Microsoft Word - 476hernandez.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 43, 2015 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Chief Editors: Sauro Pierucci, Jiří J. Klemeš Copyright © 2015, AIDIC Servizi S.r.l., ISBN 978-88-95608-34-1; ISSN 2283-9216 Optimal Loading of a MW Susceptible Catalysed DPF Vincenzo Palma, Eugenio Meloni*, Paolo Ciambelli University of Salerno, Department of Industrial Engineering, Via Giovanni Paolo II, 132 - 84084 Fisciano (SA), Italy emeloni@unisa.it Diesel engines have low fuel consumption and enough torque compared with equivalent gasoline engines. Diesel engines emitted less CO2 which is known well as the greenhouse gas, and a percentage of new diesel passenger car registrations is increasing in EU year by year. However, NOx and soot (particulate matter, PM) emitted by diesel engines are a source of pollution with impact on the environment and human health. Since the automobile regulations have been stricter, new technologies for diesel emissions abatement have been proposed, including the addition of fuel additives for diesel smoke reduction. A diesel particulate filter (DPF) is one of the most important technologies for the above strict PM regulations, consisting in alternately plugged parallel square channels, so that the exhaust gases are forced to flow through the porous inner walls: in this way the particles are collected on the surface and in the porosity of the channel walls, progressively blocking the pores. Since the pressure loss increases by the formation of a thick soot cake as the PM is accumulated, the DPF needs to be periodically regenerated by burning off the accumulated soot. In our previous works we showed that the simultaneous use of a microwave (MW) applicator and a specifically CuFe2O4 catalysed DPF, allows to reduce the temperature, the energy and the time required for the filter regeneration. Starting by these very promising results, the objectives of this work are to modify the active species formulation in order to simultaneously further reduce the PM oxidation temperature and keep low the pressure drop. 1. Introduction Diesel engines combine a high fuel economy with high durability and low maintenance costs and are, therefore, used on a large scale for transportation purposes. However, their exhausts environmental pollution has become more and more serious in the last decade, and currently the regulation of diesel emissions becomes tightened especially with respect to NOx and PM. Since the reduction of both NOx and PM to the admitted level cannot be accomplished by engine modifications alone, aftertreatment processes for the reduction of diesel emissions should be developed (Liu et al., 2003). The difficulty to reduce NOx emissions, led to their reduction as much as possible through engine management known as EGR (Exhaust Gas Recirculation), while emissions of PM are generally controlled through the use of a diesel particulate filter (DPF), that consists of a silicon carbide (SiC) structure in the Wall Flow configuration, with alternately plugged parallel square channels (Palma et al., 2012). As a consequence, the exhaust gases are forced to flow through the porous channel walls that act as filters, so achieving a PM trapping efficiency >95 % (Palma et al., 2011). Since the pressure loss increases by the formation of a thick soot cake as the soot is accumulated, the DPF needs to be periodically regenerated by burning off the accumulated soot. As the exhaust gas temperature of diesel is typically about 150 °C, this temperature is far too low compared with the minimum combustion temperature of 600 °C needed to burn PM. As a result, some type of system is necessary in order to forcibly regenerate the PM (Ohno, 2008). The oxidation step is promoted by the so-called ‘passive’ (at temperatures upstream the DPF of about 200 – 400 °C) and ‘active’ regeneration (temperatures upstream the DPF of about 550 – 600 °C) (Liati et al., 2012). The regeneration of loaded DPF using microwaves belongs to the active systems of regeneration, because, depending