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Microwave assisted regeneration of a catalytic diesel soot trap
Fuel 181 (2016) 421–429
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Full Length Article
Microwave assisted regeneration of a catalytic diesel soot trap Vincenzo Palma, Eugenio Meloni ⇑ University of Salerno, Department of Industrial Engineering, Via Giovanni Paolo II, 132, 84084 Fisciano, SA, Italy
h i g h l i g h t s Improvement of an innovative catalytic diesel soot trap. The catalytic soot trap in SiC is able to absorb microwaves and burn soot at once. The catalytic soot trap is totally cleaned by the MW assisted regeneration. The combination of microwaves and catalytic soot trap allows a higher energy saving. The MW assisted DPF regeneration is independent of the engine operating conditions.
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Article history: Received 14 September 2015 Received in revised form 14 January 2016 Accepted 5 May 2016 Available online 10 May 2016 Keywords: Soot removal Diesel particulate filters Microwave regeneration Catalysis Catalytic diesel soot trap
a b s t r a c t Diesel engines have low fuel consumption and enough torque compared with equivalent gasoline engines, so the registration of new diesel cars is increasing in EU year by year. On the contrary, diesel engines exhaust contains large amount of hazardous substances, such as soot and NOx. Because of increasingly stringent emission regulations, various filters are commonly used for soot abatement in diesel exhaust, among which diesel particulate filter (DPF) is one of the most important. It consists of a bundle of small axial parallel channels, which are of small and, typically, square cross-section. Adjacent channels are alternatively plugged at each end, so that the gas enters into the monolith through the open channels in the inlet monolith cross-section (inlet channels) and is 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 and increasing the pressure loss. So a periodic regeneration is necessary, 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 ignition temperature, the energy and the time required for the filter regeneration. Starting by these very promising results, the objective of this work is to modify the active species formulation in order to simultaneously further reduce the ignition temperature and keep low the pressure drop. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The diesel internal combustion engine is one of the most environmentally-friendly vehicle devices, because it emits less carbon dioxide and is more fuel efficient than the universal gasoline stoichiometric engine. However, considerable challenges still exist regarding the emission control of particulate matter (PM), commonly known as soot, and NOx emitted by the diesel combustion process [1]. Since the reduction of both NOx and PM to the admitted level cannot be accomplished by engine modifications alone, after treatment processes for the reduction of diesel emissions should be developed [1]. Currently, PM emissions are generally ⇑ Corresponding author. E-mail address: [email protected] (E. Meloni). http://dx.doi.org/10.1016/j.fuel.2016.05.016 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
controlled through the use of a diesel particulate filter (DPF), that consists of a silicon carbide (SiC) structure in the Wall Flow configuration, characterized by alternately plugged parallel square channels [2]. As a consequence, the exhaust gases entering in the monolith are forced to flow through the porous channel walls that act as filters; in this way high particulate trapping efficiency (>95%) can be achieved, as reported in our previous work [3]. As the particulates are accumulated in the pores of the filter, the pressure drop increases, and, after some time, burning of soot particles inside the filter is necessary. Soot burns at 550–600 °C with oxygen, while the diesel exhaust temperature is only 200–400 °C, therefore to force the soot burning two different systems are used: the first ones are continuous systems lowering the temperature of the soot oxidation using a catalyst [4], the second ones are active systems boosting the soot temperature up to the ignition
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temperature [5]. The microwave assisted regeneration of a loaded DPF is an active system of regeneration, because, depending on the filter load, the soot is heated up to its oxidation temperature; furthermore it differs from conventional systems in the point of heat introduction, since differently from conventional systems in which the heat is transferred to the filter indirectly by heating the exhaust gas [6], in the microwave regeneration system the heat is coupled directly into the soot. In this way combining the good dielectric properties of SiC, catalyst and soot, with MW heating and catalytic combustion, the effective oxidation of diesel soot at lower temperature and higher reaction rate may be reached [7]. In this way the independence of the MW assisted regeneration technology by the operating conditions of the engine, and the consequent energy saving in the regeneration step due to the combination of this technology with the catalytic DPF, could confirm the ability of our proposed technology as a potential alternative to actually employed technologies (such as fuel post-injection) in the DPF regeneration phase. The comparison of the performances of not catalysed and Copper-Ferrite loaded DPFs during the regeneration step, reported in our previous works, showed that the increase of catalyst load up to 30 wt.% and the simultaneous use of microwaves at lower gas flow rate, allows to reduce the energy supplied and the regeneration time than that required for the not catalysed filter [8]. 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: in particular the aim of this work is to verify the effect of K addition to our catalyst formulation, since Liu et al. [8] 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.
were then wrapped in an expanding intumescent ceramic-mat (Interam by 3M) which expands with heat and enclosed in a stainless steel wave guide. Further geometrical characteristics of the filters are reported in Table 1.
2. Materials and methods
The thermal surface pre-treatment of the bare monoliths allowed the coating of the SiC particles with SiO2 streaks, that can greatly help the adherence of the active species to the filter, even in the absence of a washcoat [9]. Furthermore the optimized preparation procedure allowed to avoid the occurrence of the filter fractures shown in literature for the thermal shock of SiC monoliths [12].
In this work Cu0.95K0.05Fe2O4 catalysed DPFs with different loads were prepared and characterized by several techniques, as described below. 2.1. Catalyst and Wall Flow Filters preparation The catalyst (Cu0.95K0.05Fe2O4) was prepared starting from the precursors salts properly mixed with the right molar ratio, and distilled water, continuously stirred at 60 °C. Silicon Carbide (SiC) Wall Flow monoliths (Pirelli Ecotechnology, 150 cpsi) were selected as supports for the preparation of the catalytic filters. The catalytic monoliths were prepared by repeated impregnation phases in the precursors solution, drying at 60 °C and calcination at 1000 °C after each impregnation, following the previously optimized preparation procedure [9], so obtaining various loads of active species up to 30 wt.%. The prepared filters, previously opportunely conformed in rectangular shape, showed in Fig. 1, and calcined at 1000 °C for 48 h,
a
c b
Fig. 1. Geometrical features of the 150 cpsi Pirelli Ecotechnology SiC Wall Flow Filter.
