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Ce0.67Zr0.33O2-Al2O3 three-way catalyst
JOURNAL OF RARE EARTHS, Vol. 30, No. 8, Aug. 2012, P. 748
Steam effects over Pd/Ce0.67Zr0.33O2-Al2O3 three-way catalyst WANG Jianqiang (⥟ᓎᔎ)1,2, SHEN Meiqing (≜㕢ᑚ)1, WANG Jun (⥟ )ݯ1, GAO Jidong (催㒻ϰ)2, MA Jie (偀 ᵄ)2, LIU Shuangxi (߬ঠ୰)2 (1. Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering& Technology, Tianjin University, Tianjin 300072, China; 2. China Automotive Technology & Research Center, Tianjin 300162, China) Received 28 November 2011; revised 22 December 2011
Abstract: Ceria-zirconia-alumina (CZA) solid solution was prepared by sol-gel method in the present study. 0.5 wt.% Pd supported on CZA was prepared by incipient wetness impregnation. The steam effects for CO and C3H8 oxidation, three-way catalytic activity and stoichiometric window property were studied. The light-off temperature of the CO oxidation reaction shifted to a lower temperature due to the water-gas shift (WGS) reactions. The oxidation of C3H8 was enhanced due to the steam reforming (SR) reactions. The steam promoted the C3H8 oxidation and NO reduction in three-way catalytic reaction. The amplitude of stoichiometric window was amplified by the addition of water to the feed stream. Keywords: Pd/Ce0.67Zr0.33O2-Al2O3; three-way catalytic activity; water-gas shift; steam reforming; stoichiometric window; rare earths
Concerns regarding environmental protection are increasing, and the demand for improvement in clean exhaust for automobiles is increasing each year all over the word. The three-way catalysts (TWCs) for gasoline-fuelled automobiles can simultaneously remove CO, HC (hydrocarbons) and NOx efficiently at the theoretical air/fuel (A/F) ratio of around 14.6[1]. Ceria-based materials are an indispensable ingredient in TWCs, which emerge from its reversible oxygen storage and release capacity (OSC) in response to fluctuating oxygen concentration, ensuring the targeted air/fuel ratios and emission control[2]. CeO2 is used as the primary component of oxygen storage material. However, pure ceria has fatal drawbacks, such as the severe deactivations after high temperature ageing or redox treatments. Much effort has been made to synthesize ceria-based materials with better stability and performance[3,4]. Recently, ceria-zirconiaalumina (CZA) has been wildly researched as an outstanding oxygen storage material due to its superior thermal stability and oxygen storage/release capacity[5,6]. In practice, the role of ceria is not limited to the storage/release of oxygen. Indeed, the picture is much more complex since ceria is both stabilizer of noble metal dispersion and alumina sintering, and a promoter of the low-temperature water-gas shift reaction, CO and alkane oxidation[2]. Further, exhaust gas is not only composed of three major air pollutants (CO, HC and NOx) but also H2, O2, H2O and CO2, etc. So the activity of three-way catalysts is highly dependent on the reactants present in the automobile exhaust gases as well as their relative concentration. Thus, the influence of each reactant on the kinetic behavior of the whole mixture is important. The ef-
fects of steam on the catalytic performance have been the subject of some studies[7–12]. To the best of our knowledge, few literatures have been reported on the steam effects over Pd/Ce0.67Zr0.33O2-Al2O3 three-way catalyst. The present paper reports intensive study of the steam effects over Pd/ Ce0.67Zr0.33O2-Al2O3 catalysts with a simulated exhaust gas. The CO oxidation, C3H8 oxidation, three-way catalytic activity and the stoichiometric window performance with and without water in the feed stream were investigated to understand the steam effects over Pd/Ce0.67Zr0.33O2-Al2O3 catalysts.
1 Experimental 1.1 Sample preparation Ce0.67Zr0.33O2-Al2O3 (Ce0.67Zr0.33O2:Al2O3 mass ratio=1:1) were synthesized by citric acid sol-gel method. Ce(NO3)3 (Zibo Rongruida), Zr(NO3)4 (Zibo Rongruida) and Al(NO3)3 (Tianjin Kewei) were used as starting materials, citric acid was added as complex agent, and glycol was used as additive. After continuous stirring for 2 h, the mixed solution was treated at 80 ºC overnight to form the sponge yellow gel. Then, the gel was dried at 100 ºC for 3 h and milled before calcination. The dried gel was calcined at 700 ºC for 5 h to obtain the as-prepared support oxides. The 0.5 wt.% Pd loading was achieved by impregnating support oxides with Pd(NO3)2 aqueous solutions, followed by calcination at 500 ºC for 5 h. These as-prepared samples were called fresh samples, and were designated as Pd/CZA. Aged samples were obtained by hydrothermally aging at 1050 ºC for 10 h in a 10% steam-air condition, which were designated as Pd/CZAa.
