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Accepted Manuscript Title: K/CeO2 Catalysts Supported on Cordierite Monoliths: Diesel Soot Combustion Study Authors: C.A. Neyertz, E.E. Mir´o, C.A. Querini PII: DOI: Reference:

S1385-8947(11)01389-1 doi:10.1016/j.cej.2011.11.010 CEJ 8612

To appear in:

Chemical Engineering Journal

Received date: Revised date: Accepted date:

5-5-2011 29-9-2011 4-11-2011

Please cite this article as: C.A. Neyertz, E.E. Mir´o, C.A. Querini, K/CeO2 Catalysts Supported on Cordierite Monoliths: Diesel Soot Combustion Study, Chemical Engineering Journal (2010), doi:10.1016/j.cej.2011.11.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.


*Highlights

K/CeO2 Catalysts Supported on Cordierite Monoliths: Diesel Soot Combustion Study.

RESEARCH HIGHLIGHTS

A study of K/CeO2 catalysts supported on cordierite for the reaction of diesel soot

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combustion is reported.  The combustion activity was studied by temperatureprogrammed oxidation (TPO) in the soot combustion with loose contact.  The optical microscopy and scaning electron microscope determined a redistribution of the crystals

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in the monolith by heating or soot combustion reaction. The activity depends on the

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potassium precursor used, decreasing in the order KNO3 > K2CO3 > KOH.  The nondeactivation of the samples during the reaction allowed us to discard the volatilization of the active species in the temperature range (20 – 600 ºC) in which the tests were

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performed.  The aging treatments at 800 ºC determined a deactivation of K/CeO2

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catalysts due to potassium loading lost.

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*Revised Manuscript

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K/CeO2 Catalysts Supported on Cordierite Monoliths: Diesel Soot Combustion Study

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C. A. Neyertz*, E. E. Miró, C. A. Querini

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Instituto de Investigaciones en Catálisis y Petroquímica, INCAPE (FIQ, UNL - CONICET) Santiago del Estero 2654, 3000 Santa Fe, Argentina.

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*Corresponding author: Tel./FAX: 54-342- 4533858, e-mail: cneyertz@fiq.unl.edu.ar

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Abstract The combination of filters and oxidation catalysts is one of the most effective aftertreatment techniques to eliminate soot particles from the exhaust gases of diesel engines. The activity of powder K/CeO2 catalysts has been extensively reported in the literature but few studies refer to K/CeO2 supported on monoliths. This work presents a study of K/CeO2

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catalysts supported on cordierite for the reaction of diesel soot combustion. The catalysts were prepared by sequential impregnation of the monolith with ceria and different

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potassium precursors. The activity was studied by temperature-programmed oxidation (TPO) in the soot combustion with loose contact. No deactivation was observed after six

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cycles of combustion-soot charge. The activity depends on the potassium precursor used, decreasing in the order KNO3 > K2CO3 > KOH. The structural effects caused by the different precursors of potassium determine the material activity. The X-ray diffraction

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characterization does not indicate a crystalline structure change before and after reaction. The optical microscopy and scanning electron microscope determined a redistribution of

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the crystals in the monolith by heating or by the soot combustion reaction. A weight loss during the reaction was ascribed to the degradation of the catalytic monolith. The nondeactivation of the samples during the reaction allowed us to discard the volatilization of

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the active species in the temperature range of 20 to 600 ยบC, in which the tests were performed. On the other hand, the ageing treatments at 800 ยบC led to the deactivation of

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K/CeO2 catalysts due to a partial loss of potassium loading.

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Keywords: diesel soot, cerium, potassium, cordierite

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1. Introduction The combination of filters and oxidation catalysts is one of the most effective aftertreatment techniques to eliminate soot particles from the exhaust gases of diesel engines.

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The deposited catalyst lowers the soot ignition temperature from 550 ºC (uncatalyzed reaction) to typical values of diesel exhaust gases (180–400 ºC). Our research group has

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studied the K/CeO2 powder catalysts [1-6]. The addition of K to CeO2 notably decreases the soot combustion temperature due to a synergistic effect between the catalyst

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components. Cerium oxide has oxygen storage capacity and with the addition of potassium, a more active catalyst can be obtained. It is known that potassium favors the surface mobility, thus improving the soot-catalyst contact and increasing the surface

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basicity. On the other hand, the degree of physical contact between the soot and the combustion catalyst influences the oxidation reactivity [7, 8]. The tight or close contact in

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the catalytic activity analysis prevails in a number of studies in the literature. Although the tight contact between soot and catalyst defines the intrinsic catalytic activity under optimal conditions, the loose contact between catalysts and soot is a convenient method to simulate

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real conditions of filter traps in diesel engines [7]. Loose contact always presents higher combustion temperatures than tight contact.

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There are few publications related to the activity of K/CeO2 on a porous filter. However, some patents related to catalyzed soot filter containing cerium and potassium have been recently reported [9-11]. In the patent published by F. Mao et al. [10], a porous filter is coated with a catalytic agent that is a mixture of alkali metal, preferably potassium, and

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cerium oxides. The porous filter can be porous silicon carbide, cordierite or mullite. The mole ratio of alkali metal to cerium is from 0.5 to 5. The amount of potassium in the catalytic agent is preferably in the range 10 to 25 weight percent of the catalytic agent, and the amount of cerium in the catalytic agent is preferably in the range of seventy to thirty weight percent of the catalytic agent. The catalytic agent can contain a platinum group metal if desired. In this patent, the authors note that the exact nature of the catalitytic agent is not known. In the present work, we focused on the study of K/CeO2 catalysts, supported on a cordierite monolith. Cordierite is considered a properly structured support due to its high melting point, high temperature and thermalshock resistance, as well as to its high

