- Email: [email protected]
Effect of cobalt oxide on performance of Pd catalysts for lean-burn natural gas vehicles in the presence and absence of water vapor
Journal of Natural Gas Chemistry 19(2010)134–138
Effect of cobalt oxide on performance of Pd catalysts for lean-burn natural gas vehicles in the presence and absence of water vapor Enyan Long, Xiaoyu Zhang, Yile Li, Zhimin Liu, Yun Wang, Maochu Gong, Yaoqiang Chen∗ Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China [ Manuscript received August 11, 2009; revised October 10, 2009 ]
Abstract Pd-based catalysts modified by cobalt were prepared by co-impregnation and sequential impregnation methods, and characterized by X-ray powder diffraction (XRD), N2 adsorption/desorption (Brunauer-Emmet-Teller method), CO-chemisorption and X-ray photoelectron spectroscopy (XPS). The activity of Pd catalysts was tested in the simulated exhaust gas from lean-burn natural gas vehicles. The effect of Co on the performance of water poisoning resistance for Pd catalysts was estimated in the simulated exhaust gas with and without the presence of water vapor. It was found that the effect of Co significantly depended on the preparation process. PdCo/La-Al2 O3 catalyst prepared by co-impregnation exhibited better water-resistant performance. The results of XPS indicated that both CoAl2 O4 and Co3 O4 were present in the Pd catalysts modified by Co. For the catalyst prepared by sequential impregnation method, the ratio of CoAl2 O4 /Co3 O4 was higher than that of the catalyst prepared by co-impregnation method. It could be concluded that Co3 O4 played an important role in improving water-resistant performance. Key words Co; Pd-only catalyst; lean-burn natural gas vehicles; methane oxidation; water vapor
1. Introduction Natural gas vehicles (NGVs) appear attractive to reduce environmental pollutions compared with gasoline and diesel engines owing to their less CO2 emission, less NOx and particulates in the exhaust [1]. Also, natural gas engines can be operated under lean conditions, which is more effective compared with under the stoichiometric conditions [2]. Under lean-burn conditions, nitrogen oxide (NOx ) emissions are much lower than that in stoichiometric condition due to lower combustion temperature resulting from the high ratios of air to fuel (A/F about 20−27) [3]. However, there is still unburned methane and CO in the tail gas of lean-burn NGVs. CO can be easily converted due to the high amount of oxygen. Methane, a potent greenhouse gas, is recognized to contribute more to global atmosphere warming than carbon dioxide at equivalent emission rates [4]. Moreover, methane is the hydrocarbon with the lowest reactivity and therefore the most difficult to be oxidized [5]. Among noble metals, palladium is widely considered to be capable of guaranteeing high activity towards methane combustion. Although being the most active, Pd catalysts exhibit strong sensitivity to water and sulphur contain-
ing compounds, which represent severe inhibition effect on the treatment of exhaust gas from NGVs [4]. There are a few literatures focusing on the catalysts inhibited by reaction products, such as water [1,6,7], and much fewer on transition metal catalysts. Li et al. [8] reported the promoting effect of water vapor on methane conversion on cobalt/manganese mixed oxides. They found that after pretreatment in water vapor, cobalt species could segregate on the catalyst surface, which were possibly related to the activity increase in the presence of water. However, there are few investigations on the mechanism of the promoting effect of water vapor. In this paper, the effect of Co on the phenomenon of reducing water poisoning rate for Pd/La-Al2 O3 catalysts prepared by different methods was investigated by comparing methane conversion in the presence and absence of water vapor. 2. Experimental 2.1. Supports and catalysts preparation Al2 O3 modified by La2 O3 was prepared by peptizing
∗
Corresponding author. Tel/Fax: 028-85418451; E-mail: [email protected] This work was supported by the National Natural Science Foundation of China (20773090) and the Ph. D. Programs Foundation of Ministry of Education of China (200806100009) Copyright©2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(09)60044-X
Journal of Natural Gas Chemistry Vol. 19 No. 2 2010
method. La(NO3 )3 ·6H2 O solution and HNO3 (65 wt%) were added into the mixture to make pseudo boehmite peptize at pH = 4. After that, the colloid was dried. As-obtained powder was calcined at 1000 ◦ C for 3 h. The content of La2 O3 was 3 wt%. For catalyst Pd/Co/La-Al2O3 prepared by sequential impregnation, La-Al2 O3 was impregnated in Co(NO3 )2 ·6H2 O solution followed by drying and calcination at 550 ◦ C for 2 h, and then the yielding powder was impregnated in Pd(NO3 )2 solution. For catalyst PdCo/La-Al2O3 prepared by co-impregnation, Co(NO3 )2 ·6H2 O and Pd(NO3 )2 solutions were mixed first, and then La-Al2 O3 was impregnated in the mixed solutions. In order to compare, catalyst Pd/La-Al2 O3 without Co was also prepared by impregnation method. All the prepared catalysts mentioned above were dried and then calcined at 550 ◦ C for 2 h. The loadings of Pd and Co were 1.25 wt% and 2 wt%, respectively. The resulting powder were milled with proper amount of de-ionized water to form homogenous slurry and then coated onto a honeycomb cordierite (2.5 cm3 , Corning, Shanghai, China). Excess slurry was blown away by compressed air. The loadings of washcoat and Pd were kept about 180 g/L and 2.25 g/L, respectively. 