Nanosized Pt-perovskite catalyst for the regeneration of a wall-flow filter for soot removal from diesel exhaust gases Emanuele Cauda, Debora Fino*, Guido Saracco, and Vito Specchia Materials Science and Chemical Engineering Department, Politecnico di Torino, Corso Duca degli Abruzzi, 24-10129 Torino, Italy Perovskite-type catalysts have been investigated for diesel soot combustion: (i) the LaCr 0.9 O 3)d substoichiometric perovskite, (ii) K–La partially substituted chromites; (iii) Pt added ii-type perovskites. The catalysts prepared showed a progressively higher activity and potential for practical application in diesel particulate traps. Engine bench tests performed on a SiC wall-flow trap (Ibiden) lined with the La 0.9 K 0.1 Cr 0.9 O 3-d + 1 wt%Pt catalyst showed that the catalyst not only speeds up soot combustion on occasional trap heating (regeneration phase) but also prolongs the trap loading phase (soot accumulation during normal operation) as Pt active sites promote NO–NO 2 oxidation, followed by the non-catalytic reaction of NO 2 with the trapped soot. KEY WORDS: diesel particulate combustion; wall-flow monolith; Pt–La–K–Cr catalyst; perovskite. 1. Introduction Soot particles and nitrogen oxides are undesirable products of diesel engines. Moreover, carbonaceous aerosols catch special attention due to their possible effect on public health. Recent European directives concerning the quality of air in urban areas are more and more often forcing local authorities to limit car circulation on selected days (see www.comune.torino.it/ ambientecgi/inquinamento.html for the situation of a large city like Turin, Italy). Particulate matter smaller than 10 lm (the so called PM10), largely produced by diesel engines, is the pollutant more often responsible for this occurrence. The current concentration limit in urban areas is 50 lg/m 3 , a limit that is exceeded routinely. Diesel engines dominate the heavy-duty market but are steadily increasing their penetration in the passen- ger-vehicle one. However, this trend can face a halt owing to the carcinogenic effect of diesel particulates [1]. Most car manufacturers are spending extensive efforts in the development of soot traps and trap-regeneration systems so as to accomplish the pending law limits (EURO IV, to be enforced in 2005, will impose to never exceed 0.025 g/km on a ECE-EUDC standard driving cycle). The most promising technology in the field is based on wall-flow type catalytic traps, periodically regenerated by a peculiar use of last generation Com- mon-Rail engines (i.e. post-injection of HCs burned out by a specific catalytic converter so as to heat the downstream trap up to ignition of the trapped soot [2]). The role of the catalyst is here to reduce soot ignition temperature and enhance the combustion rate of the soot collected on the filter during trap regeneration [3]. The main target of the present paper is to check the performance of a new Pt-perovskite catalyst on wall- flow particulate filter; this multifunctional catalyst should in fact combine the direct combustion activity of chromium-based perovskites [2], guaranteed by their strong attitude to chemisorb active oxygen species in the temperature range of interest (300–450 °C), with the non-catalytic combustion activity of NO 2 , easily gener- ated by NO oxidation over the Pt active sites [4]. 2. Experimental 2.1. Catalyst preparation and characterisation A large screening of soot-combustion perovskite cat- alysts was presented in a previous paper [5]. The best catalyst composition was found to be the perovskite La 0.9 K 0.1 Cr0 0.9 O 3-d . Some of this material was here pre- pared by combustion synthesis [6]. A concentrated aqueous solution of various precursors and an organic fuel was poured in a crucible located in an oven at 600 °C for few minutes, so as to ignite the very fast overall synthesis process, which can be formally split into two steps: 0:1KNO 3 þ 0:9LaðNO 3 Þ 3 þ 0:9CrðNO 3 Þ 3 ! La 0:9 K 0:1 Cr 0:9 O 3d þ 6:75O 2 þ 2:75N 2 ð1Þ 4:5COðNH 2 Þ 2 þ 6:75O 2 ! 4:5CO 2 þ 9H 2 O þ 4:75N 2 ð2Þ The catalyst was then ground in a ball mill at room temperature. X-ray diffraction (PW 1710 Philips diffrac- tometer) was used to check the achievement of the perovskite structure. The specific surface area of the prepared catalyst were evaluated using a Micrometrics ASAP 2010 BET analyser. A part of the prepared perovskite was then impreg- nated with 1.0 wt% Pt by the incipient wetness method. AH 2 PtCl 6 Æ 6H 2 O aqueous solution was used as a precursor, so as to reach an even Pt distribution over the support: the perovskite was placed into a becker and the impregnating solution corresponding to the total pore * To whom correspondence should be addressed. E-mail: debora.fino@polito.it Topics in Catalysis Vols. 30/31, July 2004 (Ó 2004) 299 1022-5528/04/0700–0299/0 Ó 2004 Plenum Publishing Corporation