Zeolite supported Pt catalysts for reduction of NO under oxygen excess: a comparison of C 3 H 6 , HNCO and NH 3 as reducing agents Filip Acke a,b,* , Magnus Skoglundh a a Competence Centre for Catalysis, Chalmers University of Technology, S-412 96 Go Èteborg, Sweden b Department of Inorganic Chemistry, Chalmers University of Technology and Go Èteborg University, S-412 96 Go Èteborg, Sweden Received 1 August 1998; received in revised form 8 November 1998; accepted 8 November 1998 Abstract The effect of the zeolite SiO 2 /Al 2 O 3 ratio of ZSM-5 supported Pt catalysts on the reduction of NO under oxygen excess has been investigated. The activity and selectivity of three reducing agents were compared, i.e. C 3 H 6 , HNCO and NH 3 . The maximum NO reduction with both C 3 H 6 and HNCO as the reducing agent was affected by the SiO 2 /Al 2 O 3 ratio of the support. Although no effect was observed on the maximum NO reduction for NH 3 , the yields of N 2 and N 2 O were affected. The results are put in perspective of reaction mechanisms proposed in the literature. # 1999 Elsevier Science B.V. All rights reserved. Keywords: NO x reduction; Platinum; ZSM-5; Lean conditions; Catalysis; Ammonia; Propene; Isocyanate 1. Introduction The need for active and selective catalysts reducing NO from internal combustion engines under oxygen excess justi®es efforts to characterise important fea- tures of the reaction mechanism involved. Through the years, a number of catalysts have been suggested. Among these, supported noble metal catalysts have given promising results by means of combining rea- sonable activity with good (hydro) thermal stability and resistance to SO 2 [1,2]. A high N 2 O formation is, however, a disadvantage when Pt is used as the noble metal [1±4]. Both the selectivity, N 2 versus N 2 O formation, and the activity were shown to be affected by the platinum support material, g-Al 2 O 3 , ZSM-5 and SiC, when C 3 H 6 is used as the reducing agent [5]. Effects of experimental conditions were also observed, i.e. heating ramp versus steady state experi- ments. Independent of experimental conditions, the inert support (SiC) systematically showed the lowest activity and the highest N 2 O yield. The role of the support was also investigated by Takami et al. [6], who studied MFI zeolites with varying SiO 2 /Al 2 O 3 ratios as support for a noble metal mixture of Pt, Ir and Rh on the lean reduction of NO x by propene. A maximum NO x conversion was found for the zeolite with a ratio of 80. This maximum was related to the ability of the zeolite to adsorb propene. A coupling of nitrogen atoms is a prerequisite for formation of N 2 and N 2 O during reduction of NO over noble metal surfaces. This is allegedly enhanced by the presence of an unpaired nitrogen atom in the reducing agent [7]. The pairing of nitrogens between Applied Catalysis B: Environmental 20 (1999) 235±242 *Corresponding author. Tel.: +46-031-772-28-86; fax: +46-031- 772-28-53; e-mail: filip@inoc.chalmers.se 0926-3373/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-3373(98)00114-3