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Ind. Eng. Chem. Res. 2008, 47, 3834–3839
Zirconia-Supported Cu-KNO3 Catalyst: Characterization and Catalytic Behavior in the Catalytic Combustion of Soot with a NO/O2 Mixture Ileana D. Lick,† Alfredo L. Carrascull,‡ Marta I. Ponzi,‡ and Esther N. Ponzi*,† CINDECA (CONICET-UNLP), Departamento de Química, Fac. de Ciencias Exactas, Calle 47, N° 257, 1900 La Plata, Buenos Aires, Argentina, and INTEQUI (CONICET-UNSL), Facultad de Ingeniería y Ciencias Económico Sociales, 25 de Mayo 38, 5730 Villa Mercedes, San Luis, Argentina
The catalytic combustion of diesel soot was studied in the presence of catalysts: CuZrO2, KNO3ZrO2, and CuKNO3ZrO2. The catalysts were prepared by impregnation of ZrO2 · nH2O into an aqueous solution of Cu(NO3)2, KNO3, or both salts simultaneously. The catalysts were characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and temperature-programmed reduction (TPR). Cu(n)KNO3ZrO2 catalysts present high activity, and the value of the combustion temperature decreases 200 °C with respect to the temperature of the process without catalyzing, with Tmax values between 370 and 380 °C. The activity is associated with KNO3 presence, and the role of KNO3 can be attributed to (i) an increase of contact between soot and the surface of the catalyst and (ii) a redox reaction in which the nitrate is reduced to nitrite by reaction with carbon and again reoxidized to nitrate. The copper contribution is associated with the reaction selectivity toward CO2 formation. Introduction Emission limits of particulate matter coming from diesel emissions and diesel soot are more severe. These emissions are dangerous for human health in general, especially for lungs, because the particles have diameters smaller than five microns and they cannot be retained by the breathing tract. The use of regenerable traps is a way to decrease these emissions. The soot is collected in traps, and the regeneration consists of performing the combustion of collected soot. The use of catalysts is proposed to decrease the combustion temperature from 600 °C to temperature values included in the operation range of diesel exhausts (170–400 °C). Several formulations of catalysts that are able to decrease the soot combustion temperature to a major or lesser degree are present in the bibliography, and there also exist diverse mechanistic hypotheses that explain this temperature decrease. Some authors maintain the hypothesis that the soot combustion occurs via a redox mechanism,1,2 principally in catalysts where the active phase is an oxide: MyOx + C f MyOx + CO2
(1)
MyOx-2 + O2 f MyOx
(2)
Other investigations sustain that the limiting step of the combustion process rate is the contact between soot and catalyst (soot-catalyst), which improves if the catalyst has meltable phases. Mouljin et al.3 use Cs2SO4 · V2O5 catalysts that present mobility and adhere to the soot once the system melting temperature is reached. According to this hypothesis, it is stated that, when improving the interaction soot-catalyst, the oxygen transfer process of the reducible species to the soot is improved to give COx. A large number of studies report that salts of alkaline metals favor soot oxidation.4–20 Among these catalysts, it is possible to find those ones that contain alkaline nitrates, which melt * To whom correspondence should be addressed. E-mail: eponzi@ quimica.unlp.edu. † CINDECA (CONICET-UNLP). ‡ INTEQUI (CONICET-UNSL).
within the temperature range that presents gases in exhaust pipes of automobiles and have the property of increasing the contact between the catalyst and the soot. It is also reported that the nitrate ion participates in soot combustion14–20 by means of a reaction between the nitrate ion and the soot, to give a nitrite ion that is oxidized by the oxygen of the gaseous phase, thus completing the catalytic cycle: NO3- + C f CO2 + NO2-
(3)
NO2- + ½O2 f NO3-
(4)
It is well-known that catalysts containing copper have the capacity to transform NO to NO21,21 and to oxidize CO to CO2.22 In this study, results of the activity of copper and/or potassium nitrate catalysts in the soot combustion reaction, in the presence of NO/O2, are presented. 2. Experimental Section 2.1. Catalyst Preparation. The hydrated zirconium oxide (ZrO2 · nH2O) was obtained by hydrolysis of zirconium oxychloride, ZrOCl2 · 6H2O (Fluka). The necessary amount of ammonium hydroxide (tetrahedron 28%) was added to the zirconium oxychloride to reach a pH ) 10. The product obtained by hydrolysis was filtered and washed until it was free from chloride ions, as determined by the silver nitrate test. The hydrous zirconium (ZrO2 · nH2O) thus obtained was dried at 80 °C for 24 h. Precursors of catalysts containing copper and/or potassium nitrate were prepared by the impregnation of hydrous zirconium with an aqueous solution of Cu(NO3)2 and/or KNO3, using the necessary amount of solution to fill the pore volume of the support. The precursors were dried at 80 °C for 24 h and were named PCu(n)ZrO2, PKNO3ZrO2, and PCu(n)KNO3ZrO2, where n represents Cu wt %. A series of catalysts with different copper content (1.0–10.0 wt %) and/or potassium nitrate (11.5 wt %) were prepared by calcination of precursor PCu(n)KNO3ZrO2 at 600 °C for 2 h. Catalysts were represented generically by Cu(n)ZrO2, KNO3ZrO2, and Cu(n)KNO3ZrO2.
