SO x Storage Materials under Lean-Rich Cycling ConditionssPart II: Influence of Pt, H 2 O, and Cycling Time Hendrik Dathe, Peter Haider, Andreas Jentys, and Johannes A. Lercher* Technische UniVersita ¨t Mu ¨nchen, Department of Chemistry, Lichtenbergstrasse 4, D-85747 Garching, Germany ReceiVed: July 12, 2006; In Final Form: October 20, 2006 The role of Pt and the influence of the reaction conditions during lean-rich cycling experiments were studied on a second generation SO x trapping material. The combination of the Generalized 2-D Correlation Analysis, 2-D Sample-Sample Correlation Analysis, and Factor Analysis using the MCR-ALS technique was applied to identify the reactive species. Transient surface sulfate species were formed under oxidative reaction conditions (lean mode) and decomposed under reducing reaction conditions (rich operation mode). The reduction of this species was identified to be the main contribution to the SO 2 release observed under dynamic flow conditions. Pt facilitates the formation of sulfates but also catalyzes the reduction of the transient surface sulfate species leading to a higher amount of SO 2 released under rich conditions. In the presence of water, this effect was diminished, which was found to be mainly a result of the suppressed formation of surface sulfate species caused by the faster transport of SO 2 into the bulk phase of the SO x trapping component (BaCO 3 ). Increasing the time under reducing conditions in the cycles leads to an enhanced reduction of the surface during rich conditions. The presence of water did not influence the bulk type species. It is proposed that for effective SO 2 storage materials, strong SO x adsorption sites on the surface, the presence of water, and a short time under reducing conditions are essential. Introduction Lean burn engines offer significant advantages with respect to the gasoline efficiency and emission of the greenhouse gas CO 2 as compared to Otto engines (operating at stoichiometric fuel/air ratios). Nevertheless, for meeting EURO IV criteria, new techniques for emission control are necessary, especially for the reduction of the NO x emissions. The implementation of the Nitrogen Storage-Reduction concept (NSR), which is based on a cyclic engine operation mode, is mainly limited by the presence of sulfur in the exhaust gas stream due to the formation of a thermodynamically favored sulfate species leading to the blockage of NO x sorption sites. 1-3 A promising approach to overcome this limitation is the implementation of a SO x trap to remove the sulfur species present in the exhaust gas stream before reaching the NSR catalyst. An important parameter of S trapping material, the fast oxidation of SO 2 to SO 3 is typically realized by adding Pt as an oxidation component. Recent publications indicate that first row transition metals (Mn and Cu) also offer a sufficient oxidation capacity and could, therefore, be used instead of Pt. 4,5 Alkaline or rare earth metals (Ca, Ba, and Mg) are usually used for the irreversible adsorption of SO 3 under oxidizing conditions. 6,7 Apart from a high sulfur storage capacity, an essential feature of S trap materials is the stability of the sulfur species under reducing (fuel rich) reaction conditions to avoid the release of SO x during the periodic regeneration cycles of the NSR catalyst. 8,9 The investigations performed under reducing conditions mainly addressed the regeneration potential of the trapping components and suggested that only partial regeneration is possible at high temperatures. 7,10 Nevertheless, a similar study on a second generation SO x trap indicated even the reduction of a sulfate/sulfite type species under real cycling operation conditions at low temperatures causing SO 2 release under these conditions. 11 In part one of this publication, we investigated the stability of the sulfate species under reducing and oxidizing conditions and showed that the combination of advanced chemometric techniques and in situ IR spectroscopy allows the unequivocal identification of transient surface sulfate species formed under lean-rich conditions, which can be reduced and contribute to SO 2 in the exhaust gas stream. 12 In part two, we address the effect of the time under reducing conditions, the presence of water, and the influence of Pt on the formation of the transient sulfur species on a second generation SO x trapping material. Plug flow reactor experiments in combination with the results obtained from time-resolved in situ IR spectroscopy at typical diesel exhaust gas conditions will lead to an identification of the key parameters for the dynamic storage of sulfur on advanced trap materials. Experimental Procedures Materials. A CuO-Al 2 O 3 mixed oxide was synthesized by a proprietary sol-gel procedure of Venezia Tecnologia and calcined in air at 823 K for 3 h. The support was further impregnated with Ba 2+ (10%) and calcined again in air at 823 K for 1 h. Finally, the material was impregnated with Pt (1%) followed by a recalcination at 823 K for 1 h. The catalysts are denoted as Ba/CuO-Al 2 O 3 for the Pt free sample and Ba/CuO- Al 2 O 3 Pt for the sample impregnated with Pt. In Situ IR Experiments. IR experiments were carried out in a flow cell in transmission mode with a Perkin-Elmer 2000- FTIR spectrometer. For the IR measurements, the samples were pressed into self-supporting wafers. The gas composition of the mixture under lean and rich conditions is shown in Table 1. All experiments were carried out at 523 K. The cycling times * Corresponding author. Fax: +49 89 289 13544; tel.:+49 89 289 13540; e-mail: johannes.lercher@ch.tum.de. 26024 J. Phys. Chem. B 2006, 110, 26024-26032 10.1021/jp064387p CCC: $33.50 © 2006 American Chemical Society Published on Web 11/30/2006