on the filter load, the soot is heated up to the temperature for regeneration. The microwave regeneration differs from conventional systems in the point of heat introduction. In conventional systems the heat is transferred to the filter indirectly by heating the exhaust gas (Imenokhoyev, 2011). In the MW regeneration system the heat is coupled directly into the soot: the good dielectric properties of SiC, catalyst and soot, in combination with MW heating and catalytic combustion may DOI: 10.3303/CET1543340 Please cite this article as: Palma V., Meloni E., Ciambelli P., 2015, Optimal loading of a mw susceptible catalysed dpf, Chemical Engineering Transactions, 43, 2035-2040 DOI: 10.3303/CET1543340 DOI: 10.3303/CET1543340 Please cite this article as: Palma V., Meloni E., Ciambelli P., 2015, Optimal loading of a mw susceptible catalysed dpf, Chemical Engineering Transactions, 43, 2035-2040 DOI: 10.3303/CET1543340 2035 result in the effective oxidation of diesel soot at lower temperature and higher reaction rate (Palma et al., 2013). The results of our previous deposition and on-line regeneration tests on uncatalysed and Copper- Ferrite catalysed DPF, showed that the increase of catalyst load up to 30 %wt and the simultaneous use of MW during the regeneration step at lower gas flow rate, allows to reduce the energy supplied and the regeneration time (Palma et al., 2015). Starting by these very promising results, the objectives of this work are to modify the active species formulation, with the aim to verify the effect of K addition to our catalyst formulation, since Liu et al. (2003) observed that in the case of a DPF loaded with a K-doped copper ferrite (Cu0.95K0.05Fe2O4), the NOx presence in the exhaust stream had a positive effect on the catalytic activity. 2. Materials and methods In this work Cu0.95K0.05Fe2O4 catalysed DPFs with different %wt of active species are prepared; the prepared powder of Cu0.95K0.05Fe2O4 were characterized by X-Ray Diffraction (XRD) and TG-DTA analysis, while the catalysed DPFs were characterized by Scanning Electron Microscopy (SEM), Energy dispersive spectroscopy (EDAX), Hg porosimetry tests, H2-Temperature Programmed Reduction (TPR) measurements, N2 adsorption at 196 °C, applying BET method for the calculation of sample’s surface area, and catalytic activity tests. 2.1 Catalyst preparation The K-doped Copper Ferrite (Cu0.95K0.05Fe2O4) was prepared starting from iron nitrate, copper nitrate mixed in a 2:1 molar ratio, potassium nitrate, and distilled water, continuously stirred at 60 °C. The catalytic monoliths were prepared according to the previously optimized preparation procedure (Palma et al., 2013), by repeated impregnation phases in the prepared solution, drying at 60 °C and calcination at 1000 °C after each impregnation, in order to obtain a load of active species up to 30 %wt. 3. Results and discussion 3.1 Prepared samples The preparation procedure allowed to obtain an uniform and homogeneous distribution of the active species on the monolith walls and inside the porosity, avoiding the occurrence of the filter fractures shown in literature for the thermal shock of SiC monoliths (Blissett et al., 1997). Furthermore, XRD analysis showed the presence in our prepared Copper Ferrite of the typical peaks of CuFe2O4 in its tetragonal and cubic form (Palma et al., 2012), and the absence of mixed oxides peaks. 3.2 TG-DTA analysis The catalytic activity of differently loaded monoliths in powder, mixed with soot in a mortar, was evaluated by simultaneous TG-DTA analysis (SDT Q600 TA Instruments). Samples were heated in air (flow rate = 100 Ncm3 min-1, ) from 20 to 700 °C with a rate of 10 °C min-1. The results are reported in Figure 1, as Derivative Weight (in % min-1), referred to the total amount of soot in the sample, in function of Temperature. The curve of soot alone shows its typical behavior (ignition temperature of about 530 °C and maximum reaction rate at about 620 °C). The results relevant to soot mixed with the Cu0.95K0.05Fe2O4 loaded monoliths powder show that the ignition