Article pubs.acs.org/EF
Impact of the Catalyst/Soot Ratio on Diesel Soot Oxidation Pathways K. Leistner,†,‡ A. Nicolle,*,† and P. Da Costa‡ †
IFP Energies nouvelles, 1-4 Avenue de Bois-Préau, 92852 Rueil-Malmaison, France Institut Jean Le Rond d’Alembert, Université Pierre et Marie Curie, UMR CNRS 7190, 2 Place de la Gare de Ceinture, 78210 Saint Cyr l’Ecole, France
‡
ABSTRACT: The transition between catalytic and thermal soot oxidation mechanisms is of importance in diesel particulate filters (DPFs). The objective of this study is to develop and validate a new detailed kinetic model predicting reactivity and selectivity of soot oxidation by NOx + O2 over Pt/Ce0.73Zr0.27O2 over a wide range of catalyst/soot (c/s) ratios. On the basis of a hierarchical development approach, we merged previously developed kinetic subsets with reactions describing oxygen spillover between ceria and soot. The kinetic parameters for these new steps were estimated by fitting concentration profiles of the major product species. This mechanism allows us to obtain insight into the interplay between catalytic and purely thermal oxidation pathways as a function of the c/s ratio. Overall, our results stress the importance of non-catalytic reactions even at high c/s ratios. It is shown that, in the case of a NO2-rich gaseous feed, a purely thermal reaction between NO2 and soot contributes significantly to soot oxidation, even at high c/s ratios. Among the CO2 production reactions, the reaction of spilt-over oxygen with molecular oxygen shows a complex behavior with respect to the c/s ratio, which can be explained by a competition of O2 with NO2 for spiltover oxygen. NOx consumption pathways over the catalyst were also investigated. For low NO2/NOx ratios, it was found that oxygen spillover plays a more important role than NO2 in soot oxidation.
1. INTRODUCTION
2. EXPERIMENTAL SECTION
In the context of tightening air-quality standards, much research is devoted to the development and control of aftertreatment technologies for diesel engines, which commonly include CeO2−ZrO2 supports.1−3 The kinetics of some steps of interest for ceria-based diesel particulate filter (DPF) formulations has not been studied extensively. Although the diffusion of bulk oxygen to the ceria surface,4 the interaction of ceria lattice oxygen with soot,5,6 and carbonate formation on ceria7 have been put forward as important pathways in the presence of O2, the soot oxidation mechanism involved over Pt/CexZr1−xO2 in the presence of NOx + O2 remains unclear.8 The respective contribution of platinum, ceria, and non-catalytic NOx/soot reactions has not yet been quantitatively determined. To our knowledge, no model capable of predicting global reactivity and species selectivity of soot oxidation by NOx + O2 over Pt/ CexZr1−xO2 has been published. There are however several significant experimental studies of the latter subject.3,8−10 These have yielded sufficient data on reaction products and intermediates to be able to conceive of a number of likely reaction steps. Some of these, such as the reaction of soot with NOx + O2 and NO + O2 interaction with Pt/CexZr1−xO2, have been modeled previously in our group.11,12 In the present study, we merge these kinetic sets with a description of oxygen spillover between ceria and soot to predict soot reactivity with Pt/Ce0.73Zr0.27O2. The new parameters are estimated by fitting model equations to temperature-programmed oxidation (TPO) experiments. It is shown that such a mechanism is able to describe some of the major trends of Pt/CeO2−ZrO2-catalyzed diesel soot oxidation with NOx and O2 in conditions of interest to DPFs (25−850 °C), including the effect of a variation of the catalyst/soot (c/s) ratio.
2.1. Experimental Methodology. The ceria−zirconia (Ce0.73Zr0.27O2, Rhodia) supported platinum (0.45 wt % Pt) catalyst was prepared by the incipient wetness method with an aqueous solution containing the appropriate amount of the platinum precursor H12N6O6Pt. After impregnation, the catalyst was left at room temperature for approximately 1 h and then dried at 373 K for ca. 12 h. It was then calcined for 3 h at 773 K in air. Although the use of carbon black would allow for a systematic comparison of our catalytic system to others, this material is not fully representative of diesel soot in terms of composition and reactivity.13 Therefore, soot was generated by a multicylinder common rail diesel engine (DW10) from PSA, using conventional diesel fuel at operating conditions typical of the ECE urban driving cycle (1500 revolutions/min and 5 bar). The displacement, maximum rated power, maximum rated torque, and compression ratio values were 1998 cm3, 100 kW at 4000 revolutions/min, 320 N.m at 1750 revolutions/min, and 18, respectively.14 Elementary composition of the soot was determined as follows: 86.30 wt % C, 8.78 wt % O, 1.07 wt % H, and 0.14 wt % N. The Brunauer−Emmett−Teller (BET) surface area was found to be 414 m2 g−1 for the soot and 105 m2 g−1 for the ceria−zirconiasupported platinum catalyst. The reactivity tests performed in this study are TPOs with diesel soot and/or Pt/Ce0.73Zr0.27O2 and different mixtures of O2 and NO diluted in Ar. Soot and catalyst samples were mixed by shaking in a closed container (loose contact) and then placed on a porous frit inside a quartz reactor (inner diameter = 8 mm). In all samples, the mass of soot was ca. 5 mg. The total flow rate through the reactor was fixed at 250 mL/min [standard temperature and pressure (STP)]. The reactor was positioned inside a thermally isolated furnace, and after stabilization of the signals, the feed was switched to the flow of reactive gas and a temperature ramp of 10 °C/min was applied. More detail on the experimental setup is given in refs 14 and 15. Table 1 summarizes the operating conditions screened in this study.