2.2. Catalytic activity tests Catalytic activity tests were carried out on monolithic samples in a multiple fixed-bed continuous flow micro-reactor by passing a gas mixture which simulated exhaust from leanburn natural gas vehicles. All the catalysts were treated at 550 ◦ C for 1 h in reacting gases regulated by mass-flow controller before the test. The simulated exhaust contained 630 ppm CH4 , 0.4 vol% CO, 5 vol% O2 , 12 vol% CO2 , and N2 as balance gas. The gas hourly space velocity (GHSV) was 34000 h−1 . Catalytic activities of each catalyst were tested in the absence and presence of 10 vol% water vapor. The concentrations of CH4 before and after reaction were detected by an on-line gas chromatograph equipped with FID detector. 2.3. Supports and catalysts characterization The BET specific surface area and pore size of catalysts and support were obtained by adsorption/desorption of N2 at the liquid nitrogen temperature on a ZXF-06 automatic surface analyzer (Xibei Chemical Institute, China). Before measurement, the sample was degassed in vacuum at 400 ◦ C for 1 h. CO-chemisorption tests of catalysts were carried out with 0.2 g samples at room temperature after reduced by hydrogen at about 300 ◦ C for 1 h and then purged with pure Ar for 1 h. The sample was placed in a U-shaped quartz tube connected with a thermal conductivity detector (TCD). The crystal structures of the samples were determined by X-ray powder diffraction (XRD), on a DX-2500 diffractometer with Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 25 mA. The XRD data were recorded for 2θ values between 10o and 90o with an interval of 0.03o . The X-ray photoelectron spectroscopy (XPS) data were
135
collected on a spectrometer (XSAM-800, KRATOS Co.) with Al Kα radiation under UHV, calibrated internally by the carbon deposit C1s binding energy (BE) at 284.8 eV. 3. Results and discussion 3.1. Catalytic activity Catalytic activities of methane conversion were measured in the presence or absence of 10 vol% water vapor in the simulated exhaust gas. Methane conversion as a function of reaction temperature on the three catalysts in the absence and presence of water vapor respectively is shown in Figure 1. It was observed that, in the absence of water vapor, the reaction activity was in the order of Pd/La-Al2O3 >Pd/Co/La-Al2 O3 >PdCo/La-Al2O3 . However, in presence of water and below 404 ◦ C, the order was Pd/LaAl2 O3 >PdCo/La-Al2 O3 >Pd/Co/La-Al2 O3 . The difference of activity was little between the three catalysts. It could be seen that, in the presence of water vapor, as temperature increased, activity of Pd/La-Al2 O3 increased more slowly than that of PdCo/La-Al2 O3 . While above 404 ◦ C, the catalytic activity order turned to be: PdCo/La-Al2 O3 >Pd/LaAl2 O3 >Pd/Co/La-Al2 O3 . Figure 2 shows methane conversion as a function of temperature in the simulated exhaust gas in the absence and presence of water vapor over the catalysts. Methane conversion activity in the presence of water vapor decreased compared to that in the absence of water vapor for all the three catalysts. It also could be seen from Figure 2 that the difference of activities over each catalyst in the absence and presence of water vapor decreased as the temperature increased. For Pd/La-Al2 O3 , Pd/Co/LaAl2 O3 and PdCo/La-Al2 O3 the activity under the two conditions reached the same at about 470 ◦ C, 469 ◦ C and 433 ◦ C, respectively. It could be concluded that the co-impregnation of Co reduced the rate of water poisoning for the catalyst. The data of T50 , T90 , ΔT50 and ΔT90 in the presence and absence of water vapor are presented in Table 1. The lightoff temperature (T50 ) and completely conversion temperature (T90) were the temperatures at which a given pollutant conversion reached 50% and 90%, respectively. ΔT50 = T50 (in the presence of water) − T50 (in the absence of water) and ΔT90 = T90 (in the presence of water) − T90 (in the absence of water). The data of ΔT50 and ΔT90 can partly reflect the water poisoning effect of catalysts. It could be observed in the absence of water vapor that PdCo/La-Al2 O3 exhibited the highest T50 and T90 , which were 325 ◦ C and 376 ◦ C, respectively. For Pd/La-Al2 O3 , T50 and T90 were 297 ◦ C and 328 ◦ C, which were much lower than those of PdCo/LaAl2 O3 ; and for Pd/Co/La-Al2 O3 , T50 was a little higher than that of Pd/La-Al2 O3 but 20 ◦ C lower than that of PdCo/LaAl2 O3 . In the presence of water vapor, the order of T50 and T90 turned to be Pd/Co/La-Al2O3 >PdCo/La-Al2 O3 >Pd/LaAl2 O3 . It could be observed that, in the presence of water, the differences of T50 and T90 between PdCo/La-Al2O3 and Pd/La-Al2O3 should be in the range of experimental error and
136
Enyan Long et al./ Journal of Natural Gas Chemistry Vol. 19 No. 2 2010
could be neglected, although Pd/La-Al2 O3 exhibited much better performance than PdCo/La-Al2 O3 in the absence of water. The order of ΔT50 and ΔT90 was Pd/La-Al2 O3 >Pd/Co/LaAl2 O3 >PdCo/La-Al2O3 , which suggested that water had the greatest inhibition effect on Pd/La-Al2 O3 while the least on PdCo/La-Al2 O3 . For the three catalysts Pd/La-Al2 O3 , Pd/Co/La-Al2 O3 and PdCo/La-Al2 O3 , ΔT50 was 68 ◦ C, 66 ◦ C
and 42 ◦ C, respectively, and ΔT90 was 74 ◦ C, 56 ◦ C and 26 ◦ C, respectively. These data reflects that the inhibition effect of water vapor followed the order Pd/La-Al2 O3 >Pd/Co/LaAl2 O3 >PdCo/La-Al2 O3 . The addition of Co enhanced resistant effect to water vapor depending on the preparation method. Co-impregnation of cobalt performed much better than sequential impregnations.
