Influence of Potassium Doping on the Activity and the Sulfur Poisoning Resistance of Soot Oxidation Catalysts
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Catalysis Letters Vol. 109, Nos. 1–2, June 2006 (Ó 2006) DOI: 10.1007/s10562-006-0055-5
Influence of potassium doping on the activity and the sulfur poisoning resistance of soot oxidation catalysts Kirill Tikhomirov, Oliver Kro¨cher,* and Alexander Wokaun Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
Received 13 February 2006; accepted 24 February 2006
Soot oxidation on potassium nitrate impregnated cerium oxide and manganese cerium mixed oxide was investigated using TG-FTIR. It was found, that potassium nitrate promotes the soot oxidation by enhancing the contact between the soot and the catalyst, as soot ignition temperature corresponds to the melting point of potassium nitrate for both the individual and the mixed oxide. Since potassium nitrate is easier sulfatized than cerium or manganese, the susceptibility of the catalyst to sulfur poisoning is considerably decreased. KEY WORDS: soot oxidation catalyst; sulfur poisoning; potassium doping; manganese cerium mixed oxide.
1. Introduction In order to reduce the costs of diesel particulate filters, numerous cheap noble metal free soot oxidation catalysts have been investigated over the last 20 years. In these investigations it was shown that tight contact between catalyst and soot is crucial for achieving high activity [1]. However, only a loose contact is achieved between soot and catalyst under real conditions in a diesel particulate filter [2]. One option to overcome this problem is to use molten salts as catalytic material [3]. The high mobility of the catalyst at elevated temperatures leads to an intimate contact of catalyst and soot particles resulting in a high catalytic activity even if the soot was not tightly mixed with the catalyst [4]. Alkali metal salts as well as cobalt and lead compounds were used in order to decrease the melting point and to improve the mobility of the catalysts [5–8]. Potassiumloaded lanthana and ceria were proposed for the simultaneous abatement of nitrogen oxides and soot [9, 10]. Sulfur poisoning poses another problem for the development of noble metal free catalysts. For instance, ceria and manganese were found to be highly active soot oxidation catalysts due to their oxygen storage ability and strong oxidative properties [11,12]. However, these materials loose their oxidative properties rapidly when exposed to sulfur due to reductive sulfatization. Regeneration requires high temperatures and the catalytic activity is not or only partly recovered due to changes in the catalyst structure [11,13,14]. In the present work we tried to increase the catalytic activity of pure ceria as well as manganese cerium mixed *To whom correspondence should be addressed. E-mail: [email protected]
oxide by addition of potassium nitrate and investigated the influence of the dopant on the sulfur poisoning of the catalysts.
2. Experimental 2.1. Catalyst preparation and characterization MnOx–CeO2 with a molar ratio of Mn:Ce = 1:3 was prepared by co-precipitation using acetates of Ce(III) and Mn(II) as starting materials as describe
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