Journal of Catalysis 183, 196–209 (1999) Article ID jcat.1999.2415, available online at http://www.idealibrary.com on NO x Storage in Barium-Containing Catalysts Erik Fridell, ∗,1 Magnus Skoglundh, ∗ Bj¨ orn Westerberg, ∗, † Stefan Johansson, ∗, ‡ and Gudmund Smedler ∗ ∗ Competence Centre for Catalysis, Chalmers University of Technology, SE 412 96 G ¨ oteborg, Sweden; †Department of Chemical Reaction Engineering, Chalmers University of Technology, SE 412 96 G ¨ oteborg, Sweden;and ‡Department of A pplied Physics, Chalmers University of Technology, and G ¨ oteborg University, SE 412 96 G ¨ oteborg, Sweden Received October 14, 1998; revised January 4, 1999; accepted January 5, 1999 The effect of key parameters on the characteristics of barium oxide-based NO x storage catalysts was systematically investiga- ted. Model Pt/BaO/Al 2 O 3 , BaO/Al 2 O 3 , Pt–Rh/Al 2 O 3 , and Pt–Rh/ BaO/Al 2 O 3 catalysts were prepared and evaluated with respect to NO x storage capacity using transient flow reactor studies, temperature-programmed desorption studies (TPD), and in situ Fouriertransform infrared (FTIR) absorption spectroscopy. The in- fluence of temperature, storage and regeneration times, NO x source (NO or NO 2 ), oxygen concentration, reducing agent (C 3 H 6 ,C 3 H 8 , CO, or H 2 ), and carbon dioxide concentration on NO x storage capac- ity was studied. Significant amounts of NO x were found to be stored in the catalysts containing both barium oxide and noble metals. For these catalysts the following observations were made:(1) maximum NO x storage was observed at about 380 ◦ C;(2) around this tempera- ture no significant differences between NO and NO 2 on NO x storage capacity could be observed; (3) a slow increase in stored NO x could be observed with increasing oxygen concentration during the lean phase; (4) significant NO x desorption peaks, mainly of NO, were observed immediately after the switch from lean to rich conditions; and (5) at about 380 ◦ C the in situ FTIR spectra show characteristic nitrate peaks in the region 1300–1400 cm −1 when NO x was stored under lean conditions and isocyanate peaks around 2230 cm −1 when the catalysts were regenerated under rich conditions in the presence of hydrocarbons. The step leading to stored NO x is believed to involve NO 2 and the presence of atomic oxygen. During the rich period, the noble metal surfaces are probably reduced, leading to breakthrough peaks when NOdesorbs. c  1999 Academic Press 1. INTRODUCTION Increasing awareness of the need to reduce emissions of carbon dioxide into the atmosphere hasled to great pres- sure on automobile manufacturers to reduce the fuel con- sumption of their products. One way to improve the fuel economy of gasoline-fueled cars is to use engines that op- erate at lean conditions, rather than at the normal stoichio- metric air/fuel ratios. Depending on driving conditions a lean-burn engine can decrease fuel consumption by up to 30% compared with a stoichiometric engine (1). 1 To whom correspondence should be addressed. Fax: +46 31 7722967. E-mail: fridell@fy.chalmers.se. When the exhaust contains a mixture of oxygen and the pollutants hydrocarbons, carbon monoxide, and nitrogen oxides, close to stoichiometric conditions, as is the case for most gasoline-fueled cars today, the pollutants can be al- most completelytransformed to carbon dioxide,water,and dinitrogen in a three-way catalyst. However, the exhaust from a lean-burn engine contains a large surplus of oxygen which prevents the reduction of nitrogen oxides. This re- quires the development of new catalytic techniques for re- duction of lean-burn engine NO x emissions. Different cata- lyticsystemshave been developed for continuousreduction of NO x under lean conditions using specific combinations of reducing agents and catalyst compositions (2–4). A dif- ferent concept to solve this problem is the NO x storage catalyst (5–7). This catalyst is used in an engine that oper- atesalternativelyunder lean or rich conditions.Duringlean operation, the nitrogen oxides in the exhaust are stored in the catalyst. As the NO x storage capacity of the catalyst becomes saturated it is necessary to regenerate the catalyst by turning the engine to rich conditions for a short period whereupon the stored NO x is released and subsequently reduced over noble metal sites. NO x reduction for mixed lean conditions was reported byTakahashi et al. (6) using a NO x storage catalyst with no- ble metals (mainly Pt), alkaline earth metals (mainly Ba), and alumina. They found that the amount of NO x stored increased with increasing oxygen content in the gas. They also reported evidence of stored NO x in the form of ni- trates from IR studies. It was also concluded that the re- duction of NO x to N 2 during rich conditions takes place on noble metal sites. B¨ ogner et al. (7) investigated the NO x conversion properties of a NO x storage catalyst using both syntheticgasmixturesand engine exhaust.Theyconcluded that their catalyst stores NO x as a surface metal nitrate and that the catalyst becomes fully regenerated under rich con- ditions. A model NO x storage catalyst washcoat comprises three essential parts: (i) a high-surface-area substrate material, (ii) noble metals that catalyze oxidation and reduction reactions, and (iii) a NO x storage component. The sub- strate isnormally γ -alumina with a surface area of typically 0021-9517/99 $30.00 Copyright c  1999 by Academic Press All rights of reproduction in any form reserved. 196