2.2. Textural and morphological characterization The prepared powders 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-TPR measurements, N2 adsorption at 196 °C, applying BET method for the calculation of sample’s specific surface area (SSA). In addition the adherence of the catalyst to the filter was evaluated measuring the weight loss caused by exposing the monoliths to ultrasound, according to the following experimental procedure [10]: a beaker containing the samples immersed in petroleum ether (Carlo Erba), was placed in an ultrasonic bath CP104 (EIA SpA) filled with distilled water, at a temperature of 25 °C, operating at 60% of rated power, for regular intervals of 5 min. Before the test, compressed air was blown through the monoliths in order to remove any possible residue. The weight changes were recorded after monoliths drying at 120 °C and cooling up to room temperature at the end of any interval. Furthermore catalytic activity tests in oxidizing atmosphere and some preliminary tests of soot deposition and on line MW assisted regeneration of catalytic DPFs were performed by means of our diesel emission control laboratory plant [9]. 3. Results and discussion
3.1. X-ray diffraction analysis In order to verify the formation of the desired active species, the catalyst powder was analysed by X-ray Diffraction (XRD), performed with a micro diffractometer Rigaku D-max-RAPID, using Cu Ka radiation. XRD analysis showed the presence in our prepared catalyst of the typical peaks of CuFe2O4 in its tetragonal and cubic form [2], and the absence of mixed oxides peaks. 3.2. TG-DTA analysis The catalytic activity of the prepared catalysts was evaluated starting from powders, by simultaneous TG-DTA analysis (SDT Q600 TA Instruments) of soot mixed in a mortar with milled catalysed monolith samples at different active species load. The results were compared with the same analysis performed on soot alone.
Table 1 Geometrical characteristics of the filters. Total channels
Open channels
Channel length (L) [mm]
Filter wall thickness [mm]
a [mm]
b [mm]
c [mm]
585
277
1.5
0.6
36
80
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3.3. SEM–EDAX results
Fig. 2. TG tests performed on soot alone, soot mixed with SiC monolith loaded with 15 wt.%, 20 wt.%, 25 wt.% and 30 wt.% of Cu0.95K0.05Fe2O4.
All the samples were heated in air (flow rate = 100 N cm3 min 1) from 20 to 700 °C with a heating rate of 10 °C min 1. The results are reported in Fig. 2, as Derivative Weight (in % min 1), referred to the total amount of soot in the sample, in function of Temperature. While the TG curve of soot alone showed its typical behaviour (ignition temperature of about 530 °C and maximum reaction rate at about 620 °C), the results relevant to the TG of soot mixed with the powder of the catalysed samples showed that the presence of the catalyst allowed the decrease of the soot ignition temperature to about 420 °C. From Fig. 2 we can observe that the increase of the active species load results in the increase of the catalytic combustion reaction rate, even if it has no effect on the ignition temperature.
Catalysed and not catalysed samples were 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), performed in an Energy Dispersive X-ray analyser (EDX mod. INCA Energy 350, Oxford Instruments, Witney, UK): the good results reported in our previous works [2] for the Copper Ferrite loaded monoliths, were confirmed also after the K addition to our formulation, as evident in Fig. 3. The comparison of the different samples showed in Fig. 3 highlighted that the catalyst creates a very homogeneous coating on the SiC granules and that the increase of the catalyst load doesn’t result in the occlusion of the inner wall pores but only in the decrease of their diameter, since the catalyst deposit on the external surface of the granules, and then on another layer of catalyst: in this way the monolith can be used as catalytic filter. Fig. 4, in particular, evidences the good adherence and the tight contact between the active species and the SiC granules; furthermore we can see that the active species granules have an average diameter lower than 1 lm and that the average thickness of the catalyst layer is of about 3 lm. From Fig. 4 we can see more clearly the coating of the SiC granules surface with the catalyst and that it doesn’t deposit inside the inner wall pores but only on the external surface, and so on another layer of Copper Ferrite. In Fig. 5 the SEM image and the elements distribution as obtained by EDX element mapping are reported, for the 30 wt.% catalysed monolith: the encountered elements are the structural elements of the filter (C, O and Si) and the catalyst active species (Cu, K and Fe). An accurate analysis of Fig. 5 evidenced the almost complete coating of the SiC granules with the active species without any washcoat, since the signals relevant to the structural elements (in particular Carbon) are very low. 3.4. Catalyst adherence testing The results of the tests are reported in Fig. 6, in terms of weight loss (%) vs number of cycles.
Fig. 3. SEM images of a not catalysed monolith (a), a 15 wt.% of Cu0.95K0.05Fe2O4 (b) and 30 wt.% of Cu0.95K0.05Fe2O4 (c) catalysed monolith.
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The above reported results, characterized by weight losses lower than that reported in literature for washcoated supports [11], demonstrate the good adhesion of the active species on the SiC granules even in absence of a washcoat. This is due to the preliminary thermal treatment of the bare supports (calcination at 1000 °C for 48 h) that results in a growing coating of the SiC particles with SiO2 streaks, that can greatly help the adherence of the active species to the filter, even in the absence of a washcoat [9].
3.5. Specific surface area
Fig. 4. SEM images of a 30 wt.% of Cu0.95K0.05Fe2O4 catalysed monolith.
The SSA of the filters and of the Cu0.95K0.05Fe2O4 powder, obtained by means of the SORPTOMETER Kelvin 1040 Costech instrument, applying BET method for its calculation, are reported in Table 2. The values in Table 2 show that the increase of the catalyst load over the bare monoliths results in the increase of the BET specific surface area; this is an expected consequence, since the deposition
Fig. 5. SEM image and distribution of elements, as obtained by EDX element mapping, for the 30 wt.% Cu0.95K0.05Fe2O4 catalysed filter.
Fig. 6. Ultrasonic tests performed on 18 wt.%, 24 wt.% and 30 wt.% catalyst loaded monoliths. A beaker containing the samples immersed in petroleum ether (Carlo Erba), was placed in an ultrasonic bath CP104 (EIA SpA) filled with distilled water, at a temperature of 25 °C, operating at 60% of rated power, for regular intervals of 5 min. The weight changes were recorded after monoliths drying at 120 °C and cooling up to room temperature at the end of any interval.
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3.7. H2-TPR analysis
Table 2 Specific surface area of not catalysed and catalysed SiC monoliths. Sample
BET surface area [m2/g]
Powder of Cu0.95K0.05Fe2O4 Bare SiC monolith Catalytic monolith with 15 wt.% of Cu0.95K0.05Fe2O4 Catalytic monolith with 20 wt.% of Cu0.95K0.05Fe2O4 Catalytic monolith with 25 wt.% of Cu0.95K0.05Fe2O4 Catalytic monolith with 30 wt.% of Cu0.95K0.05Fe2O4
1.35 0.30 1.20 1.30 1.60 2.30
Table 3 Porosimetric characteristics of the catalytic and non-catalytic filters. Median pore diameter (lm) Non-catalytic SiC monolith Catalytic monolith with 15 wt.% Catalytic monolith with 20 wt.% Catalytic monolith with 25 wt.% Catalytic monolith with 30 wt.%
of of of of
Cu0.95K0.05Fe2O4 Cu0.95K0.05Fe2O4 Cu0.95K0.05Fe2O4 Cu0.95K0.05Fe2O4
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17.0 16.0 15.5 15.0 11.0
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.
