Energy Fuels 2010, 24, 4766–4780 Published on Web 08/09/2010
: DOI:10.1021/ef100581z
Experimental Microkinetic Approach of the Catalytic Oxidation of Diesel Soot by Ceria Using Temperature-Programmed Experiments. Part 1: Impact and Evolution of the Ceria/Soot Contacts during Soot Oxidation Badr Bassou,† Nolven Guilhaume,† Karine Lombaert,‡ Claude Mirodatos,† and Daniel Bianchi*,† †
Institut de Recherche sur la Catalyse et l’Environnement de Lyon (IRCELYON), UMR 5256 CNRS, Universit e Claude Bernard Lyon I, Bat. Chevreul, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne, France, and ‡Renault, Diesel Innovative Catalytic Materials, Direction de l’ing enierie Mat eriaux, 1 All ee Cornuel, 91510 Lardy, France Received May 8, 2010. Revised Manuscript Received July 19, 2010
The present study is dedicated to an experimental microkinetic approach of the catalyst oxidation of the diesel soot using a filter coated with ceria. To mimic the situation encountered in this process, mechanical ceria/soot mixtures have been prepared according to the tight and loose contact concepts described in the literature with ceria/soot ratio R > 1. Diesel soots prepared on an engine test bench and commercial soot have been used. The evolution of the ceria/soot contacts (via the amount of oxygen transferred from ceria to soot) with the progressive oxidation of the soot is followed using temperature-programmed experiments (denoted as TPEs) that provide the rate of CO2 and CO productions [denoted as RCO2(T) and RCO(T)] during the increase in the temperature in helium in the range of 300-1100 K. During the first TPE, different surface processes implying pure soot and ceria contribute to RCO2(T) and RCO(T), making the evaluation of the oxygen transfer difficult. It is shown that these difficulties are suppressed by performing, on the same ceria/ soot sample, successive cycles constituted by a TPE followed by adsorption of O2 at 300 K that leads to the progressive oxidation of the soot. After three cycles, it is shown that, whatever the ceria/soot mixtures, the amount of oxygen that can be transferred from ceria to soot remains constant. This indicates that the ceria/ soot surface contacts do not change during the soot oxidation, which is a conclusion consistent with recent literature data on environmental transmission electron microscopy (TEM). However, the amount of oxygen provided by ceria and available for the soot oxidation is dependent upon the type of ceria/soot mixtures, and this controls the performances of the catalyst evaluated by the decrease of the light-off temperature of the soot in a flow rate of 30% O2/He. These conclusions are used in part 2 (10.1021/ef100582w) to provide a detailed kinetic modeling of the TPE experiments for the different ceria/soot mixtures, focusing on the key role of the ceria/soot contacts on the rate of soot oxidation. This provides a consistent formalism to understand the impact of the types of catalyst/soot mixtures on the performances.
accumulation of the diesel soot is due to the fact that the temperature of the filter is lower than that of the non-catalytic oxidation of the soot during different driving conditions. To limit the pressure drop in the exhaust line to an acceptable level, a periodic regeneration of the filter must be performed via the oxidation of the carbonaceous chemical species.1-3 The physicochemical properties of the diesel soot (i.e., its structure, texture, and composition, such as the carbon/oxygen ratio and the SOF/VOF) are dependent upon parameters linked to the engine (its design, the composition and injection method of the fuel, and the engine load),5-7 and the filter (its temperature and the presence upstream of a conventional diesel oxidation catalyst, denoted as DOC).1-3,8 However, these differences have a limited impact on the dependence of the rate of the non-catalytic oxygen oxidation of the diesel soot on the reaction temperature; the rate of oxidation is significant at T g ≈723 K.1,2 This explains that commercial soots, such as Printex U from Degussa (hexane-flame soot),
1. Introduction The different legislations dedicated to diesel particulate matter (denoted as PM) emissions of passenger cars (i.e., for Europe, 0.005 g of particulates/km since 2009) have led to the development of different post-treatment technologies mainly based on the use of a filter (i.e., wall-flow filter constituting a monolith of either silicon carbide or cordierite).1-3 The composition of the PM deposit is complex2,3 with different C- and metal-oxide-containing particles from nano- to micrometer size associated with adsorbed/condensed chemical species (i.e., hydrocarbon, sulfuric acid, and water), constituting the soluble and/or volatile organic fractions of the PM (denoted as SOF/VOF, respectively).4 Elementary analysis indicates that C-containing particles (denoted as diesel soot) constitute often the main fraction of the PM deposit. The *To whom correspondence should be addressed. Telephone: 0033472431419. Fax: 0033472448114. E-mail: daniel.bianchi@ ircelyon.univ-lyon1.fr. (1) Koltsakis, G. C.; Stamatelos, A. M. Prog. Energy Combust. Sci. 1997, 23, 1–39. (2) van Setten, B. A. A. L.; Makkee, M.; Moulijn, J. A. Catal. Rev. 2001, 43, 489–564. (3) Fino, D.; Specchia, V. Powder Technol. 2008, 180, 64–73. (4) Collura, S.; Chaoui, N.; Azambre, B.; Finqueneisel, G.; Heintz, O.; Krzton, A.; Koch, A.; Weber, J. V. Carbon 2005, 43, 605–613. r 2010 American Chemical Society
(5) Tree, D. R.; Svensson, K. I. Prog. Energy Combust. Sci. 2007, 33, 272–309. (6) Virtanen, A. K. K.; Ristimaki, J. M.; Vaaraslahti, K. M.; Keskinen, J. Environ. Sci. Technol. 2004, 38, 2551–2556. (7) Maricq, M. M. Aerosol Sci. 2007, 38, 1079–1118. (8) Stanmore, B. R.; Brilhac, J. F.; Gilot, P. Carbon 2001, 39, 2247– 2268.
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can be used as a model of diesel soot. However, the amount and composition of the SOF/VOF are cited as the possible source of the erratic low-temperature regenerations of the filter.1,10 Considering practical conditions, two main options (that are not exclusive) are proposed for the regeneration of the filter:1-3 (i) the periodic increase of the temperature of the soot by a heat injection using, for instance, either an electrical heater or the catalytic oxidation of hydrocarbons using a DOC and/or (ii) the decrease in the temperature of the soot oxidation via a solid catalyst. This last method is favorable for different reasons, such as (i) a continuous oxidation of the soot can be realized, decreasing the frequency of the regeneration that improves the fuel efficiency of the vehicle, and (ii) the durability of the filter is increased because of a limitation of the temperatures during the regeneration periods.1-3 The contact between the catalyst and soot can be obtained according to different technologies. The catalyst can be (i) synthesized in the engine cylinder simultaneously to the formation of the soot using a fuel with a precursor catalyst additive according to the fuel-borne catalyst process (denoted as FBC),11-13 (ii) deposited on the soot by the vaporization of molten salts placed up stream of the filter,14-16 and (iii) coated on the filter material.17-19 The two first methods favor the soot/catalyst weight ratio and their contacts, whereas a coated filter permits using a large variety of solid catalysts. The catalyst-coated filter process (denoted as CCF) is a candidate for application in the near future.3 In previous studies, an experimental microkinetic approach of the FBC process (using a cerium-based additive) has been performed to reveal the surface elementary steps that control the rate of CO2 and CO productions.20-22 The first step of this approach is the selection of the elementary steps of a plausible kinetic model. For the soot oxidation, we have selected20-22 the unified mechanism proposed by the Moulijn group for the non-catalytic and catalytic oxidation of carbonaceous materials.23,24 It has been shown that (i) the rate of the oxygen transfer from the catalyst particles controls the oxidation of the soot20,21 and (ii) the oxygen species come from Ce- and S-containing particles denoted as CexSyOz because of the presence of 350 ppm (by weight) of sulfur in the fuel.20-22 The conclusion of the microkinetic approach was that the
oxidation temperature of the soot was limited by the oxygen transfer from the CexSyOz particles, and in line with this view, we have shown how and why other Ce-containing solids (without S) significantly decreased the oxidation temperature of the soot.22 The present study is a contribution to the experimental microkinetic approach of the CCF process using ceria as the catalyst to establish a comparison to the FBC process.20-22 A priori, the role of ceria is expected to be similar for the two processes; it provides oxygen species to the soot at a temperature lower than that of the activation of O2 by the soot. However, significant differences in the rate of this transfer are likely considering the origin of the ceria/soot contacts that may affect their natures, numbers, and evolutions during the soot oxidation. The understanding via experimental procedures of the contribution of these parameters on the soot oxidation constitutes a crucial step toward the mastering of the CCF process. This is the main aim of the present study using temperature-programmed experiments (denoted as TPEs) in the presence and absence of O2, as performed previously by the FBC process.20-22 These experiments provide the rates of CO2 and CO productions [denoted as RCO2(T) and RCO(T), respectively] as a function of the reaction temperature that are exploited using C and O mass balances to obtain the rate of the oxygen transfer from ceria to the soot.20-22 However, the ceria/soot weight ratios (denoted as R) of the CCF and FBC processes are significantly different R > 125,26 and R , 1 (i.e., R = 0.14)20-22, respectively, leading to difficulties in the exploitation of the TPE data for the CCF process; different surface processes on ceria may contribute to the RCO2(T) and RCO(T) profiles, such as the adsorption/desorption of CO2 and the oxidation of CO. The characterization of the oxygen transfer from CeO2 to soot imposes differentiating the contributions of the different surface processes to the experimental data. The aims of part 1 of the present study are, first, to define the experimental conditions, allowing for the characterization of this oxygen transfer, and second, to show how the ceria/soot contacts, which control the oxygen transfer, evolve during the progressive catalytic oxidation of the soot. This allows us in part 2 (10.1021/ef100582w) to develop the experimental microkinetic approach of the CCF process using a detailed kinetic modeling of RCO2(T) during the TPE in the absence and presence of O2 that considers explicitly the ceria/soot contacts as key kinetic parameters of the soot oxidation.
(9) Neeft, J. P. A.; Makkee, M.; Moulijn, J. A. Fuel 1998, 77, 111–118. (10) Kandylas, I. P.; Stamatelos, A. M. Ind. Eng. Chem. Res. 1999, 38, 1866–1876. (11) Krishna, K.; Makkee, M. Top. Catal. 2007, 42-43, 229–236. (12) Campenon, T.; Wouters, P.; Blanchard, G.; Macaudiere, P.; Seguelong, T. SAE Tech. Pap. 01-0071, 2004. (13) Ntziachristos, L.; Samaras, Z.; Zervas, E.; Dorlhene, P. Atmos. Environ. 2005, 39, 4925–4936. (14) van Setten, B. A. A. L.; van Dijk, R.; Jelles, S. J.; Makkee, M.; Moulijn, J. A. Appl. Catal., B 1999, 21, 51–61. (15) van Setten, B. A. A. L.; van Gulijk, C.; Makkee, M.; Moulijn, J. A. Top. Catal. 2001, 16/17, 275–278. (16) van Setten, B. A. A. L.; Spitters, C. G. M.; Bremmer, J.; Mulders, A. M. M.; Makkee, M.; Moulijn, J. A. Appl. Catal., B 2003, 42, 337–347. (17) Ambrogio, M.; Saracco, G.; Specchia, V. Chem. Eng. Sci. 2001, 56, 1613–1621. (18) Seipenbusch, M.; van Erven, J.; Schalow, T.; Weber, A. P.; van Langeveld, A. D.; Marijnissen, J. C. M.; Friedlander, S. K. Appl. Catal., B 2005, 55, 31–37. (19) Gorsmann, C. Monatsh. Chem. 2005, 136, 91–105. (20) Retailleau, L.; Vonarb, R.; Perrichon, V.; Jean, E.; Bianchi, D. Energy Fuels 2004, 18, 872–882. (21) Vonarb, R.; Hachimi, A.; Jean, E.; Bianchi, D. Energy Fuels 2005, 19, 35–48. (22) Bianchi, D.; Jean, E.; Ristori, A.; Vonarb, R. Energy Fuels 2005, 19, 1453–1461. (23) Moulijn, J. A.; Kapteijn, F. Carbon 1995, 33, 1155–1165. (24) Chen, S. G.; Yang, R. T.; Kapteijn, F.; Moulijn, J. A. Ind. Eng. Chem. Res. 1993, 32, 2835–2840.