© 2012 American Chemical Society
Received: July 30, 2012 Revised: September 26, 2012 Published: September 26, 2012 6091
dx.doi.org/10.1021/ef301275p | Energy Fuels 2012, 26, 6091−6097
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Table 1. Experimental Conditions Considered in the Present Study
a
soot weight (mg)
catalyst weight (mg)
10 0 5 5 5 5
50 25 0 2 5 25
catalyst
inlet flow rate (N cm3 min−1)
type
reference
Pt/Ce0.68Zr0.32O2 Pt/Ce0.73Zr0.27O2 Pt/Ce0.73Zr0.27O2 Pt/Ce0.73Zr0.27O2 Pt/Ce0.73Zr0.27O2 Pt/Ce0.73Zr0.27O2
60 250 250 250 250 250
TPO TPO TPO TPO TPO TPO
8 TSa TS TS TS TS
gas feed 700 200 200 200 200 200
ppmv ppmv ppmv ppmv ppmv ppmv
NO2 + 10% O2 NO + 10% O2 NO + 10% O2 NO + 10% O2 NO + 10% O2 NO + 10% O2
TS denotes experimental data obtained in this study.
2.2. Kinetic Modeling. Extrapolation of kinetics from temperature-programmed experiments (TPEs) requires a description of the reactive gaseous flow through a fixed bed of soot/catalyst particles. For details on the code and solver, see refs 14 and 16. The fixed bed can be modeled by a single continuously stirred reactor because of the low axial Peclet number.16 The pressure and temperature are deduced from mass and energy balances, and Ergun’s law for flow in porous media is used to compute mass and enthalpy flow rates. Diffusion limitations have been shown not to be significant.11,12 The reaction rate of each reaction step j is calculated according to the Arrhenius equation. Subsequently, the source terms ωi (kg s−1) for each gasphase species i are
Table 2. Surface Reaction Mechanism Considered in the Present Studya reference Reactions on Ceria (R1) O2 + 2Ce = 2CeO (R2) NO2 + Ce = CeNO2 (R3) CeNO2 = NO + CeO (R4) CeNO2 + CeO = CeNO3 + Ce (R5) CeNO2 + Ce → CeNO + CeO (R6) 2CeNO → N2 + 2CeO Reactions on Platinum (R7) O2 + 2Pt = 2Pt−O (R8) NO + Pt = Pt−NO (R9) Pt−O + Pt−NO = Pt−NO2 + Pt (R10) NO2 + Pt = Pt−NO2 Reactions on Carbon (R11) O2 + 2C → 2C(O) (R12) C(O) → CO (R13) 2C(O) + O2 → 2CO2 (R14) C + NO = C(NO) (R15) C + NO2 = C(NO2) (R16) C + C(NO) = C(N) + C(O) (R17) 2C(N) → N2 + 2C (R18) C(NO2) + C = C(NO) + C(O) (R19) C(NO2) + C(O) → C + C(ONO2) (R20) C(ONO2) → NO + CO2 (R21) CeO + C → C(Ocat) + Ce (R22) C(Ocat) + NO2 → C(ONO2) (R23) 2C(Ocat) + O2 → 2CO2
N
ωi = SaMi ∑ νi , jrj j=1
where Sa and Mi are the active surface area of soot/catalyst and the molar mass of the ith gas-phase species, respectively, N is the total number of reactions considered, rj is their reaction rates, and νi,j is the corresponding stoichiometric coefficients. Surface coverage θk of the kth surface species is dθk r = k dt Γ in which Γ represents the site density of soot or catalyst. Its value is 2.0 × 10−5, 2.7 × 10−5, and 1.7 × 10−5 mol m−2 for soot, CexZr1−xO2, and Pt, respectively. In all calculations, the dispersion of ceria−zirconia is assumed to be 45%, whereas that of platinum is set to 10%. The rate of generation or consumption of species k is calculated as follows: N
Ns ⎛ E ⎞ g rk = Ak exp⎜− k ⎟[∏ νi , kxi ∏ νj , kθj] ⎝ RT ⎠ i=1
j=1
11 11 11 11 TSb TS 11 11 11 11 12 12 12 12 12 12 12 12 12 12 TS TS TS
a
Ocat denotes the spilt-over oxygen from ceria−zirconia. bTS denotes the reaction added in this study.