Figure 1. Methane conversion as a function of temperature over Pd/La-Al2 O3 , PdCo/La-Al2 O3 and Pd/Co/La-Al2 O3 in the simulated exhaust gas. (a) In the absence of water vapor, (b) in the presence of water vapor
Figure 2. Methane conversion as a function of temperature in the simulated exhaust gas in the absence (open symbols) and presence of water vapor (filled symbols) over (a) Pd/La-Al2 O3 , (b) PdCo/La-Al2 O3 and (c) Pd/Co/La-Al2 O3 Table 1. T 50 , T 90 , ΔT 50 and ΔT 90 in the presence and absence of water vapor Samples Pd/La-Al2 O3 PdCo/La-Al2 O3 Pd/Co/La-Al2 O3
T50 (◦ C, without water) 297 325 305
T50 (◦ C, with water) 365 367 371
3.2. Specif ic surface area and CO-chemisorption The specific surface area is very important for catalytic activity of catalysts. The BET surface area and pore volume of support and catalysts are shown in Table 2. As shown in the Table, specific surface area and pore volume were similar for the support and the catalysts except Pd/Co/La-Al2 O3 . It indi-
ΔT50 (◦ C) 68 42 66
T90 (◦ C, without water) 328 376 349
T90 (◦ C, with water) 402 402 405
ΔT90 (◦ C) 74 26 56
cated that except Pd/Co/La-Al2 O3 , impregnation of Co or Pd and calcination of the catalysts resulted in almost no changes of the specific surface area of the catalyst. Maybe it was because calcination temperature of the support was much higher and calcination time was longer than that of the catalysts. Further more, the addition amounts of Pd and Co were both little. It could be concluded that the specific surface area of the cat-
Journal of Natural Gas Chemistry Vol. 19 No. 2 2010
137
alyst was decided mainly by the support prepared in present work. The decrease of specific surface area of Pd/Co/LaAl2 O3 might be due to the sequential impregnation method. As described in the preparation part of support and catalyst in this paper, after each impregnation, the support or catalyst was calcined at 550 ◦ C for 2 h. That meant there was one more impregnation and calcination step for Pd/Co/La-Al2 O3 , which led to a decrease in the specific surface area. Table 2. Specific surface area and the pore volume of the support and the catalysts Samples La-Al2 O3 Pd/La-Al2 O3 PdCo/La-Al2 O3 Pd/Co/La-Al2 O3
SBET (m2 /g) 136 135 132 116
Pore volume (ml/g) 0.38 0.37 0.37 0.39
CO-chemisorption was performed over the three catalysts and Co/La-Al2 O3 for comparison. The results are listed in Table 3. It is shown that very little carbon monoxide was adsorbed on Co/La-Al2 O3 , which is the CO adsorbed by Co contained material. Subtracting the amount of CO adsorbed by Co contained composite, it is the amount of CO adsorbed by Pd. It could be seen that CO amount adsorbed by Pd followed the order Pd/La-Al2 O3 >Pd/Co/La-Al2O3 >PdCo/LaAl2 O3 . It reflects that the dispersion of Pd on the catalyst surface followed the order above. It is consistent with the activity order in the absence of water vapor. The dispersion of Pd decreased with Co loading for the co-impregnated catalysts, due to the competition of Co with Pd to disperse on the support. Table 3. Results of CO-chemisorption of the catalysts Samples Co/La-Al2 O3 Pd/La-Al2 O3 PdCo/La-Al2 O3 Pd/Co/La-Al2 O3
CO volume chemisorbed (μmol/g) 4 30 25 30
CO amount adsorbed by Pd (μmol/g) − 30 21 26
3.3. XRD The XRD patterns are depicted in Figure 3. α-Al2 O3 can be observed due to the high temperature calcination of the support. However, as the results shown in Table 2, the specific surface area and pore volume of the support did not decrease much, reflecting that there was little Al2 O3 transforming to α phase and Al2 O3 mainly presented in γ-Al2 O3 . It should be owing to the addition of La2 O3 that had excellent stable effect on Al2 O3 as reported in Ref. [9]. No peaks ascribed to Pd were observed, indicating that the dispersion of Pd was good and maybe the amount of Pd was below the limitation of detection. It could be seen that there was just little difference among the XRD patterns of the three catalysts. A weak peak of CoAl2 O4 for Pd/Co/La-Al2 O3 was detected.