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chemical stability. The K/CeO2/cordierite catalysts were prepared by successive impregnations of Ce(NO3)3 and potassium was added using different promoters: KNO3, K2CO3 and KOH, in order to analyze the effect of potassium anion. Considering that in a previous work it was observed that the activity had a smooth maximum as a function of the K content, being around 7 wt. % for KNO3/CeO2 [3], the samples of K/CeO2/cordierite

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were prepared with a similar K/CeO2 mass ratio, approximately 0.07. We studied the effect of the potassium precursor in soot combustion by temperature-programmed

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oxidation (TPO). This work reports the effect of several cycles of soot oxidation carried out with the same sample. Since the real diesel engine could reach very high temperatures

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during operative conditions, the activity of the samples in soot combustion was also studied after ageing treatments at 800 °C.

Morphology changes during the reaction were analyzed by infrared spectroscopy (FTIR),

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and electron probe microanalysis (EPMA).

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X-ray diffraction (XRD), optical microscope (OM), scanning electron microscope (SEM)

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2. Experimental

2.1 Structured Catalysts Preparation

A cordierite honeycomb monolith, with a general composition of 2MgO:2Al2O3:5SiO2,

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provided by Corning, with 400 cps, 0.17 mm average wall thickness, was used as structured support. The apparent monolith density and its geometric surface were 0.42 g/cm3 and 27.4 cm2/cm3, respectively. Portions of monolith containing 21 channels were used in these preparations, with dimensions of 0.6 cm x 0.6 cm of section and length of 1.2

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cm. Before use, the monoliths were washed with ethanol, dried at 120 ºC and finally calcined at 500 ºC for 2 h. The ceria was fixed to the ceramic support by successive impregnations in a concentrated aqueous solution (aprox 1.6 g/ml) of Ce(NO3)3.6H2O (Anedra, analytic reagent) at 50 ºC, while stirring the solution. After each immersion, the solution excess contained in the channels was blown with N2 gas, and then the samples were calcined at 700 ºC during 2 h. The number of impregnations with cerium solutions depends on the final desired concentration, but in this case the maximum number was 10. CeO2/monolith samples obtained after 10 min of treatment in an ultrasonic bath were impregnated with potassium. The samples were immersed in aqueous solutions of different

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potassium precursor: KNO3 (Anedra, analytic reagent, 0.35 g/ml), KOH (Anedra, analytic reagent, 0.12 g/ml) or K2CO3 (Cicarelli, proanalytic reagent, 0.26 g/ml). After each immersion at 50 ºC under stirring, the solution excess was blown with N2 gas and dried in a stove. The K/CeO2 mass ratio was close to 0.07. Finally, the samples were calcined at 400 ºC during 2 h.

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During cerium and potassium impregnation, the loadings over cordierite monoliths were determined by gravimetric difference. In the case of potassium, the loading was obtained

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before calcination, in order to introduce a minor error in the calculus of weight percent by precursor transformation.

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The CeO2/cordierite catalyst was prepared with a loading of 0.218 g CeO2 and it will be referred to as M1. The resulting K/CeO2/cordierite catalysts have a nominal K/CeO2 mass ratio of 0.076, 0.093 and 0.070 for samples prepared with KNO3 (M2), KOH (M3) and

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K2CO3 (M4), respectively. These catalysts were compared with a sample of pure cordierite (Mc).

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Additional samples of KNO3/CeO2/cordierite were prepared in order to be used in stability studies. The M5 catalyst was prepared with a nominal K/CeO2 mass ratio of 0.06 in order to study the potassium loading stability during several thermal treatments in static

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air at 600ºC. The M6, M7 and M8 samples (with nominal K/CeO2 mass ratio of 0.103, °C.

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0.041 and 0.077, respectively) were prepared for activity tests after thermal ageing at 800

2.2 Coating adherence test

The adherence of ceria coating on monoliths was evaluated by exposing the samples to

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ultrasonic waves at ambient temperature [12, 13]. The coated monoliths were immersed in acetone inside a glass vessel, which was placed in an ultrasonic bath (Testlab TB04, 40 kHz and 160 W) during 10 min. After this treatment, the samples were dried overnight at 120 °C. The measurement of the weight loss indicates the adherence of the ceria coating to the cordierite support. The same test was performed with the catalysts after potassium impregnation and calcination. In all cases, the coating adherence was determined by loss weight measurement.

2.3 Catalytic soot combustion measurements

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The soot was obtained by burning a sample of diesel fuel in a glass vessel. After being collected from the vessel walls, the soot was dried in a stove at approximately 120 °C during 24 h and milled in a mortar. The structured catalysts were impregnated with soot by immersion into a methanol suspension under vigorous stirring. In all cases, the soot loading was lower than 1 mg, over

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a monolith section of approximately 0.2 g. A loose contact was obtained with this impregnation method [7].

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The activity of the structured samples for the soot combustion was studied by Temperature-programmed Oxidation (TPO) tests, using a gas mixture of 6% O2 in N2, and

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a heating rate of 12 ºC/min. A modified TPO technique was used [14]. The gases coming from the reactor outlet were introduced into a methanation reactor, where the CO, CO2 and the hydrocarbons formed during the reaction were totally transformed to CH4 by a Ni

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catalyst at 400 ºC. The CH4 was continuously measured by a FID detector. Several cycles of TPO were studied for each catalyst. The samples were impregnated

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with soot, analyzed by TPO, and then a new cycle of soot loading and TPO analysis was carried out again. The weight changes were determined after each impregnation and TPO

2.4 Stability Test

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analysis steps.

