ZrO2 Catalysts. Effect of

Co-KNO3-ZrO2 and Co-ZrO2 were prepared. The concentration effect of potassium nitrate on the activity was studied for the oxidation reaction of soot. ...
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Ind. Eng. Chem. Res. 2003, 42, 692-697

KINETICS, CATALYSIS, AND REACTION ENGINEERING Catalytic Combustion of Soot on KNO3/ZrO2 Catalysts. Effect of Potassium Nitrate Loading on Activity Alfredo L. Carrascull,† Marta I. Ponzi,† and Esther N. Ponzi*,‡ INTEQUI (CONICET-UNSL), Facultad de Ingenierı´a y Ciencias Econo´ mico Sociales, 25 de Mayo 384, (5730) Villa Mercedes, San Luis, Argentina, and CINDECA (CONICET-UNLP), Calle 47 No. 257, (1900) La Plata, Buenos Aires, Argentina

Co-KNO3-ZrO2 and Co-ZrO2 were prepared. The concentration effect of potassium nitrate on the activity was studied for the oxidation reaction of soot. Catalysts were characterized using the techniques BET, XRD, DSC, TPR, and SEM and the catalytic activity was measured in a microbalance. An important decrease of the combustion temperature of soot is observed in experiments carried out with zirconium oxide alone and also a significant enhancement of the catalytic combustion rate of soot on KNO3/ZrO2 catalysts. The activity increase can be attributed to the alkaline metal ion and also to the increase of the catalyst/soot contact by the molten potassium nitrate. 1. Introduction The combustion process occurring in diesel engines is frequently incomplete and undesired byproducts such as NOx and soot are formed. Nitrogen oxides are gaseous pollutants that contribute to acid rain formation and depletion of the stratospheric ozone layer. The size of the soot is so small that it can penetrate into the breathing tract and be deposited in the lung region, producing adverse effects for human health.1 To reduce emissions of soot and NOx, catalytic systems can be used in which hydrocarbons and part of the soot are oxidized by the emission gases. The temperature range of the exhaust gases of a diesel engine is 425-673 K, and to oxidize the soot at that temperature, it is necessary to have active combustion catalysts within this temperature range.2,3 Several catalysts are mentioned in the references for oxidation of particulate matter, the oxides of transition metals such as V2O5, CuO, and MnO2 and their mixtures.4-8 For some catalytic systems the high catalytic performance has been attributed to the formation of a liquid phase.9,10 SO3 can be generated during combustion by SO2 oxidation and then it can be retained by the support. The alumina used in some cases to make the washcoat of monolithic catalysts has the disadvantage of retaining SO3, and from this, the importance of using other supports such as Nb2O5, WO3, SnO2, and SiO211 and TiO2/SiC and ZrO2/SiC.12 The oxidizing and reducing properties are characteristic of ZrO2 and besides it is a bifunctional acid-base catalyst. This material (ZrO2) is found among the moderately active supports for the combustion reaction * To whom correspondence should be addressed. Fax: 054 221 4254277. E-mail: [email protected]. † INTEQUI (CONICET-UNSL). ‡ CINDECA (CONICET-UNLP).

of particulate matter.13-15 Potassium has been reported in the literature as a promoter since this metal increases the activity for the combustion reaction.8,15 The aim of this work is to analyze the effect of the potassium nitrate concentration in catalysts of the KNO3/ZrO2 type on the combustion rate of the particulate matter, as well as the influence of the transition metal impregnated in the promoted support. 2. Experimental Section 2.1. Catalyst Sample Preparation. The zirconium hydroxide 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 of 10. The product obtained by hydrolysis was filtered and washed up to the nonidentification of the ion chloride in the washing water, and finally the material was dried at 353 K for 24 h. Catalysts containing potassium nitrate and/or cobalt were prepared by impregnation of the zirconium hydroxide with an aqueous solution of the respective salt, KNO3 or Co(NO3)2, using the necessary amount of solution to fill the pore volume of the support. The zirconium hydroxide impregnated with the corresponding salts was dried at 353 K for 24 h. When materials are dried at 353 K, they are named precursors and the water molecule (H2O) appears together with a zirconium oxide molecule (ZrO2‚nH2O) in the nomenclature. To obtain catalysts, precursors are calcined at 873 K for 2 h and only the zirconium oxide molecule (ZrO2) appears in the denomination. Catalysts were prepared with a variable potassium nitrate content between 0.25 and 44%, which will be named generically KnZrO2, where n is the nominal concentration of potassium nitrate. The salt concentration is based on the catalyst (g of KNO3/g of catalyst).

