Intrinsic reactivity of alkaline and alkaline-earth metal oxide catalysts for oxidation of soot Lidia Castoldi, Roberto Matarrese, Luca Lietti *, Pio Forzatti Dipartimento di Energia, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milano, Italy 1. Introduction The diesel engines market has been significantly increased in recent years in Europe. The reasons of such spreading are essentially related to the good driving characteristics of the modern diesel engines, and to their lower fuel consumptions (up to 30%) if compared to the traditional gasoline power units. However, diesel exhausts have been blamed for urban and global air pollution due to the emissions of nitrogen oxides (NO x ) and of particulate matter (PM or soot). For these reasons, more stringent regulations to reduce pollutants emissions from diesel engines (and from Otto engines as well) are progressively coming in force. The recently adopted Euro V rules requires for light-weight motor vehicles (i.e. with a weight up to 2500 kg) a fivefold decrease of soot emissions if compared to the previous Euro IV regulations, from 0.025 g/km (Euro IV) to 0.005 g/km (Euro V). An even more drastic reduction is expected for NO x with the coming Euro VI rules. Remarkable efforts are being made by car manufacturers to explore innovative technologies that may allow the reduction of polluting emissions from diesel engines, and in particular of soot. The development of new and more efficient engines equipped with sophisticated diesel injection systems and with advanced combus- tion technologies has impacted the reduction of pollutant emissions, but after-treatment technologies must be applied as well to comply with the strict emissions standards. At present, the common after-treatment technology used to reduce soot emis- sions consists in the employment of diesel particulate filters (DPF), which traps the soot particles contained in the exhaust stream. These filters must be periodically regenerated, generally by a temperature increase of the filter which leads to the oxidation of the soot particles. The temperature increase is generally attained by combustion on the filter of extra-fuel injected in the exhausts; for this reason the filter might be catalyzed (generally with noble metals) to favor the fuel combustion. The catalytic layer deposited onto the filter may also favor the soot oxidation. Different kinds of catalysts have been proposed for this purpose, including single and mixed metal oxides, perovskite- and spinel-type oxides [1–5]. Many catalyst formulations include alkali metal oxides, which have been claimed to bring substantial benefits for soot oxidation [6–11]. It has been suggested that alkali oxides may favor the reaction through the formation of low melting point compounds, or of eutectics with other catalyst components, thus improving the surface mobility of the active species and hence favoring the soot– catalyst contact [12–14] which has been claimed as a key factor in the oxidation of soot with oxygen [2,3,11,15]. Among alkalis, the catalytic activity of potassium salts and containing mixed oxides Applied Catalysis B: Environmental 90 (2009) 278–285 ARTICLE INFO Article history: Received 4 November 2008 Received in revised form 9 March 2009 Accepted 16 March 2009 Available online 25 March 2009 Keywords: Soot combustion Loose contact Tight contact Alkaline oxides Alkaline-earth oxides ABSTRACT The reactivity of selected alkaline (Na, K, Cs) and alkaline-earth (Ca, Ba, Mg) oxide catalysts in the oxidation of a model soot sample (Printex U) is investigated in this study. With the aim of obtaining information about the intrinsic reactivity of these elements, samples have been prepared in which the active elements have been directly deposited on the soot (‘‘full contact’’ conditions). In this way the reactivity of the catalytic elements is not controlled by the type of contact with the soot. It is found that the combustion of soot is greatly enhanced by the presence of either alkaline or alkaline-earth oxides, with Cs and Mg exhibiting the highest and the lowest activity, respectively. Notably, the reactivity in the soot combustion is found to nicely correlate with the electropositivity of the investigated metal ions, in line with literature indications pointing out the relevance of the electron-donor characteristics of the active elements. The correlation between electropositivity and activity in the soot combustion is not apparent in the absence of a close contact, i.e. under loose contact conditions. Under these conditions it is suggested that the catalytic activity is governed by the mobility of the surface species, which favors the soot–catalyst contact. Accordingly the poor performances observed in the case of the alkaline-earth metal oxides suggest for these elements a poor mobility of the active surface species, as opposite to alkaline oxide based catalysts showing a high reactivity even under loose contact conditions. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +39 0223993272; fax: +39 0270638173. E-mail address: luca.lietti@polimi.it (L. Lietti). Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb 0926-3373/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2009.03.022