Diesel Particulate Filtration and Combustion in a Wall-Flow Trap

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Ind. Eng. Chem. Res. 2005, 44, 9549-9555

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Diesel Particulate Filtration and Combustion in a Wall-Flow Trap Hosting a LiCrO2 Catalyst Emanuele Cauda, Davide Mescia, Debora Fino, Guido Saracco,* and Vito Specchia Materials Science and Chemical Engineering Department, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy

This paper concerns the development of an innovative LiCrO2 catalyst (synthesis, characterization, mechanistic analysis, and aging). This catalyst already showed appreciable activity at 300 °C toward the catalytic combustion of soot even under loose contact conditions. Furthermore, its limited sensitivity to hydrothermal aging in the presence of SO2 at 650 °C demonstrated a favorable result for its possible application in catalytic traps for diesel particulate removal from car exhaust gases. In this perspective, an in situ combustion synthesis method was tailored to the preparation of a LiCrO2-catalyzed trap based on a SiC wall-flow monolith. Engine bench tests on this catalytic trap (trap loading and regeneration inducing a temperature increase by the catalytic combustion of suitably postinjected fuel) showed that the presence of the catalyst in the wall-flow trap enabled both a more complete regeneration and a 2-fold reduction of the regeneration time compared to the case of a noncatalytic trap, with the consequent saving of postinjected fuel. 1. Introduction Diesel engines have traditionally governed the heavyduty market. However, in recent years, they have increased their market share in the passenger car field, due to their high efficiency, the lower cost of fed fuel, and their longer durability compared with gasoline engines. Diesel engines produce low emissions of NOx, CO, and unburnt hydrocarbons (HC). However, their emission of particulate matter is about 10 times higher than that of gasoline engines.1 The negative effects of diesel particulates on health have forced more and more severe legislation limits and stimulated the development of emission reduction technologies. Despite the complex composition of diesel particulates, the main challenge is the elimination of their carbonaceous fraction, also called soot. Notwithstanding the efforts to improve fuel injector design in the past decade (e.g., multi-jet common rail engines), soot emissions could not be reduced, in most cases, down to values lower than the limits of the pending Euro IV legislation (to be enforced in 2005). A promising abatement technique lies in the development of a catalytic filter that combines filtration and oxidation of the emitted particulate matter. In this context, the key challenge is to find a stable catalyst that decreases the combustion temperature of soot down to the diesel exhaust temperature. This could lead to a sort of continuously regenerating trap.2 However, to the authors’ knowledge, no completely “passive” system (i.e., not requiring energy consumption for localized heating purposes) has ever proved to be sufficiently reliable and stable for practical application.3-7 Catalytic traps based on wall-flow ceramic monoliths (shallow-bed filtration), combined with an oxidation catalyst deposited onto their inlet channel walls, are under development in several R&D centers worldwide.8-10 These traps can be periodically regenerated by a peculiar use of last-generation Common Rail Diesel engines: some fuel can indeed be postinjected in the * To whom correspondence should be addressed. Tel.: +39011-5644654. Fax: +39-011-5644699. E-mail: guido.saracco@ polito.it.

engine and then burned out by a specific catalytic converter so as to heat up the downstream trap until catalytic combustion of soot is ignited. This paper describes the encouraging results obtained in this context with an innovative catalyst based on lithium chromite. 2. Experimental Section Catalyst Preparation. After a large screening of soot-combustion catalysts based on perovskite11 and delafossite12 oxide materials, not reported here for the sake of briefness, the most active catalyst found was the delafossite oxide LiCrO2. Delafossite ceramics have generated a lot of interest for their potential as transparent conducting oxides (TCOs) for uses as transparent electrodes for photovoltaic devices, display technologies, and smart windows.13 Their use in catalysis is much less investigated compared with perovskites. Lithium chromite was obtained via the combustion synthesis method.14 A concentrated aqueous solution of various precursors (metal nitrates and urea) was placed in an oven at 600 °C for a few minutes in a crucible, so as to ignite the highly exothermic and self-sustaining reaction:

3LiNO3 + 3Cr(NO3)3 + 10N2H4CO f 3LiCrO2 + 20H2O + 10CO2 + 16N2 As part of the urea gets hydrolyzed and thermally decomposed during preparation, an excess of this reactant had to be used. The optimum excess level was found to be 100% for the sake of maximizing the specific surface area. A reference catalyst, LaCrO3, was also prepared by an analogous route. This catalyst was found to display a good activity toward the catalytic oxidation of soot and was employed in earlier investigations.10 The obtained catalysts were then ground in a ball mill at room temperature and submitted to physical and chemical characterization. Catalyst Characterization. X-ray diffraction (PW 1710 Philips diffractometer) was used to check the achievement of the desired oxide structure.