Figure 3. XRD patterns of (1) Pd/La-Al2 O3 , (2) PdCo/La-Al2 O3 and (3) Pd/Co/La-Al2 O3
3.4. XPS In order to investigate the key factor resulting in the different performance to water resistant effect for the catalysts, XPS was used to characterize the electronic state of Pd and Co on the catalyst surface. Binding energy (BE) values of Pd 3d 5/2 and the Pd/Al ratios on the catalyst surface calculated from XPS for three catalysts are summarized in Table 4. The higher and the lower BE values of Pd 3d5/2 are assigned to PdO (336.8 eV [10]) and Pd (335.3 eV [11]), respectively. It was found that the addition of Co, no matter by co-impregnation and sequential impregnations, decreased the amount of Pd or PdO on the catalyst surface. Surface Pd/Al ratios followed the order Pd/LaAl2 O3 >Pd/Co/La-Al2 O3 >PdCo/La-Al2O3 , which was consistent with the methane conversion order of the three catalysts in the absence of water. Table 4. Binding energy (BE) of Pd 3d5/2 and surface Pd/Al ratios calculated from XPS for the three catalysts Samples Pd/La-Al2 O3
BE of Pd 3d5/2 (eV) Pd/Al (×100) Total Pd/Al (×100) 335.2 0.57 1.96 336.9 1.39 335.1 0.35 1.16 PdCo/La-Al2 O3 336.8 0.81 335.3 0.38 1.32 Pd/Co/La-Al2 O3 336.9 0.94
Figure 4 displays the XPS lines of Co over the two catalysts contained Co. It could be seen from Figure 4 that peaks of Co 2p3/2 and Co 2p1/2 exhibited strong shoulder at their higher binding energy side for both catalysts. Xiong et al. [12] compared XPS lines of pure Co3 O4 and CoAl2 O4 , and ascribed the shoulder at higher binding energy of Co 2p3/2 and Co 2p1/2 of pure CoAl2 O4 to the shake-up process of Co2+ compound in the high spin state, while that of pure Co3 O4 was remarkably weak. In this paper, binding energy of Co 2p3/2 of the two catalysts were between that of Co3 O4 (780.0 eV) and CoAl2 O4 (781.9 eV) [13], and there were strong shake-up shoulder observed from the two catalysts. For
138
Enyan Long et al./ Journal of Natural Gas Chemistry Vol. 19 No. 2 2010
the reasons above, there should be presence of both CoAl2 O4 and Co3 O4 for the two catalysts. In order to calculate the ratios of the two cobalt species for the catalysts, the Co 2p regions were fitted with 80% Gaussian and 20% Lorentzian. The results of XPS lines are presented in Figure 5. Analytical results of Co 2p3/2 binding energies are presented in Table 5. The higher BE (for PdCo/La-Al2 O3 is 782.2 eV, for Pd/Co/La-Al2 O3 is 781.8 eV) was ascribed to CoAl2 O4 and the lower BE (780.0 eV for both the catalysts) could be ascribed to Co3 O4 . The ratio of CoAl2 O4 /Co3 O4 was calculated by the relative ratio of their peak area. For Pd/Co/LaAl2 O3 , the ratio of CoAl2 O4 /Co3 O4 was higher than that of PdCo/La-Al2 O3 as shown in Table 5. This was consistent with the results of XRD. It could be explained that the interaction between Co and support La-Al2 O3 by sequential impregnations was stronger than by the co-impregnation method. The resistant effect of water vapor on catalyst prepared by coimpregnation of Pd and Co was much stronger than that on the catalysts prepared by sequential impregnations. It was Co3 O4 (not CoAl2 O4 ) played an important role in resisting the water vapor. Cullis et al. [7] found that the addition of water vapor strongly inhibited the reaction over a Pd/A12 O3 catalyst. They suggested that a competition of H2 O with CH4 on surface active sites led to its reaction with PdO to form Pd(OH)2
at the PdO surface, effectively blocking the access of methane to the active PdO phase. Co3 O4 might be more active to react with water than PdO, which explained that the catalyst coimpregnated by Pd and Co acted better water-resistant effect than Pd/La-Al2 O3 . Table 5. Binding energy (BE) of Co 2p3/2 and the ratio of CoAl2 O4 /Co3 O4 Sample PdCo/La-Al2 O3 Pd/Co/La-Al2 O3 a
Co3 O4 BEa Area 780.0 109.5 780.0 64.7
CoAl2 O4 BEa Area 782.2 305.0 781.8 397.0
Ratio of CoAl2 O4 /Co3 O4 b 2.785 6.136
BE of Co 2p3/2 , b Calculated by peak area
4. Conclusions In this paper, Pd catalysts were prepared and investigated. Co was added by co-impregnation and sequential impregnations, respectively. From the results of catalytic activity tests in the absence and presence of water vapor, it was found that the addition of Co did not improve the catalytic activity for methane conversion, but reduced the water poisoning rate for Pd catalysts which were sensitive to water. It was notable that the promoting effect of Co significantly affected by the impregnation methods. The resistant effect to water was much higher for PdCo/La-Al2 O3 prepared by coimpregnation method. The results of XRD and XPS showed that the ratio of CoAl2 O4 /Co3 O4 for Pd/Co/La-Al2 O3 was higher than that for PdCo/La-Al2 O3 , and Co3 O4 played an important role in resisting the water vapor. It could be explained that Co3 O4 was more active with water than PdO. So the water poisoning of PdO was partially inhibited by the addition of Co, especially if it was prepared by co-impregnation method. References
Figure 4. XPS lines of Co for PdCo/La-Al2 O3 (1) and Pd/Co/La-Al2 O3 (2)
Figure 5. XPS lines of Co 2p3/2 and Co 2p1/2 for PdCo/La-Al2 O3 and Pd/Co/La-Al2 O3
[1] G´elin P, Urfels L, Primet M, Tena E. Catal Today, 2003, 83: 45 [2] Holmgreen E M, Yung M M, Ozkan U S. Appl Catal B, 2007, 74: 73 [3] Lampert J K, Kazi M S, Farrauto R J. Appl Catal B, 1997, 14: 21l [4] G´elin P, Primet M. Appl Catal B, 2002, 39: 1 [5] Liotta L F, Carlo D, Pantaleo G, Venezia A M, Deganello G, Borla E M, Pidria M. Appl Catal B, 2007, 75: 182 [6] Butch R, Urbano F J, Loader P K. Appl Catal A, 1995, 123: 173 [7] Cullis C F, Trimm D L, Nevell T G. J Chem Soc-Faraday Trans 1, 1972, 68: 1406 [8] Li W B, Lin Y, Zhang Y. Catal Today, 2003, 83: 239 [9] Zhang L J, Dong W P, Guo J X, Yuan S H, Zhang L, Gong M C, Chen Y Q. Acta Phys-Chim Sin (Wuli Huaxue Xuebao), 2007, 23: 1738 [10] Monteiro R S, Noronha F B, Dieguez L C, Schmal M. Appl Catal A, 1995, 131: 89 [11] Kobayashi T, Yamada T, Kayano K. Appl Catal B, 2001, 30: 287 [12] Xiong H F, Zhang Y H, Liew K Y, Li J L. J Mol Catal A, 2005, 231: 145 [13] Zsoldos Z, Guczi L. J Phys Chem, 1992, 96: 9393