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2.2. Catalyst Characterization. Transformations occurring during calcination of ZrO2‚nH2O and of the precursors of catalysts K/ZrO2‚nH2O, Co/ZrO2‚nH2O, and Co/K/ZrO2‚nH2O were studied using differential scanning calorimetry (DSC) and thermogravimetry (TGA). Also, experiments using the DSC technique were performed with catalysts containing variable potassium concentration to detect the melting temperature of the potassium nitrate in these catalysts. The temperature-programmed reduction (TPR) tests were carried out in conventional equipment. In a typical TPR experiment the sample was loaded in a quartz microreactor installed in a furnace provided with a temperature controller/programmer and a thermocouple in the catalytic bed to measure the temperature. The sample was heated in a gaseous mixture of 5% H2 in N2 up to 1123 K with a heating rate of 10 K/min. The surface area of the support and catalysts was determined by using the BET method, in Micromeritics Accusorb 2100E equipment. The crystalline structure of the catalysts was determined by X-ray diffraction studies (XRD) utilizing Rigaku D-Max III equipment with Cu KR radiation (λ ) 1.5378 Å, 40 K, 30 mA). The 2θ range analyzed was the one between 15° and 70°. The crystal size of the monoclinic zirconium oxide was determined utilizing a scanning rate of 1/8°/min. To calculate the crystal size, the Scherrer equation that relates the peak width at half-height to the crystal size was used. The morphology of the catalysts was observed by micrographs utilizing a Microscope Philips SEM 505. 2.3. Activity Test. To carry out catalytic tests, the soot (Printex-U, Degussa) and the catalyst, with 1/100 and 1/10 ratios, were carefully mixed in agate mortar. Both components were ground up to achieve an intimate contact between soot and catalyst. The oxidation was performed in a Shimadzu thermobalance, model TGA50, with a heating rate of 10 K/min in an air/He flow (2:1). The weight loss and the temperature were recorded as a function of time. Starting with the information of weight loss as a function of time, the derivative curve (DTGA) was obtained and from there the temperature of the maximum oxidation rate.

Figure 1. Thermal analysis of KNO3.

3. Results and Discussion 3.1. Characterization of Precursors Using DSC. DSC and TGA diagrams of potassium nitrate were obtained by performing experiments with 20 mg of sample, 30 cm3/min of inert gas, and a heating rate of 20 K/min and results are shown in Figure 1. In the DSC diagram two endothermic signals at 405 and 608 K are observed. The potassium nitrate can be found in two crystalline forms, rhombic or trigonal. A transition of crystalline phases at 402 K and the melting point at 607 K are indicated in the references.16 According to the above-mentioned data, the first peak of the DSC diagram corresponds to the crystalline transformation and the second one corresponds to the potassium nitrate melting. In the TGA diagram, it is observed that the sample begins to lose weight from 840 K. This phenomenon can be associated with the potassium nitrate evaporation. Figure 2 shows DSC diagrams of precursors ZrO2‚ nH2O, ZrO2‚nH2O impregnated with KNO3 with different concentration values (curves a-g), and ZrO2‚nH2O impregnated with Co(NO3)2/KNO3 (curve h), and Co-

Figure 2. DSC of precursors promoted with potassium nitrate and/or cobalt nitrate. (a) ZrO2‚nH2O; (b) K0.25ZrO2‚nH2O; (c) K1.25ZrO2‚nH2O; (d) K4.5ZrO2‚nH2O; (e) K11.5ZrO2‚nH2O; (f) K20.5ZrO2‚nH2O; (g) K44ZrO2‚nH2O; (h) CoK4.5ZrO2‚nH2O; (i) Co/ ZrO2‚nH2O.