10.1021/ie071194v CCC: $40.75 2008 American Chemical Society Published on Web 05/08/2008
Ind. Eng. Chem. Res., Vol. 47, No. 11, 2008 3835
2.2. Characterization. Crystalline phases within the catalysts were identified by powder X-Ray diffraction (XRD) analysis using a Rigaku D-Max III diffractometer equipped with Nifiltered Cu KR radiation (λ ) 1.5378 Å). The melting point of the potassium nitrate in the catalysts was studied by differential scanning calorimetry (DSC) using a Shimadzu DSC 50 equipment. R-Al2O3 was used as reference and N2 as a carrier. TPR (temperature-programmed reduction) experiments were carried out with the conventional equipment. The TPR was performed using 10% hydrogen in nitrogen (flow rate ) 20 cm3 min-1) with a heating rate of 10 °C/min up to 950 °C. The sample loaded was 20 mg. The presence of nitrate and nitrite anions on catalysts was studied by means of Fourier transform infrared (FTIR) spectroscopy using Bruker equipment. 2.3. Activity Tests. Catalytic tests were performed in a temperature-programmed oxidation (TPO) apparatus, a quartz microreactor with analysis of reaction gases. The microreactor had a fixed bed constructed in quartz (i.d. ) 0.8 cm) and heated electrically. The reaction mixture was obtained from three feed lines individually controlled: NO/He, O2/He, and He to close the balance. To study the soot-combustion reaction, the reactor was fed with the following mixture: 1500 ppm of NO and 8% of O2 (Qtotal ) 50 mL/min). The microreactor was loaded with 30 mg of catalyst and 3 mg of soot (Printex-U), and the combustion was carried out in the range of 200–650 °C with a heating rate of 2 °C/min. Before the reaction, the soot was mixed with the catalyst, with a spatula (loose contact). The reaction products were monitored with a gas chromatograph, Shimadzu model GC-8A, provided with a thermal conductivity detector (TCD). The sampling was carried out approximately every 8 min. The separation of products was performed with a concentric column CTRI of Alltech. This system permitted the identification and quantification of the peaks of O2, N2, CO2, and CO. The concentration of CO2 and CO was determined from the area of the CO2 and CO peaks obtained by chromatographic analysis. 3. Results and Discussion 3.1. Characterization of Catalysts. 3.1.1. X-ray Diffraction, XRD. The bulk structure of Cu(n)ZrO2 catalysts was determined by powder X-ray diffraction (Figure 1a). The XRD pattern (not shown in this study) of the hydrated zirconium oxide calcined at 600 °C for 2 h showed lines of the monoclinic phase (2-theta ) 28.2, 31.5, 34.5, 35.3, 49.3, 50.6, 54.1, 55.3). XRD patterns of Cu(n)ZrO2 catalysts showed a mixture of monoclinic and tetragonal phases (2-theta ) 30.5, 35.2, 50.7, 60.3, 63.2), and the intensity of the tetragonal phase increased with increasing Cu loading. Cu(5)ZrO2 and Cu(10)ZrO2 catalysts contained the metastable tetragonal phase as the predominant phase. Copper addition to the hydrated zirconia led to stabilization of the metastable tetragonal crystalline phase. This stabilization was observed by other authors23,24 when they added di- or trivalent cations to zirconium oxide. The crystalline phase of the copper oxide (2-theta ) 38.5, 35.3, 48.6, 61.4, 67.9) was only observed in the Cu(10)ZrO2 catalyst. XRD patterns of Cu(n)KNO3ZrO2 catalysts (Figure 1b) show that the KNO3 presence strongly influences the crystallization of amorphous hydrated zirconium oxide. Catalysts containing KNO3 crystallize predominantly in the tetragonal form, whereas zirconia without doping crystallizes in the monoclinic form. The KNO3 presence also influences copper segregation. In the XRD patterns of Cu(n)KNO3ZrO2 catalysts, it is possible to observe lines of copper oxide, CuO, from a concentration of 2.5%. The copper oxide is segregated, forming observable
Figure 1. (a) XRD diagrams: a ) Cu(1)ZrO2, b ) Cu(2.5)ZrO2, c ) Cu(5)ZrO2, and d ) Cu(10)ZrO2; (—) tetragonal ZrO2, (b) monoclinic ZrO2, (O) copper oxide. (b) XRD diagrams: a ) Cu(1)KNO3ZrO2, b ) Cu(2.5) KNO3ZrO2, c ) Cu(5)KNO3ZrO2, and d ) Cu(10)KNO3ZrO2; (—) tetragonal ZrO2, (b) monoclinic ZrO, (O) copper oxide.