Effect of cobalt oxide on performance of Pd catalysts for lean-burn natural gas vehicles in the presence and absence of water vapor Enyan Long, Xiaoyu Zhang, Yile Li, Zhimin Liu, Yun Wang, Maochu Gong, Yaoqiang Chen∗ Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China [ Manuscript received August 11, 2009; revised October 10, 2009 ]
Abstract Pd-based catalysts modified by cobalt were prepared by co-impregnation and sequential impregnation methods, and characterized by X-ray powder diffraction (XRD), N2 adsorption/desorption (Brunauer-Emmet-Teller method), CO-chemisorption and X-ray photoelectron spectroscopy (XPS). The activity of Pd catalysts was tested in the simulated exhaust gas from lean-burn natural gas vehicles. The effect of Co on the performance of water poisoning resistance for Pd catalysts was estimated in the simulated exhaust gas with and without the presence of water vapor. It was found that the effect of Co significantly depended on the preparation process. PdCo/La-Al2 O3 catalyst prepared by co-impregnation exhibited better water-resistant performance. The results of XPS indicated that both CoAl2 O4 and Co3 O4 were present in the Pd catalysts modified by Co. For the catalyst prepared by sequential impregnation method, the ratio of CoAl2 O4 /Co3 O4 was higher than that of the catalyst prepared by co-impregnation method. It could be concluded that Co3 O4 played an important role in improving water-resistant performance. Key words Co; Pd-only catalyst; lean-burn natural gas vehicles; methane oxidation; water vapor
1. Introduction Natural gas vehicles (NGVs) appear attractive to reduce environmental pollutions compared with gasoline and diesel engines owing to their less CO2 emission, less NOx and particulates in the exhaust [1]. Also, natural gas engines can be operated under lean conditions, which is more effective compared with under the stoichiometric conditions [2]. Under lean-burn conditions, nitrogen oxide (NOx ) emissions are much lower than that in stoichiometric condition due to lower combustion temperature resulting from the high ratios of air to fuel (A/F about 20−27) [3]. However, there is still unburned methane and CO in the tail gas of lean-burn NGVs. CO can be easily converted due to the high amount of oxygen. Methane, a potent greenhouse gas, is recognized to contribute more to global atmosphere warming than carbon dioxide at equivalent emission rates [4]. Moreover, methane is the hydrocarbon with the lowest reactivity and therefore the most difficult to be oxidized [5]. Among noble metals, palladium is widely considered to be capable of guaranteeing high activity towards methane combustion. Although being the most active, Pd catalysts exhibit strong sensitivity to water and sulphur contain-
ing compounds, which represent severe inhibition effect on the treatment of exhaust gas from NGVs [4]. There are a few literatures focusing on the catalysts inhibited by reaction products, such as water [1,6,7], and much fewer on transition metal catalysts. Li et al. [8] reported the promoting effect of water vapor on methane conversion on cobalt/manganese mixed oxides. They found that after pretreatment in water vapor, cobalt species could segregate on the catalyst surface, which were possibly related to the activity increase in the presence of water. However, there are few investigations on the mechanism of the promoting effect of water vapor. In this paper, the effect of Co on the phenomenon of reducing water poisoning rate for Pd/La-Al2 O3 catalysts prepared by different methods was investigated by comparing methane conversion in the presence and absence of water vapor. 2. Experimental 2.1. Supports and catalysts preparation Al2 O3 modified by La2 O3 was prepared by peptizing
∗
Corresponding author. Tel/Fax: 028-85418451; E-mail: [email protected] This work was supported by the National Natural Science Foundation of China (20773090) and the Ph. D. Programs Foundation of Ministry of Education of China (200806100009) Copyright©2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(09)60044-X
Journal of Natural Gas Chemistry Vol. 19 No. 2 2010
method. La(NO3 )3 ·6H2 O solution and HNO3 (65 wt%) were added into the mixture to make pseudo boehmite peptize at pH = 4. After that, the colloid was dried. As-obtained powder was calcined at 1000 ◦ C for 3 h. The content of La2 O3 was 3 wt%. For catalyst Pd/Co/La-Al2O3 prepared by sequential impregnation, La-Al2 O3 was impregnated in Co(NO3 )2 ·6H2 O solution followed by drying and calcination at 550 ◦ C for 2 h, and then the yielding powder was impregnated in Pd(NO3 )2 solution. For catalyst PdCo/La-Al2O3 prepared by co-impregnation, Co(NO3 )2 ·6H2 O and Pd(NO3 )2 solutions were mixed first, and then La-Al2 O3 was impregnated in the mixed solutions. In order to compare, catalyst Pd/La-Al2 O3 without Co was also prepared by impregnation method. All the prepared catalysts mentioned above were dried and then calcined at 550 ◦ C for 2 h. The loadings of Pd and Co were 1.25 wt% and 2 wt%, respectively. The resulting powder were milled with proper amount of de-ionized water to form homogenous slurry and then coated onto a honeycomb cordierite (2.5 cm3 , Corning, Shanghai, China). Excess slurry was blown away by compressed air. The loadings of washcoat and Pd were kept about 180 g/L and 2.25 g/L, respectively. 