(NO3)2 (curve i). In curve a, ZrO2‚nH2O, it is observed that the amorphous phase transformation of hydrous zirconia to crystalline phase occurs at 720 K. The addition of potassium nitrate increases the transformation temperature. Thus, in the precursor with 0.25% potassium nitrate (curve b), the transformation occurs at 725 K whereas in the precursor with 44% potassium nitrate (curve g), the transformation occurs at 790 K. The endothermic transformation of the potassium nitrate melting (610 K) is observed in curve g. In precursors containing KNO3 and Co(NO3)2, the transformation of the amorphous-crystalline phase also occurs at higher temperature than in ZrO2‚nH2O.

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Figure 3. (A) XRD of catalyst samples with variable KNO3 concentrations. (a) K0.25ZrO2; (b) K1.25ZrO2; (c) K4.5ZrO2; (d) K11.5ZrO2; (e) K20.5ZrO2; (f) K44ZrO2. (B) XRD of samples of support and catalysts promoted with Co(NO3)2 and KNO3. (a) CoK4.5ZrO2; (b) K4.5ZrO2; (c) Co/ZrO2; (d) ZrO2. Table 1. Specific Surface Area of the Support and Catalysts (Precursor Calcined at 873 K)

a

catalyst

crystal size (Å)

S (m2/g)

ZrO2 K0.25ZrO2 K1.25ZrO2 K4.5ZrO2 K11.5ZrO2 K20.5ZrO2 K44ZrO2 CoZrO2a CoK4.5ZrO2a

124

45 45 52 28 15 11 36 7

179

324

Co concentration 5% based on the catalyst.

3.2. Characterization of Catalysts Using DSC, XRD, SEM, and TPR. The specific surface area of the support and catalysts containing potassium and cobalt determined by the BET method is shown in Table 1. The support of zirconium oxide (ZrO2) and catalysts containing only potassium nitrate in lower concentration (K0.25ZrO2 and K1.25ZrO2) present the largest surface. On the other hand, catalysts obtained by impregnation of individual potassium (with concentration higher than 4.5%) and cobalt salts or by simultaneous impregnation present smaller specific surface area. The X-ray diffraction diagrams obtained for the support and catalysts, ZrO2, CoZrO2, KnZrO2, and K4.5CoZrO2, are presented in Figure 3A,B. In Figure 3A, diagrams for catalysts with different potassium nitrate contents are shown. It is observed that when the potassium nitrate concentration increases up to 20.5%, the tetragonal phase of zirconium oxide (2θ 30.2) also increases and the monoclinic crystalline phase decreases (2θ 28.3 and 31.6). When the concentration

is 44% potassium nitrate, the signal corresponding to tetragonal zirconium oxide decreases and KNO3 is observed. Figure 3B shows results of catalysts promoted with cobalt and potassium nitrate. The ZrO2 support (curve d) contains principally the monoclinic phase and the CoZrO2 catalyst (curve c) presents only tetragonal phase of ZrO2 (the monoclinic phase is completely removed), whereas the potassium nitrate with a concentration of 4.5% (curve b) only moderately increases the formation of tetragonal phase and the monoclinic phase coexists. The catalyst impregnated simultaneously with cobalt and potassium (curve a), contrary to what occurs with each individual element, promotes the crystallization in the monoclinic phase. In this catalyst two signals at 28.3° and 31.6° with intense peaks are observed and this indicates that ZrO2 crystals of the monoclinic phase are large. The crystal sizes of the ZrO2 crystallized in monoclinic phase are shown in Table 1. An increase of the ZrO2 crystal size is observed when promoters are incorporated into the zirconium oxide. There exists a good correlation between the crystal size of the monoclinic phase and the specific surface area of these catalysts; the catalyst promoted with potassium nitrate and cobalt presents the largest crystal size, 324 Å, and the smallest area of the group, 7 m2/g. DSC diagrams of catalysts with variable potassium concentration are shown in Figure 4. In the catalyst with higher potassium nitrate concentration (K44ZrO2, curve f) the endothermic transformation of the potassium nitrate melting (610 K) is clearly observed, when the test is performed by increasing the temperature. During the cooling stage of the catalyst, the exothermic