3.6. Hg porosimetry tests The porosimetric characteristics of the filters were measured by means of Hg penetration technique using the ‘‘PASCAL 140” and ‘‘PASCAL 240” Thermo Finnigan instruments and are reported in Table 3. The data of Table 3 highlighted that the increase of the active species load results in the pores diameter decrease from 17 lm to 11 lm, and not in their occlusion. These results could be likely due to a behaviour deposition that seems to depend from the catalyst load. SEM images showed in a previous paragraph evidenced that the deposition of the active species mainly occurs inside the pores and then on the walls of the monolith at the lower load, while it mainly occurs on the external surface and then inside the pores when the load of active species is higher than 15 wt.%.
The H2-TPR analysis was carried out heating in 5% H2/N2 flow two SiC monoliths, with a load of 15 wt.% and 30 wt.% of Cu0.95K0.05Fe2O4, from room temperature to 900 °C at a heating rate of 5 °C min 1. The reaction parameters (temperature and concentrations) have been monitored by means of an HIDEN Analytical system, including a mass spectrometer. Fig. 7 shows the H2-TPR profiles: two pronounced reduction peaks were observed in the range 200–375 °C and 450–700 °C for the two samples; as evident, the increase of the catalyst load results in the shift of the peaks to higher temperature values. The comparison between the results showed in Fig. 6 and the H2-TPR tests performed on CuFe2O4 loaded monoliths [9], evidenced that isn’t possible to clearly identify the reduction of potassium, that at these experimental conditions is very difficult to reduce. About the discussion of the TPR profile, on the base of the reduction peaks of copper ferrite based catalysts showed by Tasca et al. [13], we can propose the following steps: the first one, appearing in the temperature range 200–375 °C was assigned to the reduction of CuFe2O4 to metallic Cu and Fe2O3, and to the subsequent reduction of Fe2O3-hematite to Fe3O4-magnetite, while the second one, appearing at higher temperature, is attributed to the reduction of Fe3O4 to FeO, followed by the reduction to metallic Fe. The hydrogen consumed for Cu mole (H2/Cu), calculated for the samples with a nominal catalyst load of 15 wt.% and 30 wt.% was of 5.28 and 4.35 respectively, corresponding to real load of Cu0.95K0.05Fe2O4 of about 19 wt.% and 31 wt.%. As shown in literature in the case of copper ferrite [14], after the reduction, also mixture of Cu, K and Fe is favourable for the formation of K doped CuFe2O4 at about 800 °C in air, even if not all in the same crystalline form, so confirming its redox behaviour and its use as oxidation catalyst.
3.8. Catalytic activity tests The performances of our catalytic monoliths towards soot oxidation after K addition to our previous formulation were evaluated by means of Temperature Programmed Oxidation (TPO) tests. These tests were performed heating two catalytic monoliths loaded with 15 wt.% and 30 wt.% of catalyst from room temperature to 800 °C at a heating rate of 5 °C min 1 in 5% O2/N2 flow with
Fig. 7. H2-TPR profiles of a SiC monolith catalysed with 15 wt.% and 30 wt.% of Cu0.95K0.05Fe2O4.
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Fig. 8. TPO profiles of a soot loaded SiC monolith catalysed with 15 wt.% and 30 wt.% of Cu0.95K0.05Fe2O4 (test without NOx).
Fig. 9. TPO profiles of a 30 wt.% Cu0.95K0.05Fe2O4 loaded monolith in presence of 5% O2/N2 flow (dot line) and 5% O2/N2 with 550 ppm NOx flow (bold line).
and without the addition of about 550 ppm of NOx (typical composition of a diesel engine exhaust). The monoliths, on which was previously deposited the soot (about 5 g l 1), were opportunely shaped and entrapped in a tubular reactor. The temperature and CO2 concentration were monitored by means of an HIDEN Analytical system, including a mass spectrometer. The results of the tests performed without NOx are reported in Fig. 8. The data reported in Fig. 8 confirm the results obtained in the TG tests, evidencing the good oxidation property of this formulation, with an ignition temperature of about 350 °C, and that the
increase of the catalyst load doesn’t result in the decrease of this value. The comparison of the performances of the samples in a stream with and without NOx, is reported in Fig. 9. The TPO profiles reported in Fig. 9 showed the ignition temperature decrease of about 80 °C and the faster reaction rate due to NOx presence in the gas stream: these results confirmed the positive effect of NOx on the catalytic activity of the catalyst towards the soot oxidation, since the coexistence of NO and O2 in the gas phase improved the catalytic activity for PM oxidation, probably because NO2 is a better oxidant than O2.
V. Palma, E. Meloni / Fuel 181 (2016) 421–429 Table 4 Engine exhaust stream composition. Component
Average concentration
CO CO2 NO NO2 O2 PM SO2
0.02 vol.% 6.00 vol.% 70.00 ppm 4.00 ppm 12.00 vol.% 45.00 mg/m3 <1.00 ppm
3.9. Deposition and on-line MW assisted regeneration tests Starting by the very promising results reported in the previous paragraph, some preliminary tests of soot deposition and on line MW assisted regeneration of catalytic DPFs were performed by
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means of our diesel emission control laboratory plant [9]. As a first step, the composition of the engine exhaust stream was characterized by means of the on-line gas analysis system, with which we are able to continuously monitor CO, CO2, NO, NO2, SO2 and O2, and to directly acquire the data by a specifically set-up acquisition system in Lab-View Ambient. The results are reported in Table 4. All the deposition tests were performed at the operating engine conditions of 920 rpm and Poil = 30 bar, with a fixed flow rate into the filter of about 110 l min 1, with the exhaust gas temperature of 200 °C, and with a soot concentration in the exhaust gas of about 45 mg (m3) 1. The deposition and MW assisted regeneration phases were performed following the previously optimized procedure [9]. The behaviour of the pressure drop through the filter (DP) and the temperature profile during the last 15 min of the deposi-
Fig. 10. DP and temperature profiles during the last 15 min of the deposition phase and the complete MW assisted regeneration phase of 20 wt.% loaded DPFs with Cu0.95K0.05Fe2O4, performed with the microwave generator set at 50% of its nominal power and with the exhaust gas flow rate of 30 l min 1.