Foundation item: Project supported by National High-Tech Research and Development Program of China (2011AA03A405) and the Key Program of Tianjin Natural Science Foundation (09JCZDJC26600) Corresponding author: SHEN Meiqing (E-mail: [email protected]; Tel.: +86-22-27892301) DOI: 10.1016/S1002-0721(12)60123-9
WANG Jianqiang et al., Steam effects over Pd/Ce0.67Zr0.33O2-Al2O3 three-way catalyst
749
1.2 Characterization The X-ray powder diffraction (XRD) patterns were acquired on an X’Pert Pro diffractometer operating at 40 kV and 40 mA with ferrum-filtered Co K radiation. The 2 ranges from 20º to 90º with a 0.03 step size. Diffractometer was calibrated using a standard Si sample. The lattice parameters were calculated according to Bragg’s equation 2dsin=k. The average particle size was determined by Scherrer equation. All the calculations were done by Jade 5.0 software. The specific surface area of the sample was measured by the BET (Brunauer-Emmett-Teller) method, using N2 adsorption/desorption (Quantachrome NOVA2000) at 77 K. 1.3 Activity test Catalytic activity tests were conducted in a quartz micro-reactor using the stoichiometric gas mixture (2%CO+ 0.1%C3H8+0.1%NO+1.5%O2+12%CO2, N2 balance) at space velocity of 50000 h–1. Steam (5%) was injected into the inlet of the reactor system to simulate the real working situation of TWC. The catalyst powder (200 mg) was diluted with quartz pellets to 0.9 ml, and this mixture was placed at the bottom of reactor. A standard activity test was conducted from room temperature to 600 ºC at rate of 10 ºC/min. The furnace temperature was kept within a maximum variation of ±2 ºC by an automatic temperature controller. Oxygen concentrations were determined by oxygen gas sensor. The effluents of the reactor (CO, CO2, NO, C3H8) were determined by an IR analyzer (Nicolet380). Removal of some component in the mixture was substituted by nitrogen to balance for investigating the steam effect at a variety of conditions. The stoichiometric window experiments were tested with the simulated exhaust gas containing 2% CO, 0.1% NO, 0.1% C3H8, 12% CO2, O2 and N2 (balance)[1]. The space velocity was 50000 h–1. The -value (defined as: =(2O2+NO)/ (CO+10C3H8)) was adjusted by altering the concentration of O2 in the feed stream. These different oxygen percentages from 1.15% to 1.75% in the feed stream allow us to experiment with different -values. The stoichiometric window experiment was carried out at 550 ºC for all samples, respectively. From those conversion data we can determine the amplitudes of stoichiometric windows, defined as the interval of -values inside which the conversion is above 80% for all contaminants.
2 Results and discussion 2.1 Texture analysis The XRD patterns of all samples are shown in Fig. 1. For fresh samples, the main diffraction peaks represent a fluorite cubic structure, and have shown the decent symmetry. This confirms the formation of ceria-zirconia-alumina semi-solid solutions or micromorphology grains. The absence of the diffraction peaks for Al2O3 indicates that Al2O3 incorporates into the CZ lattice to form the semi-solid solution. Another possibility is that Al2O3 particles exist as micromorphology
Fig. 1 XRD pattern of Pd/Ce0.67Zr0.33O2-Al2O3 samples
grains that are too small to be detected. The peaks for metallic Pd (40.4° and 41.9°) can not be detected, because the loading of Pd is low, and/or the dispersion of Pd is high. After hydrothermal aging, the sharpened peaks of the aged sample indicate the larger crystallite sizes. It should be noted that Pd/CZAa sample remains single fluorite lattice structure without Al2O3 or ZrO2 phases being detected. It suggests that the interaction between CZ and Al2O3 can prevent the phase segregation of the CZ and phase transition of Al2O3. Therefore, such an interaction is going to improve the thermal stability of the mixed oxides, which is in agreement with findings in the literatures[5,6,13]. The structural and textural properties of different samples are summarized in Table 1. As indicated by the results of SBET and crystallite size, the hydrothermal ageing treatment induces seriously sintering of samples, which is a major driving force for crystal growth. The interaction between CZ and Al2O3 prevents the particle sintering. Further, both fresh and aged Pd/CZA shows higher surface areas, which is favorable for the superior catalytic activity. 2.2 Activity tests 2.2.1 Direct oxidation and WGS of CO Fig. 2(a) shows the CO conversion profiles due to the direct oxidation, when only CO and O2 are present in the feed streams. The light-off temperatures at 50% conversion(T50) and 90% conversion (T90) are compiled in Table 1. The CO light-off temperature— needed to reach 50% conversion—resulted in T50=196 ºC, and its conversion is increased to 245 ºC—needed to reach 90% conversion. The CO WGS reaction profile is represented in Fig. 2(b) where CO and H2O are the only reactants. This reaction begins around 150 ºC and increases slowly up to 50% at 212 ºC and 90% at 375 ºC. According to Bunluesin et al.[14], CO adsorbed on the metal sites reacts with oxygen atoms from the oxidized ceria Table 1 Results of texture characterizations for different samples Samples