10.1021/ie050250u CCC: $30.25 © 2005 American Chemical Society Published on Web 05/26/2005

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Compositional analysis (dissolution in HNO3/HCl followed by atomic absorption analysis with a PerkinElmer 1100B spectrometer), performed on all prepared samples, confirmed that the overall amount of the various elements of interest (La, Li, and Cr) was consistent with that used in the precursors and was compatible with the phases detected by X-ray diffraction (sensitivity: 4%). A field emission scanning electron microscope (FESEM, Leo 50/50 VP with Gemini column) was employed to analyze the microstructure of the crystal aggregates of the catalysts as prepared and after aging. Conversely, transmission electron microscopy (TEM, Philips CM 30 T) was employed to assess the size and morphology of the oxide crystals themselves. The specific surface areas of the prepared catalysts were evaluated from the linear parts of the BET plot of N2 isotherms, using a Micromeritics ASAP 2010 analyzer. For bulk and nonporous catalysts, such as the perovskite or delafossite ones, the specific surface area can be directly related to the average crystal size. As a further characterization procedure, oxygen temperature programmed desorption experiments were performed on the catalysts in a Thermoquest TPD/R/O 1100 analyzer, equipped with a thermal conductibility (TCD) detector. A fixed bed of catalyst powders was enclosed in a quartz tube and sandwiched between two quartz wool layers; prior to each temperature programmed desorption (TPD) run, the catalyst was heated under an O2 flow (40 mL/min) up to 750 °C. After a 30 min stay in O2 flow at this temperature as a common pretreatment, the reactor temperature was then lowered down to room temperature by keeping the same flow rate of oxygen, thereby allowing complete oxygen adsorption over the catalyst. Afterward, helium was fed to the reactor at a 10 mL/min flow rate and kept up for 1 h at room temperature in order to purge out any excess oxygen molecules. The catalyst was then heated to 1100 °C at a constant heating rate of 10 °C/min using helium at a flow rate of 10 mL/min. The O2 desorbed during the heating was measured by the TCD detector. Catalytic Activity Assessment. The activity of the prepared catalyst toward soot combustion was analyzed by temperature programmed combustion (TPC) carried out in a fixed-bed microreactor, according to the standard operating procedures described in detail by Fino et al.:11 air was fed at the constant rate of 50 mL‚min-1 to the fixed bed constituted of a mixture of carbon and powdered catalyst (1:9 mass basis). All experiments were performed by using, instead of real diesel soot, an amorphous carbon by Cabot Ltd (particle average diameter ) 45 nm in diameter, BET specific surface area ) 200 m2/g, ashes content after calcination at 800 °C ) 0.34%, adsorbed water moisture at room temperature ) 12.2 wt %, and no adsorbed hydrocarbons and sulfates). This allows us to regard the results obtained as conservative, because amorphous carbon is more difficult to burn than real diesel soot. The catalyst-carbon mixture was prepared according to two different procedures: (1) tight contact, by intimate mixing in an agate mortar for 15 min; and (2) loose contact, by gentle shaking in a polyethylene sample bottle for 1 h. Tight contact mixtures lead to more reproducible results. However, loose contact is more representative of the real contact conditions occurring in a catalytic trap for diesel particulate removal.2 The reaction temperature was controlled through a PID-regulated oven and varied from 200 to 700 °C at a