2.2. Catalytic activity tests Catalytic activity tests were carried out on monolithic samples in a multiple fixed-bed continuous flow micro-reactor by passing a gas mixture which simulated exhaust from leanburn natural gas vehicles. All the catalysts were treated at 550 ◦ C for 1 h in reacting gases regulated by mass-flow controller before the test. The simulated exhaust contained 630 ppm CH4 , 0.4 vol% CO, 5 vol% O2 , 12 vol% CO2 , and N2 as balance gas. The gas hourly space velocity (GHSV) was 34000 h−1 . Catalytic activities of each catalyst were tested in the absence and presence of 10 vol% water vapor. The concentrations of CH4 before and after reaction were detected by an on-line gas chromatograph equipped with FID detector. 2.3. Supports and catalysts characterization The BET specific surface area and pore size of catalysts and support were obtained by adsorption/desorption of N2 at the liquid nitrogen temperature on a ZXF-06 automatic surface analyzer (Xibei Chemical Institute, China). Before measurement, the sample was degassed in vacuum at 400 ◦ C for 1 h. CO-chemisorption tests of catalysts were carried out with 0.2 g samples at room temperature after reduced by hydrogen at about 300 ◦ C for 1 h and then purged with pure Ar for 1 h. The sample was placed in a U-shaped quartz tube connected with a thermal conductivity detector (TCD). The crystal structures of the samples were determined by X-ray powder diffraction (XRD), on a DX-2500 diffractometer with Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 25 mA. The XRD data were recorded for 2θ values between 10o and 90o with an interval of 0.03o . The X-ray photoelectron spectroscopy (XPS) data were
135
collected on a spectrometer (XSAM-800, KRATOS Co.) with Al Kα radiation under UHV, calibrated internally by the carbon deposit C1s binding energy (BE) at 284.8 eV. 3. Results and discussion 3.1. Catalytic activity Catalytic activities of methane conversion were measured in the presence or absence of 10 vol% water vapor in the simulated exhaust gas. Methane conversion as a function of reaction temperature on the three catalysts in the absence and presence of water vapor respectively is shown in Figure 1. It was observed that, in the absence of water vapor, the reaction activity was in the order of Pd/La-Al2O3 >Pd/Co/La-Al2 O3 >PdCo/La-Al2O3 . However, in presence of water and below 404 ◦ C, the order was Pd/LaAl2 O3 >PdCo/La-Al2 O3 >Pd/Co/La-Al2 O3 . The difference of activity was little between the three catalysts. It could be seen that, in the presence of water vapor, as temperature increased, activity of Pd/La-Al2 O3 increased more slowly than that of PdCo/La-Al2 O3 . While above 404 ◦ C, the catalytic activity order turned to be: PdCo/La-Al2 O3 >Pd/LaAl2 O3 >Pd/Co/La-Al2 O3 . Figure 2 shows methane conversion as a function of temperature in the simulated exhaust gas in the absence and presence of water vapor over the catalysts. Methane conversion activity in the presence of water vapor decreased compared to that in the absence of water vapor for all the three catalysts. It also could be seen from Figure 2 that the difference of activities over each catalyst in the absence and presence of water vapor decreased as the temperature increased. For Pd/La-Al2 O3 , Pd/Co/LaAl2 O3 and PdCo/La-Al2 O3 the activity under the two conditions reached the same at about 470 ◦ C, 469 ◦ C and 433 ◦ C, respectively. It could be concluded that the co-impregnation of Co reduced the rate of water poisoning for the catalyst. The data of T50 , T90 , ΔT50 and ΔT90 in the presence and absence of water vapor are presented in Table 1. The lightoff temperature (T50 ) and completely conversion temperature (T90) were the temperatures at which a given pollutant conversion reached 50% and 90%, respectively. ΔT50 = T50 (in the presence of water) − T50 (in the absence of water) and ΔT90 = T90 (in the presence of water) − T90 (in the absence of water). The data of ΔT50 and ΔT90 can partly reflect the water poisoning effect of catalysts. It could be observed in the absence of water vapor that PdCo/La-Al2 O3 exhibited the highest T50 and T90 , which were 325 ◦ C and 376 ◦ C, respectively. For Pd/La-Al2 O3 , T50 and T90 were 297 ◦ C and 328 ◦ C, which were much lower than those of PdCo/LaAl2 O3 ; and for Pd/Co/La-Al2 O3 , T50 was a little higher than that of Pd/La-Al2 O3 but 20 ◦ C lower than that of PdCo/LaAl2 O3 . In the presence of water vapor, the order of T50 and T90 turned to be Pd/Co/La-Al2O3 >PdCo/La-Al2 O3 >Pd/LaAl2 O3 . It could be observed that, in the presence of water, the differences of T50 and T90 between PdCo/La-Al2O3 and Pd/La-Al2O3 should be in the range of experimental error and
136
Enyan Long et al./ Journal of Natural Gas Chemistry Vol. 19 No. 2 2010
could be neglected, although Pd/La-Al2 O3 exhibited much better performance than PdCo/La-Al2 O3 in the absence of water. The order of ΔT50 and ΔT90 was Pd/La-Al2 O3 >Pd/Co/LaAl2 O3 >PdCo/La-Al2O3 , which suggested that water had the greatest inhibition effect on Pd/La-Al2 O3 while the least on PdCo/La-Al2 O3 . For the three catalysts Pd/La-Al2 O3 , Pd/Co/La-Al2 O3 and PdCo/La-Al2 O3 , ΔT50 was 68 ◦ C, 66 ◦ C
and 42 ◦ C, respectively, and ΔT90 was 74 ◦ C, 56 ◦ C and 26 ◦ C, respectively. These data reflects that the inhibition effect of water vapor followed the order Pd/La-Al2 O3 >Pd/Co/LaAl2 O3 >PdCo/La-Al2 O3 . The addition of Co enhanced resistant effect to water vapor depending on the preparation method. Co-impregnation of cobalt performed much better than sequential impregnations.
