Ag/alumina catalysts for the selective catalytic reduction of NO x using various reductants E.F. Iliopoulou * , A.P. Evdou, A.A. Lemonidou, I.A. Vasalos Laboratory of Petrochemical Technology, Department of Chemical Engineering, Aristotle University of Thessaloniki, Center for Research and Technology Hellas/Chemical Process Engineering Research Institute, P.O. Box 1517, 54006 University City, Thessaloniki, Greece Received in revised form 24 June 2004; accepted 28 June 2004 Available online 27 August 2004 Abstract A series of Ag-based catalysts supported on unpromoted or Ce-promoted g-alumina were prepared and studied for the selective catalytic reduction (SCR) of NO x with various reductants. The activity for NO reduction increased as the oxygen-content in the reaction mixture increased from 2% to 10%. The influence of Ag loading, the catalytic support used and the type of the reductant on NO reduction was examined. Moderate silver loadings (3%) exhibit the most efficient deNO x activity, while increasing the metal loading or using a Ce-promoted alumina support inhibits the catalytic activity. Among C 3 H 6 , CH 4 and CO used as reducing agents, C 3 H 6 has the higher reducing activity. C 2 H 4 ,C 2 H 6 ,C 3 H 8 ,C 4 H 10 , 1-C 4 H 8 and 1,3-C 4 H 6 were also tested as reducing agents, suggesting that use of higher and less saturated hydrocarbons results in an enhanced deNO x performance. Higher concentrations of the optimum reductant (1-C 4 H 8 ) significantly improved the deNO x performance, achieving over 80% NO x reduction and an interestingly broad active temperature window (300–550 8C). NO x is reduced by reacting with intermediates generated from partial oxidation of hydrocarbons used as reductants. Increased availability of these species either by more C atoms in higher hydrocarbons, their easier formation from less saturated hydrocarbons or higher reductant concentration explains the improvement of NO x reduction. The performance of Ag catalysts was linked with the formation of different crystal phases, which are stabilized through a strong interaction with the alumina support. Existence of Ag in oxidation state +1 seems to be the active phase that favors most the NO reduction. # 2004 Elsevier B.V. All rights reserved. Keywords: Ag; Ce; Alumina; NO reduction; Reductant type 1. Introduction Limitations on the nitrogen oxides (NO x ) emissions are becoming more and more stringent worldwide. The selective catalytic reduction of NO by hydrocarbons (hc-SCR) has been extensively studied as a potential method for the removal of NO x under oxygen-rich conditions from various exhausts [1,2]. The activity of silver loaded catalysts for the selective catalytic reduction (SCR) of NO x by a reductant in the presence of excess O 2 has been previously studied [3– 34,38,40]. It has been reported that the preparation method of Ag- based catalysts affects their catalytic performance. Besides conventional silver nitrate, lactic silver was also used as precursor salt leading to homogeneous distribution of Ag metal on the alumina support [3,4]. Ag/alumina catalysts were also synthesized from inverse microemulsions [5] or from boehmite needles, which were formed by hydrolysis of aluminium tri-isopropoxide (AIP) [6]. In general, various preparation techniques were applied for the synthesis of Ag/ alumina catalysts including multiple impregnations [7], single-step co-gelation process [8], polyol process usually applied for synthesis of monodisperse metal powders [9], co-precipitation [10] and sol–gel methods [10–12]. The ion-exchange method was also used for the preparation of www.elsevier.com/locate/apcata Applied Catalysis A: General 274 (2004) 179–189 * Corresponding author. Tel.: +30 2310 996175; fax: +30 2310 996184. E-mail address: eh@cperi.certh.gr (E.F. Iliopoulou). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.06.052