crystals with the XRD technique. XRD patterns do not show diffraction lines associated with the crystalline KNO3 presence (2-theta ) 25.06, 31.1, 27.86, 36.05) nor with the presence of K2O (2-theta ) 28.87, 31.13, 31.70, 36.7). 3.1.2. Differential Scanning Calorimetry, DSC. Figure 2a shows DSC diagrams of Cu(n)KNO3ZrO2 catalysts and of potassium nitrate. The DSC diagram of KNO3 shows endothermic signals at 350 °C assigned to salt melting.25 This signal of KNO3 melting is observed in all catalysts. The melting temperature of KNO3 in catalysts is lower than that in pure salt. This phenomenon can be originated in the salt dispersion on the support, which will weaken the intermolecular salt bond. To corroborate that the endothermic signal in the DSC of catalysts corresponds to salt melting, DSC experiments are complemented with a cooling stage. Figure 2b shows the DSC diagram for the Cu(10)KNO3ZrO2 catalyst, where the endothermic signal corresponding to the salt melting is observed during the heating stage and the solidification exothermic signal is observed during the cooling stage. 3.1.3. Temperature-Programmed Reduction (TPR). Figure 3 shows TPR diagrams of catalysts Cu(5)ZrO2, KNO3ZrO2, and Cu(n)KNO3ZrO2. For quantitative analysis, the reduction peak was integrated and the values of H2 consumed (mmol) are given in Table 1.
3836 Ind. Eng. Chem. Res., Vol. 47, No. 11, 2008
Figure 3. TPR patterns of catalysts: (a) KNO3ZrO2, (b) Cu(5)ZrO2, (c) Cu(1)KNO3ZrO2, (d) Cu(2.5)KNO3ZrO2, (e) Cu(5)KNO3ZrO2, and (f) Cu(10) KNO3ZrO2. Table 1. Integrated H2 Uptake Values from TPR
Figure 2. (a) DSC diagrams of catalysts: a ) Cu(1)KNO3ZrO2, b ) Cu(2.5) KNO3ZrO2, c ) Cu(5)KNO3ZrO2, d ) Cu(10)KNO3ZrO2, and e ) KNO3; KNO3 mass ) 1.95 mg and catalyst mass ) 10 mg. (b) DSC diagram of Cu(5)KNO3ZrO2 catalyst; during stages of heating (line a) and cooling (line b).