Figure 1. Methane conversion as a function of temperature over Pd/La-Al2 O3 , PdCo/La-Al2 O3 and Pd/Co/La-Al2 O3 in the simulated exhaust gas. (a) In the absence of water vapor, (b) in the presence of water vapor
Figure 2. Methane conversion as a function of temperature in the simulated exhaust gas in the absence (open symbols) and presence of water vapor (filled symbols) over (a) Pd/La-Al2 O3 , (b) PdCo/La-Al2 O3 and (c) Pd/Co/La-Al2 O3 Table 1. T 50 , T 90 , ΔT 50 and ΔT 90 in the presence and absence of water vapor Samples Pd/La-Al2 O3 PdCo/La-Al2 O3 Pd/Co/La-Al2 O3
T50 (◦ C, without water) 297 325 305
T50 (◦ C, with water) 365 367 371
3.2. Specif ic surface area and CO-chemisorption The specific surface area is very important for catalytic activity of catalysts. The BET surface area and pore volume of support and catalysts are shown in Table 2. As shown in the Table, specific surface area and pore volume were similar for the support and the catalysts except Pd/Co/La-Al2 O3 . It indi-
ΔT50 (◦ C) 68 42 66
T90 (◦ C, without water) 328 376 349
T90 (◦ C, with water) 402 402 405
ΔT90 (◦ C) 74 26 56
cated that except Pd/Co/La-Al2 O3 , impregnation of Co or Pd and calcination of the catalysts resulted in almost no changes of the specific surface area of the catalyst. Maybe it was because calcination temperature of the support was much higher and calcination time was longer than that of the catalysts. Further more, the addition amounts of Pd and Co were both little. It could be concluded that the specific surface area of the cat-
Journal of Natural Gas Chemistry Vol. 19 No. 2 2010
137
alyst was decided mainly by the support prepared in present work. The decrease of specific surface area of Pd/Co/LaAl2 O3 might be due to the sequential impregnation method. As described in the preparation part of support and catalyst in this paper, after each impregnation, the support or catalyst was calcined at 550 ◦ C for 2 h. That meant there was one more impregnation and calcination step for Pd/Co/La-Al2 O3 , which led to a decrease in the specific surface area. Table 2. Specific surface area and the pore volume of the support and the catalysts Samples La-Al2 O3 Pd/La-Al2 O3 PdCo/La-Al2 O3 Pd/Co/La-Al2 O3
SBET (m2 /g) 136 135 132 116
Pore volume (ml/g) 0.38 0.37 0.37 0.39
CO-chemisorption was performed over the three catalysts and Co/La-Al2 O3 for comparison. The results are listed in Table 3. It is shown that very little carbon monoxide was adsorbed on Co/La-Al2 O3 , which is the CO adsorbed by Co contained material. Subtracting the amount of CO adsorbed by Co contained composite, it is the amount of CO adsorbed by Pd. It could be seen that CO amount adsorbed by Pd followed the order Pd/La-Al2 O3 >Pd/Co/La-Al2O3 >PdCo/LaAl2 O3 . It reflects that the dispersion of Pd on the catalyst surface followed the order above. It is consistent with the activity order in the absence of water vapor. The dispersion of Pd decreased with Co loading for the co-impregnated catalysts, due to the competition of Co with Pd to disperse on the support. Table 3. Results of CO-chemisorption of the catalysts Samples Co/La-Al2 O3 Pd/La-Al2 O3 PdCo/La-Al2 O3 Pd/Co/La-Al2 O3
CO volume chemisorbed (μmol/g) 4 30 25 30
CO amount adsorbed by Pd (μmol/g) − 30 21 26
3.3. XRD The XRD patterns are depicted in Figure 3. α-Al2 O3 can be observed due to the high temperature calcination of the support. However, as the results shown in Table 2, the specific surface area and pore volume of the support did not decrease much, reflecting that there was little Al2 O3 transforming to α phase and Al2 O3 mainly presented in γ-Al2 O3 . It should be owing to the addition of La2 O3 that had excellent stable effect on Al2 O3 as reported in Ref. [9]. No peaks ascribed to Pd were observed, indicating that the dispersion of Pd was good and maybe the amount of Pd was below the limitation of detection. It could be seen that there was just little difference among the XRD patterns of the three catalysts. A weak peak of CoAl2 O4 for Pd/Co/La-Al2 O3 was detected.