2. Experimental Section 2.1. Materials: Preparation and Pretreatment Procedures. 2.1.1. Diesel Soots. Diesel soots, collected on SiC filters (200 cpsi with walls of 0.4 mm and a porosity of 45%), were obtained on an engine test bench fitted with a Renault 2 L diesel engine during a new European driving cycle (NEDC) using the fuel EN590 containing less than 10 ppm sulfur. At the inlet of the filters, the temperatures were in the range of 373-623 K. The soots have been collected in the absence and presence of a DOC that was placed in the front of the filter. High temperatures in the filter limited the amount of volatile organic compounds (VOCs) in the soot mainly formed (at least 95%) by of unburned lubricating oil,27 (25) Neeft, J. P. A.; Makkee, M.; Moulijn, J. A. Chem. Eng. J. 1996, 64, 295–302. (26) Neeft, J. P. A.; van Pruissen, O. P.; Makkee, M.; Moulijn, J. A. Appl. Catal., B 1997, 12, 21–31. (27) Sakurai, H.; Tobias, H. J.; Park, K.; Zarling, D.; Docherty, K. S.; Kittelson, D. B.; McMurry, P. H.; Ziemann, P. J. Atmos. Environ. 2003, 37, 1199–1210.
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gases because of either different surface processes or the desorption/decomposition of the adsorbed species only, respectively, (ii) temperature-programmed oxidation (TPO) and temperature-programmed reduction (TPR) using a flow rate of either a x% O2/He or y% CO/He gas mixture, and (iii) isothermal adsorption of a gas, such as O2, using x% O2/y% Ar/He gas mixtures (Ar was a tracer showing the beginning of the consumption of O2). Two experiments have been particularly used: (i) TPO of soot and ceria/soot mixtures in the presence of oxygen (weight of soot ≈ 0.02 g; 500 cm3/min of 30% O2/He; R = 30 K/ min) for the determination of the light-off temperatures of the non-catalytic and catalytic oxidation of the soots and (ii) successive TPE of ceria/soot mixtures followed by O2 adsorption at 300 K (denoted as TPE/O2 adsorption cycles). The procedure of these cycles was as follows: a C-containing sample (weight in the range of 0.1-0.3 g) was introduced into the microreactor and heated in helium (100 cm3/min) to T = 473 K for 2 h. Then, the sample was cooled to 300 K in helium, and the temperature was progressively increased (R = 30 K/min), leading to different surface processes, such as (i) the desorption (as CO2) of the carbonate species adsorbed on ceria, (ii) the desorption/decomposition (as CO and CO2) of the surface-oxygenated complexes of the soot (denoted as SOCs), (iii) the oxidation of CO by ceria, (iv) the oxygen transfer from ceria to soot, and (v) the soot oxidation. After the TPE, the solid was cooled to 300 K and a switch He f 1% O2/1% Ar/He (gas flow rate of 100 cm3/min) was performed to adsorb O2 (see more details on the quantification of the data in refs 20 and 21). After a purge in helium, a second TPE was performed and compared to the first TPE. These TPE/O2 adsorption cycles were repeated on the same solid sample to mimic the progressive oxidation of the soot. 2.2.2. Raman Spectrometry. Raman spectra of the samples were recorded with a LabRam HR Raman spectrometer (Horiba-Jobin Yvon) equipped with a confocal microscope. The 514.53 nm exciting line of a 2018 RM Arþ-Krþ laser (Spectra Physics) was focused using a 50 long working distance objective. The spectra collected using an 1800 grooves mm-1 grating were accurate within 2 cm-1. The spectral evolution with the laser power was previously examined to determine the most suitable power; under the microscope, the spectra at room temperature in air were recorded with a power of 2 mW to avoid dehydration. 2.2.3. Fourier Transform Infrared (FTIR) Spectroscopy of the Adsorbed Species on the Different Solids. The FTIR spectra, in the diffuse reflectance mode, were obtained using a hightemperature diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell with CaF2 windows associated with a collector assembly from Harrick Scientific Corporation (HVC-DR3, Praying Mantis type). The cell was connected to a classical gas flow regulation, control, and purification system.33 For strongly absorbing solids, such as soot, a dilution with KBr was used (ceria was studied without dilution). The analytical procedure was as follows: the solids (diluted or not) were slightly compressed and then crushed and sized to obtain particles of ≈100 μm diameter to reduce the specular reflectance participation to a reasonable low level.33,34 Then, ≈100 mg of particles were introduced in the sample holder of the DRIFTS cell positioned on top of an heating cartridge (T < 700 K), forming a layer >3 mm that is the limit to assume a “semi-infinite thick” sample.34,35 The sample was treated using gas flow rates (100 cm3/ min) at atmospheric pressure. The FTIR spectrometer was a Nicolet 6700 from ThermoScientific, with a MCT detector cooled to 77 K. The intensity of the spectra was provided in pseudoabsorbance; however, only semi-quantitative analysis was permitted because of the relative reflectance of the samples.33
while it is considered that the oxidation of a fraction of SO2 into SO3 leads to the presence of sulfuric acid.28 A commercial soot, Printex U from Degussa, has been used for a comparison to the homemade diesel soots. The soots were used either as prepared (or received for Printex U) or after a standardization procedure, 8 h at 673 K in air (details provided below). 2.1.2. Ceria. The CeO2 solid used for the catalytic oxidation of the soots was provided by Rhodia [HSA; BrunauerEmmett-Teller (BET) area, 325 m2/g]. The temperatureprogrammed desorption (TPD) in helium using a mass spectrometer as a detector (see below) of the as-received ceria led in addition to H2O, CO2, and CO peaks to the observation of two concomitant peaks of NOx (quantified as NO, 380 μmol/g) and O2 (212 μmol/g) at TM = 780 K. It has been shown that the decomposition of cerium nitrate led to similar observations but at a lower temperature TM = 566 K.22 This indicated that either adsorbed or occluded NOx species remain after the preparation of CeO2. To prevent their contribution to the TPE, CeO2 was treated in air at 873 K for 3 h as performed by other authors using a similar ceria material.29,30 However, this standardization led to a decrease in the surface area of ceria (156 m2/g) because of the limited stability of unpromoted CeO2.30 2.1.3. Ceria/Soot Mixtures. To facilitate the experimental procedure, mechanical ceria/soot mixtures have been used to mimic the situation encountered in the CCF process, in line with the “tight” and “loose” contact concepts defined by Neeft et al.25,26 Known amounts of soot and ceria were either grinded firmly in an agate mortar to obtain the tight contact (denoted as TC) or mixed with a spatula until an “apparent” homogenization (on the basis of the color of the mixture) to obtain the loose contact (denoted as LC). Two ceria/soot weight ratios have been studied (R = 10 and 1), leading to three mixtures denoted as TC-R = 10, LC-R = 10, and TC-R = 1. The R values were of the order of magnitude of the effective ratios used by Neeft et al. in their TGA analysis, in the range of 2.5-35.26 The ceria/soot mixtures were prepared using 0.1 g of soot. According to Neeft et al.,25,26 the loose contact was representative of coated filters. However, the a priori better homogeneity of the tight contact is favorable for the microkinetic approach. 2.2. Analytical Procedures. 2.2.1. Analytical System for Transient Experiments. This system has been described in detail previously.31,32 Mainly various valves allowed us to perform controlled switches between regulated gas flows in the range of 100-1000 cm3/min (at the atmospheric pressure), which passed through a quartz microreactor (volume of ≈2 cm3), filled by the C-containing sample (mass in the range of 20-300 mg). The gas composition (molar fractions) at the outlet of the reactor was determined using a quadrupole mass spectrometer (with an analysis frequency of 0.66 Hz) after a calibration procedure with gas mixtures of known compositions (the accuracy was dependent upon the complexity of the gas mixture; the range was 2-10%). Water was analyzed but not quantified because of different difficulties linked to its adsorption in the analytical system. The temperatures of the furnace and the solid sample were simultaneously recorded, using coaxial K-type thermocouples (diameter Φ = 0.5 mm). This analytical system allowed us to quantify the amount of gases either produced or consumed under different experimental conditions, such as (i) TPE and TPD designating experiments in a helium flow rate, at a increasing temperature (R in K/s), leading to the production of (28) Tobias, H. J.; Beving, D. R.; Ziemann, P. J.; Sakurai, H.; Zuk, M.; McMurry, P. H.; Zarling, D.; Waytulonis, R.; Kittelson, D. B. Environ. Sci. Technol. 2001, 35, 2233–2243. (29) Daturi, M.; Binet, C.; Lavalley, J. C.; Vidal, H.; Kaspar, J.; Graziani, M.; Blanchard, G. J. Chim. Phys. 1998, 95, 2048–2060. (30) Perrichon, V.; Laachir, A.; Abouarnadasse, S.; Touret, O.; Blanchard, G. Appl. Catal., A 1995, 129, 69–82. (31) Bourane, A.; Bianchi, D. J. Catal. 2002, 209, 114. (32) Bourane, A.; Dulaurent, O.; Bianchi, D. J. Catal. 2000, 195, 406– 411.
(33) Couble, J.; Gravejat, P.; Gaillard, F.; Bianchi, D. Appl. Catal., A 2009, 371, 99–107. (34) Li, B.; Gonzalez, R. D. Appl. Spectrosc. 1998, 52, 1488–1491. (35) Moradi, K.; Depecker, C.; Barbillat, J.; Corset, J. Spectrochim. Acta, Part A 1999, 55, 43–64.
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Figure 2. Evolution of the molar fractions of CO2 as a function of the furnace temperature during TPO of different ceria/Printex-U mixtures using the experimental conditions of Figure 1: (a) Pure Printex U, (b) TC-R = 10, (c) TC-R = 1, and (d) LC-R = 10.
