In situ visible microscopic study of molten Cs 2 SO 4 V 2 O 5 –soot system: Physical interaction, oxidation rate, and data evaluation Agus Setiabudi, Niels K. Allaart, Michiel Makkee * , Jacob A. Moulijn Section Reactor and Catalysis Engineering, Faculty of Applied Sciences, Delft University of Technology, Julianalaan 136, NL 2628 BL Delft, The Netherlands Received 6 October 2004; received in revised form 15 March 2005; accepted 21 March 2005 Available online 27 April 2005 Abstract In situ visible microscopy has been used to characterise the physical interaction between soot and Cs 2 SO 4 V 2 O 5 , a potentially attractive catalyst, under oxidation conditions. Upon heating to its melting point, Cs 2 SO 4 V 2 O 5 shows both mobility and attachment to the soot particle. This creates a liquid catalyst–soot–gas interface that is favourable for the oxidation reaction. Microscope image data can be used to estimate the oxidation rate, which is in good agreement with data obtained from TGA and fixed bed experiments. # 2005 Elsevier B.V. All rights reserved. Keywords: Cs 2 SO 4 V 2 O 5 ; Liquid catalyst; Diesel soot oxidation; Contact soot catalyst 1. Introduction For the catalytic reaction between a solid reactant and a gaseous reactant in the presence of a solid catalyst, contact between the catalyst and the solid reactant is a key factor to initiate reaction that takes place at the solid reactant–solid catalyst–gas interface. Catalysed diesel soot oxidation is a good example of this. Earlier, it was demonstrated that when soot was mixed with the metal oxide catalyst in a ball mill (‘‘tight contact’’), the oxidation rate is much higher than when soot and the catalyst was mixed by spatula (‘‘loose contact’’) [1]. It has been shown that under practical conditions in a catalytic filter loose contact rather than tight contact is encountered. Therefore, a soot–catalyst mixture prepared for example by ball mill mixing is not representative for practical application. To solve the problem, the catalyst–soot contact should be created in situ without exerting mechanical forces. A completely different route is the following. So-called mobile catalysts can establish mobility either via volatilisation or surface migration. They enable an intense interaction between reactants and catalyst. In the oxidation of soot with Cu/K/V/Cl or Cu/K/Mo/Cl catalysts, the presence of volatile compounds dispersed over the soot particle has been concluded to produce activity [2,3]. Another class of catalysts that are capable to create a catalyst–soot contact are molten salt catalyst, for example, Cs 2 SO 4 V 2 O 5 , CsVO 3 MoO 3 , and Cs 2 O V 2 O 5 of which surface migration is thought to create the catalyst mobility [4–6]. These catalysts show high activities at and above their melting points, i.e. 625, 635, and 655 K, respectively. The mobility of the catalyst particle and the wetting of carbon surface have been acknowledged to increase the catalyst activity in the oxidation of carbonaceous material due to a better catalyst–carbonaceous contact [7,8]. Molten salts diesel soot oxidation can have some drawback in some practical application. These drawbacks can be related to either thermal degradation or selective leaching in condensed water in the soot oxidation process (one of the components of the molten salt is partially evaporated or dissolved, respectively). Although catalyst–soot contact has been generally acknowledged as a very important factor for the catalytic soot oxidation with mobile catalyst, the chemical mechan- ism and the physical interaction between catalysts and soot www.elsevier.com/locate/apcatb Applied Catalysis B: Environmental 60 (2005) 233–243 * Corresponding author. Tel.: +31 15 2781391; fax: +31 15 2785006. E-mail address: m.makkee@tnw.tudelft.nl (M. Makkee). 0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2005.03.005