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The thermal stability was studied for KNO3/CeO2/cordierite samples: M6, M7 and M8. The M6 and M7 catalysts were calcined in air at approximately 25 ml/min and 800 ºC during 1 and 24h, respectively. The M8 catalyst was treated with stream of wet air at 800 ºC during 1h. In the latter case, the stream of air was saturated with water at 40°C and then

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flowed through the monolith catalyst. The resulting water concentration in the air stream was 10%, which is a typical value in diesel engine exhausts. In all cases, the stability of the ageing samples of KNO3/CeO2/cordierite was analyzed by TPO test and compared with the fresh samples. The final K concentration was determined by ICP.

2.5 Characterization BET area: The surface area was determined with a Micromeritics Accusorb 2100 sorptometer using N2 gas. The samples were previously degassed during 2h at 200°C.

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IR spectroscopy (FTIR): Infrared spectra were obtained using a Shimadzu IR Prestige-21 spectrometer. The samples were crushed in an agate mortar and then prepared in the form of pressed wafers (ca. 1% sample in KBr). All spectra involved the accumulation of 80 scans at 4 cm-1 resolution. X-ray diffraction (XRD): The X-ray diffractograms were obtained with a Shimadzu XD-

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D1 instrument with a monochromator using Cu Kα radiation at a scan rate of 4 °/min, with 2θ from 10 to 100°. The monoliths were supported on the edge of an aluminum sample

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holder window for the XRD analysis.

Scanning electron microscope (SEM): The sample morphology of the different coatings

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was examined with a Jeol JSM-35C scanning electron microscope operated at accelerating voltages of 20–25 kV. Samples were glued to the sample holder with Ag paint and then coated with a thin layer of Au in order to improve the quality of the images.

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Electron probe micro analysis (EPMA): The distribution of the chemical elements in the coating was analyzed by X-ray spectra with the EDAX software. Semi quantitative results

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were obtained with the theoretical quantitative method (SEMIQ), which does not require standards. X-ray spectra were obtained with an acceleration of 20 kV. Optical Microscope (OM): The surface morphology of monolith catalysts was studied

LAS EZ software.

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with a Leica S8 APO stereo-microscope equipped with a Leica LC3 digital camera and

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The characterization by FTIR, DRX, SEM, EPMA and OM was carried out on both fresh and used samples after several cycles of TPO.

3. Results and Discussion

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The nominal concentrations of potassium and ceria (expressed as K/CeO2 mass ratio), and the corresponding potassium precursors of the structured catalysts are presented in Table 1. In order to analyze if the cerium precursor (Ce(NO3)3) was completely transformed into CeO2 during calcination at 700 °C, two samples were evaluated by FTIR spectroscopy. Two monoliths of cordierite were impregnated with an aqueous solution of Ce(NO3)3, and then they were compared after different treatments, including drying and calcination. One of these samples was simply dried at 120 ºC in a stove after impregnation (MCed). The other sample was calcined at 700 °C (MCec) during 2 h. In both cases, the loading after each treatment was approximately 0.039 g. The monolith cerium coatings were studied by FTIR. The spectra are shown in Fig. 1, which includes spectra of pure

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cordierite (Mc) and of the solid precursor Ce(NO3)3. According to Vratny [15], the Ce(NO3)3 presents bands at 733, 813, 1040, 1325-1465, 1630 and 1770 cm-1. The MCed spectrum reveals that some IR peaks can be correlated with Ce(NO3)3 at 1640, 1460 and 1330 cm-1. However, the MCed bands at wave numbers lower than 1400 cm-1 are similar to those of pure cordierite and hide the possible signals of Ce(NO3)3. The bands observed for

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the precursor Ce(NO3)3 spectrum and the dried sample (MCed) are not present in the calcined sample (MCec). This latter sample only displays bands that are very similar to the

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cordierite base. These results demonstrate the disappearance of the nitrate precursor after calcination, and consequently the formation of CeO2. Therefore, all the samples studied in

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this work have CeO2 as coating species.

The coating adherence was determined for CeO2/cordierite catalysts, before potassium impregnation, by measuring the weight loss after 10 min in an ultrasonic bath. In these

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samples, the weight loss was lower than 7% of the CeO2 charge.

Each CeO2/cordierite sample was impregnated with potassium aqueous solutions several

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times in order to obtain a similar K/CeO2 mass ratio in all samples. The effect of potassium species on activity was analyzed by the impregnation of CeO2/cordierite samples with different potassium precursors: KNO3, K2CO3 or KOH. The adherence test

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in acetone was repeated after potassium loading. In this case, it was observed that the weight loss depended on the potassium precursor. In catalysts prepared with KNO3 and

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K2CO3, the weight loss was lower than 0.4%. However, sonication produced a weight loss of 2.8% with KOH as precursor. This indicates that the selection of the precursor is important regarding the interaction with the support, and consequently it affects the mechanical stability.