Fig. 11. CO, CO2 and NO2 profiles during the regeneration phase of the 20 wt.% loaded DPFs with Cu0.95K0.05Fe2O4, performed with the microwave generator set at 50% of its nominal power and with the exhaust gas flow rate of 30 l min 1.
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Fig. 12. DP/DP0 and temperature profiles during the regeneration phase of 20 wt.% loaded DPFs with CuFe2O4 and Cu0.95K0.05Fe2O4, performed with the microwave generator set at 50% of nominal power and with the exhaust gas flow rate of 30 l min 1.
tion phase and the complete MW assisted regeneration phase of a 20 wt.% Cu0.95K0.05Fe2O4 loaded DPF is reported in Fig. 10. As evident from Fig. 10, the MW application results in the contemporary increase of the outlet gas temperature and of the slope of DP-time curve; in particular it is possible to emphasize that when the temperature reaches the value of about 300 °C (the catalyst threshold temperature), the DP curve shows a plateau indicating that the catalytic soot combustion rate is comparable to the soot deposition rate. Furthermore, we can observe that when the temperature increases, the DP curve decreases, so indicating that the filter regeneration occurs. These results showed that also after the K addition to our formulation, the system composed by SiC filter, active species and soot is able to absorb microwaves and to completely regenerate in about 20 min. During the test the opacity of the exhaust stream at the filter inlet and outlet have the mean values of 22% and 0.2% respectively, with an average filtration efficiency of about 99.0%. In Fig. 11 the data related to the exhaust gas composition in terms of CO, CO2 and NO2 as functions of time during the regeneration phase are reported. From Fig. 11 is evident that during the regeneration phase, when the exhaust gas stream convoyed into the filter reach the soot ignition temperature (about 300 °C), the soot starts to burn: so we have the oxygen consumption and the CO and CO2 formation, showed by their upper peaks. An interesting result is the contemporary consumption of NO2 (lower peak), that probably is due to the oxygen donation capacity of NO2 during the oxidation of soot. In Fig. 12 the comparison of catalytic activity tests performed on a 20 wt.% K-doped Copper Ferrite and 20 wt.% copper ferrite loaded DPF at the exhaust of a diesel engine, regenerated by means of microwaves, is shown. It is evident from Fig. 12 that DP and temperature profiles are practically specular; in particular the temperatures reach their maximum value around about 750 °C, corresponding to the maximum combustion rate of accumulated soot. Furthermore Fig. 12 shows that the K addition to copper ferrite allows to decrease the temperature ignition from 400 °C to about 300 °C: the comparison of the MW energy supplied for DPF litres during the regeneration phase (about 4500 kJ and 3400 kJ for the 20 wt.% CuFe2O4 loaded and 20 wt.% Cu0.95K0.05Fe2O4 loaded filters, respectively),
indicates that the K-addition results in a higher catalytic activity and a further lower energy consumption during the regeneration phase. 4. Conclusions In this work, K-doped copper ferrite (Cu0.95K0.05Fe2O4) loaded SiC Wall Flow Filters (with various catalyst loads) were prepared and their performances towards diesel soot oxidation were verified, even in presence of NOx. The preliminary characterization tests performed on the powders of the prepared samples, showed the presence of the typical peaks of CuFe2O4 in its tetragonal and cubic form and the absence of mixed oxides peaks, and that the increase in the load of active species up to 30 wt.% resulted in increased reaction rate, even if not in lower soot oxidation temperature. As a consequence the characterization tests of the catalytic monoliths were performed. The Hg porosimetry tests and the specific surface area analysis showed that the increase of the catalyst load results in decreased median pores diameter (but not in their complete occlusion), and in increased BET specific surface areas. The SEM–EDX analysis evidenced the presence on the catalytic filter not only of the structural elements of the filter (C, O and Si), but also of the catalyst active species (Cu, K and Fe), homogeneously distributed on the support and inside the pores. The H2-TPR measurements showed two pronounced reduction peaks attributed to the reduction of Cu0.95K0.05Fe2O4 to Cu and Fe in two steps, while it isn’t possible to clearly identify the reduction of potassium, that is very difficult in the reduction conditions used in the experimental tests. As shown in literature in the case of copper ferrite, after the reduction, also mixture of Cu, K and Fe is favourable 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, Cu0.95K0.05Fe2O4 is a very good oxidation catalyst. The preliminary catalytic activity tests (TPO tests) with and without NOx performed using catalytic DPFs with 15 wt.% and 30 wt.% catalyst load, on which soot was previously deposited, showed that the growing catalyst load resulted in a higher reaction rate, even if it seems to have no effect on the ignition temperature (about 350 °C), and that the presence of NOx had a positive effect on the catalytic activity, allowing an ignition temperature decrease of about 80 °C. The tests performed
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directly at the exhaust of a diesel engine showed that the K addition to copper ferrite allowed to decrease the temperature ignition from 400 °C to about 300 °C, so obtaining a further energy saving during the regeneration phase. In conclusion the independence of the MW assisted regeneration technology by the operating conditions of the engine, and the energy saving in the regeneration step due to the combination of this technology with the Cu0.95K0.05Fe2O4 loaded DPF, confirm the ability of our proposed technology as a potential alternative to actually employed technologies (such as fuel post-injection) in the DPF regeneration phase. References [1] Lee C, Park Joo-Il, Shul Y-G, Einaga H, Teraoka Y. Appl Catal B 2015;174175:185–92. [2] Palma V, Ciambelli P, Meloni E. Chem Eng Trans 2012;29:637–42.