Figure 1. Evolutions of the molar fractions of the gases and the temperatures of the solid sample TS and the furnace TF during the TPO (30 K/min) of Printex U using 500 cm3/min of 30% O2/He for TC-R = 10.
because of the ignition process.8 The ignition, characterized by the temperature Ti = 684 K (the corresponding temperature of the furnace is TFi = 649 K), is associated with (i) a strong consumption of O2 (1058 μmol), (ii) a strong production of CO2 (1457 μmol), and (iii) a small production of CO (8 μmol), leading to a CO2/CO ratio of 182. Similarly, data are obtained for the non-catalytic oxidation of Printex U; however, the ignition is detected at Ti = 852 K with a CO2/ CO ratio of 1.9. These data clearly show that ceria in TC-R = 10 decreases Ti by 168 K and favors the selectivity in CO2. The Ti values for ceria/soot mixtures are easily measured, and this parameter has been used to compare the effectiveness of various catalysts for soot oxidation via a highthroughput approach.36 However, Ti is dependent upon different chemical and physical processes; the ignition is provoked when the rate of energy production because of the soot oxidation becomes higher than the rate of the heat transfer.8,37 This allows us to reach rapidly a TS value for the non-catalytic oxidation of the soot. Considering this point, it is the evolution of CO2 and CO productions before the ignition that is more representative of the impact of CeO2 on the soot oxidation. However, the mass spectrometer (MS) background signal used to measure the molar fractions of CO and CO2 (m/e 28 and 44), associated with the low rates of CO and CO2 productions at low temperatures, render the determination of the exact temperature of the beginning of the soot oxidation difficult. This temperature (measured by either TF or TS) cannot be used as a clear and simple criterion for the comparison of the catalytic performances of different catalyst/soot mixtures. This explains the interest in the choice of Ti as a criterion, even if it is partially dependent upon physical properties of the solids (such as heat capacities and thermal conductivity).36 Figure 2 illustrates this point; it gives the evolution of the molar fraction of CO2 at the outlet of the reactor for different ceria/Printex U mixtures as a function of TF. The temperature at the beginning of the CO2 production can be only estimated, whereas the ignition temperature is determined with accuracy. For the noncatalytic oxidation of the soot (curve a), the CO2 production is detected at TFS ≈ 650 K and increases progressively until the ignition at TFi = 810 K (ΔT = TFi - TFS ≈ 160 K). For TC-R = 10, the CO2 production (curve b) is detected at
3. Results The experimental microkinetic approach of a heterogeneous gas/solid catalytic process is based on a plausible kinetic model that fixes the elementary steps that are studied by experimental procedures for the evaluation of their kinetic parameters (i.e., coverages of the adsorbed species, rate constants, and activation energies).20,21 This allows us to determine the elementary steps that control the global rate of the reaction in the absence of limitations by physical processes, such as diffusion, which is the situation at low temperatures for low rates of reaction [this fixes the window validity of the microkinetic, at low conversions of the reactants (< ≈15%)]. In previous studies dedicated to the FBC process using ceria,20-22 the plausible kinetic model was based on the unified mechanism for the non-catalytic and catalytic oxidation of C-containing materials proposed by the Moulijn group.23,24 Briefly, the kinetic model considers that (i) the soot oxidation proceeds by the desorption/decomposition of the SOCs and (ii) the role of the catalyst is to produce SOCs at temperatures lower than the pure soot via an oxygen transfer at the catalyst/soot contacts. This signifies that the evolution of these contacts with the soot conversion constitutes a key parameter of the microkinetic approach of the catalytic soot oxidation. For the FBC process, the ceria/soot contacts were assumed constant because of the high dispersion of Cecontaining particles on the soot.20,21 For the CCF process, these contacts are more dependent upon the experimental conditions (i.e., tight and loose contacts). The aim of the present study is to characterize these contacts and, particularly, their evolution during the soot oxidation by TPE procedures. 3.1. Impact of the Ceria/Soot Contacts on the Catalytic Oxidation of the Soots with O2. The impact of the nature of (i) the soot and (ii) the soot/ceria mixture on the O2 oxidation of the soots has been studied using TPO (30 K/min) procedures (weight of soot in the range of 18-22 10-3 g; 30% O2/He; 500 cm3/min). Figure 1 gives the evolutions of (i) the molar fractions of the gases and (ii) the temperatures TF and TS of the furnace and the sample, respectively, as a function of the duration of the TPO for TC-R = 10 using Printex-U (TC-R = 10 means tight contact with a ceria/soot weight ratio of R = 10). At low temperatures, the difference TF - TS is slightly positive (≈ þ3 K) because of the heat transfer, and then for TF > ≈600 K, this value decreases because of the soot oxidation detected by the CO2 production and the O2 consumption. At TS = 684 K, a sharp increase in TS is observed, corresponding to the rapid oxidation of the soot
(36) Iojoiu, E. E.; Bassou, B.; Guilhaume, N.; Farrusseng, D.; Desmartin-Chomel, A.; Lombaert, K.; Bianchi, D.; Mirodatos, C. Catal. Today 2008, 137, 103–109. (37) Lahaye, J.; Boehm, S.; Chambrion, P.; Ehrburger, P. Combust. Flame 1996, 104, 199–207.
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bands of the soot. This shows that there is an inhomogeneity in the composition of the loose contact mixture at the micrometer scale. The TC1 and TC2 Raman spectra (inset of Figure 3) of the TC-R = 10 display the bands of ceria, 460 cm-1, and soot, 1358 or 1594 cm-1, and this is the situation regardless of the position of the microprobe. However, the ratio of the intensities of the bands of the two solids, thus, their weight ratio, varies slightly with the position. This shows that the homogeneity of TC-R = 10 is better than that of LC-R = 10. This favors, a priori, the average number of contact points between soot and ceria. Note that the limited homogeneity of the present ceria/soot mixtures is consistent with the TGA data of Neeft et al., considering the oxidation of small amounts of catalyst/soot mixtures of R = 2; the effective ratio measured from TGA was in the range of 2.5-36.26 3.3. TPE/O2 Adsorption Cycles Using Pure Ceria. The comparison of the surface area of the soots (i.e., for the asreceived Printex-U, 110 m2/g) and ceria (150 m2/g after the standardization procedure) indicates that the surface properties of ceria (i.e., adsorption/desorption of CO2 and oxidation of CO) may contribute significantly to the RCO2(T) and RCO(T) curves during TPE of the ceria/soot mixtures (particularly for R = 10). This explains that ceria has been characterized using TPD and O2 adsorption at 300 K. Table 1 summarizes the different amounts of CO, CO2, and O2 detected during TPD/TPE experiments on ceria, soots, and ceria/soot mixtures. 3.3.1. TPD Using the MS System: Amount of Adsorbed Species on the Standardized Ceria. Figure 4 shows the evolutions of the gases, using the MS system, during the TPD of the standardized ceria (He = 100 cm3/min; R = 50 K/min). The two main compounds formed are (i) H2O (not shown) detected by a large m/e 18 signal in the Td range of 300-750 K and (ii) CO2 detected by a strong peak at 460 K (Figure 4), 200 μmol/g, followed by a smaller broad peak (total amount of 46 μmol of CO2/g) with shoulders at 680, 714, and 1040 K, indicating the presence of different adsorbed species with a large range of activation energies of desorption. The H2O and CO2 productions are associated (Figure 4) with three small hydrogen peaks at 424 K ( 473 K. The identification of the adsorbed species associated with the different TPD peaks in Figure 4 has been obtained using the DRIFTS cell. 3.3.2. TPD Using the DRIFTS Cell: Identification of the Adsorbed Species on Ceria. Figure 5 provides a selection of the recorded IR spectra during the TPD of the standardized ceria (He = 100 cm3/min; R = 12 K/min). The evolutions of the spectra are complex because of (i) the presence of several adsorbed species characterized by IR bands in the same wavenumber range (1700-1100 cm-1) and (ii) the formation of new adsorbed species. To facilitate the presentation, the spectra are discussed, from the highest (small number of IR
Figure 3. Raman spectra of (a) pure diesel soot and (b) pure fresh ceria. (Inset) TC1 and TC2 and LC1 and LC2, two Raman spectra of the TC-R = 10 and LC-R = 10 samples, respectively, at two locations.
TFS ≈ 560 K increases progressively until TFi = 649 K (ΔT ≈ 89 K). The presence of ceria allows for the production of CO2 at lower temperatures, and the faster rate of oxidation decreases ΔT. Curves c and d in Figure 2 show the CO2 productions during the TPO of LC-R = 10 and TC-R = 1 using Printex U; the CO2 productions are detected at TFS ≈ 625 and 650 K, respectively, whereas the ignition is detected at TFi = 752 K for the two mixtures, indicating that the oxidation rates of the soot are similar. The repeatability of the Ti values for TC-R = 10 (three experiments) is (10 K because of the fact that the average “homogeneity” of the mixture is satisfied using 20 mg of soot. Similar TPO experiments have been performed using the homemade diesel soots without observing significant differences with Printex U. 3.2. Homogeneity of the Soot/Ceria Mixtures. The TPO data in Figure 2 show that the catalytic oxidation of soot is dependent upon how the ceria/soot mixture is prepared. We have studied the homogeneity of the different soot/ceria mixtures at the micrometer range, using Raman spectrometry with microprobe analysis of 1 μm diameter. The Raman spectra of pure diesel soot and pure ceria (before the standardization) are shown in Figure 3 (spectra a and b, respectively). The two strong and broad overlapping bands at 1358 and 1594 cm-1 in spectrum a are similar to those observed by Sadezky et al.38 for different soots and ascribed to the first-order D (defect) and G (graphite) bands of disordered graphite structure, respectively. The strong absorption band at 460 cm-1 in spectrum b is characteristic of the first-order Raman-active mode of the cubic fluorite structure of ceria,39 whereas the band at 1030 cm-1 is probably due to the residual nitrate species in the fresh ceria.40,41 These bands are used to evaluate the homogeneity of the LC-R = 10 and TC-R = 10 ceria/soot mixtures by studying their intensities at different positions of the microprobe. The inset of Figure 3 shows two Raman spectra (denoted as LC1 and LC2) of LC-R = 10: LC1 reveals the presence of the bands at 1358 and 1594 cm-1 (soot) and 460 cm-1 (ceria), whereas LC2 displays only the characteristic (38) Sadezky, A.; Muckenhuber, H.; Grothe, H.; Niessner, R.; P€ oschl, U. Carbon 2005, 43, 1731–1742. (39) Choi, Y. M.; Abernathy, H.; Chen, H. T.; Lin, M. C.; Liu, M. Chem. Phys. Chem. 2006, 7, 1957–1963. (40) Uy, D.; O’Neill, A. E.; Weber, W. H. Appl. Catal., B 2002, 35, 219–225. (41) Sergent, N.; Epifani, M.; Pagnier, T. J. Raman Spectrosc. 2006, 37, 1272–1277.