Pd/CZA
BET/
Lattice parameters/
Particle diameter/
(m2/g)
nm
nm
Fresh
92.5
0.5414
4.6
Aged
17.1
0.5351
14.4
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JOURNAL OF RARE EARTHS, Vol. 30, No. 8, Aug. 2012
Fig. 2 Oxidation of CO on various catalysts using feed streams (a) 2%CO+1%O2; (b) 2%CO+5%H2O; (c) 2%CO+1%O2+5%H2O
which is reduced and subsequently again oxidized by O2 and/or H2O. The presence of noble metal on the ceria surface makes its possible oxidation by water at low temperature with hydrogen production. At higher temperatures, the CO oxidation is catalyzed by the metal active phase. Comparison of Fig. 1(a) and (c) reveals the enhancement effect of water on the CO oxidation reactions over Pd/Ce0.67Zr0.33O2-Al2O3 three-way catalyst. The role of water on the CO conversion is not only limited to the WGS reaction when oxygen is present in the medium. It was considered that the presence of water enhances the oxidation of CO by modifying the catalyst surface, by weakening the adsorption of CO and advancing the CO direct oxidation[10–12]. Thus all the gas-phase oxygen is already consumed before the WGS reaction begins. After hydrothermal ageing treatment, the catalytic activity is seriously deteriorated due to the sintering of palladium particles and agglomeration of support oxides. The promotion effect of water on the CO oxidation reactions can also be observed as that for fresh samples. Table 2 T50 and T90 for the CO conversion (ºC) Samples
H2O (absent)
WGS
H2O (present)
T50
T90
T50
T90
T50
T90
Pd/CZA
196
245
212
375
193
231
Pd/CZAa
266
340
290
ˉ
283
303
2.2.2 Direct oxidation and SR of C3H8 Fig. 3(a) shows the C3H8 oxidation in an oxidizing environment, and the lightoff temperatures at 50% conversion (T50) and 90% conversion (T90) are compiled in Table 3, where it is significantly converted at 354 ºC and totally at 439 ºC. The SR reaction
can be observed in Fig. 3(b) to take place only above 350 ºC. The steam effects on the C3H8 can be observed in Fig. 3(c). It can be seen that steam virtually does not react in excess oxygen as C3H8 is totally oxidized at lower temperature, below 300 ºC (Fig. 3(c)); conversely in an oxygen-deficient medium, very small amounts of water are required to convert C3H8. This is the case for water which is produced in the first stage of oxidation and is able to increase significantly the conversion of not-yet-oxidized propane, in agreement with results of Botas et al.[10] Similar to the oxidation of CO, the presence of water increased the propane oxidation rate, as can be seen by comparison of the slope between Fig. 3(a) and (c) at lower temperatures (300–400 ºC), where SR reactions are negligible. This fact can be attributed to the effect of water weakening the adsorption strength of propane or intermediates on the catalyst surface. According to Shinjoh et al.[15], either propene or intermediates (such as partial oxidation products) are adsorbed strongly on the catalyst surface causing self-inhibition in propene oxidation over Pt catalysts. After hydrothermal ageing treatment, the catalytic activity is seriously deteriorated due to the sintering of palladium particles. The steam effects on the C3H8 oxidation can also be observed as that for fresh samples. 2.2.3 Three-way catalytic activity In the process of purification automobile exhaust, water existing plays an important role as an oxidant, which converts CO and hydrocarbons (HCs) by the water-gas shift (WGS) and the steam reforming (SR) reactions, respectively, especially at high temperature. Meanwhile it is major component of the exhaust which can affect significantly the thermal stability of the three-way
Fig. 3 Oxidation of C3H8 on various catalysts using feed streams (a) 0.1%C3H8+1.5%O2; (b) 0.1%C3H8+5%H2O; (c) 0.1%C3H8+1.5%O2+5%H2O
WANG Jianqiang et al., Steam effects over Pd/Ce0.67Zr0.33O2-Al2O3 three-way catalyst