Figure 3 shows a reduction diagram of Cu(5)ZrO2 catalyst with reduction signals between 170 and 300 °C, characteristic of CuO.23,26,27 All copper catalysts, Cu(n)ZrO2, present a similar reduction profile, and there exists a lineal correlation between the H2 consumption and the copper content in Cu(n)ZrO2 catalysts. The KNO3ZrO2 catalyst presents a reduction peak at 594 °C attributed to nitrate ion reduction. Cu(n)KNO3ZrO2 catalysts present a unique signal and a marked decrease in copper reducibility, with respect to that of Cu(n)ZrO2 catalysts. This phenomenon is attributed to the copper-potassium interaction.28,29 On the other hand, Cu(n)KNO3ZrO2 catalysts show higher reducibility than the KNO3ZrO2 catalyst. The decrease of the reduction temperature of nitrate species is attributed to the generated Cu° contribution. As the copper content increases in Cu(n)KNO3/ZrO2 catalysts, the reducibility increases, showing a reduction signal with a maximum at 435, 424, 422, and 394 °C for catalysts with 1%, 2.5%, 5%, and 10% copper, respectively. Cu(n)KNO3ZrO2 catalysts show almost equal H2 consumption for reduction of nitrate ion. Moles of H2 consumed in the reduction of the nitrate ion were calculated as the difference
catalysts
mmol of H2
mmol of H2NO3-
KNO3ZrO2 Cu(1)ZrO2 Cu(2.5)ZrO2 Cu(5)ZrO2 Cu(10)ZrO2 Cu(1)KNO3ZrO2 Cu(2.5)KNO3ZrO2 Cu(5)KNO3ZrO2 Cu(10)KNO3ZrO2
0.028 0.0030 0.0078 0.0171 0.0323 0.0454 0.0474 0.0549 0.0720
0.028
0.0408 0.0381 0.0363 0.0397
between the total mmoles consumed and the mmoles corresponding to CuO reduction. 3.1.4. FTIR Spectroscopy. Figure 4 shows FTIR spectra of Cu(n)KNO3/ZrO2 catalysts. Spectra of catalysts present antisymmetric N-O stretching bands typical of free nitrate ions (1385 cm-1).30 Catalysts presented in this work were calcined at 600 °C and present KNO3 in their composition. These results are in agreement with results obtained by TPR and DSC. 3.2. Catalytic Results. Catalytic tests were performed in a temperature-programmed oxidation (TPO) apparatus with a mixture of 1500 ppm of NO and 8% of O2. Catalysts (30 mg) and Printex-U (3 mg) were mixed together with a spatula to obtain “loose contact” between the catalyst and the soot. TPO
Figure 4. FTIR spectra of catalysts: (a) Cu(1) KNO3ZrO2, (b) Cu(2.5) KNO3ZrO2, (c) Cu(5) KNO3ZrO2, (d) Cu(10) KNO3ZrO2, and (e) KNO3ZrO2.
Ind. Eng. Chem. Res., Vol. 47, No. 11, 2008 3837 Table 2. Tmax Obtained with Cu(n)KNO3ZrO2 Catalysts catalyst
Tmax (°C)
KNO3ZrO2 Cu(1)KNO3ZrO2 Cu(2.5)KNO3ZrO2 Cu(5)KNO3ZrO2 Cu(10)KNO3ZrO2
380 375 380 380 370
Table 3. Influence of the NO Presence in the Maximum Burning Rate; Soot/Catalyst (1:10) Mixed with Spatula
Figure 5. TPO results of catalyst-soot (10/1) mixture, “loose contact” in NO/O2 presence: (∆) soot, (9) Cu(1)ZrO2, (π) Cu(2.5)ZrO2, (9) Cu(5)ZrO2, (9) Cu(10)ZrO2, and (O) KNO3ZrO2.
Figure 6. TPO results of catalyst-soot (10/1) mixture, loose contact in NO/O2 presence: (9) Cu(1)KNO3ZrO2, (O) Cu(2.5) KNO3ZrO2, (9) Cu(5) KNO3ZrO2, and (9) Cu(10) KNO3ZrO2.