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Table 1. Amounts of CO, CO2, and O2 Formed during TPD/TPE and O2 Re-adsorbed at 300 K after TPD/TPE on the Ceria, Soots, and Ceria/Soot Mixtures solid and experiment
CO (μmol/g)
CO2 (μmol/g)
O2 (μmol/g)
O2 adsorbed at 300 K (μmol/g)
CeO2, TPD-1 Printex U, TPD-1 Printex U standardized,a TPD-1 Printex U, standardized,a TPD-2 diesel soots standardized,a TPD-1 TC-R = 10,b TPE-1 TC-R = 10,b TPE-2 TC-R = 10,b TPE-9 LC-R = 10,b TPE-1 LC-R = 10,b TPE-2 LC-R = 10,b TPE-9 TC-R = 1,b TPE-1 TC-R = 1, TPE-2 TC-R = 1,b TPE-8
0 800 4010 115 4410 2608c 232c 134c 2568c 72c 40c 4400c 71c 4c
246 285 944 8 928 3700c 910c 653c 3987c 220c 66c 1550c 68c 80c
0 0 0 0 0 1d 15d 8d 0 12d 9d 0 ≈13d ≈7d
22 0 0 0 0 132d 93d 64d 128d 103d 46d 260d 110d 94d
Standardization = treatment in air at 673 K for 8 h. b Ceria/standardized Printex U mixture. c In μmol/g of soot. d In μmol/g of ceria (the amount of O available by gram of soot is obtained considering R). a
that IR bands of small intensities are detected at 2930, 2840, and 2723 cm-1 for Td = 473 K (Figure 5c), increasing until Td = 578 K (Figure 5e) and then decreasing and disappearing at 673 K (Figure 5f). These IR bands are characteristic of a small amount of formate species HCO2 ads (2840 cm-1 for ν-CH; 2930 and 2723 cm-1 combination IR bands)44,47 formed in the course of the TPD. The other IR bands of the formate species at ≈1365 cm-1 (νs-OCO), ≈1380 cm-1 (δ-CH), and ≈1570 cm-1 (νas-OCO)44,47 contribute to the different shoulders observed in the 1600-1200 cm-1 range. Other carbonate species are present on the surface at Td > 413 K; a shoulder of low intensity is detected at 1215 cm-1 that increases until 473 K (detected as a distinct IR band), decreases progressively, and disappears at 528 K. This IR band is characteristic of the δ-OH of a small amount of bicarbonate species HOCO2 ads. The other IR bands of this species at ≈3620 cm-1 (ν-OH), ≈1620 cm-1 (νas-OCO), ≈1400 (νs-OCO)43-45,48 contribute to the different shoulders. Between 413 K (Figure 5b) and 473 K (Figure 5c), two IR bands at 1564 and 1300 cm-1 decrease strongly; they can be ascribed to the νas-OCO and νs-OCO of bidentate carbonate45,48 detected at 300 K. The increase in Td from 300 to 413 K leads to the following main modifications (Figure 5): (i) a strong decrease in the IR bands at 1635 and 3390 cm-1, indicating the desorption of undissociated adsorbed water, H2Oads, (ii) a shift of the IR band from 1550 to 1564 cm-1, tentatively ascribed to the modifications of the interaction between the carbonate species (i.e., the bidentate) and adsorbed water H2Oads, (iii) the decrease in broad shoulders at 1353 and 1430 cm-1, and (iv) the appearance of an IR band at 1387 cm-1. This new IR band is not significantly modified by the increase in Td up to 473 K. Then, it decreases progressively in parallel to the IR bands of the bidentate carbonate. Its position corresponds to either the lowest or highest values of the νas-OCO and νs-OCO of carbonate species, respectively.44,45,48 Therefore, this IR band can be associated with an IR band situated at either 1300 cm-1 (νs-CO) or 1550 cm-1 (νas-CO). This last assignment seems consistent with the complex profile of the IR spectra in the range of 1650-1400 cm-1 (spectra a-c of Figure 5). We ascribe tentatively the IR band at 1387 cm-1 to the νs-OCO of a unidentate carbonate formed in parallel to the desorption of the bidentate carbonate. 3.3.3. Impact of the Surface Properties of Ceria on the Study of Ceria/Soot Mixtures. TPD experiments of ceria using MS and DRIFTS lead to the following views of the
Figure 4. Molar fractions of the gases during the TPD in helium (50 K/min) of the standardized ceria and the temperature of the solid TS as a function of the time on stream.
bands) to lowest Td values, using literature data on the identification of the adsorbed species.42-48 At 723 K, the spectrum g in Figure 5 reveals two IR bands at 3628 and 3485 cm-1, with a shoulder at 3690 cm-1, that are ascribed to isolated OH groups on different cerium sites42 and two IR bands at 1455 and 1374 cm-1 that can be ascribed to νas-OCO and νs-OCO of either monodentate43,44 or polydentate45 carbonate species, respectively. The contribution of ionic carbonate, CO32- (an IR band at ≈1460-1450 cm-1)44,46, to the IR band at 1455 cm-1 can be considered. In the Td range of 473-723 K, the IR bands of the OH groups (spectra c-g) decrease with the increase in Td associated with a slight shift to lower wavenumbers because of the progressive dehydroxylation of the ceria surface. Spectra c-f show (42) Laachir, A.; Perrichon, V.; Badri, A.; Lamotte, J.; Catherine, E.; Lavalley, J. C.; El Fallah, J.; Hilaire, L.; le Normand, F.; Quemere, E.; Sauvion, G. N.; Touret, O. J. Chem. Soc., Faraday Trans. 1991, 87, 1601–1609. (43) Li, C.; Sakata, Y.; Arai, T.; Domen, K.; Maruya, K.; Onoshi, T. J. Chem. Soc., Faraday Trans. 1989, 85, 929–943. (44) Bianchi, D.; Chafik, T.; Khalfallah, M.; Teichner, S. J. Appl. Catal., A 1993, 105, 223–249. (45) Binet, C.; Daturi, M.; Lavalley, J. C. Catal. Today 1999, 50, 207– 225. (46) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds, 2nd ed.; Wiley-Interscience, John Wiley and Sons: New York, 1969. (47) Li, C.; Sakata, Y.; Arai, T.; Domen, K.; Maruya, K.; Onishi, T. J. Chem. Soc., Faraday Trans. 1989, 85, 1451–1461. (48) Bianchi, D.; Chafik, T.; Khalfallah, M.; Teichner, S. J. Appl. Catal., A 1994, 112, 219–235.
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Figure 5. Evolution of the IR spectra in diffuse reflectance mode of the adsorbed species on the standardized ceria during TPD in helium: (a) 300 K, (b) 413 K, (c) 473 K, (d) 528 K, (e) 578 K, (f) 678 K, and (g) 725 K.
peaks of CO2, total amount of 285 μmol/g, (ii) a broad CO peak (not ending at 1200 K) with a maximum at 1073 K, 800 μmol/g (Table 1), and (iii) a hydrogen production at Td> 950 K (20 μmol/g). These results are consistent with the literature data on the desorption/decomposition of SOCs on C-containing materials;50-53 they indicates that different SOCs with a broad range of activation energies of desorption are adsorbed on the Printex U surface. Before TPD, the diesel soots are treated 1 h at 373 K in helium to remove the VOF.20 After this pretreatment, TPD spectra of the diesel soot are dependent upon the presence or absence of a DOC before the filter. In the absence of DOC, CO, CO2, and H2 productions (Figure 6B) are similar to those of the diesel soot studied previously (see Figure 3 in ref 20), 379, 1130, and 11 μmol/g, respectively. However, the SO2 peak observed at 585 K in ref 20 is absent in Figure 6B because of the low sulfur content of the present fuel. Panels A and B of Figure 6 show that the SOCs are different on Printex U and the diesel soots because of the conditions of their preparation. Panels A and B of Figure 7 give CO2 and CO productions, respectively (H2 not shown) during the TPD of the different diesel soots; curves a and b correspond to diesel soot collected in an exhaust line without and with DOC, respectively. The comparison indicates that the DOC modifies the nature and amount of SOCs on the surface of the diesel soot. The production of CO2 at low temperatures is suppressed, and those of CO2 and CO at high temperatures are increased (curves b); the total amount of CO2 and CO productions are 452 and 2330 μmol/g, respectively, as compared to 379 and 1130 μmol/g without DOC (the hydrogen production for T > 1153 K is 57 μmol/g). The impact of the DOC can be ascribed to the fact that it allows for the partial oxidation of the soot (Figure 2a shows that the non-catalytic oxidation of the soot starts at ≈650 K). This is supported by the following experiment: the diesel soot collected without a DOC in the exhaust line is treated in air at 673 K for 8 h; the weight loss is 24%, indicating a very low oxidation rate. This treatment, after cooling the sample to 300 K in air, modified the TPD spectrum by increasing significantly CO2 and CO productions (curves c in panels A and B of Figure 7 and Table 1), 928 and 4410 μmol/g, respectively (CO/CO2 = 4.34), and suppressing the hydrogen production. 3.4.2. TPD of the Standardized Soots Using MS. The pretreatment in air at 673 K for 8 h (denoted as standardization) has been applied to the different soots. Panels C and D of Figure 6 give CO and CO2 productions for the standardized Printex U and the soot collected without DOC. Clearly, the profiles are very similar, indicating that
evolution of the adsorbed species on the standardized ceria. At 300 K, undissociated water and a bidentate carbonate species dominate, leading to interactions between the two species. In the Td range of 300-473 K, the two species desorb simultaneously, and in parallel, unidentate carbonate and bicarbonate are formed. This indicates that the strong TPD CO2 peak observed at 460 K in Figure 4 is mainly due to the bidentate carbonate. To facilitate the exploitation of the TPE data using ceria/soot mixtures, a treatment in helium at 473 K for 1 h is performed to remove the contribution of the bidentate carbonate to RCO2(T). After this treatment, the remaining adsorbed species are mainly very stable carbonates (ionic carbonate and poly- and unidentate) with a small amount of formate, which desorbs before 678 K. These species may contribute to the first TPE of CeO2/soot samples for 37 μmol of CO2/g of ceria. The adsorption of O2 at 300 K after the TPD indicates a reduction of the ceria at Td > 473 K. This is probably associated with the desorption of the formate species that leads to the production of H2 (4 μmol of H2/g) by the reaction with OH groups:44 this is associated with the reduction of a site linked to the OH groups, such as Ce4þ f Ce3þ.44 Note that a fraction of H2 produced by the decomposition of the formate species is possibly not detected because of its contribution to the reduction of CeO2.49 The absence of a CO2 production simultaneously to the H2 peak (Figure 4) indicates that a strongly adsorbed carbonate is formed (such as CO32- and polydentate), similar to the observations on ZrO2.44 Note that there is neither desorption of CO2 nor adsorption of O2 at 300 K during a second TPD/ O2 cycle. 3.4. TPD of the Soots Followed by O2 Adsorption. Similar to ceria, pure soot must be characterized by TPD because it is well-known that the SOCs desorb/decompose as CO2 and CO at high temperatures,50-53 and their contributions to RCO2(T) and RCO(T) of the TPE of ceria/soot mixtures must be known. 3.4.1. TPD of the Fresh Soots Using MS. The TPD of the Printex-U in helium (Figure 6A) was similar to that observed previously (see Figure 4 in ref 20) with (i) several overlapped (49) Perrichon, V.; Laachir, A.; Bergeret, G.; Frety, R.; Tournayan, L.; Touret, O. J. Chem. Soc., Faraday Trans. 1994, 90, 773–781. (50) Kyotani, T.; Zhang, Z.; Hayashi, S.; Tomita, A. Energy Fuels 1988, 2, 136–141. (51) Du, Z.; Sarofim, A. F; Longwell, J. P. Energy Fuels 1991, 5, 214– 221. (52) Brown, T. C.; Haynes, B. S. Energy Fuels 1992, 6, 154–159. (53) Haydar, S.; Moreno-Castilla, C.; Ferro-Garcia, M. A.; CarrascoMarin, F.; Rivera-Utrilla, J.; Perrard, A.; Joly, J. P. Carbon 2000, 38, 1297–1308.
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Figure 6. TPD in helium of Printex U and diesel soots (without DOC) before and after standardization at 673 K in air for 8 h. (A and B) Printex U and diesel soots before standardization, (C and D) Printex U and diesel soots after standardization, and (E) FTIR spectra during TPD of the standardized Printex U: (a) Td = 300 K and (b) Td = 730 K.