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[3] Palma V, Ciambelli P, Meloni E. Chem Eng Trans 2011;25:1001–6. [4] Vernikovskaya NV, Pavlova TL, Mokrinskii VV, Murzin DYu, Chumakova NA, Noskov AS. Chem Eng J 2015;269:416–24. [5] Bermúdez V, Serrano JR, Piqueras P, García-Afonso O. Appl Energy 2015;140:234–45. [6] Imenokhoyev I, Matthes A, Gutte H, Walter G. Numerical 3D-FEM-simulation made by COMSOL Multiphysics of a microwave assisted cleaning system for a diesel sooty particle filter and its experimental validation. In: International COMSOL conference. Ludwigsburg 2011, October 26–28. [7] Palma V, Ciambelli P, Meloni E, Sin A. Catal Today 2013;216:185–93. [8] Liu G, Huang Z, Shangguan W, Yan C. Chin Sci Bull 2003;48:305–8. [9] Palma V, Ciambelli P, Meloni E, Sin A. Fuel 2015;140:50–61. [10] Palma V, Ciambelli P, Meloni E, Sin A. Chem Eng Trans 2013;35:727–32. [11] Giani L, Cristiani C, Groppi G, Tronconi E. Appl Catal B 2006;62:121–31. [12] Blissett MJ, Smith PA, Yeomans JA. J Mater Sci 1997;32:317–25. [13] Tasca JE, Quincoces CE, Lavat A, Alvarez AM, Gonzalez MG. Ceram Int 2011;37:803–12. [14] Kameoka S, Tanabe T, Tsai AP. Catal Lett 2005;100(1–2):89–93.
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Full Length Article
Microwave assisted regeneration of a catalytic diesel soot trap Vincenzo Palma, Eugenio Meloni ⇑ University of Salerno, Department of Industrial Engineering, Via Giovanni Paolo II, 132, 84084 Fisciano, SA, Italy
h i g h l i g h t s Improvement of an innovative catalytic diesel soot trap. The catalytic soot trap in SiC is able to absorb microwaves and burn soot at once. The catalytic soot trap is totally cleaned by the MW assisted regeneration. The combination of microwaves and catalytic soot trap allows a higher energy saving. The MW assisted DPF regeneration is independent of the engine operating conditions.
a r t i c l e
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Article history: Received 14 September 2015 Received in revised form 14 January 2016 Accepted 5 May 2016 Available online 10 May 2016 Keywords: Soot removal Diesel particulate filters Microwave regeneration Catalysis Catalytic diesel soot trap
a b s t r a c t Diesel engines have low fuel consumption and enough torque compared with equivalent gasoline engines, so the registration of new diesel cars is increasing in EU year by year. On the contrary, diesel engines exhaust contains large amount of hazardous substances, such as soot and NOx. Because of increasingly stringent emission regulations, various filters are commonly used for soot abatement in diesel exhaust, among which diesel particulate filter (DPF) is one of the most important. It consists of a bundle of small axial parallel channels, which are of small and, typically, square cross-section. Adjacent channels are alternatively plugged at each end, so that the gas enters into the monolith through the open channels in the inlet monolith cross-section (inlet channels) and is 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 and increasing the pressure loss. So a periodic regeneration is necessary, 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 ignition temperature, the energy and the time required for the filter regeneration. Starting by these very promising results, the objective of this work is to modify the active species formulation in order to simultaneously further reduce the ignition temperature and keep low the pressure drop. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The diesel internal combustion engine is one of the most environmentally-friendly vehicle devices, because it emits less carbon dioxide and is more fuel efficient than the universal gasoline stoichiometric engine. However, considerable challenges still exist regarding the emission control of particulate matter (PM), commonly known as soot, and NOx emitted by the diesel combustion process [1]. Since the reduction of both NOx and PM to the admitted level cannot be accomplished by engine modifications alone, after treatment processes for the reduction of diesel emissions should be developed [1]. Currently, PM emissions are generally ⇑ Corresponding author. E-mail address: [email protected] (E. Meloni). http://dx.doi.org/10.1016/j.fuel.2016.05.016 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
controlled through the use of a diesel particulate filter (DPF), that consists of a silicon carbide (SiC) structure in the Wall Flow configuration, characterized by alternately plugged parallel square channels [2]. As a consequence, the exhaust gases entering in the monolith are forced to flow through the porous channel walls that act as filters; in this way high particulate trapping efficiency (>95%) can be achieved, as reported in our previous work [3]. As the particulates are accumulated in the pores of the filter, the pressure drop increases, and, after some time, burning of soot particles inside the filter is necessary. Soot burns at 550–600 °C with oxygen, while the diesel exhaust temperature is only 200–400 °C, therefore to force the soot burning two different systems are used: the first ones are continuous systems lowering the temperature of the soot oxidation using a catalyst [4], the second ones are active systems boosting the soot temperature up to the ignition
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temperature [5]. The microwave assisted regeneration of a loaded DPF is an active system of regeneration, because, depending on the filter load, the soot is heated up to its oxidation temperature; furthermore it differs from conventional systems in the point of heat introduction, since differently from conventional systems in which the heat is transferred to the filter indirectly by heating the exhaust gas [6], in the microwave regeneration system the heat is coupled directly into the soot. In this way combining the good dielectric properties of SiC, catalyst and soot, with MW heating and catalytic combustion, the effective oxidation of diesel soot at lower temperature and higher reaction rate may be reached [7]. In this way the independence of the MW assisted regeneration technology by the operating conditions of the engine, and the consequent energy saving in the regeneration step due to the combination of this technology with the catalytic DPF, could confirm the ability of our proposed technology as a potential alternative to actually employed technologies (such as fuel post-injection) in the DPF regeneration phase. The comparison of the performances of not catalysed and Copper-Ferrite loaded DPFs during the regeneration step, reported in our previous works, showed that the increase of catalyst load up to 30 wt.% and the simultaneous use of microwaves at lower gas flow rate, allows to reduce the energy supplied and the regeneration time than that required for the not catalysed filter [8]. 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: in particular the aim of this work is to verify the effect of K addition to our catalyst formulation, since Liu et al. [8] 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.
were then wrapped in an expanding intumescent ceramic-mat (Interam by 3M) which expands with heat and enclosed in a stainless steel wave guide. Further geometrical characteristics of the filters are reported in Table 1.
2. Materials and methods
The thermal surface pre-treatment of the bare monoliths allowed the coating of the SiC particles with SiO2 streaks, that can greatly help the adherence of the active species to the filter, even in the absence of a washcoat [9]. Furthermore the optimized preparation procedure allowed to avoid the occurrence of the filter fractures shown in literature for the thermal shock of SiC monoliths [12].
In this work Cu0.95K0.05Fe2O4 catalysed DPFs with different loads were prepared and characterized by several techniques, as described below. 2.1. Catalyst and Wall Flow Filters preparation The catalyst (Cu0.95K0.05Fe2O4) was prepared starting from the precursors salts properly mixed with the right molar ratio, and distilled water, continuously stirred at 60 °C. Silicon Carbide (SiC) Wall Flow monoliths (Pirelli Ecotechnology, 150 cpsi) were selected as supports for the preparation of the catalytic filters. The catalytic monoliths were prepared by repeated impregnation phases in the precursors solution, drying at 60 °C and calcination at 1000 °C after each impregnation, following the previously optimized preparation procedure [9], so obtaining various loads of active species up to 30 wt.%. The prepared filters, previously opportunely conformed in rectangular shape, showed in Fig. 1, and calcined at 1000 °C for 48 h,
a
c b
Fig. 1. Geometrical features of the 150 cpsi Pirelli Ecotechnology SiC Wall Flow Filter.
