Vanadium-loaded carbon-based monoliths for on-board NO reduction: Influence of nature and concentration of the oxidation agent on activity A. Boyano a , M.C. Iritia a , I. Malpartida b , M.A. Larrubia b , L.J. Alemany b , R. Moliner a , M.J. La ´zaro a, * a Instituto de Carboquı ´mica, CSIC, c/Miguel Luesma Casta ´n, E-50018 Zaragoza, Spain b Departamento de Ingenierı ´a Quı ´mica, Facultad de Ciencias, Campus de Teatinos s/n, Universidad de Ma ´laga, E-29071 Ma ´laga, Spain Available online 1 April 2008 Abstract A series of polymeric carbon-coated monoliths oxidized with either concentrated or 2N HNO 3 ,H 2 O 2 or H 2 SO 4 and subsequently loaded with 3 wt.% vanadium were prepared. The influence of the different oxidation treatment conditions on the SCR (selective catalytic reduction) catalytic activity, texture and chemical surface properties were studied. Similar pore distribution and pore volumes were observed for the four oxidized samples, indicating that surface modification of carbon supports has been successfully made without disrupting the original textural structures of activated coated monoliths. The surface chemistry created by the oxidation treatments has two effects on the catalytic activity. One of these is that higher surface acidity results in higher NO reduction up to a certain extent. The highest acidities seem to promote a sufficiently strong NH 3 adsorption on the surface such that the lack of efficient desorption decreases the overall NO conversion efficiency. The other effect is that a low surface acidity does not seem to promote vanadium dispersion and fixation, thereby also resulting in decreased NO reduction efficiency. # 2008 Elsevier B.V. All rights reserved. Keywords: Functionalization; Carbon-based catalysts; SCR-NH 3 1. Introduction Today, increasingly stringent regulations require advanced treatment of automotive exhaust to protect the quality of the air and the atmosphere. Among the wide range of treatment technologies that are being developed and optimized, selective catalytic reduction (SCR) of NO x is one that has garnered significant attention in the past several years. Various catalysts have been studied for the SCR of NO x with NH 3 [1,2], but carbon-based catalyts present two important unique advan- tages: they are active at lower temperatures [3] and exhibit an activity promotion by SO 2 [4]. As a result, in this study these are proposed for on-board SCR. Carbon materials are becoming more interesting and promising as supports for heterogeneous catalysts because their porosity and surface chemistry can be controlled. At the same time, they exhibit relatively low reactivity. When activated carbon is used as a support for metallic catalysts, its performance is expected to be strongly influenced by functional groups present on the carbon surface since they influence metal dispersion, resistance to sintering, and the overall catalytic behaviour [5,6]. In order to form surface carbon–oxygen complexes, liquid oxidation treatments are widely used. This method creates oxygen functionalities while producing only minimal changes in the textural properties. Liquid oxidation is an alternative to methods involving oxidizing gases, which require high temperatures. According to the literature, different oxidizing agents can be used in aqueous solution [7,8]. Each oxidizing agent behaves in a different way, eliciting particular transformations that mostly depend on the characteristics of the reacting agent as well as the nature of the activated carbon [9]. Regardless of the oxidizing agent, however, oxidation treatments introduce three types of surface oxygen complexes: acidic, basic, and neutral [9]. Acidic groups correspond to surface oxides (such as carboxyls, phenolic hydroxyls, lactones, and quinone groups) whose fixation on the surface makes it more hydrophilic and acidic, decreasing the pH of their point of zero charge [10]. These changes contribute to the creation of new surface properties that directly impact activity in two primary ways: (i) provision of www.elsevier.com/locate/cattod Available online at www.sciencedirect.com Catalysis Today 137 (2008) 222–227 * Corresponding author. Tel.: +34 976733977; fax: +34 976733318. E-mail address: mlazaro@icb.csic.es (M.J. La ´zaro). 0920-5861/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2008.02.010