from >600 to 578 ºC). It was concluded that the catalytic activity for C3H8 oxidation and NO reduction was promoted in the presence of steam. After hydrothermal aging, the Pd/CZA catalyst showed lower activity for CO, NOx, and HC conversion due to sintering of precious metal and agglomeration of supports[16]. For C3H8 and CO conversion, the catalytic activity in the presence of 5 vol.% steam shifts to the lower temperature (T50,HC from 510 to 474 ºC, T90,HC from 567 to 557 ºC, T50,CO from 271 to 269 ºC, T90,NO from 313 to 289 ºC). It was concluded that the catalytic activity for C3H8 and CO oxidation was promoted in the presence of steam. With regard to NO conversion, the change is inconspicuous. In conclusion, the presence of water may influence the properties of the catalyst. So the steam has obvious effect on the TWC activity. 2.2.4 The stoichiometric window performance Fig. 5 shows the stoichiometric windows of Pd/Ce0.67Zr0.33O2-Al2O3 catalyst for the steam effect and hydrothermal aging treatment as
Table 3 T50 and T90 for the C3H8 conversion (ºC) Samples
H2O (absent)
SR
H2O (present)
T50
T90
T50
T90
T50
T90
Pd/CZA
354
439
402
440
384
476
Pd/CZAa
428
481
497
ˉ
412
474
751
catalyst. The three-way catalytic activity of Pd/CZA catalysts were examined with feed stream containing CO, C3H8, NO, O2, CO2 and N2. For comparison, an experiment was also carried out in the presence of water. The conversion profiles are shown in Fig. 4 and the light-off temperatures at 50% conversion (T50) and 90% conversion (T90) are compiled in Table 4. For fresh catalyst, it can be observed that the T50 and T90 for CO conversion keep unchanged independent of the steam. With regard to C3H8 and NO conversion, the catalytic activity in the presence of 5 vol.% steam shifts to the lower temperature (T50,HC from 541 to 455 ºC, T90,HC from 594 to 572 ºC, T50,NO from 595 to 550 ºC, T90,NO
Fig. 4 Three-way catalytic activity for Pd/Ce0.67Zr0.33O2-Al2O3 Feed gas composition: (a) 2%CO+0.1%NO+0.1%C3H8+12%CO2+1.5%O2+N2 (balance); (b) 2%CO+0.1%NO+0.1%C3H8+12%CO2+ 1.5%O2+5% H2O+N2 (balance) Table 4 Temperature required to reach 50% and 90% conversion of pollutant T50/ºC
Samples
Pd/Ce0.67Zr0.33O2-Al2O3
T90/ºC
CO
C3H8
NO
CO
C3H8
NO
Fresh
218
541
595
257
594
ˉ
Aged
271
510
565
313
567
571
Fresh (with H2O)
237
455
550
285
572
578
Aged (with H2O)
269
474
576
289
557
583
Fig. 5 Stoichiometric windows corresponding to Pd/Ce0.67Zr0.33O2-Al2O3 steam effect (a) and aging effect (b)
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JOURNAL OF RARE EARTHS, Vol. 30, No. 8, Aug. 2012
TWC. The left side of the theoretical stoichiometric value (=1) is lean and the right rich. The upper limit of the stoichiometric windows is limited by NO conversion under rich condition, the lower limit by C3H8 or CO conversion under lean conditions for all samples[17,18]. Table 5 lists the stoichiometric windows amplitude of the different samples, the upper and lower limits, and the reactants responsible for this limit. Hydrocarbon steam reforming reactions can take place in the TWC reaction as a reducing medium. Further, WGS also contributes to CO conversion when steam is present in automobile exhaust. So the optimum steam may amplify the stoichiometric window. As shown in Fig. 5, the steam effects on the stoichiometric windows performance are observed on the lower limit with CO conversion. However, the promotion effect of water in oxidizing conditions is very little. As shown in Fig. 5, under <1.0 conditions, the NO conversion reaches almost 100% but decreases sharply to zero with the -value continuously increasing. After hydrothermal aging, the lower limit of the stoichiometric window changed to C3H8 conversion. The upper limit is shifted to the higher -value. So the stoichiometric windows amplitude is almost unchanged. Noteworthy, the steam effect on the stoichiometric window performance for Pd/CZAa is very similar with Pd/CZA catalysts. The conversion profiles for Pd/CZAa are shown elsewhere. It is suggested that the Pd/Ce0.67Zr0.33O2-Al2O3 catalyst shows superior thermal stability, which is in agreement with the XRD result. Table 5 Stoichiometric windows performance of Pd/CZA Sample Pd/CZA
Limits
Amplitude
(CO)
With 5% steam
0.811
(NO)
0.203
Without steam
0.864(CO)–1.006(NO)
0.142
Aged
0.880(HC)–1.020(NO)
0.140
–1.014
3 Conclusions The steam effects were investigated over Pd/Ce0.67Zr0.33O2Al2O3 catalyst for the CO and C3H8 conversion in different feed streams. The present investigation identified that the CO conversion was greatly enhanced in the presence of steam due to the WGS, represented by the lower T50 and T90. With regard to C3H8 conversion, the oxidation rate was increased by the addition of steam. The stoichiometric windows amplitude was amplified by the presence of steam due to synergistic effect of WGS and SR.
References: [1] Shen M Q, Wang J Q, Shang J C, An Y, Wang J, Wang W L. Modification ceria-zirconia mixed oxides by doping Sr using the reversed microemulsion for improved Pd-only three-way catalytic performance. J. Phys. Chem. C, 2009, 113(4): 1543. [2] Trovarelli Alessandro. Catalytic properties of ceria and CeO2containing materials. Catal. Rev.-Sci. Eng., 1996, 38(4): 439.