2.2. Textural and morphological characterization The prepared powders 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-TPR measurements, N2 adsorption at 196 °C, applying BET method for the calculation of sample’s specific surface area (SSA). In addition the adherence of the catalyst to the filter was evaluated measuring the weight loss caused by exposing the monoliths to ultrasound, according to the following experimental procedure [10]: a beaker containing the samples immersed in petroleum ether (Carlo Erba), was placed in an ultrasonic bath CP104 (EIA SpA) filled with distilled water, at a temperature of 25 °C, operating at 60% of rated power, for regular intervals of 5 min. Before the test, compressed air was blown through the monoliths in order to remove any possible residue. The weight changes were recorded after monoliths drying at 120 °C and cooling up to room temperature at the end of any interval. Furthermore catalytic activity tests in oxidizing atmosphere and some preliminary tests of soot deposition and on line MW assisted regeneration of catalytic DPFs were performed by means of our diesel emission control laboratory plant [9]. 3. Results and discussion
3.1. X-ray diffraction analysis In order to verify the formation of the desired active species, the catalyst powder was analysed by X-ray Diffraction (XRD), performed with a micro diffractometer Rigaku D-max-RAPID, using Cu Ka radiation. XRD analysis showed the presence in our prepared catalyst of the typical peaks of CuFe2O4 in its tetragonal and cubic form [2], and the absence of mixed oxides peaks. 3.2. TG-DTA analysis The catalytic activity of the prepared catalysts was evaluated starting from powders, by simultaneous TG-DTA analysis (SDT Q600 TA Instruments) of soot mixed in a mortar with milled catalysed monolith samples at different active species load. The results were compared with the same analysis performed on soot alone.
Table 1 Geometrical characteristics of the filters. Total channels
Open channels
Channel length (L) [mm]
Filter wall thickness [mm]
a [mm]
b [mm]
c [mm]
585
277
1.5
0.6
36
80
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3.3. SEM–EDAX results
Fig. 2. TG tests performed on soot alone, soot mixed with SiC monolith loaded with 15 wt.%, 20 wt.%, 25 wt.% and 30 wt.% of Cu0.95K0.05Fe2O4.
All the samples were heated in air (flow rate = 100 N cm3 min 1) from 20 to 700 °C with a heating rate of 10 °C min 1. The results are reported in Fig. 2, as Derivative Weight (in % min 1), referred to the total amount of soot in the sample, in function of Temperature. While the TG curve of soot alone showed its typical behaviour (ignition temperature of about 530 °C and maximum reaction rate at about 620 °C), the results relevant to the TG of soot mixed with the powder of the catalysed samples showed that the presence of the catalyst allowed the decrease of the soot ignition temperature to about 420 °C. From Fig. 2 we can observe that the increase of the active species load results in the increase of the catalytic combustion reaction rate, even if it has no effect on the ignition temperature.
Catalysed and not catalysed samples were 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), performed in an Energy Dispersive X-ray analyser (EDX mod. INCA Energy 350, Oxford Instruments, Witney, UK): the good results reported in our previous works [2] for the Copper Ferrite loaded monoliths, were confirmed also after the K addition to our formulation, as evident in Fig. 3. The comparison of the different samples showed in Fig. 3 highlighted that the catalyst creates a very homogeneous coating on the SiC granules and that the increase of the catalyst load doesn’t result in the occlusion of the inner wall pores but only in the decrease of their diameter, since the catalyst deposit on the external surface of the granules, and then on another layer of catalyst: in this way the monolith can be used as catalytic filter. Fig. 4, in particular, evidences the good adherence and the tight contact between the active species and the SiC granules; furthermore we can see that the active species granules have an average diameter lower than 1 lm and that the average thickness of the catalyst layer is of about 3 lm. From Fig. 4 we can see more clearly the coating of the SiC granules surface with the catalyst and that it doesn’t deposit inside the inner wall pores but only on the external surface, and so on another layer of Copper Ferrite. In Fig. 5 the SEM image and the elements distribution as obtained by EDX element mapping are reported, for the 30 wt.% catalysed monolith: the encountered elements are the structural elements of the filter (C, O and Si) and the catalyst active species (Cu, K and Fe). An accurate analysis of Fig. 5 evidenced the almost complete coating of the SiC granules with the active species without any washcoat, since the signals relevant to the structural elements (in particular Carbon) are very low. 3.4. Catalyst adherence testing The results of the tests are reported in Fig. 6, in terms of weight loss (%) vs number of cycles.
Fig. 3. SEM images of a not catalysed monolith (a), a 15 wt.% of Cu0.95K0.05Fe2O4 (b) and 30 wt.% of Cu0.95K0.05Fe2O4 (c) catalysed monolith.
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The above reported results, characterized by weight losses lower than that reported in literature for washcoated supports [11], demonstrate the good adhesion of the active species on the SiC granules even in absence of a washcoat. This is due to the preliminary thermal treatment of the bare supports (calcination at 1000 °C for 48 h) that results in a growing coating of the SiC particles with SiO2 streaks, that can greatly help the adherence of the active species to the filter, even in the absence of a washcoat [9].
3.5. Specific surface area
Fig. 4. SEM images of a 30 wt.% of Cu0.95K0.05Fe2O4 catalysed monolith.
The SSA of the filters and of the Cu0.95K0.05Fe2O4 powder, obtained by means of the SORPTOMETER Kelvin 1040 Costech instrument, applying BET method for its calculation, are reported in Table 2. The values in Table 2 show that the increase of the catalyst load over the bare monoliths results in the increase of the BET specific surface area; this is an expected consequence, since the deposition
Fig. 5. SEM image and distribution of elements, as obtained by EDX element mapping, for the 30 wt.% Cu0.95K0.05Fe2O4 catalysed filter.
Fig. 6. Ultrasonic tests performed on 18 wt.%, 24 wt.% and 30 wt.% catalyst loaded monoliths. A beaker containing the samples immersed in petroleum ether (Carlo Erba), was placed in an ultrasonic bath CP104 (EIA SpA) filled with distilled water, at a temperature of 25 °C, operating at 60% of rated power, for regular intervals of 5 min. The weight changes were recorded after monoliths drying at 120 °C and cooling up to room temperature at the end of any interval.