The model describes the chemical structure via an active surface and the mean field approximation. Soot/catalyst atoms are assumed to exist in a monolayer, and their contact is assumed to remain constant during soot oxidation, in line with the experimental data from Simonsen et al.17 Table 2 presents the surface mechanism used in this study. The kinetic parameters evaluated in this study by fitting the experimental gas-phase concentration profiles are listed in Table 3. Reactions R5 and R6 were introduced to account for NOx reduction to N2 occurring on ceria at low temperatures. These steps are based on the formation of a nitrosyl species on ceria already observed by Azambre et al.18 In line with these authors, we postulated a reduction of nitrosyl species to N2. Temporal analysis of products (TAP) experiments have shown that soot is oxidized by lattice oxygen from ceria.9 On the basis of this result, reaction steps were proposed to describe oxygen spillover from ceria to soot and subsequent COx production by the spilt-over oxygen. Following the dependency suggested by Issa et al.,19 the rate of spillover reaction R21 was assumed to linearly increase with the c/s ratio. For c/s ratios above 0.4, the following rate law
Table 3. Kinetic Parameters of the New Reactions Included in the Surface Mechanism reaction
Ai (mol m−2 s−1)
Ei (kJ mol−1)
R5 R6 R21 R22 R23
9.00 × 10 1.00 × 1017 see the text 3.00 × 1013 2.67 × 1016
25.0 50.0 70.0 105.0 118.0
23
correctly reproduced the impact of the c/s ratio on COx and NOx concentration profiles observed during soot oxidation (see the next section). θCeO/ceria and θC/soot denote the coverage of lumped CeO species on the ceria surface and that of carbon sites on the soot surface, respectively. Note that the optimized activation energy is lower than that obtained by Bassou et al.4 In the present study, we have no experimental measurement pertaining to the separate behaviors of the oxygen species on ceria. Our choice is therefore to make a more global but meaningful representation rather than a detailed but purely mathematical fit. Bulk oxygen, surface oxygen, and active surface oxygen are therefore merged into one ceria species (CeO)
⎞ ⎛ catalyst ⎛ 8420 ⎞ ⎟θ r21 = ⎜7.54 − 2.69⎟ × 1013 exp⎜− θ ⎝ T ⎠ CeO/ceria C/soot ⎠ ⎝ soot (mol m−2 s−1) 6092
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Figure 1. Experimental and simulated COx profiles for different c/s ratios. Inlet feed: 200 ppmv NO, 10% O2, and balance Ar, at 10 °C/min. Symbols represent experimental data, and lines correspond to modeling results.
Figure 2. Major (left) C consumption and (right) CO2 production reactions. Inlet feed: 200 ppmv NO, 10% O2, and balance Ar, at 10 °C/min. Black, orange, red, and green lines refer to c/s ratios of 0, 0.4, 1, and 5, respectively (same legend as Figure 1). incorporating the impact of zirconia. The comparison to the data from Bassou et al. is thus not straightforward. For the sake of simplicity, we did not explicitly include the transfer of oxygen from platinum to the carbon surface proposed by Jeguirim et al.20 Finally, it is important to mention that the soot used in the experiments contains a significant amount of soluble organic fraction (SOF), which alters reactivity21 by providing additional reactive hydrocarbons. However, the modeling of the interaction of SOF with soot and its volatilization lie outside of the scope of this work.