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Due to the low weight loss of potassium observed during the ultrasonic bath treatments, the sample concentration can be expressed as the nominal potassium content without any significant error in the K/CeO2 mass ratio. In Table 1, the concentration of CeO2 corresponds to the loading obtained after the adherence test. The potassium and cerium concentrations were not analytically determined. It has to be kept in mind that in order to determine the global concentration, the sample should be destroyed. The surface area was determined for bare cordierite and K/CeO2/cordierite catalysts. The BET area was 0.5 m2/g for cordierite while the K/CeO2/cordierite catalysts had a surface area of approximately 2 m2/g. The last value indicates a very low surface area for the monolith catalysts compared with the powder samples. However, Mir贸 et al. [16] have

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concluded that the surface area has little influence on the soot combustion because the soot particles are too large to fit inside any pore and only the catalytic external surface takes place in the reaction In order to analyze the catalytic behavior of K/CeO2/cordierite catalysts in soot combustion, the activity of the samples KNO3/CeO2/cordierite, KOH/CeO2/cordierite and

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K2CO3/CeO2/cordierite (M2 to M4 respectively) were compared with pure cordierite (Mc) and CeO2/cordierite (M1) by TPO analyses. The samples were impregnated with soot by

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immersion in a soot-methanol suspension under vigorous stirring. With this procedure, the soot loading obtained over the structured catalysts was 1 mg. We determined that this

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loading favors a combustion reaction in a kinetically controlled regime. This was verified by loading different amounts of soot, following the procedure described by Peralta et al. [4]. The TPO profiles shown in Fig. 2 correspond to the sixth cycle of soot combustion. It

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has to be emphasized that there is practically no difference between the first and the sixth cycle (not shown). Therefore, the TPO results indicate that there is no catalyst deactivation

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during the different cycles. Moreover, even though the final TPO temperature (600 ºC) is higher than the calcination temperature of K/CeO2/cordierite after the potassium impregnation (400°C), there is no change in the soot combustion activity. In consequence,

temperatures of 600 ºC.

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the TPO results confirm the catalytic stability of K/CeO2/cordierite samples at least up to

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The characteristic temperatures of soot combustion during the sixth TPO cycle are presented in Table 2 for several catalysts. The initial (Ti) and final (Tf) temperatures are used to compare the catalytic activities. Besides, the maximum peak temperature (Tmax) is presented as a reference temperature of the maximum reaction rate. In Fig. 2, it can be

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observed that the Tmax temperature obtained with soot supported on cordierite (Mc) is 540 ºC. On the other hand, the Tmax for soot combustion on CeO2/monolith catalyst (M1) is 518 ºC. In the case of tight contact mode using powder catalyst, it was reported that CeO2 acts as an active support with temperatures (Tmax) near 479 ºC. For non-catalytic soot combustion the maximum temperatures varied from 533 ºC to 600°C [1, 4, 5]. Although cordierite contains MgO as component, which could act as catalyst in this reaction, its effect on soot combustion is negligible compared with that of CeO2. In the case of the K/CeO2/monolith, the catalytic soot combustion occurs between 400 and 550 ºC (Fig. 2). As previously demonstrated [1, 3], a synergistic effect between K and CeO2 takes place during soot combustion. CeO2 supplies the oxygen for the redox

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mechanism, while potassium increases the catalyst-soot contact due to a high mobility of surface species. The activity data for K/CeO2/cordierite catalysts prepared using KNO3 (M2), KOH (M3) and K2CO3 (M4) as precursors, are shown in Table 2. These three catalysts have similar K/CeO2 mass ratios. It can be seen that the Tmax are similar for the catalysts prepared with KNO3 (M2) and K2CO3 (M4), being 443 and 448 ºC, respectively.

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These temperatures are lower than in the case of the sample prepared with KOH (M3) which is 467 ºC. As it is shown in Table 2, the lowest Ti temperature was obtained with

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the structured catalyst prepared with KNO3 (330 ºC), as compared to the catalyst prepared with K2CO3 (355 ºC) or KOH (385 ºC). On the other hand, the Tf temperatures are

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between 520 and 550 ºC for all the K/CeO2/cordierite catalysts. Therefore, the activity of these three catalysts follows the order: KNO3 > K2CO3 > KOH. Similar results were obtained in previous studies using powder catalysts [17]. On the other hand, Aneggi et al.

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[18] found that the activity of K2CO3 is similar to KOH for catalysts supported on CeO2 by wetness impregnation. Using MgO as support, Jiménez et al. [19] found that KNO3 is

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more active than KOH in tight and loose contact modes. In tight contact, the difference between Tmax of KNO3 and KOH is 19 °C, while in loose contact this difference increases to 105 °C. These results have been attributed to the presence of adsorbed oxygen

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species, more reactive and mobile in the KNO3/MgO catalyst than in the KOH/MgO catalyst. Each anion modifies the surface basicity differently. In addition, it affects the

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interaction of the active species with the support surface. The higher basicity of KOH compared to KNO3 decreases the adsorption of gaseous oxygen and its dissociation, and also decreases the reactivity of the lattice oxygen [19]. In our case, it is known that the CeO2 redox capacity leads to a more active catalyst than MgO.

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The activity of all these catalysts supported on cordierite with a loose contact is lower than in the case of a tight contact, as can be concluded by comparison of these results with those shown in previous studies [3, 4]. Our group studied the powder K/CeO2 catalysts and observed higher combustion temperatures under loose contact mode. It was observed that a soot burning range of 300 - 450 °C and Tmax of approximately 380 ºC in a TPO profile obtained with tight contact, shifted to 350 - 500 °C in loose contact with Tmax of 469 °C. The powder catalyst studied was a K/CeO2 sample with 7 wt.% of K, prepared with KNO3, and using a catalyst:soot mass ratio of 20:1. According to those previous studies [3, 4], in loose contact some soot particles react with oxygen by a non catalytic reaction mechanism. The temperatures obtained with loose contact with powder catalysts

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are similar to the results obtained using monolith supported catalysts. On the other hand, the temperatures obtained for catalyst KNO3/CeO2/cordierite are similar to the results reported by F. Mao et al. [10], namely, Tmax of 435 °C and 433 °C for K/Ce mole ratios of 0.25 and 0.5 respectively, while we found a Tmax of 443 °C for a mole ratio of 0.33 in M2 sample.

