Effect of chromium migration from metallic supports on the activity of diesel exhaust catalysts W. Kaltner, M. Veprek-Heijman, A. Jentys *, J.A. Lercher Department Chemie, Technische Universita ¨t Mu ¨nchen, Lichtenbergstrabe 4, 85748 Garching, Germany 1. Introduction Catalytic converters for the oxidation of CO and residual hydrocarbon emissions are necessary for diesel cars to comply with current emission legislation. In typical diesel oxidation catalysts platinum supported on ceramic or metallic monoliths is used as active component for the oxidation of CO, NO and hydrocarbons [1,2]. Recently, novel structures based on stainless steel hollow spheres [3] were proposed as catalyst supports for future applications, having the potential to combine different functionalities such as catalysis and noise reduction. While most studies of diesel oxidation catalysts have investi- gated reaction mechanisms and tried to improve the catalytic activities properties of the fresh catalysts, the deactivation during long-term application at elevated temperatures has not been studied in great detail. In general, the following processes for deactivation of exhaust catalyst have been reported [4–10]: (i) loss of metal surface area due to sintering of Pt after long-term operation at high temperature (thermal deactivation), (ii) accu- mulation of poisons such as Ca and P arising from the lubricant oil on the surface of the active sites, (iii) formation of sulfates from sulfur species in the fuel, (iv) coking and (v) the migration or diffusion of elements such as Cr from the support into the washcoat or into the active noble metal phase. The deactivation depends strongly on the combination of parameters such as time, temperature, atmosphere, support and catalyst composition. In the present study Pt-based oxidation catalysts supported on metallic hollow sphere structures and on a ceramic monolith are thermally aged under laboratory conditions. The light-off temperature for the CO oxidation and the activity for NO oxidation were used as indicators for the catalytic activity. Both are typical reactions for the emission reduction in car exhaust systems. The CO oxidation is required to lower the emissions, while the oxidation of NO to NO 2 is used to enhance the rate in a subsequent NH 3 -SCR reaction [11] and for the effective removal of soot from a particulate filter [12]. 2. Experimental 2.1. Catalysts The support structures were of cylindrical shape. For catalyst A (diameter 21 mm  length 25 mm) and catalyst B (diameter 21 mm  length 28 mm) sintered hollow sphere structures (sphere diameter of 3 mm) made of Fe–Cr–Ni and of Fe–Cr–Al stainless steel, respectively, were used (for compositions see Table 1) as support. The hollow spheres were prepared by coating PE spheres with metal particles, followed by a thermal treatment to remove the inner core and to sinter the particles to a stable shell. These intermediates were packed into the desired shape (e.g. a torus or a sheet) and sintered to obtain the final structure. For catalyst C (diameter 21 mm  length 25 mm) a ceramic monolith (cordierite 400 cpsi) was used as support. The metallic structures were pre-treated in air at 680 8C, coated with a standard washcoat based on g-Al 2 O 3 and impregnated with Pt (loading 3.5  10 3 g/m 3 ), a formulation typically used for diesel Applied Catalysis B: Environmental 89 (2009) 123–127 ARTICLE INFO Article history: Received 25 August 2008 Received in revised form 1 December 2008 Accepted 6 December 2008 Available online 24 December 2008 Keywords: Metallic support structures Oxidation catalysts Cr diffusion ABSTRACT Fe–Cr–Ni and Fe–Cr–Al sintered spheres are explored as alternative support structures for automotive exhaust catalysts. Initially the activity of Pt supported on the metal support and on ceramic monoliths (cordierite) for the oxidation of CO and NO is comparable. However, after thermal aging at elevated temperatures (800 8C) and extended reaction times (24 h) the activity for the oxidation of NO of the catalysts supported on the Fe–Cr–Ni spheres decreases significantly, which is attributed to the migration of chromium from the metal bulk phase into washcoat. In the contrary, the aluminium present in the Fe– Cr–Al steel alloy formed a protective alumina barrier inhibiting the degradation of the catalyst after aging by blocking the migration of chromium to the surface. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +49 89 289 135 38; fax: +49 89 289 135 44. E-mail address: Andreas.Jentys@ch.tum.de (A. Jentys). Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb 0926-3373/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2008.12.006