[3] Zhao M W, Shen M Q, Wang J. Effect of surface area and bulk structure on oxygen storage capacity of Ce0.67Zr0.33O2. J. Catal., 2007, 248(2): 258. [4] Fan J, Weng D, Wu X D, Wu X D, Ran R. Modification of CeO2-ZrO2 mixed oxides by coprecipitated/impregnated Sr: Effect on the microstructure and oxygen storage capacity. J. Catal., 2008, 258(1): 177. [5] Di Monte R, Fornasiero P, Desinan S, Kašpar J. Thermal stabilization of CexZr1–xO2 oxygen storage promoters by addition of Al2O3: Effect of thermal aging on textural, structural, and morphological properties. Chem. Mater., 2004, 16(22): 4273. [6] Morikawa A, Suzuki T, Kanazawa T, Kikuta K, Suda A, Shinjo H. A new concept in high performance ceria-zirconia oxygen storage capacity material with Al2O3 as a diffusion barrier. Appl. Catal. B, 2007, 78(3-4): 210. [7] Barbier J, Duprez D. Steam effects in three-way catalysis. Appl. Catal. B, 1994, 4(2-3): 105. [8] Maillet T, Barbier J, Duprez D. Reactivity of steam in exhaust gas catalysis III. Steam and oxygen/steam conversions of propane on a Pd/Al2O3 catalyst. Appl. Catal. B, 1996, 9(1-4): 251. [9] Whittington B I, Jiang C J, Trimm D L. Vehicle exhaust catalysis: I. The relative importance of catalytic oxidation, steam reforming and water-gas shift reactions. Catal. Today, 1995, 26(1): 41. [10] Botas J A, Gutiérrez-Ortiz M A, González-Marcos M P, González-Marcos J A., González-Velasco J R. Kinetic considerations of three-way catalysis in automobile exhaust converters. Appl. Catal. B, 2001, 32(4): 243. [11] Kwon H J, Baik J H, Kwon Y T, Nam In-Sik, Oh Se H. Enhancement effect of water on oxidation reactions over commercial three-way catalyst. Chem. Eng. J., 2008, 141(1-3): 194. [12] Wang J Q, Shen M Q, Wang J, Wang W L. Steam effects over Pd/Ce0.67Zr0.33O2 three-way catalyst. J. Rare Earths, 2011, 29(3): 217. [13] Wang J, Wen J, Shen M Q. Effect of interaction between Ce0.7Zr0.3O2 and Al2O3 on structural characteristics, thermal stability, and oxygen storage capacity. J. Phys. Chem. C, 2008, 112(13): 5113. [14] Bunluesin T, Gorte R J, Graham G H. Studies of the watergas-shift reaction on ceria-supported Pt, Pd, and Rh: Implications for oxygen-storage properties. Appl. Catal. B, 1998, 15(1-2): 107. [15] Shinjoh H, Muraki H, Fujitani Y. Periodic operation effects in propane and propylene oxidation over noble metal catalysts. Appl. Catal., 1989, 49(2): 195. [16] Han Z Q, Wang J Q, Yan H J, Shen M Q, Wang J, Wang W L. Performance of dynamic oxygen storage capacity, water-gas shift and steam reforming reactions over Pd-only three-way catalysts. Catal. Today, 2010, 158(3-4): 481. [17] González-Velasco J R, Botas J A, Ferret R, González-Marcos M P, Marc J L, Gutiérrez-Ortiz M A. Thermal aging of Pd/Pt/Rh automotive catalysts under a cycled oxidizingreducing environment. Catal. Today, 2000, 59(3-4): 395. [18] Guo Y, Lu G Z, Zhang Z G, Zhang S H, Qi Y, Liu Y. Preparation of CexZr1–xO2 (x=0.75, 0.62) solid solution and its application in Pd-only three-way catalysts. Catal. Today, 2007, 126(3-4): 296.
Steam effects over Pd/Ce0.67Zr0.33O2-Al2O3 three-way catalyst WANG Jianqiang (⥟ᓎᔎ)1,2, SHEN Meiqing (≜㕢ᑚ)1, WANG Jun (⥟ )ݯ1, GAO Jidong (催㒻ϰ)2, MA Jie (偀 ᵄ)2, LIU Shuangxi (߬ঠ୰)2 (1. Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering& Technology, Tianjin University, Tianjin 300072, China; 2. China Automotive Technology & Research Center, Tianjin 300162, China) Received 28 November 2011; revised 22 December 2011
Abstract: Ceria-zirconia-alumina (CZA) solid solution was prepared by sol-gel method in the present study. 0.5 wt.% Pd supported on CZA was prepared by incipient wetness impregnation. The steam effects for CO and C3H8 oxidation, three-way catalytic activity and stoichiometric window property were studied. The light-off temperature of the CO oxidation reaction shifted to a lower temperature due to the water-gas shift (WGS) reactions. The oxidation of C3H8 was enhanced due to the steam reforming (SR) reactions. The steam promoted the C3H8 oxidation and NO reduction in three-way catalytic reaction. The amplitude of stoichiometric window was amplified by the addition of water to the feed stream. Keywords: Pd/Ce0.67Zr0.33O2-Al2O3; three-way catalytic activity; water-gas shift; steam reforming; stoichiometric window; rare earths
Concerns regarding environmental protection are increasing, and the demand for improvement in clean exhaust for automobiles is increasing each year all over the word. The three-way catalysts (TWCs) for gasoline-fuelled automobiles can simultaneously remove CO, HC (hydrocarbons) and NOx efficiently at the theoretical air/fuel (A/F) ratio of around 14.6[1]. Ceria-based materials are an indispensable ingredient in TWCs, which emerge from its reversible oxygen storage and release capacity (OSC) in response to fluctuating oxygen concentration, ensuring the targeted air/fuel ratios and emission control[2]. CeO2 is used as the primary component of oxygen storage material. However, pure ceria has fatal drawbacks, such as the severe deactivations after high temperature ageing or redox treatments. Much effort has been made to synthesize ceria-based materials with better stability and performance[3,4]. Recently, ceria-zirconiaalumina (CZA) has been wildly researched as an outstanding oxygen storage material due to its superior thermal stability and oxygen storage/release capacity[5,6]. In practice, the role of ceria is not limited to the storage/release of oxygen. Indeed, the picture is much more complex since ceria is both stabilizer of noble metal dispersion and alumina sintering, and a promoter of the low-temperature water-gas shift reaction, CO and alkane oxidation[2]. Further, exhaust gas is not only composed of three major air pollutants (CO, HC and NOx) but also H2, O2, H2O and CO2, etc. So the activity of three-way catalysts is highly dependent on the reactants present in the automobile exhaust gases as well as their relative concentration. Thus, the influence of each reactant on the kinetic behavior of the whole mixture is important. The ef-
fects of steam on the catalytic performance have been the subject of some studies[7–12]. To the best of our knowledge, few literatures have been reported on the steam effects over Pd/Ce0.67Zr0.33O2-Al2O3 three-way catalyst. The present paper reports intensive study of the steam effects over Pd/ Ce0.67Zr0.33O2-Al2O3 catalysts with a simulated exhaust gas. The CO oxidation, C3H8 oxidation, three-way catalytic activity and the stoichiometric window performance with and without water in the feed stream were investigated to understand the steam effects over Pd/Ce0.67Zr0.33O2-Al2O3 catalysts.
