2 species, the oxidation catalyst also catalyses the
combustion of un-burned hydrocarbons and the oxidation of carbon monoxide,
which have not been consumed during the SCR reaction.
The main objective of this current study is to
investigate the role of the CoOx/CeO2 as an oxidation
catalyst in a dual catalyst bed. Initial studies examined NO to NO2
oxidation over CoOx/CeO2 with different Co metal loading
levels under steady state conditions. This is necessary as NO can form through
the partial reduction of NO2 during SCR, thus it is important for it
to be converted back into NO2. The
kinetics of the oxidation reactions is examined. The role of the oxidation
catalyst on the water tolerance of the catalytic system is also
investigated. The interaction of
hydrocarbons with the surfaces of CoOx/CeO2 and Pd/SZ
catalysts is studied using in-situ DRIFTS technique.
 B. Mirkelamoglu, M. Liu, U.S.
Ozkan, Catalysis Today 151 (2010) 386-394.
Ozkan, Applied Catalysis B: Environmental 96 (2010) 421-433.
E.M. Holmgreen, M.M. Yung, U.S.
Ozkan, Catalysis Letters 111 (2006) 19-26.
 E.M. Holmgreen, M.M. Yung, U.S.
Ozkan, Journal of Molecular Catalysis A: Chemical 270 (2007) 101-111.
 E.M. Holmgreen, M.M. Yung, U.S.
Ozkan, Applied Catalysis B: Environmental 74 (2007) 73–82.
 P. Gawade, A-M.C. Alexander, R. Clark, U.S. Ozkan, Catalysis Today (2012)