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In Fig. 2, the peak that comes out below 200 °C corresponds to CO2 desorption [20].

The X-ray diffractograms corresponding to the prepared catalysts are shown in Fig. 3,

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both for fresh (A) and used (B) catalysts. The cordierite (Mc) pattern shows the peaks that correspond to the support, as reference. The spectra obtained with CeO2 supported on the

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monolith, containing 0.218 g CeO2 (M1), displays peaks corresponding to cerium oxide, with peaks corresponding to the cordierite support with lower intensity. The diffractogram obtained with ceria supported on the cordierite (not shown) displays peaks at

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approximately 28.57, 33.11, 47.5, 56.4, 59.15, 69.48, 76.78, 79.16, 88.53 and 95.6 º. The diffractograms of the K/CeO2/cordierite samples (M2, M3 and M4) are very similar to M1.

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The potassium-containing compounds were not detected by XRD because of their extremely low content in the catalysts. The presence of potassium or the use of the catalysts in a TPO analysis does not affect the phases detected by XRD, as indicated by

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the results shown in Fig. 3 A and B.

The FTIR spectra obtained with fresh and used K/CeO2/cordierite catalysts are presented

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in Fig. 4. As mentioned above, the spectrum of cordierite displays strong signals at wave numbers lower than 1300 cm-1, and this introduces interference in the analysis of other species present in these catalysts and, therefore, only some bands can be identified. The KNO3 characteristic IR vibrational wave numbers are 1433 (sh), 1384 (s), 1354 (sh), 1273

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(sh), and 826 (m) cm-1 [2]. The FTIR spectra obtained with KNO3/CeO2/cordierite fresh catalyst (M2), displays bands at 1385 and 826 cm-1 that correspond to KNO3. These bands have very low intensity after using the catalyst in a TPO experiment. The catalyst prepared using K2CO3 (M4) presents bands at 1640 cm-1 that correspond to the vibrational mode of adsorbed water molecules but does not present the characteristic bands due the potassium precursor. According to Aneggi et al. [18], the band at 1400 cm-1 could indicate the formation of C-O-K. This band has a lower intensity in the M4 catalyst after being used in a TPO experiment. The M3 catalyst, prepared using KOH, has a very similar spectrum as the catalyst prepared using K2CO3 (M4). This could be related to a carbonation that occurs

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during catalyst preparation. In the M3 and M4 samples, the band of adsorbed H2O molecules was observed at 1640 cm-1. The K/CeO2/cordierite samples were also analyzed by optical microscopy before and after soot combustion. The images of M2, M3 and M4 can be observed in Fig. 5. A heterogeneous distribution is observed in all fresh catalysts (M2f, M3f and M4f), with an

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accumulation of crystals in the channel mouth. After soot combustion, these crystals cannot be observed. It has to be kept in mind that these samples were calcined at 400 째C,

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and during the TPO experiments the temperature was increased up to 600 째C. In all cases, similar results were observed, even though the potassium precursors were different. In

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consequence, the Optical Microscopy indicates that there is a redistribution of the catalytic components inside the cordierite, due to the treatment at high temperature. Since the K/CeO2/cordierite catalysts used in this study do not deactivate during the TPO analyses,

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the potassium loss in a significant amount can be neglected at temperatures lower than 600 째C.

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In order to obtain more information regarding the redistribution of the active phase observed by Optical Microscopy, the catalysts were studied by SEM. Results are shown in Figs. 6 and 7. The fresh M2 and M4 samples, prepared using KNO3 and K2CO3

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respectively, show crystals deposited on the channel walls (see Fig. 6). However, these crystals disappeared in both samples after soot combustion by TPO. The crystals are not

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observed in the catalyst M3 prepared with KOH. In Fig. 6, it can be observed that there is a difference in surface crystal morphology inside the channels, depending upon the potassium precursor used in the preparation. Therefore, the crystal structures depend on the potassium precursor. Cross sections of channel walls are shown in Fig. 7, for samples

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M2, M3 and M4, before and after soot combustion. This figure shows the surface of inner walls near the channel edges. For the fresh M2 catalyst, the penetration of ceria in the cordierite porous (point A of Fig. 7) can be observed, detected by coupling the SEM observation with microprobe analysis. The solution wets the channels walls and can easily diffuse into the pores by capillary action, forming the catalyst inside the pores. This penetration establishes a better anchoring of the layer on the substrate surface, as demonstrated with the low loading loss during the mechanical stability test. On the other hand, the view of channel edges shows a clear decrease in crystal size in the M2 catalyst after the TPO experiments (Fig. 7). However, the other two catalysts do not show a trend regarding the crystals size before and after the TPO analyses. A similar analysis (SEM

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coupled

with

microprobe

analysis)

is

shown

for

KNO3/CeO2/cordierite

and

KOH/CeO2/cordierite (M2 and M3 respectively) in Fig. 8 for the channel surface. Different points of the sample were analyzed by EPMA to determine the relative concentration of potassium and cerium, as indicated in Figs. 7 and 8. Table 3 presents the relative concentrations at these points (A-Q). The presence of cerium is observed in the

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entire surface. A similar relative concentration of cerium can be seen at surface points with different morphology. For example, different points as C, K, L, N and Q have the highest

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cerium relative concentration for samples M2 and M3, respectively. On the other hand, the surface exposed by a coating crack at point H has a similar concentration of cerium as