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Figure 4. DSC of catalysts promoted with KNO3. (a) K0.25ZrO2; (b) K1.25ZrO2; (c) K4.5ZrO2; (d) K11.5ZrO2; (e) K20.5ZrO2; (f) K44ZrO2.

signal corresponding to solidification of the liquid potassium nitrate is also observed. In the catalysts with lower concentration, the signal corresponding to the melting is less pronounced. In images obtained by SEM, Figure 5A-C, in the catalyst with higher potassium nitrate concentration, the presence of a molten species covering crystals is observed (Figure 5A). Instead, for intermediate concentrations of potassium nitrate, lengthened forms with rounded ends are observed (Figure 5B,C). During the exposition of the catalyst sample to electron rays, crystals as prisms are clearly observed initially and they continue losing rigidity during exposition, finishing with rounded forms in the ends. Some of these crystals present fissures during exposition to electron rays as shown in Figure 5B. TPR profiles of samples CoZrO2 and CoK4.5ZrO2 are shown in Figure 6 and TPR profiles of standard substances as CoO and Co3O4 are also shown. The Co3O4 was prepared by following the technique suggested in Cotton and Wilkinson17 and van Steen et al.18 Cobalt nitrate was calcined in an air current at 773 K for 2 h. In turn, CoO was prepared by calcination at 973 K for the same period. The reduction of Co3O4 (curve a) shows a 610 K peak attributed to the Co3+-to-Co2+ transformation and another signal at 673 K where all the Co2+ reduces to metallic cobalt.19 The reduction of CoO (curve b) shows a unique 673 K peak attributed to the Co2+ reduction to metallic cobalt. The sample containing potassium nitrate and cobalt, CoK4.5ZrO2 (curve c), and the catalyst containing only cobalt supported on zirconium, CoZrO2 (curve d), present two peaks at 673 and 650 K, attributed to the reduction of CoO to metallic Co. The signal of 610 K, in the CoZrO2 catalyst, is attributed to the reduction of Co3O4 to CoO. Figure 7A presents results of soot combustion experiments for the complete series of catalysts with variable potassium concentration KnZrO2. These results indicate that the combustion rate is increased up to a potassium

Figure 5. (A) SEM photograph of the K44ZrO2 catalyst. (B) SEM photograph of the K20.5ZrO2 catalyst. (c) SEM photograph of the K11.5ZrO2 catalyst.

nitrate concentration of 11.5% and then it remains constant up to 20.5% and decreases at higher concentration. Figure 7B presents results of soot combustion for K11.5ZrO2 and K20.5ZrO2 catalysts using a lower catalyst/soot ratio (1/10). The temperature of the maximum combustion rate for these catalysts is 676 K when the catalyst/soot ratio is 1/100 and 695 K when the catalyst/soot ratio is 1/10. The soot combustion was

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Figure 6. TPR of catalysts promoted with Co(NO3)2 and KNO3. (a) Co3O4; (b) CoO; (c) CoK4.5ZrO2; (d) CoZrO2.

