Hydrothermal Stability of Fe-BEA as an NH 3 ‑SCR Catalyst Soran Shwan,* ,† Radka Nedyalkova, † Jonas Jansson, ‡ John Korsgren, ‡ Louise Olsson, † and Magnus Skoglundh † † Competence Centre for Catalysis, Chalmers University of Technology, SE-41296, Gothenburg, Sweden ‡ Volvo Group Trucks Technology, SE-40508, Gothenburg, Sweden ABSTRACT: The hydrothermal stability of Fe-BEA as a selective catalytic reduction (SCR) catalyst was experimentally studied. Cordierite supported Fe-BEA samples were hydrothermally treated at 600 and 700 °C for 3-100 h to capture the effect of aging time and temperature. Before and after aging the samples were characterized with BET, XPS, XRD, and NH 3 -TPD. The catalytic performance of the samples with respect to NO oxidation, NH 3 oxidation, and NO reduction (NH 3 -SCR) was studied by flow reactor experiments to correlate changes of the catalytic performance with structural changes of the zeolite and the iron phases. The NH 3 -SCR experiments did not show any significant decrease in activity after a short time of aging (3 h at 700 °C) even though the ammonia storage capacity decreased by 40% and the oxidation state of iron slightly increased. A longer time of aging resulted in decreased activity for NO reduction during low temperatures (150-300 °C), while at higher temperatures (400-500 °C) the activity remained high. The results indicate that the NO reduction is more sensitive at low temperatures to changes in the oxidation state of iron caused by hydrothermal aging than at higher temperatures. Furthermore, a maximum in activity for NO oxidation and increased oxidation state of iron (Fe 3+ ) indicate Fe 2 O 3 particle growth. 1. INTRODUCTION Catalyst deactivation is a significant and inevitable problem in many important applications. The deactivation causes negative economic as well as environmental effects, and there is considerable motivation for understanding the mechanisms for catalyst decay to design stable catalysts and catalytic processes. Selective catalytic reduction with ammonia (NH 3 - SCR) is a well-established and effective method to eliminate nitrogen oxides (NO X ) under oxygen excess for stationary and, more recently, mobile applications. 1 Vanadia-based catalysts were initially used for SCR applications. However, problems including decrease in activity and selectivity, and also the toxicity of vanadia, which may form volatile compounds at high temperatures, have promoted the development of alternatives to vanadia-based catalysts, especially for mobile applications. 1 Metal-ion-exchanged zeolites, particularly based on copper 2-7 or iron, 1,3,8-11 have in this connection proven to be very active and promising alternatives to vanadia in SCR catalysts. 1,12,13 Iron-based zeolites are generally more active for SCR at higher temperatures compared to zeolites based on copper, which have good low-temperature performance but lower high- temperature activity. Furthermore, Kamasamudram et al. have compared iron- and copper-based zeolites and shown higher SCR performance for zeolites based on iron under transient conditions. 14 Recently, promising results have been shown for simultaneously exchanged Fe/Cu zeolites combining the advantages of both metals in the same catalyst. 15 Several challenges arise when using these materials in exhaust after-treatment systems for lean burn or diesel vehicles. One problem is catalyst deactivation by poisoning caused by hydrocarbons, 16-18 especially during cold start conditions. The resistance of metal-exchanged zeolites toward hydrocarbon poisoning can be enhanced by the addition of a protection layer of mordenite. 16 Another problem is thermal deactivation due to the high-temperature conditions in connection with the regeneration of the particulate filter, which usually is placed close to the SCR catalyst. The regeneration occurs typically between 600 and 700 °C. 19 Hence, the hydrothermal stability of metal-based zeolites is crucial. Previous studies of Fe-ZSM-5 as an SCR catalyst have shown stable catalytic properties below 500 °C under hydrothermal conditions. However, at higher temperatures deactivation becomes a problem. 1,10,20,21 Different mechanisms have been proposed for the deactivation caused by hydro- thermal treatment. The two major mechanisms are (i) the loss of catalytic surface area due to pore collapse of crystallites of the active phase and (ii) the loss of support area due to support collapse and crystallite growth. 22 At temperatures above 500 °C Brønsted acidity has been shown to decrease due to dealumination. 1,3,23-26 Water also favors migration of iron in Fe-exchanged zeolites which results in formation of metal oxide clusters and particles with reduced activity. 27 Brandenberger et al. proposed a mechanism for the hydrothermal deactivation of Fe-ZSM-5 which is dominated by three parallel processes: (i) rapid dealumination of Brønsted sites (i.e., Al-OH-Si), (ii) depletion of dimeric iron species, and (iii) migration of isolated iron ions followed by dealumination. 23 The impact from loss of active iron sites and Brønsted acidity on the SCR activity has been thoroughly investigated, but no general conclusions have been able to be drawn. Recent studies show that high numbers of Brønsted acid sites are not required for high SCR activity. 23,28 It has been concluded that NO oxidation is the rate-determining step 3,29 in the SCR reaction at low temper- atures where the reaction preferentially proceeds over the ion- exchanged metal. 1,27 Devadas et al. showed that Fe 2 O 3 particles Received: June 11, 2012 Revised: September 11, 2012 Accepted: September 12, 2012 Published: September 12, 2012 Article pubs.acs.org/IECR © 2012 American Chemical Society 12762 dx.doi.org/10.1021/ie301516z | Ind. Eng. Chem. Res. 2012, 51, 12762-12772