temperature is lowered to about 420 °C; it is important to highlight that the increase of the active species load, even if it doesn’t influence the ignition temperature, results in the increase of the catalytic combustion reaction rate. Figure 1: TG tests performed on soot alone, soot mixed with SiC DPF loaded with 15 %wt, 20 %wt, 25 %wt and 30 % wt of Cu0.95K0.05Fe2O4 2036 3.3 SEM-EDAX results Catalysed and uncatalysed samples have been investigated by Scanning Electron Microscopy (SEM), using a Scanning Electron Microscope (SEM mod. LEO 420 V2.04, ASSING), and Energy Dispersive X-ray Spectroscopy (EDX), perfomed in an Energy Dispersive X-Ray analyzer (EDX mod. INCA Energy 350, Oxford Instruments, Witney, UK): the results reported in our previous works (Palma et al., 2012) showed the very homogeneous distribution of the active species on the filter surface, and the comparison among the uncatalysed filter and the catalysed filter with various Cu0.95K0.05Fe2O4 loads showed that the active species cover all the SiC granules surface and that increasing their load, they don’t deposit inside the inner wall pores but only on the external surface, and so on another layer of catalyst: in this way there is only the decrease of the pore diameter, but not their occlusion, so allowing its use as catalytic filter. In Figure 2 is reported the SEM image and the elements distribution as obtained by EDX element mapping, for the 15 %wt catalysed filter: on the catalytic filter the encountered elements are, apart from C, O and Si (the structural elements of the filter), also Cu, K and Fe, the catalyst active species. These results confirm that with our catalytic filter preparation procedure, we can obtain the deposition of the active species on the support without any washcoat. O Fe Cu K Si Figure 2: SEM image and distribution of elements, as obtained by EDX element mapping, for the the 15 %wt Cu0.95K0.05Fe2O4 catalysed filter 3.4 Specific Surface Area The Specific Surface Area (SSA) of the filters has been obtained by means of the SORPTOMETER Kelvin 1040 Costech instrument, applying BET method for its calculation. The values are summarized in Table 1. Table 1: Specific Surface Area of uncatalytic and catalysed SiC monoliths Load of Cu0.95K0.05Fe2O4 BET Surface Area [m2/g] 0 % 0.3 15 % 1.2 20 % 1.3 25 % 1.6 30 % 2.3 The values in table 1 show that the growing catalyst load over the bare monoliths induces an increase of the BET specific surface areas; this is an expected result, since the deposition of the active species on a support characterized by low values of specific surface area, such as SiC, results in the increase of surface roughness, without plugging pores. 3.5 Hg porosimetry tests The porosimetric characteristics of the filters have been measured by the Hg penetration technique using the “PASCAL 140” and “PASCAL 240” Thermo Finnigan instruments: the results are reported in Table 2. 2037 Table 2: porosimetric characteristics of the catalysed and uncatalysed filters Median pore diameter (μm) Uncatalytic SiC DPF 17.0 Catalytic DPF with 15%wt of Cu0.95K0.05Fe2O4 15.7 Catalytic DPF with 25%wt of Cu0.95K0.05Fe2O4 14.9 Catalytic DPF with 30%wt of Cu0.95K0.05Fe2O4 11.0 From these data, it is evident that the increase of the active species load results in the median pores diameter decrease. In particular, by analyzing the changes of this value, is evident that the increase of the active species load doesn’t result in the occlusion of the pores, but only in their diameter decrease, from 17 μm to 11 μm. These results can be likely due to the behavior deposition of the catalyst that occurs inside the pores and on the walls of the DPF at the lower load, while only on the external surface at the higher catalyst load. 3.6 H2 TPR Analysis The H2-TPR analysis was carried out using a SiC monolith catalysed with 15 %wt and 30 %wt of Cu0.95K0.05Fe2O4 from room temperature to 900 °C at a heating rate of 5 °C min-1 in 5 % H2/N2 flow. The reaction parameters (temperature and concentrations) have been monitored by means of an HIDEN Analytical