Journal of Catalysis 214 (2003) 308–316 www.elsevier.com/locate/jcat Elementary steps of NO x adsorption and surface reaction on a commercial storage–reduction catalyst Ch. Sedlmair, a,b K. Seshan, b A. Jentys, a and J.A. Lercher a,∗ a Technische Universität München, Institut für Technische Chemie II, Lichtenbergstr. 4, D-85747 Garching, Germany b University of Twente, Faculty of Chemical Technology, PO Box 217, 7500 AE Enschede, The Netherlands Received 16 August 2002; revised 15 September 2002; accepted 28 September 2002 Abstract The surface species formed during adsorption of NO x on a commercial NSR catalyst (containing barium oxide, Pt, and alumina as the main components) were investigated by in situ IR spectroscopy. During adsorption of NO, mainly linear and bridged bonded nitrites of Ba–O–N– O–Ba type were formed on Al and Ba oxide components. Nitrites were detected during the initial phase of the NO/O 2 and NO 2 adsorption, whereas with further exposure nitrates were the dominant surface species. Using the different surface species and reaction intermediates identified by IR spectroscopy a series of sequential reaction steps during the sorption of NO x on a NSR catalyst was derived. Initially, NO is stored in the form of nitrites on the storage component (Ba oxide). NO 2 formed by oxidation on the noble metal component (Pt) sorbs either molecularly by forming nitrate species or dissociatively by forming nitrites. After a certain concentration of NO x is adsorbed, the transformation and further oxidation of the surface nitrites into surface nitrates by NO 2 occur. The stability of the NO x surface species was found to increase in the order Al nitrites < Ba nitrites < Al nitrates < Ba nitrates.  2003 Elsevier Science (USA). All rights reserved. Keywords: Nitrates; Nitrites; In situ FTIR spectroscopy; NO x storage; Adsorption; Barium-oxide; Mechanism 1. Introduction Conventional Pt–Rh-based three-way catalysts (TWCs) are very efficient in reducing nitrogen oxide (NO x ), CO, and unburned hydrocarbon (HC) emissions from gasoline engines. However, the general demand for lower CO 2 emis- sions and, thus, the requirement of more fuel-efficient gaso- line engines led to the development of lean-burn engines operating at significantly higher air-to-fuel ratios then used by traditional engines [1]. Under these oxygen-rich exhaust- gas conditions three-way catalysts cannot efficiently reduce NO x . The most promising approach to the reduction of NO x under lean-burn conditions is based on the concept of NO x storage-reduction (NSR), where the engine is operated in a mixed lean/rich operation mode [2,3]. NSR catalysts con- tain storage components, typically alkali or alkaline earth metals, such as barium, and noble metal components pro- viding oxidation/reduction functionality. Under lean opera- tion conditions (i.e., oxidizing atmosphere), NO is oxidized * Corresponding author. E-mail address: johannes.lercher@ch.tum.de (J.A. Lercher). to NO 2 over the noble metal component and stored on the storage component. By periodic changes to short cycles of rich operating conditions (i.e., reducing atmosphere), NO 2 is released from the storage component and converted to N 2 over the metal component. The major drawback of this process at present is the high susceptibility of the NSR cata- lysts to sulfur poisoning, which lowers the NO x storage ca- pacity [4,5]. A detailed understanding of the overall NO x storage mechanism is one of the basic steps required to improve the efficiency, as well as the sulfur resistance, of the catalysts. Several studies have focused recently on the mechanism of NO x storage on model NSR catalysts [5–11]. In an early study, Takahashi et al. [5] investigated commercial NSR catalysts and found indications of oxidation of NO on metal sites and subsequent storage on adjacent storage sites in the form of nitrates. Mahzoul et al. [8] suggested that two different Pt sites are involved in the storage process, one close to BaO related to the nitrate formation and another (further away) acting as oxidation catalysts for NO. The authors also reported the formation of nitrates in the absence of gas phase oxygen and in the absence of Pt. 0021-9517/03/$ – see front matter  2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0021-9517(02)00085-4