1 Experimental 1.1 Sample preparation Ce0.67Zr0.33O2-Al2O3 (Ce0.67Zr0.33O2:Al2O3 mass ratio=1:1) were synthesized by citric acid sol-gel method. Ce(NO3)3 (Zibo Rongruida), Zr(NO3)4 (Zibo Rongruida) and Al(NO3)3 (Tianjin Kewei) were used as starting materials, citric acid was added as complex agent, and glycol was used as additive. After continuous stirring for 2 h, the mixed solution was treated at 80 ºC overnight to form the sponge yellow gel. Then, the gel was dried at 100 ºC for 3 h and milled before calcination. The dried gel was calcined at 700 ºC for 5 h to obtain the as-prepared support oxides. The 0.5 wt.% Pd loading was achieved by impregnating support oxides with Pd(NO3)2 aqueous solutions, followed by calcination at 500 ºC for 5 h. These as-prepared samples were called fresh samples, and were designated as Pd/CZA. Aged samples were obtained by hydrothermally aging at 1050 ºC for 10 h in a 10% steam-air condition, which were designated as Pd/CZAa.
Foundation item: Project supported by National High-Tech Research and Development Program of China (2011AA03A405) and the Key Program of Tianjin Natural Science Foundation (09JCZDJC26600) Corresponding author: SHEN Meiqing (E-mail: [email protected]; Tel.: +86-22-27892301) DOI: 10.1016/S1002-0721(12)60123-9
WANG Jianqiang et al., Steam effects over Pd/Ce0.67Zr0.33O2-Al2O3 three-way catalyst
749
1.2 Characterization The X-ray powder diffraction (XRD) patterns were acquired on an X’Pert Pro diffractometer operating at 40 kV and 40 mA with ferrum-filtered Co K radiation. The 2 ranges from 20º to 90º with a 0.03 step size. Diffractometer was calibrated using a standard Si sample. The lattice parameters were calculated according to Bragg’s equation 2dsin=k. The average particle size was determined by Scherrer equation. All the calculations were done by Jade 5.0 software. The specific surface area of the sample was measured by the BET (Brunauer-Emmett-Teller) method, using N2 adsorption/desorption (Quantachrome NOVA2000) at 77 K. 1.3 Activity test Catalytic activity tests were conducted in a quartz micro-reactor using the stoichiometric gas mixture (2%CO+ 0.1%C3H8+0.1%NO+1.5%O2+12%CO2, N2 balance) at space velocity of 50000 h–1. Steam (5%) was injected into the inlet of the reactor system to simulate the real working situation of TWC. The catalyst powder (200 mg) was diluted with quartz pellets to 0.9 ml, and this mixture was placed at the bottom of reactor. A standard activity test was conducted from room temperature to 600 ºC at rate of 10 ºC/min. The furnace temperature was kept within a maximum variation of ±2 ºC by an automatic temperature controller. Oxygen concentrations were determined by oxygen gas sensor. The effluents of the reactor (CO, CO2, NO, C3H8) were determined by an IR analyzer (Nicolet380). Removal of some component in the mixture was substituted by nitrogen to balance for investigating the steam effect at a variety of conditions. The stoichiometric window experiments were tested with the simulated exhaust gas containing 2% CO, 0.1% NO, 0.1% C3H8, 12% CO2, O2 and N2 (balance)[1]. The space velocity was 50000 h–1. The -value (defined as: =(2O2+NO)/ (CO+10C3H8)) was adjusted by altering the concentration of O2 in the feed stream. These different oxygen percentages from 1.15% to 1.75% in the feed stream allow us to experiment with different -values. The stoichiometric window experiment was carried out at 550 ºC for all samples, respectively. From those conversion data we can determine the amplitudes of stoichiometric windows, defined as the interval of -values inside which the conversion is above 80% for all contaminants.