predicted. For c/s = 0.4, the model predicts a slightly bimodal shape, which is not observed experimentally, with the two peaks being due to Ocat (spilt-over oxygen) and C*(O), respectively. The reasonable reproduction of the temperature shift seems to suggest that spillover of a single oxygen species from the catalyst may indeed be responsible for such a shift. However, other factors should presumably be taken into account to achieve a better fit of CO2 peak heights, including, possibly, the evolution of the contact area during the course of the reaction. With regard to the CO concentration profile, its peak value is estimated quite well, but the shift in the peak temperature with an increasing c/s is not reproduced by the model. Whether this shift could be reproduced by considering CO production from C(Ocat) or CO oxidation on the catalyst23,24 lies beyond the scope of this study. Because the mechanism was found to predict CO and CO2 concentration profiles correctly over a range of c/s ratios, we can use it with confidence to perform rate-of-reaction as well as sensitivity analyses. Figure 2 shows major temperature dependency of soot consumption pathways highlighted by rate-of-production analyses. According to the present mechanism, the rates of non-catalytic (reactions R11 and R14) and catalytic (reaction R21) carbon oxidation pathways are similar for c/s = 0.4. Non-catalytic (reactions R13 and R20) CO2
3. RESULTS AND DISCUSSION 3.1. Impact of the c/s Ratio. The calculated CO2 profiles are shown in comparison to experimental CO2 profiles in Figure 1. As expected, the addition of Pt/Ce0.73Zr0.27O2 to soot enhances the rate of oxidation. The experimental CO2 peak shifts by ca. 100 K as 5 mg of catalyst is added to soot (c/s = 1). Bassou et al.22 found a shift of 60 K for a ceria catalyst in a flow of O2 under loose contact conditions. The more catalyst added, the more marked the effect, and the TPO curves become taller and narrower. The general trend of a shift to lower temperatures with an increasing value of c/s is reproduced quite well. The trend of a decreasing peak height with a decreasing value of c/s is also captured to some extent. However, the absolute peak heights are somewhat under6093
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Figure 3. (On the left) Simulated SOC coverages for different c/s ratios. Inlet feed: 200 ppmv NO, 10% O2, and balance Ar, at 10 °C/min. Dotted lines and continuous lines correspond to C(Ocat) and C(O), respectively. (On the right) Normalized sensitivity of the temperature at which 50% of soot conversion is achieved (T50). S(ceria) and S(Pt) correspond to the exposed surface of ceria−zirconia and platinum, respectively. Normalized sensitivities are calculated as Si = (Ai/T50)(∂T50/∂Ai), where Ai is the pre-exponential factor of the ith reaction.
Figure 4. Experimental and simulated NOx profiles for different c/s ratios. Inlet feed: 200 ppmv NO, 10% O2, and balance Ar, at 10 °C/min. Symbols represent experimental data, and lines denote modeling results.
of the rate constant of reaction R11 results in the increase of C(O) buildup, which is much less reactive than spilt-over oxygen C(Ocat). Thus, the sensitivity of T50 with respect to the rate constant of reaction R11 is positive. The sensitivities of T50 with respect to reactions R22 and R23 show opposite trends with an increasing c/s ratio. This could be attributed to the fact that the amount of NO2 available decreases strongly with an increasing c/s ratio, because less platinum sites remain available for NO oxidation. This point is addressed below. The ability to predict not only soot burnoff and COx concentrations but also the NOx concentration is of crucial importance for multifunctional exhaust aftertreatment components.26,27 Figure 4 shows NOx concentration profiles obtained at different c/s ratios. As seen, the NO2 concentration increases with the c/s ratio. The contribution of NO2 to soot oxidation can be quantified by calculating the difference between the blue and green symbols. From integration of the concentration measurements, the amount of oxygen contained in NO2 consumed by soot is found to represent only 6% of the oxygen found in the products. This calculation supports the idea that oxygen spillover plays a more important role than NO2 in soot oxidation in our experimental conditions.
production pathways are also significant for this low c/s ratio. The transition from a quasi-exclusively catalytic (high c/s) to a fully thermal (low c/s) soot oxidation mechanism is of particular importance because, in a real DPF, soot cake oxidation has been found to be mainly driven by the thermal oxidation rate.25 Unexpectedly, the peak rate of reaction R23 appears to evolve non-monotonically with respect to the c/s ratio. This observation can be rationalized by the surface coverage profiles on the left panel of Figure 3. In the case of c/s = 5, the earlier onset of C(Ocat) consumption on soot surface results in a lower C(Ocat) coverage in the 450−650 °C range. Because reaction R23 becomes only active at temperatures higher than 450 °C, it is affected by C(Ocat) depletion through the concurrent reaction R22, which grows in importance as c/s increases. The right panel of Figure 3 shows the normalized sensitivities of T50 with respect to kinetic parameters as well as ceria and platinum active surfaces. As seen, platinum parameters [A8f, A9f, and S(Pt)] are important for soot oxidation prediction, regardless of the value of the c/s ratio (provided that c/s ≠ 0). On the contrary, the spillover step R21 from ceria to soot is seen to have an impact on T50 only at low c/s ratios only. With regard to the non-catalytic soot oxidation pathway, the increase 6094
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Figure 5. (On the left) Experimental and simulated mass profiles for different c/s ratios. Inlet feed: 700 ppmv NO2, 10% O2, and balance Ar, at 10 °C/min. Symbols represent experimental data,8 and lines denote modeling results. (On the right) Coverages on the soot surface in the same operating conditions. Dotted lines and continuous lines correspond to C(Ocat) and C(O), respectively.
Figure 6. Integrated contributions of (left) C oxidation reactions and (right) CO2 production reactions.
simulations. As seen, the agreement between model and experiment is adequate, even if the model does not predict the reactivity of soot at low temperatures (
