2011 Chinese Journal of Catalysis Vol. 32 No. 5 Article ID: 0253-9837(2011)05-0736-10 DOI: 10.1016/S1872-2067(10)60230-6 Article: 736–745 NO x Reduction on Fully Formulated Lean NO x Trap Catalysts Subjected to Simulated Road Aging: Insights from Steady-State Experiments Jin WANG, Yaying JI, Uschi GRAHAM, Caio CESAR SPINDOLA DE OLIVEIRA, Mark CROCKER * Center for Applied Energy Research, University of Kentucky, Lexington, KY 40511, USA Abstract: Fully formulated lean NO x trap (LNT) catalysts of the type Pt/Rh/BaO/Al 2 O 3 were prepared with and without incorporation of CeO 2 -ZrO 2 in the washcoat, and their NO x reduction behavior was evaluated in steady-state, continuous flow experiments. In the fresh state, the CeO 2 -ZrO 2 addition was found to exert little effect on NO x reduction activity using H 2 , CO, and NH 3 as the reductants. However, after simulated road aging, NO x reduction activity was significantly impaired for the CeO 2 -ZrO 2 -free catalyst, whereas the performance of the CeO 2 -ZrO 2 -containing analog was affected to only a minor degree. These differences are explained on the basis of high-resolution transmis- sion electron microscopy measurements showing that Pt supported on CeO 2 -ZrO 2 remained highly dispersed after aging, whereas Pt sup- ported on BaO/Al 2 O 3 underwent significant sintering. In addition, the Pt/CeO 2 -ZrO 2 component did not accumulate sulfur during aging, unlike Pt/BaO/Al 2 O 3 for which significant sulfation of the Ba phase occurred. For both catalysts, selectivity to NH 3 in NO and NO 2 reduction by H 2 increased after the catalyst aging, indicative of a change in the relative surface coverages of N and H ad-atoms on the precious metal sites. Key words: lean NO x trap; NO x storage-reduction catalyst; steady state; NO x reduction; ceria-zirconia; catalyst aging CLC number: O643 Document code: A Received 28 February 2011. Accepted 8 April 2011. *Corresponding author. Tel: +1-859-257-0295; Fax: +1-859-257-0302. E-mail: crocker@caer.uky.edu This work was supported by the U.S. Department of Energy (DOE) under award No. DE-EE0000205. English edition available online at Elsevier ScienceDirect (http://www.sciencedirect.com/science/journal/18722067). Disclaimer: This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. References herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Lean-burn engines provide more efficient fuel combus- tion and lower CO 2 emissions compared with traditional stoichiometric engines [1]. However, the effective removal of NO x from lean exhaust represents a challenge to the automotive industry. Conventional three-way catalysts are highly effective for control of nitrogen oxides, CO, and hydrocarbons from stoichiometric engine exhaust but are not effective for NO x mitigation under lean conditions. Of the various approaches proposed for lean NO x removal, lean NO x trap (LNT) catalysts represent a promising technology, particularly for lean-burn gasoline and light-duty diesel applications [1,2]. LNT catalysts are typically composed of at least one precious-metal component and one alkali or alkaline-earth component which are supported on a high surface area refractory oxide. These catalysts operate in a cyclic manner, whereby the catalyst stores or "traps" NO x as nitrate species during lean period of operation. Periodically a short rich pulse is introduced so that the trapped NO x is released and reduced to N 2 , thereby regenerating the trap- ping capacity of the catalyst [3]. In previous work [4,5] we observed that the addition of a Ce-Zr mixed oxide to a fully formulated Pt/Rh/BaO/Al 2 O 3 LNT catalyst significantly improved NO x storage–reduction performance under lean–rich cycling conditions. This was particularly evident after catalyst aging under simulated road conditions [5]. Several other recent studies have like- wise shown that the addition of ceria-containing mixed ox- ides can impart beneficial properties to model LNT cata- lysts. For example, ceria possesses significant NO x storage capacity [6–10], particularly at low to moderate tempera- tures (< 400 °C), which may help to supplement the NO x storage capacity of the main alkaline earth or alkali metal