Figure 7. TPD in helium of diesel soot: (A) CO2 and (B) CO. (a) Diesel soot without DOC, (b) diesel soot with DOC, and (c) diesel soot without DOC standardized in air 8 h at 673 K.
the standardization leads to the homogenization of the nature and amount of SOCs; for Printex U, the amounts of CO and CO2 are 4010 and 944 μmol/g, respectively (Table 1). This is consistent with the increase in the surface area of the solid from 150 to 510 m2/g before and after treatment in air, in agreement with the literature data on Printex U.8,54 The reproducibility of the TPD data for the standardized Printex U has been studied, repeating the experimental procedure 3 times (including the standardization); the CO/CO2 ratios of the TPD were in the range of 4.25-4.7. The total amount of SOCs detected by the TPD of the standardized Printex U, 4954 μmol of SOCs/g, represents roughly a coverage of 0.55 of the surface (assuming 1019 SOC/m2 at full coverage). This indicates that some strongly adsorbed SOCs may remain adsorbed at the highest temperature of the TPD. In particular, a second TPD (results not shown) indicates small CO2 and CO productions, 8 and 115 μmol/g, respectively, starting at 1020 K and not ending at 1120 K. Note that, whatever the soot (fresh and standardized diesel and Printex U), there is no oxygen adsorption at 300 K after the TPD (Table 1), as observed previously.20
3.4.3. TPD of the Standardized Soot Using DRIFTS. A He-TPD of the standardized Printex U diluted by KBr has been performed using the DRIFTS cell. Figure 6E (spectrum a) shows that, at 300 K, there are three IR bands at 1740 cm-1 (broad), 1598 cm-1, and 1295 cm-1 (very broad) that decrease and some of them shift to lower wavenumbers, 1725, 1590, and 1295 cm-1, after the desorption at Td = 735 K. These results are consistent with those of Setiabudi et al.55, who suggest that the IR bands at 1740 and 1295 cm-1 characterize the CdO and C-O bonds of different SOCs (i.e., lactone, carboxylique, and anhydride), whereas that at 1590 cm-1 corresponds to quinone species. The three IR bands decrease during heating in helium (curve b in Figure 6E); however, it is that at 1740 cm-1 that decreases more strongly, indicating the low stability of CdO containing SOCs.50-53 The standardization of the soots in air at 673 K for 8 h presents clear advantages for the study of the oxygen transfer; it homogenizes the surface properties of the different soots, allowing us to use only Printex-U for the microkinetic approach.
(54) Yezerets, A.; Currier, N. W.; Kim, D. H.; Eadler, H. A.; Epling, W. S.; Peden, C. H. F. Appl. Catal., B 2005, 61, 120–129.
(55) Setiabudi, A.; Makkee, M.; Moulijn, J. A. Appl. Catal., B 2004, 50, 185–194.
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still controlled by the desorption of carbonate. At Tr = 823 K, the total CO consumption and CO2 production are 395 and 214 μmol/g, respectively, indicating that the amount of strongly adsorbed carbonates formed on the ceria surface is 181 μmol/g. This significant accumulation of carbonates during the CO oxidation is consistent with the results of Badri et al.,60 who have observed that the reduction of ceria by CO measured by magnetic susceptibility is associated with the increase in the weight of the sample until ≈680 K because of the formation of carbonates. In Figure 8, a hydrogen production is detected at Tr = 780 K, with a maximum at Tr = 922 K associated with new CO consumption and CO2 production. This hydrogen must come from the hydroxyl group of ceria. According to the Gorte group,62,63 it can be suggested that surface processes similar to those involved in the water-gas shift reaction are operant at high temperatures, involving H2O formed by dehydroxylation of ceria. At 1030 K (Figure 8), the CO molar fraction becomes higher than at the inlet of the reactor, indicating that either carbonate species desorb as CO from the reduced ceria or CO2 may oxidize the reduced ceria as suggested by Sharma et al.63 3.5.2. TPR of Ceria by CO Using DRIFTS. To confirm the accumulation of carbonates, the oxidation of 0.8% CO/He by ceria (treated 1 h in helium at 473 K) has been studied by DRIFTS (results not shown). In the range of 18001200 cm-1, the switch He f 0.8% CO/He at 300 K does not lead to significant modifications of the spectrum (see spectrum c in Figure 5). However, an IR band of low intensity is detected at 2170 cm-1 because of the reversible adsorption of CO on Ceþδ sites. There is no modification of the IR spectra for Tr < 473 K. Then, the intensities of the IR bands of carbonates at 1455 and 1387 cm-1 (Figure 5c) increase with Tr and slightly shift to lower wavenumbers probably because of the progressive reduction of ceria. At Td = 723 K, the IR spectrum shows three strongly overlapped IR bands at 1438, 1388, and 1290 cm-1, with an intensity higher by a factor ≈3.4, as compared to Figure 5c. A switch 0.8% CO/He f He at 723 K indicates a very small decrease in the intensity of the IR bands (by ≈5% after 5 min), indicating the formation of strongly adsorbed carbonates, such as polydentate and ionic carbonate, during the reduction of ceria by CO. In the range of 3100-2600 cm-1, DRIFTS spectra reveal the presence of the three IR bands of the formate (see Figure 5) that increase from 473 to 623 K and then decrease progressively. They are detected with a very low intensity at 723 K. This decomposition of the formate species is probably linked to the small hydrogen peak detected with a maximum Tr = 623 K in Figure 8. 3.5.3. Upon the Reduction of Ceria by CO during TPR. The reduction of the ceria can be quantified considering that its highest reduction level is Ce2O3 (CeO1.75).60 Calculations from the CO consumption and the CO2 production are ambiguous because of the formation of carbonates and the implication of the OH groups of ceria at high temperatures. For instance, the CO consumption before the hydrogen production at Tr < ≈823 K (Figure 8), 395 μmol/g, indicates that the reduction is 13% (CeO1.935) using PCO = 0.8 kPa, as compared to 34% (CeO1.83) for PCO = 10 kPa in ref 60. Considering, the total CO consumption (whatever the origin
Figure 8. Evolution of the molar fractions of the gases during TPR of ceria using 1% CO/1% Ar/He.
3.5. TPR of Ceria by CO. CO2 and CO formed by desorption/decomposition of the SOCs of the standardized soot (Figures 6 and 7) can be adsorbed and oxidized, respectively, by the ceria catalyst. Several studies have been dedicated to the reaction of CO on ceria with56-58 and without59-61 O2. In the absence of O2, CO is oxidized at low temperatures by surface oxygen species and at high temperatures by bulk oxygen species, leading to the reduction of ceria.59-61 The level of reduction of ceria is dependent upon different parameters, such as the partial pressure of CO, the temperature, and the surface area of ceria.59-61 During the TPE of ceria/soot mixtures, the CO produced by the SOCs can be oxidized by ceria according to a process different from the one associated with the oxygen transfer from ceria to the soot. 3.5.1. TPR of Ceria by CO Using MS. Figure 8 shows the evolution of the molar fractions of the gases and the temperature of ceria during a TPR using 100 cm3/min of 1% CO/ 1% Ar/He (PCO = 1 kPa). Before the reaction, ceria is treated 1 h at 473 K in helium to desorb undissociated water and the main part of the adsorbed carbonate species (Figures 4 and 5). The introduction of CO at 300 K (Figure 8A) indicates a small reversible CO adsorption, ≈1.5 μmol of CO/g of ceria. During the increase in the temperature, the consumption of CO and the CO2 production start at Tr ≈ 410 K (in agreement with literature data)60,61 and Tr ≈ 520 K (when the CO conversion is ≈8.2%), respectively. This indicates that CO2 remains adsorbed on the ceria surface as carbonates; the total amount of CO consumption at 520 K is 18 μmol of CO/g. The rates of CO consumption and CO2 production, RCO(T) and RCO2(T), respectively, are maximum at Tr = 750 and 785 K, respectively, and then they decrease because of either the decrease in the amount of oxygen species available in ceria and/or a limitation of the CO oxidation by the strongly adsorbed carbonates.56,57 Until Tr = 823 K, RCO > RCO2, showing that (i) the surface is not saturated by carbonates and (ii) RCO2 is (56) Breysse, M.; Guenin, M.; Claudel, B.; Latreille, H.; Veron, J. J. Catal. 1972, 27, 275–280. (57) Wilkes, M. F.; Hayden, P.; Bhattacharya, A. K J. Catal. 2003, 219, 295–304. (58) Alam, M. K.; Ahmed, F.; Nakamura, K.; Suzuki, A.; Sahnoun, R.; Tsuboi, H.; Koyama, M.; Hatakeyama, N.; Endou, A.; Takaba, H.; Del Carpio, C. A.; Kubo, M.; Miyamoto, A. J. Phys. Chem. C 2009, 113, 7723–7727. (59) Yao, H. C.; Yao, Y. F. Y. J. Catal. 1984, 86, 254–265. (60) Badri, A.; Lamotte, J.; Lavalley, J. C.; Laachir, A.; Perrichon, V.; Touret, O.; Sauvion, G. N.; Quemere, E. Eur. J. Solid State Inorg. Chem. 1991, 28, 445–448. (61) Madier, Y.; Descorme, C.; Le Govic, A. M.; Duprez, D. J. Phys. Chem. B 1999, 103, 10999–11006.
(62) Cracium, R.; Shereck, B.; Gorte, R. J. Catal. Lett. 1998, 51, 149. (63) Sharma, S.; Hilaire, S.; Vohs, J. M.; Gorte, R. J.; Jen, H. W. J. Catal. 2000, 190, 199–204.
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desorption of carbonates from CeO2, and (ii) the oxygen chemisorption at 300 K after the TPD is 26 μmol/g, as compared to 22 μmol/g for the pure ceria. Clearly, there is no transfer of active O-containing species from ceria to the soot by the gas phase (only CO formed from the SOCs can be oxidized by ceria). 3.7. TPE Using Ceria/Soot Mixtures: Case of TC-R = 10. If the accepted view of the catalytic oxidation of the soot is that the solid catalyst provides oxygen to the soot via the contacts between the two solids (Figure 9 confirms that there is no transfer via the gas phase), it must be observed that there are very few studies dedicated to the experimental characterization of these contacts and their evolutions during the reaction. This is the aims of the present study, using TPE of the ceria/soot mixtures, taking into account the occurrence of the different surface processes in parallel to the oxygen transfer. Two experimental points play in favor of the exploitation of the TPE data. First, on the pure standardized soots (i.e., Printex-U), CO2 and CO productions from the desorption of the SOCs start at Td ≈ 600 and 700 K, respectively (Figure 7). This means that, during the first TPE of ceria/soot mixtures, CO2 and CO productions before 600 K are relevant of the oxygen transfer. Second, it is shown below that performing successive TPE/O2 adsorption cycles (denoted as TPE-i, with i being the rank in the series) decreases significantly the contribution of pure soot and pure ceria to the TPE data, allowing for an easier exploitation of the data for the characterization of the oxygen transfer. The TPE/O2 cycles of TC-R = 10 are used to present how experimental data are exploited to characterize the evolution of the ceria/soot contacts with the progressive soot oxidation. The data on LC-R = 10 and TC-R = 1 are more shortly described using comparisons to TC-R = 10. 3.7.1. First TPE of the TC-R = 10 Ceria/Soot Mixture. 3.7.1.1. Experimental Data. Before TPE-1, TC-R = 10 is treated 1 h in helium at 473 K to remove the main part of the weakly adsorbed carbonate species on CeO2. After cooling to 300 K, the switch He f 1% O2/1% Ar/He reveals the adsorption of 12 μmol of O2/g of ceria (ceria is selected as the reference because soot does not adsorb O2 at 300 K). Considering that (i) pure ceria is not reduced by desorption at 473 K and (ii) SOCs of the standardized soot are not desorbed/decomposed into CO before 700 K, it must be concluded that ceria has been slightly reduced by a slow oxygen transfer to the soot; the average rate is ≈0.4 μmol of O (g of ceria)-1 min-1 at 473 K. It must be considered that this temperature, equal to the light-off temperature of the oxidation of CO by ceria (Figure 8), corresponds to the beginning of the oxygen transfer from ceria to the soot. Figure 10 shows TPE-1 of TC-R = 10 after the adsorption of O2 at 300 K. A very small oxygen peak is detected with a maximum at 450 K, ≈1 μmol of O2/g of ceria. The hydrogen production in Figure 10 is that expected for the contribution of pure ceria (Figures 4 and 8) with (i) the desorption/ decomposition of formate providing the peaks at 600 and 680 K and (ii) the reaction of the OH groups of ceria with CO formed by the desorption of the SOCs at T > 935 K. The CO2 production starts at 525 K (that is consistent with the slow reduction of ceria during the treatment in helium at 473 K and the formation of strongly adsorbed carbonate species during the TPE with CO; Figure 8), leading to a first broad peak with a maximum at 700 K, followed by a strong peak with a maximum at 1030 K, associated with a broad shoulder at 1080 K. The presence of two CO2 peaks indicates that two
Figure 9. Evolution of the molar fractions of the gases during the consecutive TPE of standardized Printex-U and ceria in a series of two reactors (panel A), followed by the adsorption of 1% O2/1% Ar/He at 300 K (panel B) (see the text for more details).
