Journal of Catalysis 244 (2006) 169–182 www.elsevier.com/locate/jcat A combined transient in situ FTIR and flow reactor study of NO X storage and reduction over M/BaCO 3 /Al 2 O 3 (M = Pt, Pd or Rh) catalysts Hussam Abdulhamid a,b,∗ , Jazaer Dawody a,c , Erik Fridell a,c,1 , Magnus Skoglundh a,b a Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Göteborg, Sweden b Department of Chemical and Biological Engineering – Applied Surface Chemistry, Chalmers University of Technology, SE 412 96 Göteborg, Sweden c Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden Received 5 May 2006; revised 27 August 2006; accepted 1 September 2006 Available online 9 October 2006 Abstract Transient in situ DRIFTS and flow-reactor experiments were performed to study the storage and reduction of NO X over Pt/BaCO 3 /Al 2 O 3 , Pd/BaCO 3 /Al 2 O 3 , and Rh/BaCO 3 /Al 2 O 3 samples using CO, H 2 ,C 3 H 6 , or C 3 H 8 as the reducing agent. The DRIFTS results show that exposure of the examined samples to NO 2 results in the formation of nitrite/nitrate peaks over alumina and barium and that the reduction of stored NO X is influenced by the type of precious metal and reducing agent. Using CO as the reductant results in a lower NO X reduction capacity for Pt/BaCO 3 /Al 2 O 3 compared with Pd- and Rh-based samples, whereas H 2 shows a significant ability to reduce the stored NO X on all samples examined. In addition, the reduction with CO and C 3 H 6 proceeds via the formation of isocyanate species over both barium and alumina sites. The intensity of barium-isocyanate species for Pt/BaCO 3 /Al 2 O 3 is significantly lower than the corresponding intensity for the Pd- and Rh- based samples, suggesting that the interaction between barium and Pt is lower than the corresponding interaction in the Pd/BaCO 3 /Al 2 O 3 and Rh/BaCO 3 /Al 2 O 3 samples.  2006 Elsevier Inc. All rights reserved. Keywords: DRIFT; NO X reduction; NO X storage; Pt; Pd; Rh; Reducing agent; H 2 ; CO; C 3 H 6 ;C 3 H 8 1. Introduction NO X storage and reduction technology offers the possibility of reducing emissions of NO X from vehicles operating under lean-burn conditions. The concept is based on incorporating a storage material (commonly Ba) in the conventional three-way catalyst to store NO X (NO + NO 2 ) under lean conditions un- til it is saturated with NO X . Subsequently the stored NO X is released and reduced to N 2 by turning the engine to rich oper- ating conditions under a short period [1]. Since the concept was introduced by Toyota in the beginning of the 1990s [2], a substantial number of studies have been per- formed to understand the mechanisms that control the operation of the NO X storage catalysts using gas-phase and surface char- * Corresponding author. Fax: +46 31 160062. E-mail address: husam@chem.chalmers.se (H. Abdulhamid). 1 Present address: IVL Swedish Environmental Research Institute, P.O. Box 5302, SE-411 33 Göteborg, Sweden. acterisation techniques. Spectroscopic techniques, including in situ Fourier transform infrared spectroscopy (FTIR), have been used to follow the evolution of different surface species under different reaction conditions [3]. Many FTIR studies are devoted to understanding the NO X storage and reduction processes [4–15]. Some of these studies have indicated that NO X storage occurs via the formation of ni- trites and nitrates over the storage component and the support. Using a Pt/BaCO 3 /Al 2 O 3 catalyst, Fanson et al. [9] observed that nitrites are transformed to nitrates when the NO pressure is increased. Vibrational bands between 2000 and 2200 cm −1 were formed when propene was used to reduce the stored NO X . These bands were correlated to isocyanato on platinum and to cyano, isocyano, and cyanato on barium [9]. For barium- based catalysts, the decrease in peak intensity for stored NO X species (in the region 1200–1650 cm −1 ) under regeneration with carbon-containing reductants is hard to distinguish due to overlap with barium carbonate peaks [9,16,17]. In contrast, regeneration with H 2 , where no carbonate species are formed 0021-9517/$ – see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2006.09.003