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3.7. H2-TPR analysis
Table 2 Specific surface area of not catalysed and catalysed SiC monoliths. Sample
BET surface area [m2/g]
Powder of Cu0.95K0.05Fe2O4 Bare SiC monolith Catalytic monolith with 15 wt.% of Cu0.95K0.05Fe2O4 Catalytic monolith with 20 wt.% of Cu0.95K0.05Fe2O4 Catalytic monolith with 25 wt.% of Cu0.95K0.05Fe2O4 Catalytic monolith with 30 wt.% of Cu0.95K0.05Fe2O4
1.35 0.30 1.20 1.30 1.60 2.30
Table 3 Porosimetric characteristics of the catalytic and non-catalytic filters. Median pore diameter (lm) Non-catalytic SiC monolith Catalytic monolith with 15 wt.% Catalytic monolith with 20 wt.% Catalytic monolith with 25 wt.% Catalytic monolith with 30 wt.%
of of of of
Cu0.95K0.05Fe2O4 Cu0.95K0.05Fe2O4 Cu0.95K0.05Fe2O4 Cu0.95K0.05Fe2O4
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17.0 16.0 15.5 15.0 11.0
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.
3.6. Hg porosimetry tests The porosimetric characteristics of the filters were measured by means of Hg penetration technique using the ‘‘PASCAL 140” and ‘‘PASCAL 240” Thermo Finnigan instruments and are reported in Table 3. The data of Table 3 highlighted that the increase of the active species load results in the pores diameter decrease from 17 lm to 11 lm, and not in their occlusion. These results could be likely due to a behaviour deposition that seems to depend from the catalyst load. SEM images showed in a previous paragraph evidenced that the deposition of the active species mainly occurs inside the pores and then on the walls of the monolith at the lower load, while it mainly occurs on the external surface and then inside the pores when the load of active species is higher than 15 wt.%.
The H2-TPR analysis was carried out heating in 5% H2/N2 flow two SiC monoliths, with a load of 15 wt.% and 30 wt.% of Cu0.95K0.05Fe2O4, from room temperature to 900 °C at a heating rate of 5 °C min 1. The reaction parameters (temperature and concentrations) have been monitored by means of an HIDEN Analytical system, including a mass spectrometer. Fig. 7 shows the H2-TPR profiles: two pronounced reduction peaks were observed in the range 200–375 °C and 450–700 °C for the two samples; as evident, the increase of the catalyst load results in the shift of the peaks to higher temperature values. The comparison between the results showed in Fig. 6 and the H2-TPR tests performed on CuFe2O4 loaded monoliths [9], evidenced that isn’t possible to clearly identify the reduction of potassium, that at these experimental conditions is very difficult to reduce. About the discussion of the TPR profile, on the base of the reduction peaks of copper ferrite based catalysts showed by Tasca et al. [13], we can propose the following steps: the first one, appearing in the temperature range 200–375 °C was assigned to the reduction of CuFe2O4 to metallic Cu and Fe2O3, and to the subsequent reduction of Fe2O3-hematite to Fe3O4-magnetite, while the second one, appearing at higher temperature, is attributed to the reduction of Fe3O4 to FeO, followed by the reduction to metallic Fe. The hydrogen consumed for Cu mole (H2/Cu), calculated for the samples with a nominal catalyst load of 15 wt.% and 30 wt.% was of 5.28 and 4.35 respectively, corresponding to real load of Cu0.95K0.05Fe2O4 of about 19 wt.% and 31 wt.%. As shown in literature in the case of copper ferrite [14], after the reduction, also mixture of Cu, K and Fe is favourable for the formation of K doped CuFe2O4 at about 800 °C in air, even if not all in the same crystalline form, so confirming its redox behaviour and its use as oxidation catalyst.
3.8. Catalytic activity tests The performances of our catalytic monoliths towards soot oxidation after K addition to our previous formulation were evaluated by means of Temperature Programmed Oxidation (TPO) tests. These tests were performed heating two catalytic monoliths loaded with 15 wt.% and 30 wt.% of catalyst from room temperature to 800 °C at a heating rate of 5 °C min 1 in 5% O2/N2 flow with
Fig. 7. H2-TPR profiles of a SiC monolith catalysed with 15 wt.% and 30 wt.% of Cu0.95K0.05Fe2O4.
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Fig. 8. TPO profiles of a soot loaded SiC monolith catalysed with 15 wt.% and 30 wt.% of Cu0.95K0.05Fe2O4 (test without NOx).
Fig. 9. TPO profiles of a 30 wt.% Cu0.95K0.05Fe2O4 loaded monolith in presence of 5% O2/N2 flow (dot line) and 5% O2/N2 with 550 ppm NOx flow (bold line).
and without the addition of about 550 ppm of NOx (typical composition of a diesel engine exhaust). The monoliths, on which was previously deposited the soot (about 5 g l 1), were opportunely shaped and entrapped in a tubular reactor. The temperature and CO2 concentration were monitored by means of an HIDEN Analytical system, including a mass spectrometer. The results of the tests performed without NOx are reported in Fig. 8. The data reported in Fig. 8 confirm the results obtained in the TG tests, evidencing the good oxidation property of this formulation, with an ignition temperature of about 350 °C, and that the
increase of the catalyst load doesn’t result in the decrease of this value. The comparison of the performances of the samples in a stream with and without NOx, is reported in Fig. 9. The TPO profiles reported in Fig. 9 showed the ignition temperature decrease of about 80 °C and the faster reaction rate due to NOx presence in the gas stream: these results confirmed the positive effect of NOx on the catalytic activity of the catalyst towards the soot oxidation, since the coexistence of NO and O2 in the gas phase improved the catalytic activity for PM oxidation, probably because NO2 is a better oxidant than O2.