of oxygen, Oce or OH), 458 μmol/g, the reduction is 15%. Another method of evaluation of the reduction of ceria consists using the adsorption of O2 at 300 K. After the TPD of Figure 8, the switch He f 1% O2/1% Ar/He at 300 K indicates the adsorption of 161 μmol O2/g, corresponding to a reduction of 10.6%. Note that the increase in the temperature to 330 K, in the presence of O2, does not reveal a new O2 consumption, indicating that the oxidation of the reduced ceria to CeO2 is achieved at 300 K in agreement with magnetic measurements.60 Considering the aim of the present study and taking into account that the SOCs present on a soot surface desorb, as CO2 and CO, at high temperatures, the conclusions of the TPO of CO on ceria in the absence of oxygen are as follows: (i) there is an accumulation of strongly adsorbed carbonates (very low desorption rate at 723 K) in parallel to the oxidation of CO; (ii) the oxygen transfer from ceria to soot can be masked by the oxidation of CO formed by desorption/ decomposition of the SOCs (except if the transfer proceeds at a temperature lower than the desorption of the SOCs); and (iii) the adsorption of O2 at 300 K can be used to quantify the level of reduction of CeO2. 3.6. TPE of CeO2 and Soot without Contact. The standardized Printex U and ceria are introduced in a series of two identical microreactors R1 and R2 heated by the same furnace. The gas flow rate enters in R1, and the gas composition is measured at the outlet of R2. The same weight of ceria and soot are used to mimic a ceria/soot mixture R = 1, and the two solids are treated in helium at 473 K for 1 h. For the soot in R1 and ceria in R2, Figure 9 shows that CO2 and CO productions are dominated by the desorption/decomposition of the SOCs from the soot (see Figures 6 and 7). However, the CO/CO2 ratio is significantly decreased to 3.28, as compared to the TPD of the pure soot in the range of 4.2-4.7. This is mainly due to the oxidation of a fraction CO formed in R1 by ceria in R2. Note that the hydrogen production at high temperatures in Figure 9 is similar to that of the TPD of ceria (Figure 8). Figure 9B shows the switch He f 1% O2/1% Ar/He at 300 K after the TPD; the O2 consumption is 188 μmol of O2/g of ceria, a value slightly higher than after the reduction of ceria with PCO = 0.8 kPa (Figure 8), 161 μmol/g, because of the highest partial PCO value during the TPE in Figure 9, ≈2 kPa. For ceria in R1 and soot in R2, CO2 and CO productions (results not shown) are almost the superposition of the data with pure ceria and soot: (i) CO/CO2 ≈ 4.1 because of the contribution of the 4775
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shown that the amount of CO2 that can be adsorbed is 181 μmol of CO2/g of ceria. Assuming that a similar amount is formed during TPE-1 of TC-R = 10, this indicates that 1810 μmol of C/g of soot remain on ceria, majoring significantly in the amount of C eliminated from the soot (1354 þ 1810) = 3164 μmol/g. However, this number is higher than QOce, probably because of the fact that the surface properties of ceria in the ceria/soot mixture are not identical to those of the pure ceria (i.e., strongly adsorbed carbonates cannot be formed on the ceria surface, which is in contact with the soot). We have not found an experimental procedure allowing us to quantify, without ambiguity, the amount of carbonates adsorbed on ceria during TPE-1 of TC-R = 10, and this prevents a clear estimation of the amount of oxygen transferred from ceria to the soot. However, lowest and highest values of this amount are obtained according to the following exploitation of the TPE-1 data. The amount of oxygen transferred from ceria to soot during TPE-1 can be used for two main reactions: (i) the transformation of the SOCs desorbing as CO on the pure soot into SOCs desorbing as CO2 according to surface processes, such as20-25
Figure 10. Evolution of the molar fractions of the gases during TPE-1 of the TC-R = 10 ceria/soot (Printex U) mixture.
reservoirs of reactants with different activation energies (either oxygen species from ceria or SOCs on soot) are involved during the TPE. The CO production starts at Tr ≈ 800 K (as compared to 700 K on the standardized soot; Figure 7), increases slightly until Tr = 935 K (corresponding to the appearance of hydrogen), and then strongly, leading to a peak with a maximum at 1107 K. The total CO2 and CO productions (not ending at the highest temperature of TPE-1) are (Table 1) of 3700 and 2608 μmol/g of soot, respectively (the soot is selected as the reference because CO2 and CO are mainly formed by this solid), which are significantly different of those of the pure standardized soot (CO2 = 944 μmol/g and CO = 4010 μmol/g in Figure 6). This leads to a CO/CO2 ratio of 0.705, as compared to 4.2-4.7 for the pure soot. The reduction of CeO2 after the TPE-1 has been quantified by O2 adsorption at 300 K according to the switch Hef 1% O2 /1% Ar/He, 132 μmol of O2/g of ceria (in comparison to 161 μmol/g after the TPE of CO). However, it is shown and discussed below that a fraction of this oxygen, 15 μmol of O2/g of ceria, desorbs as O2 without any reaction with the soot. This means that (i) the amount of oxygen species of ceria Oce available for the soot oxidation in TC-R = 10 is QOce = (132 - 15) 2 10 = 2340 μmol of O/g of soot (oxygen either transferred or used for the CO oxidation) and (ii) the reduction of the ceria after TPE-1 is 7.7%. 3.7.1.2. Exploitation of the Experimental Data. The surface processes involved during TPE-1 of TC-R = 10 are discussed on the basis of calculations involving data on pure standardized soot and ceria. CO and CO2 productions during TPE show that the total amounts of carbon eliminated from the soot are 6308 and 4954 μmol of C/g of soot for TC-R = 10 and pure soot, respectively. This indicates that the oxygen transfer allows for the elimination of an additional amount of QAC = 1354 μmol of C/g of soot, whereas ceria provides a total amount of oxygen of QOce = 2340 μmol of O/g of soot either transferred to the soot or used for the oxidation of CO produced by the desorption of the SOCs. The fact that QAC < QOce can be due to (i) the formation of strongly adsorbed carbonate species on ceria, as observed during the TPO of CO (Figure 8), (ii) the consumption of more than one oxygen for the removal of one C, and (iii) the oxidation of CO formed by the desorption of SOCs. This shows that the estimation of the amount of oxygen transferred from ceria imposes the determination of the contribution of each of those processes to the experimental data. However, this is not possible during TPE-1. For instance, considering the contribution of the formation of the strongly adsorbed carbonates on ceria, the TPO of CO on ceria (Figure 8) has
ðCeO2 Þ- Oce þ Cn CCf ðOÞ f CeO2 þ Cn CðOÞCf ðOÞ ð1Þ Cn CðOÞCf ðOÞ f Cn - 1 CCf þ CO2
ð2Þ
where Cf(O) and C(O)Cf(O) denote SOCs desorbing as CO and CO2, respectively, whereas Cf is a carbon defect site formed by the desorption of a SOC, and (ii) the oxidation of the defect sites according to ðCeO2 Þ- Oce þ Cn CCf f CeO2 þ Cn CCf ðOÞ
ð3Þ
Equation 3 can be followed by eqs 1 and 2. However, the SOC formed in eq 3 may also either remain on the surface or desorb as CO according to Cn CCf ðOÞ f Cn - 1 CCf þ CO
ð4Þ
Finally, the amount of oxygen transferred during TPE-1 is in the range of 1354 and 2340 μmol of Oce/g of soot, assuming that only CnCCf(O) species are oxidized (according to eqs 1 and 2) or that all oxygen provided by ceria is transferred to the soot (there is no oxidation of CO), respectively. Successive TPE/O2 adsorption cycles limit different surface processes, such as the desorption of the SOCs present on soot (they are removed during TPE-1) and the formation of strongly adsorbed carbonates on ceria (the surface is saturated after TPE-1), simplifying the evaluation of the oxygen transfer during TPE. 3.7.2. Second TPE of TC-R = 10 after Adsorption of O2 at 300 K. 3.7.2.1. Experimental Data. Figure 11 gives the evolution of the molar fractions of the gases during TPE-2 after adsorption of O2 at 300 K. An O2 peak higher than that in Figure 10 is observed at TM = 478 K, 15 μmol/g. The profile of RCO2(T) is similar to that in Figure 10 with a broad peak at 803 K, 307 μmol of CO2/g of soot, followed by a sharp peak with a maximum at 1130 K, 603 μmol/g of soot. The CO and H2 productions start at T > 950 K and are not ending at the highest temperature, 232 and 32 μmol/g of soot, respectively. The CO/CO2 ratio, 0.25, is lower than during TPE-1 (0.75), showing that the selectivity in CO2 is increased by the removal of the SOCs present on the standardized soot. The O2 peak at TM = 478 K is consistent with the study by 4776
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desorbing as CO have been removed from the soot during TPE-1). On the pure soot, it has been indicated that a second TPE leads to small amounts of CO2 and CO, 8 and 115 μmol/g of soot, respectively, at T > 1020 K. This means that the oxygen species Oce provided by ceria has been mainly transferred to the soot, QOce = 1660 μmol of Oce/g of soot. This allows us to provide some details on how the oxygen transferred reacts with the soot from the amounts of CO and CO2 productions during TPE-2. We assume that the SOC remaining on the soot after TPE1 produces the same quantities of CO and CO2 as during TPE-2 of the pure soot. This indicates that the Oce species transferred have allowed the formation of QCO = (232 115) = 117 μmol of CO/g of soot and QCO2 = (910 - 8) = 902 μmol of CO2/g of soot, respectively. These values are compared to the total amount of oxygen transferred QOce = 1660 μmol of O/g of soot. In a first step, the calculations are performed without considering that the Oce species may form stable SOCs remaining on the soot surface during TPE-2. The CO production, QCO = 117 μmol of CO/g of soot, indicates that a fraction of Oce (denoted as QOce(1)) species has been used to convert defect sites of the soot into SOCs providing CO according to eqs 3 and 4. This leads to the consumption of QOce(1) = 117 μmol of Oce/g of soot, indicating that the remaining fraction of Oce, QOce(2) = 1660 117 = 1543 μmol of Oce/g of soot, has been involved in the production of CO2, QCO2 = 902 μmol/g of soot. Two surface processes must be considered for the formation of CO2 via either eqs 1 and 2 that consume one oxygen (the amount of CO2 produced by this process is denoted as QCO2(1)) or (ii) the oxidation of defect sites into CO2 according to eqs 1-3 that consume two Oce (the amount of CO2 produced by this process is denoted as QCO2(2)). This leads to a system of two equations considering the amount of Oce consumed for the CO2 production QOce(2) and the total CO2 production QCO2
Figure 11. Evolution of the molar fractions of the gases during TPE-2 of the TC-R = 10 ceria/soot (Printex U) mixture.