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point F (Fig. 7, Table 3). Since the cerium impregnation is obtained by several immersions into the Ce(NO3)3 solution, the coating of CeO2 could be thought as a surface formed by different layers. Consequently, it can be assumed that there is good cerium dispersion over

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the cordierite monolith. In most of the points indicated in Figs. 7 and 8, the potassium relative concentration is over 1%. The highest concentration is observed in crystals

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identified with points J and P (Fig. 8, Table 3). According to these results, a relationship between the crystal morphology and the presence of cerium or potassium is not apparent. In order to analyze the loss of active species during the heating in TPO cycles, the

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weight loss by calcination of a sample of K/CeO2/cordierite was studied. The sample M5 with a K/CeO2 mass ratio of 0.060 corresponding to 2.7 wt.%K and 38.4 wt.%Ce was

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repeatedly heated in an oven up to 600 ÂşC, in the presence of air. The weight loss of each cycle was gravimetrically determined. The results are shown in Fig. 9 and indicate that the degradation of the catalyst M5 starts in a significant way at heating cycle 15. Although the concentration analyzed by ICP in the final state of the sample (20 heatings) indicates that

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the loading of potassium and cerium is 2.24% and 25% respectively. Therefore, the weight loss determined on this catalyst might be attributed to monolith breaking by handling and temperature, discarding the total loss of active species. This result corroborates the similar TPO profiles obtained after several cycles. In order to study the catalyst stability under more severe conditions, the activity in soot combustion of three samples of KNO3/CeO2/cordierite (M6, M7 and M8) was tested by TPO after treatment at high temperatures. Samples M6 and M7 were treated in air flow during 1 h and 24 h at 800°C, respectively. In sample M8, the water effect was analyzed by ageing the catalyst during 1 h in air-(10%) water stream at 800 °C. The K/CeO2 ratios of fresh samples are presented in Table 1. The temperatures of soot combustion for the

Page 14 of 31


14

aged catalysts are shown in Table 2. The Tmax shifted to higher temperatures in the three aged samples (532, 529 and 528 °C for M6, M7 and M8, respectively) compared to M2 catalyst (443 °C), indicating that the catalyst deactivated during the severe ageing treatments carried out in each sample. The TPO profiles obtained with the thermallytreated catalysts were very similar among them (Table 2, figure not shown). Only a little

ip t

difference in Tf is observed according to the treatment. In sample M6 calcined at 800 °C during 1 h, the Tf was 553 °C while the sample treated at the same temperature during 24

cr

h had a Tf of 587 °C. A similar temperature, 576 °C, was obtained for M8 treated in wet air stream at 800 °C during 1 h. It is interesting to note that the latter two treatment

us

methods (air at 800 °C during 24 h, wet stream at 800 °C during 1 h) had a similar effect on the behavior of the KNO3/CeO2/cordierite catalyst for soot combustion. The similar catalytic behavior of aged catalysts as M1 and even Mc, implies that the potassium

an

volatility is the reason for the deactivation. Similar conclusions were obtained by Aneggi et al. [18] for a 10%K/CeO2 powder catalyst submitted to severe ageing conditions (50 h

M

under air and (10%) water at 750 °C). However, these authors did not observe a great difference in activity when they compared the fresh samples with the catalyst after mild ageing conditions (12 h under air at 750 °C), using the TPO test with catalyst-soot in tight

ed

contact. On the other hand, Wu et al. [21] showed that the activity of a powder sample of 20%KNO3/CeO2 decreased under loose contact conditions from 398 to 545 °C of Tmax,

ce pt

after an ageing treatment at 800 °C during 24 h in flow air. Since more than half of potassium nitrate was evaporated, the authors postulated that the activity decrease could be ascribed to the loss of surface active oxygen and supported potassium salt. The weight losses of samples M6, M7 and M8 were determined in order to evaluate the

Ac

potassium sublimation rate. The values of 10.8, 9.04 and 5.5% were higher than the %K in monolith M6, M7 and M8 catalysts, respectively. In consequence, both potassium loss and monolith catalyst degradation occurred during the thermal treatment and sample handling. On the other hand, the TPO profiles show that despite the thermal treatment, the initial temperatures of the aged samples M6, M7 and M8 are similar to that of M2. These results indicate that not all the potassium loading is lost during the ageing treatments. The analysis of final potassium loading by ICP determined that the loss is approximately 60% in the M6, M7 and M8 catalysts. In consequence, a mechanical degradation of the monolith catalysts by temperature and handling occurred. Similar results were obtained by Wu et al [21], who observed a loss of 69% of potassium nitrate after ageing treatment at

Page 15 of 31


15

800 ºC during 24h in flow air. The initial temperatures (Ti) of the aged catalysts is similar to that of M2, and, consequently, there is no complete loss of the KNO3/CeO2/cordierite catalytic activity.

Conclusions

ip t

The preparation of the K/CeO2/cordierite catalysts by impregnation method in an aqueous solution of Ce(NO3)3 and different potassium precursors (KNO3, K2CO3 and

cr

KOH), produces active catalysts for soot combustion. This study shows the formation of CeO2 over cordierite after calcination at 700 ºC of the support impregnated with Ce(NO3)3

us

solution. The effect of the potassium precursor over structured catalysts was similar to that observed for K/CeO2 powder samples. The activity decreases in the following order, depending upon the potassium precursor: KNO3 > K2CO3 > KOH. On the other hand, no

an

deactivation was observed after several cycles of the soot combustion reaction. The characterization analysis by X-ray diffraction showed that there is no effect of

M

potassium over ceria structure. The optical microscopy and scanning electron microscope determine a redistribution of the crystals in the monolith. This redistribution can be produced during heating in reaction or by the reaction with impregnated soot. Despite the

ed

high volatility of potassium, its weight loss was discarded during the gravimetric analysis after heating at 600 °C. A low weight loss during reaction can be ascribed to the

ce pt

degradation of the catalytic monolith. This result confirms the stability of the K/CeO2/cordierite at temperatures lower than 600 ºC. Since a real diesel engine could reach extreme temperatures during operation, the activity of KNO3/CeO2/cordierite catalysts after ageing at 800 °C was studied. It was determined that the catalytic behavior

Ac

of aged KNO3/CeO2/cordierite is approaching CeO2/cordierite, with a low potassium loading (approximately 40% of the initial weight) that leads to a low activity at low temperatures.