system, including a mass spectrometer. Figure 3: H2-TPR profiles of a SiC monolith catalysed with 15 %wt and 30 %wt of Cu0.95K0.05Fe2O4 Figure 3 shows the H2-TPR profiles: two pronounced reduction peaks were observed in the range 200 °C - 375 °C and 450 °C - 700 °C for the two samples; these peaks are attributed to the reduction of Cu0.95K0.05Fe2O4 to Cu and Fe3O4, and subsequently to Fe and K. As evident, the increase of the catalyst load, results in the shift of the peaks to higher temperature values. The total amounts of H2 consumed for Cu mole (H2/Cu ratio) were 5 and 4.35, for the 15 %wt and 30 %wt catalyst loaded monoliths, respectively. These values are consistent with that for the complete reduction of Cu0.95K0.05Fe2O4 to Cu and Fe. The overall reaction is: Cu0.95K0.05Fe2O4 + 4H2 → 0.95Cu +0.05K+2Fe + 4H2O (1) The values of 5 and 4.35 corresponds to about 18 %wt and 31 %wt of Cu0.95K0.05Fe2O4, that are in a quite good agreement with the estimated 15 %wt and 30 %wt of Cu0.95K0.05Fe2O4 on the monolith. As shown in literature (Kameoka et al., 2005), after the reduction, mixture of Cu and Fe is favorable for the formation of CuFe2O4 at about 800 °C in air, even if not all in the same crystalline form. So we can say that through redox process, the Cu0.95K0.05Fe2O4 completely reduces to Cu, Fe and K, and then returns to Cu0.95K0.05Fe2O4, even if not in the exactly same crystalline form, confirming that it is a very good oxidation catalyst. 3.7 Catalytic activity tests Preliminary activity tests were performed on two catalytic monoliths, loaded with 15 %wt and 30 %wt of catalyst, respectively. In order to verify the effect of K addition to our catalyst formulation, in terms of catalytic activity with and without NOx in the exhaust stream, as reported in literature (Liu et al., 2003) in the case of a DPF loaded with a K-doped Copper ferrite (Cu0.95K0.05Fe2O4), some activity tests were performed using a gas 2038 stream with the typical composition of a diesel engine exhaust. In particular some Temperature Programmed Oxidation (TPO) tests were performed using a SiC monolith catalysed with 30 %wt of Cu0.95K0.05Fe2O4 from room temperature to 800 °C at a heating rate of 5 °C min-1 in 5 % O2/N2 flow and then with the addition of about 550 ppm of NOx. These tests were performed shaping and entrapping the monoliths, on which the soot was previously deposited, into a tubular reactor. The reaction parameters (temperature and concentrations) have been monitored by means of an HIDEN Analytical system, including a mass spectrometer. The comparison of the performances of the samples with different catalyst load are reported in Figure 4. Figure 4:TPO profiles of a soot loaded SiC monolith catalysed with 15 %wt and 30 %wt of Cu0.95K0.05Fe2O4 The data reported in Figure 4 confirm the results obtained in the TG tests; in particular it is evident that the increase of the active species load doesn’t result in the decrease of the ignition temperature, that is of about 350 °C, so confirming also the good oxidation property of this formulation. The comparison of the performances of the samples in a stream with and without NOx, is reported in Figure 5. Figure 5: TPO profiles of a 30 %wt Cu0.95K0.05Fe2O4 loaded DPF in presence of a 5 % O2/N2 and with 550 ppm NOx flow Figure 5 shows that the presence of NOx has a positive result on the catalytic activity of the catalyst towards the soot oxidation, since NO2 is a better oxidant than O2, so allowing an ignition temperature decrease of about 80 °C, and a faster reaction rate. These very promising results will be verified in the future by performing activity tests directly at the exhaust of a diesel engine. It is important to note that the use of our proposed 2039 catalyst, doesn’t lead to an increment of manufacture cost of the filter, that is actually of about € 1000,00 for light duty diesel vehicles. 