Laachir et al.,42 showing a similar peak at TM = 383 K during a TPD performed after adsorption of O2 at 300 K on a ceria reduced (CeO1.88) in pure H2 at 673 K. Using the comparison between the level of reduction of ceria obtained by magnetic measurements and the total amount of oxygen consumed, the authors concluded that the oxygen in the peak at TM = 383 K is in excess of that implicated in the oxidation of CeO1.88 to CeO2.42 According to literature data, the O2 peak at TM = 478 K in Figure 11 seems to correspond to a superoxide, O2-ads, and/or peroxide, O22-ads, adsorbed species (denoted as O2δ-ads) formed on defect sites of a reduced ceria (i.e., Ce3þ þ O2(g) f Ce4þ-O2-ads). These species have been detected by different spectroscopic methods,64-69 and their stabilities have been justified by density functional theory (DFT) calculations.70 A TPD performed after oxidation of ceria at 713 K with 1% O2/He followed by a cooling stage in O2 (result not shown) does not provide this O2 peak (in agreement with the TPD of the standardized ceria in Figure 4 and Table 1), indicating that the defect sites are suppressed after adsorption of O2 at high temperatures. According to Figures 10 and 11, the O2δ-ads species desorb before the appearance of CO2 and are not involved in the soot oxidation. This justifies that the amount of oxygen provided by ceria to the soot during TPE-1 of TC-R = 10 (see above) is [132 (total O2 adsorption at 300 K) - 15 (amount of O2δ-) μmol of O2/g] 117 μmol of O2/g of ceria or 2340 μmol of Oce/g of soot. After TPE-2, the switch He f 1% O2/1% Ar/He at 300 K gives an adsorption of 93 μmol of O2/ g of ceria, whereas TPE-3 indicates that 10 μmol of O2/g of ceria is adsorbed as O2δ-; the amount of Oce provided by ceria during TPE-2 is QOce = 1660 μmol of Oce/g of soot. 3.7.2.2. Exploitation of the Experimental Data. The experimental data of TPE-2 of TC-R = 10 can be more easily exploited than those of TPE-1, considering that two processes on the ceria surface are significantly suppressed after TPE-1: (i) the adsorption/desorption of CO2 (at the end of TPE-1, ceria is saturated by strongly adsorbed carbonates) and (ii) the oxidation of CO (a large amount of SOCs
QCO2 ¼ QCO2 ð1Þ þ QCO2 ð2Þ ¼ 902 μmol=g
ð5Þ
QOce ð2Þ ¼ QCO2 ð1Þ þ 2QCO2 ð2Þ ¼ 1543 μmol=g
ð6Þ
Equations 5 and 6 provide QCO2(2) = 641 μmol/g of soot and QCO2(1) = 261 μmol/g. The sum QCO2(2) þ QCO = 818 μmol/g provides the amount of defect sites Cf that have been removed from the soot during TPE-2. The above calculations can be slightly modified considering that the oxygen transfer produces SOCs remaining on the surface. To evaluate this contribution, a third TPE has been performed, after TPE-2, without adsorption of O2 at 300 K, leading to the production of 161 μmol of CO/g of soot and 126 μmol of CO2/g of soot at T > 910 K. These values are higher than those measured for TPE-2 of the pure standardized soot, 115 and 8 μmol/g of soot, respectively. The differences can be tentatively ascribed to the formation of new SOCs during TPE-2 of TC-R = 10. The amounts of Oce implicated are (161 - 115) = 46 μmol/g of soot and (126 - 8) 2 = 236 μmol Oce/g for the SOCs producing CO and CO2, respectively. This decreases the amount of Oce implicated in the formation of CO and CO2 during TPE-2, QOce(2) = 1660 - 117 - 46 - 236 = 1261 μmol/g, modifying the values provided by eqs 5 and 6, QCO2(2) = 359 μmol/g and QCO2(1) = 543 μmol/g, and leading to a number of Cf sites eliminated, 359 þ 117 = 476 μmol/g of soot. Therefore, the calculations show that, if the amount of oxygen transferred during TPE-2 is known by the adsorption of O2 at 300 K, 1660 μmol of O/g
(64) Li, C.; Domen, K.; Maruya, K.; Onishi, T. J. Am. Chem. Soc. 1989, 11 (1), 7683–7687. (65) Zhang, X.; Klabunde, K. J. Inorg. Chem. 1992, 31, 1706–1709. (66) Soria, J.; Martinez-Arias, A.; Conesa, J. C. J. Chem. Soc., Faraday Trans. 1995, 91, 1669–1678. (67) Long, R. Q.; Huang, Y. P.; Wan, H. L. J. Raman Spectrosc. 1997, 28, 29–32. (68) Guzman, J.; Carrettin, S.; Corma, A. J. Am. Chem. Soc. 2005, 127, 3286–3287. (69) Martı´ nez-Arias, A.; Gamarra, D.; Fernandez-Garcı´ a, M.; Wang, X. Q.; Hanson, J. C.; Rodriguez, J. A. J. Catal. 2006, 240, 1–7. (70) Huang, M.; Fabris, S. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, No. 081404.
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Figure 12. Evolution of the molar fractions of the gases during TPE-9 of the TC-R = 10 ceria/soot (Printex U) mixture.
of soot, its contribution for CO2 and CO formations is not known with accuracy. Considering the relationship between catalytic oxidation of the soot by ceria and the oxygen transfer, it must be noted that the fraction of QOce that may decrease the light-off temperature of the soot is that providing the broad peak of CO2 at 803 K in Figures 11 and 12. Assuming that the CO2 formed in this peak (307 μmol/g) is only due the oxidation by Oce of defect sites, this indicates that, at best, 614 μmol of Oce/g of soot are implicated in an efficient oxygen transfer from ceria to soot. The remaining Oce fraction, 1660 - 614 = 946 μmol/g of soot, is transferred at temperatures allowing for the activation of O2 by the soot. 3.7.3. Evolution of the Ceria/Soot Contacts of TC-R = 10 with the Oxidation of the Soot. CO2 and CO productions during the TPE of Figures 10 and 11 correspond to the progressive combustion of the soot by the oxidation of the Cf sites. This shows that the evolution of the ceria/soot contacts with the progressive conversion of the soot can be studied by performing successive (TPE/adsorption of O2 at 300 K) cycles. Nine successive cycles have been performed for TCR = 10, and except for TPE-1 (Figure 10), all of the following TPEs are qualitatively similar to TPE-2 (Figure 11), as shown for TPE-9 in Figure 12. The total amounts of CO2 and CO productions during TPE-9 are 653 and 134 μmol/g of remaining soot (Table 1). After 9 cycles, ≈15% of the soot present in the TC-R = 10 mixture has been oxidized. Symbols 9 in Figure 13 give the evolution of QOce, the total amount of oxygen transferred from ceria to soot as a function of the soot oxidation. For TPE-1, we have reported the total amount of oxygen provided by ceria (transferred and used for the CO oxidation). Symbols 9 show that the amount of Oce transferred decreases significantly during the first three TPEs and then it remains roughly constant considering the experimental uncertainties. The decrease for TPE-i (i e 3) is probably linked to different processes, such as (i) the saturation of the ceria surface by the strongly adsorbed carbonates may decrease the capacity of the solid to provide Oce, and (ii) the surface of the ceria/soot contacts created by the mechanical mixing may decrease, during TPE-i (i < 3) because of the decrease in the surface area of ceria in parallel to its reduction at high temperatures. This last point is confirmed by the following experiments: after TPE-9, the remaining soot fraction in TC-R = 10 is totally oxidized at 873 K (temperature of the standardization) using a 1% O2/He mixture, and then the surface area of the ceria has been measured, 50 m2/g, compared to 150 m2/g after the standardization.
Figure 13. Evolution of the amount of oxygen transferred from ceria to the soot (Printex U) as a function of the consumption of the soot during successive TPE/O2 adsorption cycles: (9) TC-R = 10, (0) LC-R = 10, and (b) TC-R = 1.
Figure 14. Evolution of the molar fractions of the gases during TPE-2 (full line) and TPE-9 (dotted line) of the LC-R = 10 ceria/soot (Printex U) mixture. (Inset) Evolution of the molar fractions of the gases during TPE-1 of LC-R = 10.
3.8. TPE/O2 Adsorption Cycles for the LC-R = 10 Ceria/ Soot Mixture. After the treatment in helium for 1 h at 473 K, the profiles of CO2 and CO productions (hydrogen not shown) during TPE-1 of LC-R = 10 (inset of Figure 14) are similar to TPE-1 of TC-R = 10 (Figure 10). The CO2 production starts at 557 K with first a broad peak with a maximum at 680 K, followed by a strong and broad peak with a maximum at ≈1020 K. The CO production starts at Tr ≈ 800 K and increases progressively, leading to a strong peak with a maximum at 1096 K. CO and CO2 productions (not ending at the highest temperature of TPE-1) are 3987 and 2568 μmol/g of soot, respectively (Table 1). The total amount of C eliminated from the soot, 6525 μmol/g of soot, is similar, considering the homogeneities of the mixtures, to that of TC-R = 10, 6308 μmol/g. However, the CO/CO2 ratios are different, 1.5 and 0.75 for LC-R = 10 and TC-R = 10, respectively. After TPE-1, the amount of oxygen adsorbed at 300 K is 128 μmol of O2/g of ceria. The following TPE/O2 adsorption cycles provide similar CO2 and CO productions as shown in Figure 14 that compares TPE-2 and TPE-9. Similar to TC-R = 10, (i) O2 peaks because of the desorption of O2δ- species are observed at T < 500 K, 12 and 10 μmol/g of ceria for TPE-2 and TPE-9, respectively, and (ii) the CO2 productions for TPE-i (i > 1) (Figure 12) present a broad peak at ≈830 K, 220 and 66 μmol/g for TPE-2 and TPE-9, respectively, followed by a sharp peak with a maximum at 4778
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LC-R = 10; however, their amounts per gram of soot are significantly lower, 68 μmol of CO2/g and 71 μmol of CO/g for TPE-2 and 80 μmol of CO2/g and 4 μmol of CO/g for TPE-8, respectively (Table 1). After TPE-8, the adsorption of oxygen is 94 μmol/g of ceria (Table 1), indicating that, even after several TPE/O2 adsorption cycles, ceria is more deeply reduced for R = 1 than R = 10. Symbols b in Figure 13 give the evolution of the amount of Oce transferred by gram of soot for TC-R = 1 as a function of the percentage of soot that has been oxidized. After the decrease during the first TPE, the amount of Oce species available for the soot oxidation remains roughly constant with the progressive oxidation of the soot, similar to the two other ceria/soot mixtures. Figure 15. Evolution of the molar fractions of the gases during TPE2 (full line) and TPE-8 (dotted line) of the TC-R = 1 ceria/soot (Printex U) mixture. (Inset) Evolution of the molar fractions of the gas during TPE-1 of TC-R = 1.
