Soot combustion over ceria-praseodymia nanocatalysts

Recent legislation has introduced more stringent NOx and particulate limits for light and heavy-duty vehicles as well as for passenger cars, and has imposed the use of catalytic devices to satisfy the required standards [1-2]. The solid carbon (soot) that forms Diesel Exhaust Particulates can be burnt off above 600 °C, although typical diesel engine exhaust temperatures fall within the 200 to 500 °C range [3]. Therefore, oxidation catalysts can be used to increase the oxidation rate of filter traps at lower temperatures. In this scenario, ceria-based materials are among the most active soot oxidation catalysts under either O2 or in a NOx/O2 atmosphere. Ceria alone, or in combination with other metal oxides, may exhibit promising soot oxidation activity [3,4]. The redox behaviour, as well as the availability of chemisorbed oxygens, are the most important factors that play a role on the oxidation activity of CeO2-based materials. However, the number of sootcatalyst contact points also affects the catalytic activity of ceria for soot oxidation and it is therefore necessary to maximize the interaction between the soot particles and the catalyst surface [1-4]. Ceria nanomaterials with well-defined reactive (100) and (110) planes are usually more active than conventional polycrystalline ceria NPs with preferred exposure of (111) planes [5,6]. Moreover, inserting aliovalent cations, like Pr 3+/ Pr 4+ into ceria framework gives more surface oxygen vacancies (structural defects) as well as redox active sites. The present work investigates the catalytic activity of a series of nanostructured ceria-praseodymia with various molar compositions, as well as their morphologies, in soot oxidation. Tailoring the morphology of nanoscale ceria confers interesting surface properties in terms of reactivity, as previously discussed in the literature [1]. Thus, a set of ceria-praseodymia catalysts with different Pr loadings (0, 10%, 25%, and 50%-mol, denoted further as Ce100, Ce90Pr10, Ce75Pr25, and Ce50Pr50 respectively) were synthesized through a hydrothermal process using nitrate metal salt precursors and concentrated sodium hydroxide. For comparison, a set of ceria-praseodymia catalysts, and a pure CeO2, were also synthesized through the solution combustion synthesis (SCS). The physico-chemical properties of the prepared catalysts have been investigated using complementary techniques. XXXVIII Meeting of the Italian Section of the Combustion Institute The diffraction peaks for mixed Ce-Pr oxide catalysts, reported in Figure 1 (left), show that all samples refer to pure CeO2 despite varying in praseodymium contents. Figure 1. Powder XRD patterns of the prepared samples (left) and their corresponding FESEM images (right) This means good praseodymium dispersion in CeO2 lattice, forming a homogeneous solid solution. Samples with higher praseodymium content show broader peaks, indicating smaller particle sizes according to Scherrer equation. These data are in agreement with findings from FESEM analysis on Figure 1 (right). Pure CeO2 sample (Ce100) takes form in nanocubes with average size of 200-300 nm. Samples with praseodymium experienced reduction of particle size to 100 nm. Both Ce90Pr10 and Ce75Pr25 still maintain the dominance of nanocubes, while in Ce50Pr50 sample a mixed phase of nanocubes and nanorods is observed. The addition of praseodymium, since it modifies the morphology, slightly increases the surface area of the catalysts, as reported on Table 1. Table 1. BET surface area of the catalysts. Samples BET surface area (m 2 g -1 ) Ce100 6 Ce90Pr10 9 Ce75Pr25 12 Ce50Pr50 20 Catalytic tests in a fixed bed micro reactor were performed to gain initial insights XXXVIII Meeting of the Italian Section of the Combustion Institute into the effect of praseodymium doping and morphology on soot combustion activity. A typical “tight contact” condition between soot and catalyst in the reactor bed was preferred since it allows better discrimination of the activity of each catalyst. The results of catalytic tests in the fixed bed reactor are summarized in Figure 2. It appears that the morphology of the catalyst plays a relevant role: hence, the nanocube morphology of pure Ceria (Ce100) is more active than the corresponding Ce-SCS, at high conversion degrees. Moreover, increasing praseodymium contents in the nanocube structure entailed lower soot combustion temperatures. 200 300 400 500 600 0 10 20 30 40 50 60 70 80 90 100 s o o t c o n v e rs io n ( % ) temperature (°C) Ce50Pr50

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