V. Palma, E. Meloni / Fuel 181 (2016) 421–429 Table 4 Engine exhaust stream composition. Component
Average concentration
CO CO2 NO NO2 O2 PM SO2
0.02 vol.% 6.00 vol.% 70.00 ppm 4.00 ppm 12.00 vol.% 45.00 mg/m3 <1.00 ppm
3.9. Deposition and on-line MW assisted regeneration tests Starting by the very promising results reported in the previous paragraph, some preliminary tests of soot deposition and on line MW assisted regeneration of catalytic DPFs were performed by
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means of our diesel emission control laboratory plant [9]. As a first step, the composition of the engine exhaust stream was characterized by means of the on-line gas analysis system, with which we are able to continuously monitor CO, CO2, NO, NO2, SO2 and O2, and to directly acquire the data by a specifically set-up acquisition system in Lab-View Ambient. The results are reported in Table 4. All the deposition tests were performed at the operating engine conditions of 920 rpm and Poil = 30 bar, with a fixed flow rate into the filter of about 110 l min 1, with the exhaust gas temperature of 200 °C, and with a soot concentration in the exhaust gas of about 45 mg (m3) 1. The deposition and MW assisted regeneration phases were performed following the previously optimized procedure [9]. The behaviour of the pressure drop through the filter (DP) and the temperature profile during the last 15 min of the deposi-
Fig. 10. DP and temperature profiles during the last 15 min of the deposition phase and the complete MW assisted regeneration phase of 20 wt.% loaded DPFs with Cu0.95K0.05Fe2O4, performed with the microwave generator set at 50% of its nominal power and with the exhaust gas flow rate of 30 l min 1.
Fig. 11. CO, CO2 and NO2 profiles during the regeneration phase of the 20 wt.% loaded DPFs with Cu0.95K0.05Fe2O4, performed with the microwave generator set at 50% of its nominal power and with the exhaust gas flow rate of 30 l min 1.
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Fig. 12. DP/DP0 and temperature profiles during the regeneration phase of 20 wt.% loaded DPFs with CuFe2O4 and Cu0.95K0.05Fe2O4, performed with the microwave generator set at 50% of nominal power and with the exhaust gas flow rate of 30 l min 1.
tion phase and the complete MW assisted regeneration phase of a 20 wt.% Cu0.95K0.05Fe2O4 loaded DPF is reported in Fig. 10. As evident from Fig. 10, the MW application results in the contemporary increase of the outlet gas temperature and of the slope of DP-time curve; in particular it is possible to emphasize that when the temperature reaches the value of about 300 °C (the catalyst threshold temperature), the DP curve shows a plateau indicating that the catalytic soot combustion rate is comparable to the soot deposition rate. Furthermore, we can observe that when the temperature increases, the DP curve decreases, so indicating that the filter regeneration occurs. These results showed that also after the K addition to our formulation, the system composed by SiC filter, active species and soot is able to absorb microwaves and to completely regenerate in about 20 min. During the test the opacity of the exhaust stream at the filter inlet and outlet have the mean values of 22% and 0.2% respectively, with an average filtration efficiency of about 99.0%. In Fig. 11 the data related to the exhaust gas composition in terms of CO, CO2 and NO2 as functions of time during the regeneration phase are reported. From Fig. 11 is evident that during the regeneration phase, when the exhaust gas stream convoyed into the filter reach the soot ignition temperature (about 300 °C), the soot starts to burn: so we have the oxygen consumption and the CO and CO2 formation, showed by their upper peaks. An interesting result is the contemporary consumption of NO2 (lower peak), that probably is due to the oxygen donation capacity of NO2 during the oxidation of soot. In Fig. 12 the comparison of catalytic activity tests performed on a 20 wt.% K-doped Copper Ferrite and 20 wt.% copper ferrite loaded DPF at the exhaust of a diesel engine, regenerated by means of microwaves, is shown. It is evident from Fig. 12 that DP and temperature profiles are practically specular; in particular the temperatures reach their maximum value around about 750 °C, corresponding to the maximum combustion rate of accumulated soot. Furthermore Fig. 12 shows that the K addition to copper ferrite allows to decrease the temperature ignition from 400 °C to about 300 °C: the comparison of the MW energy supplied for DPF litres during the regeneration phase (about 4500 kJ and 3400 kJ for the 20 wt.% CuFe2O4 loaded and 20 wt.% Cu0.95K0.05Fe2O4 loaded filters, respectively),
indicates that the K-addition results in a higher catalytic activity and a further lower energy consumption during the regeneration phase. 4. Conclusions In this work, K-doped copper ferrite (Cu0.95K0.05Fe2O4) loaded SiC Wall Flow Filters (with various catalyst loads) were prepared and their performances towards diesel soot oxidation were verified, even in presence of NOx. The preliminary characterization tests performed on the powders of the prepared samples, showed the presence of the typical peaks of CuFe2O4 in its tetragonal and cubic form and the absence of mixed oxides peaks, and that the increase in the load of active species up to 30 wt.% resulted in increased reaction rate, even if not in lower soot oxidation temperature. As a consequence the characterization tests of the catalytic monoliths were performed. The Hg porosimetry tests and the specific surface area analysis showed that the increase of the catalyst load results in decreased median pores diameter (but not in their complete occlusion), and in increased BET specific surface areas. The SEM–EDX analysis evidenced the presence on the catalytic filter not only of the structural elements of the filter (C, O and Si), but also of the catalyst active species (Cu, K and Fe), homogeneously distributed on the support and inside the pores. The H2-TPR measurements showed two pronounced reduction peaks attributed to the reduction of Cu0.95K0.05Fe2O4 to Cu and Fe in two steps, while it isn’t possible to clearly identify the reduction of potassium, that is very difficult in the reduction conditions used in the experimental tests. As shown in literature in the case of copper ferrite, after the reduction, also mixture of Cu, K and Fe is favourable 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, Cu0.95K0.05Fe2O4 is a very good oxidation catalyst. The preliminary catalytic activity tests (TPO tests) with and without NOx performed using catalytic DPFs with 15 wt.% and 30 wt.% catalyst load, on which soot was previously deposited, showed that the growing catalyst load resulted in a higher reaction rate, even if it seems to have no effect on the ignition temperature (about 350 °C), and that the presence of NOx had a positive effect on the catalytic activity, allowing an ignition temperature decrease of about 80 °C. The tests performed
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directly at the exhaust of a diesel engine showed that the K addition to copper ferrite allowed to decrease the temperature ignition from 400 °C to about 300 °C, so obtaining a further energy saving during the regeneration phase. In conclusion the independence of the MW assisted regeneration technology by the operating conditions of the engine, and the energy saving in the regeneration step due to the combination of this technology with the Cu0.95K0.05Fe2O4 loaded DPF, confirm the ability of our proposed technology as a potential alternative to actually employed technologies (such as fuel post-injection) in the DPF regeneration phase. References [1] Lee C, Park Joo-Il, Shul Y-G, Einaga H, Teraoka Y. Appl Catal B 2015;174175:185–92. [2] Palma V, Ciambelli P, Meloni E. Chem Eng Trans 2012;29:637–42.
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