4. Discussion The impact of the type of ceria/soot mixtures on the catalytic oxidation of the soot is clearly revealed by the ignition temperature Ti during the TPO (Figures 1 and 2). The experiments lead to the following order of performances: TC-R = 10 . LC-R = 10 ≈ TC-R = 1 . pure soot. For a catalyst/soot weight ratio, the fact that the tight contacts present better performances than loose contacts has been shown for different catalysts from the first works of Neeft et al.25,26 to more recent studies.71-75 This is mainly commented in term of differences in the catalyst/soot contacts without any experimental data on the contacts themselves. For instance, in the early stage of the studies dedicated to the catalytic oxidation of soot, the impact of the contacts was commented by Neeft et al.25,26 as follows: “only catalysts that are mobile under practical conditions are able to increase the soot oxidation rate, as is shown for a Cu/K/Mo/(Cl) catalyst”, and ceria was not considered as a good catalyst. However, the authors have reconsidered their view point in more recent studies dedicated to ceria-based catalysts.76,77 Similarly, Darcy et al.78 consider that the progressive oxidation of the soot is associated with the loss of the catalyst/soot contact, leading to the following situation: a catalytic process with a fast rate of oxidation is operant at low soot conversions, whereas at high conversions, the rate of oxidation is similar to that of the non-catalytic oxidation of the soot. The lack of experimental data on the catalyst/soot contacts and their evolutions during the soot oxidation were the driving forces of the present study. The contacts constitute key parameters of the microkinetic approach of the diesel soot oxidation via the CCF process. The ceria/soot contacts have been studied by the measurements of the amount of oxygen transferred from ceria to soot during TPE in the absence of O2. The experimental data used to characterize the transfer are (i) CO2 and CO productions as
1130 K, leading to a total amount of CO2 production of 648 and 340 μmol/g for TPE-2 and TPE-9, respectively (Table 1). CO and H2 (not shown) productions are 72 and 11 μmol/g of soot for TPE-2 and 40 and ≈1 μmol/g for TPE-9, respectively. Note that (i) CO2 and CO productions are not ending at the highest temperature of the TPE and (ii) CO2, CO, and H2 productions decrease progressively with the number of cycles; however, the CO/CO2 ratio remains roughly constant (≈0.11) considering the experimental uncertainties. After TPE-9, the adsorption of O2 at 300 K (Table 1) indicates an oxygen transfer from ceria to soot of QOce = 820 μmol of Oce/g of soot. Symbols 0 in Figure 13 give the evolution of the QOce for LC-R = 10, as a function of the percentage of soot oxidized. The comparison to symbols 9 shows that the evolutions are similar; after the decrease during the first two TPEs, QOce remains roughly constant. This amount is lower by a factor of ≈2, as compared to TC-R = 10, probably because of the smaller number of ceria/soot contacts for LCR = 10 (its homogeneity is limited, as shown in Figure 3). However, symbols 0 in Figure 13 confirm that a significant number of contacts are maintained during the progressive oxidation of the soot in the LC-R = 10 mixture. 3.9. TPE/O2 Adsorption Cycles for the TC-R = 1 Ceria/ Soot Mixture. After the treatment in helium for 1 h at 473 K, the profiles of CO2 and CO productions during TPE-1 (inset of Figure 15) are qualitatively similar to that of the TC-R = 10 (Figure 10). However, the CO2 production is significantly lower, particularly at low temperatures. Total CO2 and CO productions are 1550 and 4400 μmol/g of soot (Table 1), leading to a CO/CO2 ratio of 2.84. After TPE-1, the amount of oxygen adsorbed at 300 K is 260 μmol of O2/g of ceria (Table 1), a value higher than for TC-R = 10 (132 μmol/g of ceria) and LC-R = 10 (128 μmol/g of ceria). This indicated that ceria is more reduced for R = 1 than R = 10. However, the amount of Oce species available per gram of soot, 520 μmol of Oce/g of soot, is significantly lower than for R = 10. It is this number that must be considered to establish a relationship between the oxygen transfer and the decrease in the light-off temperature of the soot. The TPE profiles of the following (TPE/O2 adsorption) cycles are similar, as shown in Figure 15, which compares TPE-2 and TPE-8. Because of the low amount of ceria in TC-R = 1, the O2δspecies provide very small O2 peaks (at the limit of the detection level of the MS for Tr < 500 K). The profiles of the CO2 and CO productions are similar to those of TC-R = 10 and
(71) Wu, X.; Liu, D.; Jia, K. L.; Weng, D. Catal. Commun. 2007, 8, 1274–1278. (72) van Setten, B. A. A. L.; Schouten, J. M.; Makkee, M.; Moulijn, J. A. Appl. Catal., B 2000, 28, 253–257. (73) Attribak, I.; Bueno-Lopez, A.; Garcia-Garcia, A. Catal. Commun. 2008, 9, 250–255. (74) Issa, M.; Petit, C.; Brillard, A.; Brilhac, J. F. Fuel 2008, 87, 740– 750. (75) Peralta, M. A.; Gross, M. S.; Ulla, M. A.; Querini, C. A. Appl. Catal., A 2009, 367, 59–69. (76) Bueno-Lopez, A.; Krishna, K.; Makkee, M.; Moulijn, J. A. J. Catal. 2005, 230, 237–248. (77) Krishna, K.; Bueno-Lopez, A.; Makkee, M.; Moulijn, J. A. Appl. Catal., B 2007, 75, 189–200. (78) Darcy, P.; Da Costa, P.; Mellottee, H.; Trichard, J. M.; DjegaMariadassou, G. Catal. Today 2007, 119, 252–256.
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: DOI:10.1021/ef100581z
Bassou et al.
a function of the reaction temperature and (ii) the amount of oxygen adsorbed at 300 K after the TPE. It has been shown that different difficulties (mainly linked to the large amount of ceria used to mimic the CCF process, as compared to the FBC process) are encountered in the exploitation of the TPE data because of the occurrence, in parallel to the oxygen transfer, of other chemical processes (i.e., adsorption of CO2 and oxidation of CO formed by the SOCs on ceria), which contribute significantly to CO2 and CO productions, particularly during the first TPE. However, it has been shown that successive TPE/ adsorption of O2 cycles permit the elimination or limitation of the impact of those processes, favoring the exploitation of the data. This has allowed us to follow how the oxygen transfer (quantified by the adsorption of O2 at 300 K) is modified by the progressive oxidation of the soot contained in three ceria/soot mixtures prepared according to the tight and loose contact concepts23,24 using two ceria/soot ratios (R = 10 and 1). Figure 13 clearly shows that the evolution of the amounts of oxygen transferred (quantified per gram of soot; oxygen available for the soot oxidation) is similar for the three ceria/ soot mixtures; after a decrease during the first two or three TPEs, then it remains constant with the progressive soot oxidation. This conclusion is consistent with the experimental study by Simonsen et al.79 on the evolution of the ceria/soot contacts (to our knowledge, there is no other literature data on the experimental approach of catalyst/soot contacts). Using environmental TEM, Simonsen et al.79 have shown how the soot particles in contact with ceria particles are progressively consumed with the duration of the oxidation using 2 mbar of O2. The point of interest is that TEM reveals that the soot particles are oxidized in the close proximity of the ceria/soot contacts and that these contacts are maintained during the soot oxidation. The authors suggest that the soot/ceria interface is continuously re-established over the course of the reaction, for instance, via attractive van der Waals forces between elementary soot particles and the ceria surface. The results of the present study are in good agreement with this conclusion, and they provide additional data considering that they evaluate the amount of oxygen transferred from ceria via the catalyst/soot contacts. In particular, Figure 13 shows that this amount (linked to the number of contacts) is constant with the progressive soot oxidation, regardless of the type of ceria/soot mixtures (TC or LC) and ceria/soot weight ratios R (10 or 1). However, there is a significant decrease in the amount of oxygen provided by ceria during the first two TPEs. This can be due to a decrease in the surface of the ceria/soot contact linked to (i) the decrease in the surface area of the ceria because of its reduction at high temperatures, as observed during H2-TPR,30,80 and (ii) the removal of a large amount of SOCs by desorption and oxidation during TPE-1. Moreover, the capacity of ceria to provide oxygen may decrease because of the accumulation of strongly adsorbed carbonates on ceria (via the adsorption of CO2).56
The amount of oxygen provided by ceria via the catalyst/ soot contact, QOce, allows a qualitative comparison to the performances of the different ceria/soot mixtures for the decrease in the light-off temperature (Figures 1 and 2). Figure 13 shows that the order is TC-R = 10 > LC-R = 10 > TC-R = 1, whereas the ignition temperatures Ti of the soot are Ti(TC-R = 10) < Ti(LC-R = 10) ≈ Ti(TC-R = 1); the higher the QOce, the lower the Ti. However, this comparison between the total amount of oxygen transferred and Ti is not fully pertinent; the analysis of the TPE data must be more selective. The parameters that must be considered are, for the different ceria/soot mixtures and at low temperatures, (a) the amounts and the rates of the oxygen transfer and (b) the rate of the CO2 production during the soot oxidation in the absence of limitations by physical processes (low soot conversion). This is the aim of part 2 (10.1021/ ef100582w) of the present study, which is dedicated to a kinetic modeling of the different experimental curves RCO2(T ) observed during TPE-i (i > 2) of the three ceria/soot mixtures (Figures 12, 14, and 15) and particularly of the first CO2 peak. In line with the experimental microkinetic approach of catalytic processes,20,21 experimental procedures are used to quantify the kinetic parameters of interest and particularly the average number and the surface area of the ceria/soot contacts. 5. Conclusions The present study was dedicated to an experimental approach of the evolutions of the ceria/soot contacts during the catalytic oxidation of the soot using mechanical mixtures of the two solids mimicking the role of a catalystcoated filter in an exhaust line. In agreement with the literature data, it has been observed that, for a ceria/soot weight ratio of 10, the tight contact24,25 allows for a decrease of the ignition temperature of the soot by ≈160 K, as compared to ≈60 K for the loose contact. It has been shown that the amount of oxygen transferred from ceria to soot can be followed as a function of the soot oxidation using TPE/ O2 adsorption cycles. However, this imposes using experimental conditions, allowing us to either suppress or limit significantly different surface processes because of the large amount of ceria; this is achieved after two TPE/O2 adsorption cycles. In these conditions, the total amount of oxygen transferred during successive TPE/O2 adsorption cycles is roughly constant (considering the experimental uncertainties), regardless of the ceria/soot mixtures. This indicates that the ceria/soot contacts are maintained during the progressive oxidation of the soot. This result is in good agreement with the conclusions by Simonsen et al.79 from an environmental TEM characterization of the catalytic oxidation of the diesel soot. However, the total amount of oxygen transferred during a TPE (in the range of 500-1100 K) cannot be directly related to the decrease in the light-off temperature of the soot. It is the amount of oxygen transferred at low temperatures and the rate of this transfer that must be taken into account, as shown in part 2 (10.1021/ef100582w) of the present study.
(79) Simonsen, S. B.; Dahl, S.; Johnson, E.; Helveg, S. J. Catal. 2008, 255, 1–5. (80) Giordano, F.; Trovarelli, A.; de Leitenburg, C.; Giona, M. J. Catal. 2000, 193, 273–282.
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