repeated by using the K20.5ZrO2 catalyst and results are also shown in Figure 7B. The temperature of the maximum combustion rate in the two experiments is similar. The influence of the potassium nitrate in the process of soot combustion can be attributed to several causes. The first one is attributed to the presence of the liquid phase generated by the potassium nitrate melted from 610 K that increases the catalyst/soot contact. The activity increase can be attributed to the alkaline metal ion. The positive effect of alkali doping can be ascribed to alkali metal-carbon intermediates in the gasification

sample

Tmax (K)

sample

Tmax (K)

Printex-U ZrO2 CoZrO2

930 820 740

K4.5ZrO2 CoK4.5ZrO2

720 710

process.20 Also, the nitrate ion can be assigned to act as an oxidizing agent of the soot being transformed into some reduced species as for example the nitrite ion. Results of combustion experiments of soot without catalyst and with zirconium oxide, zirconium oxide individually promoted with potassium nitrate, or cobalt and with both promoters simultaneously are presented in Table 2. A decrease of the combustion temperature of soot around 100 K is observed in experiments carried out with zirconium oxide. The soot combustion with zirconium oxide presents its maximum rate at 823 K. It is supposed that the reaction mechanism is produced by a redox process of ZrO2 since, besides the acid-base properties, oxidizing and reducing properties are also found. The temperature of the maximum combustion rate, when catalysts containing potassium nitrate or cobalt as the promoter are used, is lower than when only the support is used. The simultaneous effect of both promoters is not very different from that generated by each promoter individually. The behavior difference between ZrO2 and the catalyst promoted with cobalt CoZrO2 can be originated in the cobalt oxide reducibility as observed in TPR profiles of this catalyst (CoZrO2) shown in Figure 6. The catalytic role of the cobalt can be explained by a redox-type mechanism in which the cobalt oxide oxidizes the soot and then it is regenerated by the oxygen as was proposed by Ciambrelli et al. 6 for the soot combustion using copper catalysts. Conclusions A decrease of the combustion temperature of soot around 100 K is observed in experiments carried out

Figure 7. (A) TPO of soot on catalysts promoted with KNO3; soot/catalyst: 1/100. (a) Without catalyst; (b) ZrO2; (c) K1.25ZrO2; (d) K4.5ZrO2; (e) K11.5ZrO2; (f) K20.5ZrO2; (g) K44ZrO2. (B) TPO of soot on catalysts promoted with KNO3, soot/catalyst: 1/10. (a) K11.5ZrO2; (b) and (c) K20.5ZrO2.

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with zirconium oxide; it is supposed that the reaction mechanism is produced by a redox process of ZrO2. A significant enhancement of the catalytic combustion rate of soot on KnZrO2 was observed up to a potassium nitrate concentration of 11.5%. This activity increase is attributed to the properties of the potassium and to the increase of soot/catalyst contact through the molten potassium nitrate. The simultaneous effect of both promoters (Co and KNO3) is not very different from that generated by each promoter individually. The decrease of the combustion temperature with respect to the combustion of the particulate matter without catalyst is of the order of 200 K. The catalytic role of the cobalt can be explained by a redox-type mechanism in which the cobalt oxide oxidizes the soot and then it is regenerated by the oxygen. Acknowledgment Financial support for this project has been obtained from CONICET, UNSL and UNLP. Nestor Bernava is acknowledged for thermal analysis. Literature Cited (1) Dockery, D. W.; Schwartz, J.; Spengler, J. D. Air pollution and daily mortality: associations with particulates and acid aerosols. Environ. Res. 1992, 59, 362. (2) Neeft, J. P. A.; Makkee, M.; Moulijn, J. A. Diesel particulate emission control. Fuel Process. Technol. 1996, 47, 1. (3) Heck, R. M.; Farrauto, R. J. Catalytic Air Pollution Control: Commercial Technology; Van Nostrand Reinhold: New York, 1995. (4) Ahlstrom, A. F.; Odenbrand, C. U. I. Catalytic combustion of soot deposits from diesel engines. Appl. Catal. 1990, 60, 143. (5) Ahlstrom, A. F.; Odenbrand, C. U. I. Combustion of soot deposits from diesel engines on mixed oxides of vanadium pentoxide and cupric oxide. Appl. Catal. 1990, 60, 157. (6) Ciambelli, P.; Corbo, P.; Parella, P.; Scialo´, M.; Vaccaro, S. Catalytic oxidation of soot from diesel exhaust gases. 1. Screening of metal oxide catalysts by TG-DTG-DTA analysis. Thermochim. Acta 1990, 162, 83. (7) Querini, C. A.; Ulla, M. A.; Requejo, F.; Soria, J.; Sedra´n, U. A.; Miro´, E. E. Catalytic combustion of diesel soot particles. Activity and characterization of Co/MgO and Co,K/MgO catalysts. Appl. Catal., B 1998, 15, 5.