Acknowledgments The authors wish to aknowledge the financial support received from ANPCyT and PICT 14-38391. Thanks are also given to Prof. Elsa I. Grimaldi for the English language editing. References

Page 16 of 31


16

[1] E.E. Mir贸, F.Ravelli, M.A. Ulla, L.M. Cornaglia, C.A. Querini, Catalytic combustion of diesel soot on Co, K supported catalysts, Catal. Today 53 (1999) 631-638. [2] V.G. Milt, C.A. Querini, E.E. Mir贸, M.A. Ulla, Abatement of diesel exhaust pollutants: NOx adsorption on Co,Ba,K/CeO2 catalysts, J. Catal. 220 (2003) 424-432. [3] M.A. Peralta, V.G. Milt, L.M. Cornaglia, C.A. Querini, Stability of Ba,K/CeO2

ip t

catalyst during diesel soot combustion: Effect of temperature, water, and sulfur dioxide, J. Catal. 242 (2006) 118-130.

cr

[4] M.A. Peralta, M.S. Gross, B.S. S谩nchez, C.A. Querini, Catalytic combustion of diesel soot: Experimental design for laboratory testing, Chem. Eng. J. 152 (2009) 234-

us

241.

[5] M.S.Gross, M.A. Ulla, C.A. Querini, Catalytic oxidation of diesel soot: New characterization and kinetic evidence related to the reaction mechanism on K/CeO2

an

catalysts, App. Catal. A: Gen. 360 (2009) 81-88.

[6] M.A. Peralta, M.S. Gross, M.A. Ulla, C.A. Querini, Catalyst formulation to avoid

M

reaction runaway during diesel soot combustion, App. Catal. A: Gen. 367 (2009) 59-69. [7] B.A.A.L. van Setten, J.M. Schouten, M. Makee, J.A. Moulijn, Realistic contact for soot with an oxidation catalyst for laboratory studies, Appl. Catal. B: Environ. 28 (2000)

ed

253-257.

[8] J.P.A. Neeft, M. Makee, JA. Moulijn, Catalysts for the oxidation of soot from diesel

ce pt

exhaust gases. I. An exploratory study, Appl. Catal. B: Environ. 8 (1996) 57-78. [9] M. Hori, M. Horiuchi, Method of the purification of the exhaust gas from a lean-burn engine using a catalyst, US Patent 6555081B2 (2003). [10] F. Mao, C.G. Li, R. Ziebarth, Catalyzed diesel soot filter and process, US Patent

Ac

7713909B2 (2010).

[11] C.G. Li, F. Mao, Zone catalyzed soot filter, US Patent 7772151B2 (2010). [12] J.M. Zamaro, M.A.Ulla, E.E. Mir贸, The effect of different slurry compositions and solvents upon the properties of ZSM5-washcoated cordierite honeycombs for the SCR of NOx with methane, Catal. Today 107-108 (2005) 86-93. [13] S. Yasaki, Y. Yoshino, K. Ihara, K. Ohkubo, Method of manufacturing an exhaust gas purifying catalyst, U.S. Patent 5,208,206 (1993). [14] S.C. Fung, C.A. Querini, A highly sensitive detection method for temperature programmed oxidation of coke deposits: Methanation of CO2 in the presence of O2, J. Catal. 138 (1992) 240-254.

Page 17 of 31


17

[15] F. Vratny, Infrared Spectra of Metals Nitrates, App. Spectrosc. 13 (1959) 59-70. [16] C.A. Querini, M.A. Ulla, F. Requejo, J. Soria, U.A. Sedrán, E.E. Miró, Catalytic combustion of diesel soot particles. Activity and characterization of Co/MgO and Co,K/MgO catalysts, Appl. Catal. B 15 (1998) 5-19. [17] M.S. Gross, Diesel engine emissions: Catalytic soot combustion mechanism and

ip t

kinetic, PhD Thesis, UNL, Santa Fe, Argentina (2010). [18] E. Aneggi, C. de Leitenburg, G. Dolcetti, A. Trovarelli, Diesel soot combustion

cr

activity of ceria promoted with alkali metals, Catal. Today 136 (2008) 3-10.

[19] R. Jiménez, X. García, C. Cellier, P. Ruiz, A. Gordon, Soot combustion with

us

K/MgO as catalyst II. Effect of K-precursor, Appl. Catal. A 314 (2006) 81-88.

[20] Y. Zhang, Y. Quin, X. Zou, Preparation, characterization and catalytic activity studies CeO2-K diesel soot oxidation catalysts loaded on porous Al2O3 substrate using

an

water-immiscible solvent, Catal. Commun. 8 (2007) 1675-1680.

[21] X. Wu, D. Liu, K. Li, J. Li, D. Weng, Role of CeO2–ZrO2 in diesel soot oxidation

Ac

ce pt

ed

M

and thermal stability of potassium catalyst , Catal. Commun. 8 (2007) 1274-1278.