diagrams resulting from the soot combustion using Cu(n)ZrO2, using KNO3ZrO2, and without catalyst are shown in Figure 5. The KNO3ZrO2 catalyst is the most active with a maximum burning rate at 378 °C (Tmax), whereas the Tmax without catalyst is 585 °C. Copper catalysts, Cu(n)ZrO2, present lower activities than the KNO3ZrO2 catalyst, showing higher Tmax as well as a wider range of combustion temperatures. The activity of Cu(n)ZrO2 catalysts can be attributed to the copper capacity of forming NO2, which is a stronger oxidizer than O2. NO + ½O2 f NO2 C + 2NO2 f CO2 + 2NO These catalysts show total selectivity toward the formation of CO2 (S ) CO2/(CO2 + CO). The activity of the KNO3ZrO2 catalyst is assigned to the carbon oxidation by the nitrate ion generating CO and CO2. NO3- + C f CO + NO22NO3- + C f CO2 + 2NO22NO2- + O2 f 2NO3TPO diagrams resulting from the soot combustion using Cu(n)KNO3ZrO2 catalysts are shown in Figure 6. Table 2 shows Tmax values obtained with Cu(n)KNO3ZrO2 catalysts. All catalysts are active with Tmax values between 370 and 380 °C and decrease 200 °C from the Tmax of the combustion without
catalyst
NO/O2
O2
without catalyst KNO3ZrO2 Cu(5)ZrO2 Cu(5)KNO3ZrO2
585 380 415 385
580 350 535 390
catalyst. Likewise, it is very important to denote that the reaction starts at temperatures of about 280 °C. The integration of production curves COx (mmol) versus temperature (Figures 5 and 6) gives similar values of mmoles of carbon burned for all experiments, showing that the interval of sample taking used is adequate and the accuracy of Tmax informed is estimated to be about (5 °C. Tmax found with Cu(n)KNO3ZrO2 catalysts are lower than the ones with Cu(n)ZrO2 catalysts. This suggests that the activity can be associated predominantly with KNO3 and with the wetting effect of the catalytic surface that this salt confers. The burning process starts at temperatures near to the temperature where the alkaline nitrate melts (about 320 °C) and continues in the temperature range where the supported oxidic phases of catalysts (350–450 °C) are reduced, as is shown in the reduction diagrams at programmed temperature, supporting in this way a redox-type mechanism. A similar redox behavior has been observed with cobalt and potassium nitrate catalysts supported on alumina, zirconia, and silica.11 It is important to denote that these catalysts do not generate CO, but this reaction product is observed in potassium nitrate catalysts. Figure 7 shows FTIR spectra of KNO3/ZrO2 catalyst fresh and extracted from the reactor after soot combustion at inert atmosphere. In this figure, it is possible to observe energy absorption bands associated to the presence of nitrate ions in the fresh catalyst, and the energy absorption band of nitrite species appears (1268 cm-1) in the spectrum of catalyst extracted from the reactor. This experiment shows evidence that the reaction of soot combustion occurs using oxygen of the nitrate ion. To study the NO effect on the feed current flow, experiments were performed using only O2 in the feed. These experiments were carried out with catalysts Cu(5)/ZrO2, KNO3ZrO2, and Cu(5)KNO3ZrO2, and in Table 3, the Tmax found are detailed. The activity of Cu(5)ZrO2 catalyst appears to be strongly influenced by the NO presence and NO2 formation is postulated, accelerating the combustion rate. This influence is not so pronounced in those catalysts containing KNO3. 3.3. Kinetic Parameters. Calculations are made assuming the behavior of a differential reactor and a kinetic expression of first order with respect to the soot (C). r ) kC where r ) reaction rate (mmol of CO2/s), k ) A exp(-Ea/RT), and C ) instantaneous value of soot mass (mg). Figure 8 shows results of ln(k) versus 1/T for the series of catalysts that only contain copper. The reaction rate (r) is estimated as the product between the COx instantaneous concentration at the reactor exit (mmol of
3838 Ind. Eng. Chem. Res., Vol. 47, No. 11, 2008
4. Conclusions
Figure 7. FTIR spectra of KNO3ZrO2 catalyst: (a) fresh and (b) extracted from the reactor after soot combustion at inert atmosphere.
Figure 8. Arrhenius plots of soot combustion in the presence of Cu(n)ZrO2 catalysts: (9) Cu(1)ZrO2, (π) Cu(2.5)ZrO2, (9) Cu(5)ZrO2, and (9) Cu(10)ZrO2. Table 4. Ea Estimated from Arrhenius Plots for Soot Combustion over the Cu(n)ZrO2 Catalysts based on TPO Data catalyst
Ea (kJ/mol)
ln A
Cu(1)ZrO2 Cu(2.5)ZrO2 Cu(5)ZrO2 Cu(10)ZrO2 KNO3ZrO2 Cu(1)KNO3ZrO2 Cu(5)KNO3ZrO2 Cu(10)KNO3ZrO2
124 118 116 107 176 198 191 174
16.6 15.1 16.9 17.1 28.8 32.6 31.9 29.2
COx/mL) and the flow (mL/s). The instantaneous value of soot mass is estimated as the difference between the initial soot mass and the soot mass burnt up to each instant. Values of activation energy (Ea) estimated for all catalysts are given in Table 4. All Cu(n)ZrO2 catalysts present a similar activation energy (107–124 kJ/mol), with a slight tendency for Ea to decrease with a copper content increase. Activation energy values of Cu(n)KNO3ZrO2 catalysts are found in the range 174–198 KJ/mol. The reaction mechanism of these catalysts can be associated with KNO3 because kinetic parameters of the KNO3ZrO2 catalyst are similar to kinetic parameters of catalysts Cu(n)KNO3ZrO2. Activation energy values estimated are comparable to the ones reported by other authors.31 Although the Ea of catalysts containing KNO3 is higher, the presence of this salt increases the value of the pre-exponential factor (A) noticeably. This phenomenon can be attributed to soot wetting by molten KNO3, thus increasing the probability of effective collisions.