2 Results and discussion 2.1 Texture analysis The XRD patterns of all samples are shown in Fig. 1. For fresh samples, the main diffraction peaks represent a fluorite cubic structure, and have shown the decent symmetry. This confirms the formation of ceria-zirconia-alumina semi-solid solutions or micromorphology grains. The absence of the diffraction peaks for Al2O3 indicates that Al2O3 incorporates into the CZ lattice to form the semi-solid solution. Another possibility is that Al2O3 particles exist as micromorphology
Fig. 1 XRD pattern of Pd/Ce0.67Zr0.33O2-Al2O3 samples
grains that are too small to be detected. The peaks for metallic Pd (40.4° and 41.9°) can not be detected, because the loading of Pd is low, and/or the dispersion of Pd is high. After hydrothermal aging, the sharpened peaks of the aged sample indicate the larger crystallite sizes. It should be noted that Pd/CZAa sample remains single fluorite lattice structure without Al2O3 or ZrO2 phases being detected. It suggests that the interaction between CZ and Al2O3 can prevent the phase segregation of the CZ and phase transition of Al2O3. Therefore, such an interaction is going to improve the thermal stability of the mixed oxides, which is in agreement with findings in the literatures[5,6,13]. The structural and textural properties of different samples are summarized in Table 1. As indicated by the results of SBET and crystallite size, the hydrothermal ageing treatment induces seriously sintering of samples, which is a major driving force for crystal growth. The interaction between CZ and Al2O3 prevents the particle sintering. Further, both fresh and aged Pd/CZA shows higher surface areas, which is favorable for the superior catalytic activity. 2.2 Activity tests 2.2.1 Direct oxidation and WGS of CO Fig. 2(a) shows the CO conversion profiles due to the direct oxidation, when only CO and O2 are present in the feed streams. The light-off temperatures at 50% conversion(T50) and 90% conversion (T90) are compiled in Table 1. The CO light-off temperature— needed to reach 50% conversion—resulted in T50=196 ºC, and its conversion is increased to 245 ºC—needed to reach 90% conversion. The CO WGS reaction profile is represented in Fig. 2(b) where CO and H2O are the only reactants. This reaction begins around 150 ºC and increases slowly up to 50% at 212 ºC and 90% at 375 ºC. According to Bunluesin et al.[14], CO adsorbed on the metal sites reacts with oxygen atoms from the oxidized ceria Table 1 Results of texture characterizations for different samples Samples
Pd/CZA
BET/
Lattice parameters/
Particle diameter/
(m2/g)
nm
nm
Fresh
92.5
0.5414
4.6
Aged
17.1
0.5351
14.4
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Fig. 2 Oxidation of CO on various catalysts using feed streams (a) 2%CO+1%O2; (b) 2%CO+5%H2O; (c) 2%CO+1%O2+5%H2O
which is reduced and subsequently again oxidized by O2 and/or H2O. The presence of noble metal on the ceria surface makes its possible oxidation by water at low temperature with hydrogen production. At higher temperatures, the CO oxidation is catalyzed by the metal active phase. Comparison of Fig. 1(a) and (c) reveals the enhancement effect of water on the CO oxidation reactions over Pd/Ce0.67Zr0.33O2-Al2O3 three-way catalyst. The role of water on the CO conversion is not only limited to the WGS reaction when oxygen is present in the medium. It was considered that the presence of water enhances the oxidation of CO by modifying the catalyst surface, by weakening the adsorption of CO and advancing the CO direct oxidation[10–12]. Thus all the gas-phase oxygen is already consumed before the WGS reaction begins. After hydrothermal ageing treatment, the catalytic activity is seriously deteriorated due to the sintering of palladium particles and agglomeration of support oxides. The promotion effect of water on the CO oxidation reactions can also be observed as that for fresh samples. Table 2 T50 and T90 for the CO conversion (ºC) Samples
H2O (absent)
WGS
H2O (present)
T50
T90
T50
T90
T50
T90
Pd/CZA
196
245
212
375
193
231
Pd/CZAa
266
340
290
ˉ
283
303
2.2.2 Direct oxidation and SR of C3H8 Fig. 3(a) shows the C3H8 oxidation in an oxidizing environment, and the lightoff temperatures at 50% conversion (T50) and 90% conversion (T90) are compiled in Table 3, where it is significantly converted at 354 ºC and totally at 439 ºC. The SR reaction
can be observed in Fig. 3(b) to take place only above 350 ºC. The steam effects on the C3H8 can be observed in Fig. 3(c). It can be seen that steam virtually does not react in excess oxygen as C3H8 is totally oxidized at lower temperature, below 300 ºC (Fig. 3(c)); conversely in an oxygen-deficient medium, very small amounts of water are required to convert C3H8. This is the case for water which is produced in the first stage of oxidation and is able to increase significantly the conversion of not-yet-oxidized propane, in agreement with results of Botas et al.[10] Similar to the oxidation of CO, the presence of water increased the propane oxidation rate, as can be seen by comparison of the slope between Fig. 3(a) and (c) at lower temperatures (300–400 ºC), where SR reactions are negligible. This fact can be attributed to the effect of water weakening the adsorption strength of propane or intermediates on the catalyst surface. According to Shinjoh et al.[15], either propene or intermediates (such as partial oxidation products) are adsorbed strongly on the catalyst surface causing self-inhibition in propene oxidation over Pt catalysts. After hydrothermal ageing treatment, the catalytic activity is seriously deteriorated due to the sintering of palladium particles. The steam effects on the C3H8 oxidation can also be observed as that for fresh samples. 2.2.3 Three-way catalytic activity In the process of purification automobile exhaust, water existing plays an important role as an oxidant, which converts CO and hydrocarbons (HCs) by the water-gas shift (WGS) and the steam reforming (SR) reactions, respectively, especially at high temperature. Meanwhile it is major component of the exhaust which can affect significantly the thermal stability of the three-way