Page 18 of 31


18

FIGURE CAPTIONS

Fig. 1. Infrared spectra of pure cordierite (Mc), dried cerium structured catalyst (MCed),

ip t

calcined cerium structured catalyst (MCec) and Ce(NO3)3 precursor. Fig. 2. Temperature-programmed oxidation (TPO) profiles of K/CeO2/cordierite samples

cr

during soot combustión.

us

Fig. 3. XRD diffractograms of Mc, M2, M3 and M4. (A) Fresh catalysts, (B) Catalysts used in TPO experiments. ( Peaks of CeO2)

an

Fig. 4. Infrared spectra of cordierite (Mc) and the different K/CeO2/cordierite catalysts (M2, M3 and M4) before and after soot combustion (Mf: fresh catalysts, Mu: catalysts

M

after TPO experience,  adsorbed H2O,  KNO3,  C-O-K).

reaction, respectively.

ed

Fig. 5. Microscopic optical images of M2, M3 and M4 sample, fresh and after TPO

ce pt

Fig. 6. SEM images of top view of channels of M2, M3 and M4 samples, fresh and after TPO reaction, respectively. Figures in the right upper corner correspond to details of the inner surface on the bottom wall.

Ac

Fig. 7. SEM images of channel surface near the edges of M2, M3 and M4 samples, fresh and after TPO reaction, respectively.

Fig. 8. SEM images and microprobe analysis (EPMA) of M2 and M3 samples in surface section: fresh and after TPO reaction, respectively.

Fig. 9. Weight loss percent of M5 catalyst with the heating cycles at 600 °C.

Page 19 of 31


Table(s)

19

Table 1. Structured catalysts prepared using cordierite support. Nominal concentrations

Catalyst

gCeO2

gK/gCeO2

ip t

of cerium and potassium, and potassium precursor used in the preparation.

Potassium

cr

precursor Mc

--

--

M1

0.218

--

M2

0.196

0.076

KNO3

M3

0.092

0.093

KOH

M4

0.103

0.070

K2CO3

M5

0.106

0.060

KNO3

M6

0.192

0.103

KNO3

0.223

0.041

KNO3

0.193

0.077

KNO3

us

an M

Ac

ce pt

M8

ed

M7

--

Page 20 of 31


20

Table 2. Initial, maximum and final temperatures (Ti, Tmax and Tf respectively)

ip t

obtained during soot combustion by temperature programmed oxidation (TPO).

Ti (ยบC)

Tmax (ยบC)

Tf (ยบC)

Mc

400

540

565

M1

405

518

M2

330

443

M3

385

467

M4

355

M6

320

M7

330

M8

320

cr

Catalyst

580

an

us

525 550 520

532

553

M

448

587

528

576

Ac

ce pt

ed

529

Page 21 of 31


21

Table 3. Relative ratio of different points of M2 and M3 samples during microprobe analysis (EPMA).

Relative concentration % (p/p) Ce

Mg

Al

A

1

78

3

9

B

1

20

8

C

1

95

0

D

2

63

4

E

<1

54

F

1

83

G

1

44

H

1

82

I

1

J

87

L

0

4

13

18

6

18

22

0

9

7

7

21

27

0

8

9

88

0

6

5

6

1

4

2

ed

an

us

42

M

9

1

96

0

1

2

0

100

0

0

0

12

71

2

7

8

Ac

M

Si

29

ce pt

K

cr

K

ip t

Point

N

2

96

0

1

1

O

6

59

7

15

13

P

83

8

2

4

3

Q

1

94

<1

3

2

Page 22 of 31


i

Figure(s)

MCed

ce pt

ed

   

MCec Mc

Ac

Transmittance (a.u.)

Ce(NO3)3

M an

us

cr

FIG 1

4000

3000

2000

1000 -1

wave number (cm ) Page 23 of 31


i ed

M4

ce pt

M3 M2

Ac

FID signal (a.u.)

M an

us

cr

FIG 2

M1 Mc

100

200

300

400

500

600

Temperature (ยบC) Page 24 of 31


i 

A



10

20

Ac

ce pt

Intensity (a.u.)

ed

30

40

50

B



M4

M3

M2

M3 M2

M12b

Mc

Mc 60

2

70

80

90

100

M4

Intensity (a.u.)

M an

us

cr

FIG 3

10

20

30

40

50

60

70

80

90

100

2

Page 25 of 31


i 

ed

M2u M3f

 

Ac

ce pt

Transmittance (a.u.)

M2f

M an

us

cr

FIG 4

M3u

M4f

  

M4u Mc

4000

3000

2000

1000

Wave number (cm-1) Page 26 of 31


i cr

M2u

M3f

M3u

M4f

M4u

Ac

ce pt

ed

M an

us

M2f

FIG 5

Page 27 of 31


FIG 6

M an

us

cr

i

Figure(s)

M2u

Ac

M3f

ce pt

ed

M2f

M3u

Page 28 of 31

M4f

M4u


Figure(s)

C

M an

us

E

B

A M2f

H

FIG 7

cr

i

D

I F

G

Ac

M3f

ce pt

ed

M2u

M3u

Page 29 of 31

M4f

M4u


i M2f

M an

J

us

cr

FIG 8

P

ce pt

10m

ed

K

10m

M

L 10m M3u

M3f

Ac

O

N

M2u

Q

P

10m

Page 30 of 31


i cr M an

us

FIG 9

ce pt

ed

10

5

Ac

Weight loss (%)

15

%K

0

0

5

10

15

20

Number of heatings at 600째C

Page 31 of 31


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