282676 Optimization of Co-Based Oxidation Catalyst in a Dual-Catalyst Bed for Lean-Burn Engine After-Treatment

Monday, October 29, 2012: 4:55 PM
320 (Convention Center )
Anne-Marie C. Alexander, Chemical and Biomolecular Engineering, Ohio State University, Columbus, OH, Preshit Gawade, Chemical and Biomolecular Engineering, The Ohio State University, Columbus, OH and Umit S. Ozkan, Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, OH

Optimization of Co-based oxidation catalyst in a dual-catalyst bed for lean-burn engine after-treatment

Anne-Marie C. Alexander, Preshit Gawade and Umit S. Ozkan

Department of Chemical and Biomolecular Engineering,

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 conditions.

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.

[1] B. Mirkelamoglu, M. Liu, U.S. Ozkan, Catalysis Today 151 (2010) 386-394.

[2] B. Mirkelamoglu, U.S. Ozkan, Applied Catalysis B: Environmental 96 (2010) 421-433.

[3] E.M. Holmgreen, M.M. Yung, U.S. Ozkan, Catalysis Letters 111 (2006) 19-26.

[4] E.M. Holmgreen, M.M. Yung, U.S. Ozkan, Journal of Molecular Catalysis A: Chemical 270 (2007) 101-111.

[5] E.M. Holmgreen, M.M. Yung, U.S. Ozkan, Applied Catalysis B: Environmental 74 (2007) 7382.

[6] P. Gawade, A-M.C. Alexander, R. Clark, U.S. Ozkan, Catalysis Today (2012) In Review.

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