4. Conclusions In this work the performances of a K-doped copper ferrite (Cu0.95K0.05Fe2O4) catalysed SiC Wall Flow Filter was verified in terms of diesel soot oxidation, at various catalyst loads, even in presence of NOx. The analysis performed on the prepared samples showed that the increase in the load of active species up to 30 %wt resulted in increased reaction rate, in decreased median pore diameter, even if not in lower soot oxidation temperature. The SEM-EDX analysis evidenced the presence on the catalytic filter not only of C, O and Si (the structural elements of the filter), but also of Cu, K and Fe, the catalyst active species, homogeneously distributed on the support and in its porosities. The specific Surface Area analysis showed that the growing catalyst load over the bare monoliths induces an increase of the BET specific surface areas, as expected, since that with the deposition of active species on a support we increase surface roughness to the composite, without plugging pores. The H2-TPR measurements showed two pronounced reduction peaks attributed to the reduction of Cu0.95K0.05Fe2O4 to Cu, K and Fe in two steps. As shown in literature, after the reduction, mixture of Cu, K and Fe is favorable for the formation of Cu0.95K0.05Fe2O4 at high temperature (about 800 °C in air), even if not all in the same crystalline form. So we can say that through redox process, the phase transition from Cu0.95K0.05Fe2O4 to Cu, K and Fe, and finally return to Cu0.95K0.05Fe2O4 implying that Cu0.95K0.05Fe2O4 is a very good oxidation catalyst. The catalytic activity tests performed using a catalytic DPF with 15 % and 30 %wt catalyst load showed that the increase in the active species load resulted in a higher reaction rate, even if it has no effect on the ignition temperature (about 350 °C), so confirming the TG-DTA results. Furthermore the tests performed in presence of NOx evidenced the positive effect of NOx on the catalytic activity of the catalyst, allowing an ignition temperature decrease of about 80 °C. In the future we will evaluate the performances of a DPF loaded with this new catalyst formulation directly at the exhaust of a diesel engine. It is important to note that the use of our proposed catalyst, doesn’t lead to an increment of manufacture cost of the filter, that is actually of about € 1000,00 for light duty diesel vehicles. References Blissett M. J., Smith P. A., Yeomans J. A., 1997, Thermal shock behaviour of unidirectional silicon carbide fibre reinforced calcium aluminosilicate, Journal of Materials Science, 32, 317-325 Imenokhoyev I., Matthes A., Gutte H. and Walter G., 2011, Numerical 3D-FEM-simulation made by COMSOL Multiphysics of a microwave assisted cleaning system for a diesel sooty particle filter and its experimental validation, International COMSOL Conference, Stuttgart , Germany, October 26 – 28., 1-7 Kameoka S., Tanabe T., Tsai A.P., 2005, Spinel CuFe2O4: a precursor for copper catalyst with high thermal stability and activity; Catalysis Letters, 100, 1–2, 89-93. Liati A., Dimopoulos Eggenschwiler P., Müller Gubler E., Schreiber D., Aguirre M., 2012, Investigation of diesel ash particulate matter: A scanning electron microscope and transmission electron microscope study, Atmospheric Environment, 49, 391-402. Liu G., Huang Z., Shangguan W. and Yan C., 2003, Simultaneously catalytic removal of NOx and particulate matter on diesel particulate filter, Chinese Science Bulletin, 48(3), 305 – 308. Ohno K., 2008, New Technology with Porous Materials: Progress in the Development of the Diesel Vehicle Business, Journal of the Korean Ceramic Society, 45(9), 497 - 506. Palma V., Ciambelli P., Meloni E., 2011, Influence of operative parameters on microwave regeneration of catalytic soot wff for diesel engines, Chemical Engineering Transactions, 25, 1001 – 1006. Palma V., Ciambelli P., Meloni E., 2012, Optimising the catalyst load for Microwave susceptible catalysed DPF, Chemical Engineering Transactions, 29, 637 – 642. Palma V., Ciambelli P., Meloni E., Sin A., 2013, Optimal CuFe2O4 load for MW susceptible catalysed DPF, Chemical Engineering Transactions, 35, 727 – 732. Palma V., Ciambelli P., Meloni E., Sin A., 2015, Catalytic DPF microwave assisted active regeneration, Fuel, 140, 50 – 61. 2040