Applied Surface Science 300 (2014) 201–207 Contents lists available at ScienceDirect Applied Surface Science jou rn al h om ep age: www.elsevier.com/locate/apsusc NH 3 -TPD-MS study of Ce effect on the surface of Mn- or Fe-exchanged zeolites for selective catalytic reduction of NO x by ammonia M. Stanciulescu ∗ , P. Bulsink, G. Caravaggio, L. Nossova, R. Burich Natural Resources Canada, CanmetENERGY, 1 Haanel Dr., Ottawa, ON, Canada K1A 1M1 a r t i c l e i n f o Article history: Received 24 September 2013 Received in revised form 25 January 2014 Accepted 26 January 2014 Available online 6 February 2014 Keywords: Zeolite acidity Catalyst acidity DeNOx NH3-SCR NH3-TPD Aftertreatment a b s t r a c t The selective catalytic reduction (SCR) of NO x with NH 3 is considered to be a promising technique for the efficient reduction of highly detrimental NO x emitted from diesel engine vehicles to N 2 . This study was focused on a series of catalysts with ZSM-5 as support, prepared by Mn- or Fe-exchange followed by wet impregnation of Ce, or Fe or Mn. These catalysts were characterized by temperature-programmed desorption coupled with a mass spectrometer using ammonia (NH 3 -TPD-MS). Specifically, NH 3 -TPD- MS was used as a means of identifying the various strengths of acid sites and their relative abundance in an attempt to explain the effect of the catalyst surface acid sites on DeNO x activity. Acid sites with adsorption energies ranging from 47.0 to 75.6 kJ/mol were detected for all of the catalysts. For the same concentration of exchanged metal it was found that the DeNO x activity depends strongly on the type of metal. Furthermore, the acid site strength and distribution depend on the active metal and correlate with the observed DeNO x catalyst activity. Additionally, SEM metal mapping images confirmed the presence of well dispersed active metal on the surface of all catalysts. The catalysts with bimetallic active phase were stable and demonstrated high NO x conversion over a broad temperature range. Impregnation of metal-exchanged zeolites with Ce enhanced the low temperature NO x conversion. Observed differences of activity between the various catalysts of this study may be due to the formation of new ammonia activation sites. The ammonia desorption profile during the elevation of temperature was correlated to the DeNO x activity. Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved. 1. Introduction During the last 30 years, great efforts have been made to limit the effect of pollutants such as SO x , NO x and VOCs from diesel engine emissions by application of various existing methods and/or new technologies to convert such pollutants to acceptable compounds [1–4]. The information regarding environmental catalysis became available and new approaches to develop and test novel catalysts have been investigated [5,6]. Most of these catalysts are based on transition metals ion-exchanged into zeolites. Novel catalysts were obtained by modifying the surface and creating Brønsted or Lewis acid sites. This approach was pursued due to the fact that activity of zeolitic catalysts is in some extent dependent on the concentration of acid sites [7,8]. The selective catalytic reduction (SCR) of NO x by ammonia occurs through many possible mechanisms on the cat- alyst surface [9,10]. The standard SCR reactions for NO, involving ∗ Corresponding author. Tel.: +1 613 943 0103; fax: +1 613 996 9400. E-mail address: mstanciu@nrcan.gc.ca (M. Stanciulescu). oxygen (which is plentiful in a lean burn exhaust stream) is shown in Eq. (1): 4NH 3 + 4NO + O 2 → 4N 2 + 6H 2 O (1) Similarly for NO 2 : 4NH 3 + 3NO 2 → 7/2N 2 + 6H 2 O (2) If both NO and NO 2 are present in the stream, the “fast” SCR reaction may occur: 2NH 3 + NO + NO 2 → 2N 2 + 3H 2 O (3) These reactions all take place on the active metal phase loaded into zeolites. NH 3 is adsorbed on Lewis acid metal sites and is sub- sequently transformed to NH 2 species, which participate in the formation of intermediates active for NO x reduction, particularly at low temperatures [11,12]. Brønsted acid sites are present on the zeolite surface as well and these reduce NO and NO 2 by different mechanisms. The exact mechanism is still a matter of discussion, but one possibility was described by Yuan et al. [13]: NH 3 + H + → NH 4 + (4) NH 4 + + NO → NH 2 NO + H + + H → N 2 + H 2 O + H 2 (5) 0169-4332/$ – see front matter. Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2014.01.175