5 °C‚min-1 rate, while air was fed at a 50 Ncm3‚min-1 flow rate. The analysis of the outlet gas was performed via NDIR analyzers (ABB). Carbon mass balances were closed in each case within a (5% error. A TPC run was also performed in the absence of the catalyst so as to set a reference for comparison. The peak temperature Tp of the TPC plot of the outlet CO2 concentration was taken as an index of the catalytic activity: the lower the Tp value, the higher the catalytic activity. Furthermore, on the grounds of the area of the TPC plots, estimates of the overall CO2 amount produced per run were calculated and the selectivity of carbon combustion toward CO2 (ηCO2) was estimated. Catalyst Aging. A catalyst used for the combustion of soot in diesel emissions may lose its activity for several reasons: temporary temperature rise up to unusual values (>600 °C, due to the sudden burning of a large soot aggregate), poisoning effect caused by some components of diesel exhausts (e.g., SO2 or H2O), or prolonged working time at high temperature. In line with earlier investigations on other catalysts4 and in order to separately consider the effect of each of these factors, aging treatments were performed under these conditions: • Thermal aging in dry air at 300 and 650 °C for 96 and 24 h, respectively. • Thermal aging at the same temperature and time values in wet air, containing 12 vol % of moisture obtained by humidification in a thermostatized bubble column. • Thermal aging at the same temperatures in air containing 200 ppmv SO2, a value much larger than current SO2 levels in exhaust gases (a few ppmv), fixed for the sake of accelerating the possible effect of this specific poison. Most of the physical and chemical characterization analyses described above were replicated on aged samples. Activation Energy Evaluation. The activation energy Ea of the catalyzed and noncatalyzed carbon combustion was measured via differential scanning calorimetry (Perkin-Elmer DSC-Pyris) by using the socalled Kissinger method.15 DSC experiments were performed on mixtures of catalysts with carbon (tight contact) so as to measure the heat released as an index of the evolution of catalytic combustion. Under an air flow rate of 100 mL‚min-1, a mixture of catalyst and carbon (2:1 mass basis, to allow a good detection of the heat released by combustion) was placed in the sample holder, whereas an equal weight of alumina was placed in the reference crucible. In a noncatalytic combustion run, alumina was placed together with carbon in the sample crucible according to the same mass ratio mentioned above. The temperature was increased from 50 to 700 °C, with different heating rates (φ ) 5, 10, 20, 30, and 50 °C/min) in different independent runs. Several plots representing exothermal combustion peaks were obtained. According to the selected method, the activation energy can be evaluated on the grounds of the following equation

ln

Ea φ )+ cost 2 RT TR R

(1)

where TR is the characteristic temperature for the given process corresponding to a fixed level R of carbon

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Figure 1. TPC plots of the catalytic combustion over LiCrO2 and LaCrO3 powder catalysts under both tight and loose contact conditions. The curve related to noncatalytic combustion of carbon is also reported as a reference including, for this case only, the CO production curve.

conversion, φ is the heating rate, Ea is the activation energy, R is the gas constant, and cost is a generic constant. By plotting the left-hand side of eq 1 vs 1/RTR at different φ values for a reference R value (50% in the present investigation), Ea can be evaluated from the slope of the interpolating line. Catalytic Trap Preparation. The LiCrO2 catalyst was deposited by in situ combustion synthesis directly over the wall-flow filters. The ceramic support was dipped in the aqueous solution of its precursors and then placed into an oven at 600 °C. The aqueous phase was rapidly brought to a boil, the precursors mixture ignited, and the synthesis reaction took place in situ. The support selected was a silicon carbide (SiC) filter produced by Ibiden (cell structure ) 14/200, diameter ) 30 mm, length ) 6 in., pore diameter of channel walls ) 9 µm, and porosity of channel walls ) 42%) which was found to be chemically compatible with the selected catalyst. The load of catalyst deposited was assessed by gravimetric analysis. The amount of perovskite deposited was 4.8 wt %, as assessed by gravimetric analysis. The morphology of the deposited catalyst layer was analyzed by FESEM observation, whereas catalyst adhesion to the monolith was assessed by a tailored ultrasonic bath test procedure.10 Diesel Engine Bench Tests. The developed trap was tested over real diesel exhaust gases on an engine bench (Kubota 1000 cm3 IDI engine, capable of up to 23.5 hp at 3000 rpm), where the temperature and gas composition before and after the trap, as well as the evolution of the pressure drop through the trap (a sign of soot accumulation therein), can be controlled and monitored. A detailed description of the plant was provided by Fino et al.16 The exhaust gas superficial velocity across the trap could be controlled at a fixed value by measuring the exhaust flow rate through a volumetric flow controller connected to a throttling valve placed on a bypass exhaust stream. The pressure drop across the trap could be measured by means of differential pressure transducers (Vika), whereas the trap inlet and outlet temperature was measured by K-type thermocouples mounted at axial positions. The soot concentration in the exhaust gases before and after the trap was measured by isokinetically sampling a small flue-gas flow rate through a pump and by collecting the suspended particulate on a 2-filter system (Pallflex 47 TX 40 HI 20-W); the two measures allow us to determine the filtration efficiency. Finally, the gas-phase composition could be monitored through a continuous analyzer by

Elsag-Bailey (nondispersive infrared (NDIR) for NO, CO, CO2, and SO2; flame ionization detector (FID) for overall HC; paramagnetic for O2). In line with the pending 2005 EU regulations, all the tests were carried out by using a low-sulfur (