(8) Querini, C. A.; Cornaglia, L. M.; Ulla, M. A.; Miro´, E. E. Catalytic combustion of diesel soot on Co, K/MgO catalysts. Effect of the potassium loading on activity and stability. Appl. Catal., B 1999, 20, 165. (9) Serra, V.; Saracco, G.; Badini, C.; Specchia, V. Combustion of carbonaceous materials by Cu-K-V based catalysts. II Reaction mechanism. Appl. Catal., B 1997, 11, 329. (10) Jelles, S. J.; Van Setten, B. A. A. L.; Makkee, M.; Moulijn, J. A. Molten salts as promising catalysts for oxidation of diesel soot: importance of experimental conditions in testing procedures. Appl. Catal., B 1999, 21, 35. (11) Oi-Uchisawa, J.; Obuchi, A.; Enomoto, R.; Liu, S.; Nanba, T.; Kushiyama, S. Catalytic performance of Pt supported on various metal oxides in the oxidation of carbon black. Appl. Catal., B 2000, 26, 17. (12) Oi-Uchisawa, J.; Obuchi, A.; Enomoto, R.; Xu, J.; Nanba, T.; Liu, S.; Kushiyama, S. Oxidation of carbon black over various Pt/Mox/SiC catalysts. Appl. Catal., B 2001, 32, 257. (13) van Doorn, J.; Varloud, J.; Me´riaudeau, P.; Perrichon, V.; Chevrier, M.; Gauthier, C. Effect of support material on the catalytic combustion of diesel soot particulate. Appl. Catal., B 1992, 1, 117. (14) Carrascull, A.; Grzona, C.; Lick, D.; Ponzi, M.; Ponzi, E. Soot combustion. Co and K catalysts supported on different supports. React. Kinet. Catal. Lett. 2002, 75, 63. (15) Yuan, S.; Me´riaudeau, P.; Perrichon, V. Catalytic combustion of diesel soot particles on copper catalysts supported on TiO2. Effect of potassium promoter on the activity. Appl. Catal., B 1994, 3, 319. (16) CRC Handbook of Chemistry and Physics, 58th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1977-78; p B-147. (17) Cotton, F. A.; Wilkinson, G. Quı´mica Inorga´ nica Avanzada; LIMUSA, Mexico, 1997. (18) van Steen, E.; Sewell, G. S.; Makhothe, R. A.; Micklethwaite, C.; Manstein, H.; Lange, M.; O’Connor, C. T. TPR Study on the Preparation of Impregnated Co/SiO2 Catalysts. J. Catal. 1996, 162, 220. (19) Jones, A.; McNicol, B. D. Temperature Programmed Reduction for Solid Materials Characterization; Marcel Dekker: New York, 1986. (20) Mross, W. D. Alkali Doping in Heterogeneous Catalysis. Catal. Rev.-Sci. Eng. 1983, 25 (4) 591.

Received for review June 26, 2002 Revised manuscript received November 12, 2002 Accepted December 13, 2002 IE020482I