Applied Catalysis B: Environmental 111–112 (2012) 58–66 Contents lists available at SciVerse ScienceDirect Applied Catalysis B: Environmental journa l h o me pa ge: www.elsevier.com/locate/apcatb Mechanistic investigation of hydrothermal aging of Cu-Beta for ammonia SCR Norman Wilken a , Kurnia Wijayanti a , Krishna Kamasamudram b , Neal W. Currier b , Ramya Vedaiyan b , Aleksey Yezerets b , Louise Olsson a,∗ a Competence Centre for Catalysis, Chemical Engineering, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden b Cummins Inc., 1900 McKinley Ave, MC 50183, Columbus, IN 47201, USA a r t i c l e i n f o Article history: Received 6 May 2011 Received in revised form 13 September 2011 Accepted 19 September 2011 Available online 29 September 2011 Keywords: Ammonia SCR Aging Hydrothermal Oxidation Calorimeter DSC Zeolite Beta Heat of adsorption XPS XRD a b s t r a c t The selective catalytic reduction of NO x with NH 3 over a Cu-BEA catalyst was studied after hydrother- mal aging between 500 and 900 ◦ C. The corresponding catalyst was characterized using XPS and XRD techniques in the aging interval of 500, 700 and 800 ◦ C. No structural changes during the aging process were observed. However, the oxidation state of copper changed during aging and more Cu 2+ was formed. We suggest that one of the deactivation mechanisms is the decrease of the Cu + species. The NO oxida- tion and NH 3 oxidation activity was decreased with increasing aging temperature. Further, we observed that the ammonia oxidation was decreased faster compared to the SCR reactions at low aging temper- atures. The experiments from the calorimeter as well as from the ammonia TPD investigations indicate a trend towards more weakly bound ammonia with higher aging temperatures. From the results of the SCR experiments using different NO 2 /NO x ratios and ammonia oxidation experiments we suggest that most of the N 2 O is coming from side reactions of the SCR mechanism and not from reactions between NH 3 and O 2 alone. Interestingly, we observe that after the 900 ◦ C aging a quite large activity remained for the case with 75% NO 2 /NO x ratio. The N 2 O production shows a maximum at 200 ◦ C, but increases again at higher temperatures. However, the N 2 O formed at low temperature is decreased after hydrothermal aging while the high temperature N 2 O is increased. We propose that the different reactions examined in this work do not all occur on the same type of sites, since we observe different aging trends for some of the reactions. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Diesel engines and lean burn gasoline engines use oxygen excess in the combustion. This results in a more complete combustion of the fuel and thereby lowers CO 2 emissions and the fuel costs. However, one problem with these techniques is that the same oxy- gen excess is also present in the exhaust stream, disabling the NO x conversion of the regular three-way catalyst. Since NO x causes acidification and increases the formation of ground level ozone it is critical to reduce the emissions of NO x . There are three major techniques being employed to remove NO x in oxygen excess: NO x storage and reduction [1,2], hydrocarbon selective catalytic reduc- tion (HC SCR) [3,4] and urea/ammonia SCR [5–9]. For practical reasons urea is used to produce the needed ammonia [10,11]. First urea decomposes to form HNCO and NH 3 . The HNCO is further hydrolyzed into CO 2 and NH 3 . These reactions are shown below. NH 2 - CO - NH 2 → NH 3 + HNCO (1) HNCO + H 2 O → NH 3 + CO 2 (2) ∗ Corresponding author. Tel.: +46 31 772 4390; fax: +46 31 772 3035. E-mail address: louise.olsson@chalmers.se (L. Olsson). Vanadia on titania was the first group of catalysts that was used for this system [6,12–14]. The downside of this group of catalysts is the toxicity of the vanadia species, the high rate of oxidation of SO 2 to SO 3 as well as the decrease in activity at high temperatures [15]. Metal exchanged zeolites became a popular alternative. Primar- ily copper [9,16–20] and iron [7,15,16,21–24] ions are exchanged into the zeolites. One problem with this type of catalyst is that they are susceptible towards hydrothermal aging. Chemical aging is also a problem that occurs [25,26], but is beyond the scope of this work. There are many factors that influence the stabil- ity of ion-exchanged zeolite catalysts. One factor is the chosen zeolite. Zeolites like Beta, mordenite and ZSM-5 [16,27,28] have, among others, shown promising results for the NH 3 SCR applica- tion, but they differ in stability while undergoing hydrothermal aging. Van Kooten et al. observed for example that a Ce-beta cata- lyst is more stable than a Ce-ZSM-5 catalyst during hydrothermal aging at 600 ◦ C [27]. Also the specific zeolite preparation plays a role. Berggrund et al. observed that using AlCl 3 as an aluminum source to prepare a ZSM-5 zeolite yields a more stable sample than using Al(NO 3 ) 3 [30]. The choice of metal also influences the stabil- ity of the catalyst. Rahkamaa-Tolonen et al. reported that using Fe as an exchange ion leads to a higher stability then Cu or Ag for a 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.09.018