XRD patterns of Cu(n)ZrO2 catalysts show a mixture of monoclinic and tetragonal phases, and the intensity of the tetragonal phase increases even as Cu loading increases. XRD patterns of Cu(n)KNO3ZrO2 catalysts show that catalysts containing KNO3 crystallize predominantly in the tetragonal form. The KNO3 presence also affects copper oxide segregation. KNO3 melting is observed by means of the DSC technique in Cu(n)KNO3ZrO2 catalysts. Copper catalysts, Cu(n)ZrO2, present reduction signal characteristics of CuO, and also catalysts containing potassium nitrate, Cu(n)KNO3ZrO2, present a decrease in the copper reducibility and a reducibility increase of nitrate ion, attributed to Cu° contribution. Cu(n)KNO3ZrO2 catalysts present high activity, and the value of the combustion temperature decreases 200 °C with respect to the temperature of the process without catalyzing. Tmax found with Cu(n)KNO3ZrO2 catalysts is lower than the ones of Cu(n)ZrO2 catalysts, which suggests that the activity can be associated predominantly to the redox properties of KNO3 and to the wetting effect of the catalytic surface conferred by this salt. The copper contribution is associated with the reaction selectivity toward the CO2 formation. The activity of the Cu(5)ZrO2 catalyst is favored by the NO presence. It is postulated that copper intervenes in NO2 formation, accelerating the combustion rate. In the estimation of kinetic parameters, it was possible to observe a noticeable increase in the pre-exponential factor (A) when potassium nitrate was used in the formulation, and this was associated with the increase of possible effective collisions between soot and catalyst, generated by the melting of potassium nitrate on the catalytic surface. Acknowledgment The financial support for this project has been obtained from CONICET, ANPCyT, UNSL, and UNLP. We thank Lic. Raúl Martino for performing TPR experiments. Literature Cited (1) Mul, G.; Kapteijn, F.; Doornkamp, C.; Moulijn, J. A. Transition Metal Oxide Catalyzed Carbon Black Oxidation: A Study with 18O2. J. Catal. 1998, 179, 258. (2) Saracco, G.; Serra, V.; Badini, C.; Specchia, V. Potential of Mixed Halides and Vanadates as Catalysts for Soot Combustion. Ind. Eng. Chem. Res. 1997, 36, 2051. (3) Setiabudi, A.; Allaart, N. K.; Makkee, M.; Moulijn, J. A. In situ visible microscopic study of molten Cs2SO4 · V2O5-soot system: Physical interaction, oxidation rate, and data evaluation. Appl. Catal., B 2005, 60, 241. (4) Zhang, Y.; Zou, X.; Sui, L. The effects of potassium halides on catalytic activities of CeO2-K based catalysts for diesel soot oxidation. Catal. Commun. 2006, 7, 855. (5) Nejar, N.; Makkee, M.; Illán-Gómez, M. J. Catalytic removal of NOx and soot from diesel exhaust: Oxidation behaviour of carbon materials used as model soot. Appl. Catal., B 2007, 75, 11. (6) Atribak, I.; Such-Basáñez, I.; Bueno-López, A.; García, A. Comparison of the catalytic activity of MO2 (M ) Ti, Zr, Ce) for soot oxidation under NOx/O2. J. Catal. 2007, 250, 75. (7) Jiménez, R.; García, X.; Cellier, C.; Ruiz, P.; Gordon, A. L. Soot combustion with K/MgO as catalyst. Appl. Catal., A 2006, 297, 125. (8) Cauda, E.; Mescia, D.; Fino, D.; Saracco, G.; Specchia, V. Diesel Particulate Filtration and Combustion in a Wall-Flow Trap Hosting a LiCrO2 Catalyst. Ind. Eng. Chem. Res. 2005, 44, 9549. (9) Zhang, Y.; Zou, X. The catalytic activities and thermal stabilities of Li/Na/K carbonates for diesel soot oxidation. Catal. Commun. 2007, 8, 760.
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ReceiVed for reView September 3, 2007 ReVised manuscript receiVed March 5, 2008 Accepted March 11, 2008 IE071194V