Fig. 3 Oxidation of C3H8 on various catalysts using feed streams (a) 0.1%C3H8+1.5%O2; (b) 0.1%C3H8+5%H2O; (c) 0.1%C3H8+1.5%O2+5%H2O
WANG Jianqiang et al., Steam effects over Pd/Ce0.67Zr0.33O2-Al2O3 three-way catalyst
from >600 to 578 ºC). It was concluded that the catalytic activity for C3H8 oxidation and NO reduction was promoted in the presence of steam. After hydrothermal aging, the Pd/CZA catalyst showed lower activity for CO, NOx, and HC conversion due to sintering of precious metal and agglomeration of supports[16]. For C3H8 and CO conversion, the catalytic activity in the presence of 5 vol.% steam shifts to the lower temperature (T50,HC from 510 to 474 ºC, T90,HC from 567 to 557 ºC, T50,CO from 271 to 269 ºC, T90,NO from 313 to 289 ºC). It was concluded that the catalytic activity for C3H8 and CO oxidation was promoted in the presence of steam. With regard to NO conversion, the change is inconspicuous. In conclusion, the presence of water may influence the properties of the catalyst. So the steam has obvious effect on the TWC activity. 2.2.4 The stoichiometric window performance Fig. 5 shows the stoichiometric windows of Pd/Ce0.67Zr0.33O2-Al2O3 catalyst for the steam effect and hydrothermal aging treatment as
Table 3 T50 and T90 for the C3H8 conversion (ºC) Samples
H2O (absent)
SR
H2O (present)
T50
T90
T50
T90
T50
T90
Pd/CZA
354
439
402
440
384
476
Pd/CZAa
428
481
497
ˉ
412
474
751
catalyst. The three-way catalytic activity of Pd/CZA catalysts were examined with feed stream containing CO, C3H8, NO, O2, CO2 and N2. For comparison, an experiment was also carried out in the presence of water. The conversion profiles are shown in Fig. 4 and the light-off temperatures at 50% conversion (T50) and 90% conversion (T90) are compiled in Table 4. For fresh catalyst, it can be observed that the T50 and T90 for CO conversion keep unchanged independent of the steam. With regard to C3H8 and NO conversion, the catalytic activity in the presence of 5 vol.% steam shifts to the lower temperature (T50,HC from 541 to 455 ºC, T90,HC from 594 to 572 ºC, T50,NO from 595 to 550 ºC, T90,NO
Fig. 4 Three-way catalytic activity for Pd/Ce0.67Zr0.33O2-Al2O3 Feed gas composition: (a) 2%CO+0.1%NO+0.1%C3H8+12%CO2+1.5%O2+N2 (balance); (b) 2%CO+0.1%NO+0.1%C3H8+12%CO2+ 1.5%O2+5% H2O+N2 (balance) Table 4 Temperature required to reach 50% and 90% conversion of pollutant T50/ºC
Samples
Pd/Ce0.67Zr0.33O2-Al2O3
T90/ºC
CO
C3H8
NO
CO
C3H8
NO
Fresh
218
541
595
257
594
ˉ
Aged
271
510
565
313
567
571
Fresh (with H2O)
237
455
550
285
572
578
Aged (with H2O)
269
474
576
289
557
583
Fig. 5 Stoichiometric windows corresponding to Pd/Ce0.67Zr0.33O2-Al2O3 steam effect (a) and aging effect (b)
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TWC. The left side of the theoretical stoichiometric value (=1) is lean and the right rich. The upper limit of the stoichiometric windows is limited by NO conversion under rich condition, the lower limit by C3H8 or CO conversion under lean conditions for all samples[17,18]. Table 5 lists the stoichiometric windows amplitude of the different samples, the upper and lower limits, and the reactants responsible for this limit. Hydrocarbon steam reforming reactions can take place in the TWC reaction as a reducing medium. Further, WGS also contributes to CO conversion when steam is present in automobile exhaust. So the optimum steam may amplify the stoichiometric window. As shown in Fig. 5, the steam effects on the stoichiometric windows performance are observed on the lower limit with CO conversion. However, the promotion effect of water in oxidizing conditions is very little. As shown in Fig. 5, under <1.0 conditions, the NO conversion reaches almost 100% but decreases sharply to zero with the -value continuously increasing. After hydrothermal aging, the lower limit of the stoichiometric window changed to C3H8 conversion. The upper limit is shifted to the higher -value. So the stoichiometric windows amplitude is almost unchanged. Noteworthy, the steam effect on the stoichiometric window performance for Pd/CZAa is very similar with Pd/CZA catalysts. The conversion profiles for Pd/CZAa are shown elsewhere. It is suggested that the Pd/Ce0.67Zr0.33O2-Al2O3 catalyst shows superior thermal stability, which is in agreement with the XRD result. Table 5 Stoichiometric windows performance of Pd/CZA Sample Pd/CZA
Limits
Amplitude
(CO)
With 5% steam
0.811
(NO)
0.203
Without steam
0.864(CO)–1.006(NO)
0.142
Aged
0.880(HC)–1.020(NO)
0.140
–1.014
3 Conclusions The steam effects were investigated over Pd/Ce0.67Zr0.33O2Al2O3 catalyst for the CO and C3H8 conversion in different feed streams. The present investigation identified that the CO conversion was greatly enhanced in the presence of steam due to the WGS, represented by the lower T50 and T90. With regard to C3H8 conversion, the oxidation rate was increased by the addition of steam. The stoichiometric windows amplitude was amplified by the presence of steam due to synergistic effect of WGS and SR.
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