NO x storage/reduction catalysts based in cobalt/copper hydrotalcites A.E. Palomares a,b, * , A. Uzca ´tegui a , A. Corma a a Instituto de Tecnologia Quimica, UPV-CSIC, Universidad Politecnica de Valencia, Avenida de los Naranjos s/n, 46022 Valencia, Spain b Departmento de Ingenierı ´a Quı ´mica y Nuclear, Universidad Politecnica Valencia, Avda. de los Naranjos s/n, 46022 Valencia, Spain Available online 4 March 2008 Abstract The use of materials based on hydrotalcites as NO x storage/reduction (NSR) catalysts has been investigated, examining their activity at low temperature and their resistance to poisons such as H 2 O and SO 2 . The results obtained show that catalysts derived from Mg/Al hydrotalcites containing copper or cobalt is active at low temperatures, specially the samples containing 10 or 15% of Co. The addition of 1 wt% of transition metals with redox properties such as Pt, Pd, V and Ru to the hydrotalcite increases its activity because the combination of the redox properties of these metals and the acid-base properties of the hydrotalcite. The best results were obtained with the catalyst derived from a hydrotalcite with a molar ratio Co/Mg/Al = 15/60/25 and containing 1 wt% V. This material shows a higher activity, at low temperatures and in the presence of H 2 O and SO 2 , than a Pt–Ba/Al 2 O 3 reference catalyst. # 2008 Elsevier B.V. All rights reserved. Keywords: NO x ; Storage reduction catalysts; Hydrotalcites; Cobalt; Vanadium 1. Introduction One of the principal environmental concerns in air pollution is the minimization of NO x emissions. These emissions come mainly from mobile sources and their control becomes an urgent necessity. Among the technologies envisaged, the most widely applied method is the three-way catalyst. This system gives excellent results when the air to fuel ratio is close to the stoichiometric ratio, but the efficiency diminishes in the presence of an excess of oxygen [1]. Nowadays the tendency is to use engines with better fuel efficiency, as lean-burn gasoline and diesel engines that works with an excess of oxygen over the stoichiometric ratio, i.e. lean conditions. These operation conditions strongly diminish the efficiency of the traditional three-way catalysts, for this reason the discovery of an alternative technology, which can achieve efficient NO x , reduction at high oxygen concentrations, without increasing fuel consumption, is an imperative need. Different alternatives have been proposed to break this challenge [2,3]. The first approach relies on the selective catalytic reduction of NO x using ammonia, or urea, as reductants [4]. However its implementation for mobile applications requires the development of a urea distribution network and it presents the problems associated with the use of a pollutant, as ammonia. Another possibility is the use of hydrocarbons as selective reductant agents employing metal exchanged zeolites as catalysts [5,6]. These materials are quite active, but present a low hydrothermal stability resulting in a fast deactivation of the catalyst [7]. In the last years a new alternative has appeared that it is attracting much attention, this is the NO x storage/reduction (NSR) technology [8–10]. It is based in the selective storage of the NO x as nitrates under lean (oxidizing) conditions and their non-selective reduction under the short, rich (reducing) excursions. NSR catalysts are typically composed of at least one basic compound (alkaline or alkaline-earth oxides) and at least one precious-metal component. One of the most common formulations used is Pt and Ba supported on Al 2 O 3 [2]. This catalyst works in two steps, in the first step, under lean conditions, NO is oxidised into NO 2 on the Pt active sites, and the NO 2 is adsorbed on the BaO as nitrate. In the rich conditions, the nitrates decompose and the NO 2 formed is reduced into N 2 with the hydrocarbons, CO and H 2 present in the exhaust gases. The problem of this material is its fast poisoning by sulphur compounds and its low activity at low temperatures [2,11,12]. In this work it is studied the www.elsevier.com/locate/cattod Available online at www.sciencedirect.com Catalysis Today 137 (2008) 261–266 * Corresponding author at: Instituto de Tecnologia Quimica, UPV-CSIC, Universidad Politecnica de Valencia, Avenida de los Naranjos s/n, 46022 Valencia, Spain. Tel.: +34 6 3877800; fax: +34 6 3877809. E-mail address: apalomar@iqn.upv.es (A.E. Palomares). 0920-5861/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2007.12.137