Figure 3. XRD patterns of (1) Pd/La-Al2 O3 , (2) PdCo/La-Al2 O3 and (3) Pd/Co/La-Al2 O3
3.4. XPS In order to investigate the key factor resulting in the different performance to water resistant effect for the catalysts, XPS was used to characterize the electronic state of Pd and Co on the catalyst surface. Binding energy (BE) values of Pd 3d 5/2 and the Pd/Al ratios on the catalyst surface calculated from XPS for three catalysts are summarized in Table 4. The higher and the lower BE values of Pd 3d5/2 are assigned to PdO (336.8 eV [10]) and Pd (335.3 eV [11]), respectively. It was found that the addition of Co, no matter by co-impregnation and sequential impregnations, decreased the amount of Pd or PdO on the catalyst surface. Surface Pd/Al ratios followed the order Pd/LaAl2 O3 >Pd/Co/La-Al2 O3 >PdCo/La-Al2O3 , which was consistent with the methane conversion order of the three catalysts in the absence of water. Table 4. Binding energy (BE) of Pd 3d5/2 and surface Pd/Al ratios calculated from XPS for the three catalysts Samples Pd/La-Al2 O3
BE of Pd 3d5/2 (eV) Pd/Al (×100) Total Pd/Al (×100) 335.2 0.57 1.96 336.9 1.39 335.1 0.35 1.16 PdCo/La-Al2 O3 336.8 0.81 335.3 0.38 1.32 Pd/Co/La-Al2 O3 336.9 0.94
Figure 4 displays the XPS lines of Co over the two catalysts contained Co. It could be seen from Figure 4 that peaks of Co 2p3/2 and Co 2p1/2 exhibited strong shoulder at their higher binding energy side for both catalysts. Xiong et al. [12] compared XPS lines of pure Co3 O4 and CoAl2 O4 , and ascribed the shoulder at higher binding energy of Co 2p3/2 and Co 2p1/2 of pure CoAl2 O4 to the shake-up process of Co2+ compound in the high spin state, while that of pure Co3 O4 was remarkably weak. In this paper, binding energy of Co 2p3/2 of the two catalysts were between that of Co3 O4 (780.0 eV) and CoAl2 O4 (781.9 eV) [13], and there were strong shake-up shoulder observed from the two catalysts. For
138
Enyan Long et al./ Journal of Natural Gas Chemistry Vol. 19 No. 2 2010
the reasons above, there should be presence of both CoAl2 O4 and Co3 O4 for the two catalysts. In order to calculate the ratios of the two cobalt species for the catalysts, the Co 2p regions were fitted with 80% Gaussian and 20% Lorentzian. The results of XPS lines are presented in Figure 5. Analytical results of Co 2p3/2 binding energies are presented in Table 5. The higher BE (for PdCo/La-Al2 O3 is 782.2 eV, for Pd/Co/La-Al2 O3 is 781.8 eV) was ascribed to CoAl2 O4 and the lower BE (780.0 eV for both the catalysts) could be ascribed to Co3 O4 . The ratio of CoAl2 O4 /Co3 O4 was calculated by the relative ratio of their peak area. For Pd/Co/LaAl2 O3 , the ratio of CoAl2 O4 /Co3 O4 was higher than that of PdCo/La-Al2 O3 as shown in Table 5. This was consistent with the results of XRD. It could be explained that the interaction between Co and support La-Al2 O3 by sequential impregnations was stronger than by the co-impregnation method. The resistant effect of water vapor on catalyst prepared by coimpregnation of Pd and Co was much stronger than that on the catalysts prepared by sequential impregnations. It was Co3 O4 (not CoAl2 O4 ) played an important role in resisting the water vapor. Cullis et al. [7] found that the addition of water vapor strongly inhibited the reaction over a Pd/A12 O3 catalyst. They suggested that a competition of H2 O with CH4 on surface active sites led to its reaction with PdO to form Pd(OH)2
at the PdO surface, effectively blocking the access of methane to the active PdO phase. Co3 O4 might be more active to react with water than PdO, which explained that the catalyst coimpregnated by Pd and Co acted better water-resistant effect than Pd/La-Al2 O3 . Table 5. Binding energy (BE) of Co 2p3/2 and the ratio of CoAl2 O4 /Co3 O4 Sample PdCo/La-Al2 O3 Pd/Co/La-Al2 O3 a
Co3 O4 BEa Area 780.0 109.5 780.0 64.7
CoAl2 O4 BEa Area 782.2 305.0 781.8 397.0
Ratio of CoAl2 O4 /Co3 O4 b 2.785 6.136
BE of Co 2p3/2 , b Calculated by peak area
4. Conclusions In this paper, Pd catalysts were prepared and investigated. Co was added by co-impregnation and sequential impregnations, respectively. From the results of catalytic activity tests in the absence and presence of water vapor, it was found that the addition of Co did not improve the catalytic activity for methane conversion, but reduced the water poisoning rate for Pd catalysts which were sensitive to water. It was notable that the promoting effect of Co significantly affected by the impregnation methods. The resistant effect to water was much higher for PdCo/La-Al2 O3 prepared by coimpregnation method. The results of XRD and XPS showed that the ratio of CoAl2 O4 /Co3 O4 for Pd/Co/La-Al2 O3 was higher than that for PdCo/La-Al2 O3 , and Co3 O4 played an important role in resisting the water vapor. It could be explained that Co3 O4 was more active with water than PdO. So the water poisoning of PdO was partially inhibited by the addition of Co, especially if it was prepared by co-impregnation method. References
Figure 4. XPS lines of Co for PdCo/La-Al2 O3 (1) and Pd/Co/La-Al2 O3 (2)
Figure 5. XPS lines of Co 2p3/2 and Co 2p1/2 for PdCo/La-Al2 O3 and Pd/Co/La-Al2 O3
[1] G´elin P, Urfels L, Primet M, Tena E. Catal Today, 2003, 83: 45 [2] Holmgreen E M, Yung M M, Ozkan U S. Appl Catal B, 2007, 74: 73 [3] Lampert J K, Kazi M S, Farrauto R J. Appl Catal B, 1997, 14: 21l [4] G´elin P, Primet M. Appl Catal B, 2002, 39: 1 [5] Liotta L F, Carlo D, Pantaleo G, Venezia A M, Deganello G, Borla E M, Pidria M. Appl Catal B, 2007, 75: 182 [6] Butch R, Urbano F J, Loader P K. Appl Catal A, 1995, 123: 173 [7] Cullis C F, Trimm D L, Nevell T G. J Chem Soc-Faraday Trans 1, 1972, 68: 1406 [8] Li W B, Lin Y, Zhang Y. Catal Today, 2003, 83: 239 [9] Zhang L J, Dong W P, Guo J X, Yuan S H, Zhang L, Gong M C, Chen Y Q. Acta Phys-Chim Sin (Wuli Huaxue Xuebao), 2007, 23: 1738 [10] Monteiro R S, Noronha F B, Dieguez L C, Schmal M. Appl Catal A, 1995, 131: 89 [11] Kobayashi T, Yamada T, Kayano K. Appl Catal B, 2001, 30: 287 [12] Xiong H F, Zhang Y H, Liew K Y, Li J L. J Mol Catal A, 2005, 231: 145 [13] Zsoldos Z, Guczi L. J Phys Chem, 1992, 96: 9393