In 2010,the annual output and ownership of motorcycles in China were 25 million and 100 million units,accounting for more than 40% and 50% of those in the world [ 1 ],respectively,indicating that China has become the largest producer and consumer in the world for motorcycles. According to statistics,motorcycle exhaust emissions of hydrocarbons (HC),CO,and NOx amounted to 7.48 million tons in China for the year 2011 [ 2 ]. What is more,in comparison with cars fueled by gasoline and diesel,the exhaust gases of motorcycles contain large proportions of small particles (~ 0.04 μm) within the PM 2.5 fraction,which are harmful to health. Therefore,it is imperative to reduce the emissions of the hazardous components in motorcycle exhaust gas [ 3, 4 ]. On July 1,2008,China began to implement the third stage of motorcycle emission standards [ 5 ],and the fourth stage,which is in the process of being drafted,will be stricter [ 6, 7 ]. In addition,although electric fuel injection technology will be applied in motorcycles,there are still many technical challenges,such as major fluctuations in the air-fuel-ratio (A/F) and working conditions of high temperature and space velocity (SV) when compared with cars fueled by gasoline [ 8 ]. Therefore,it is critical to improve the performance of catalysts to meet future motorcycle emission standards.
The support materials used in motorcycle exhaust gas catalysts typically consist of CeO2-ZrO2 oxides and Al2O3. The CeO2-ZrO2 solid solution has good oxygen storage capability (OSC) and has merit in promoting a water gas shift reaction,but its resistance to aging and durability is relatively poor [9−11]. Al2O3,however,possesses high specific surface area,large pore volume,and remarkable anti-aging performance [ 12, 13 ]. According to Morikawa et al. [ 14 ],a modified Ce-Zr rare earth oxygen storage material (OSM) combined with Al2O3 exhibits good oxygen storage capability. Furthermore,Al2O3 has played an important role as a “diffusion barrier”,inhibiting the aggregation of Ce-Zr materials and precious metals during the sintering process. Transition and rare earth elements could be doped into ceria-based oxides to change their phase structure to improve oxygen transmission at low temperature and realize high thermal stability [15−17]. In addition,the active components of motorcycle exhaust gas catalysts are usually based on Pt,Pd,and Rh,specifically Pd itself or Pd-Rh,Pt-Rh,and Pt-Pd-Rh [ 18 ].
To exploit the key advantages of OSM and Al2O3 and further improve the performance of support materials by addition of rare earth elements,a new composite oxide,CeO2-ZrO2-La2O3-PrO2-Al2O3,was synthesized by a co-precipitation method,and the Pd-Rh,Pt-Rh,and Pt-Pd-Rh supported on monolithic carrier catalysts were obtained by an impregnation route. Then the A/F,SV,and temperature characteristic of the catalysts before and after aging were investigated in a systematic manner. The results demonstrated that the new catalysts were suitable for motorcycle exhaust gas treatment,with the excellent performance characteristics meeting future emission standards.
The composite oxide,CeO2-ZrO2-La2O3-PrO2-Al2O3,was prepared by a co-precipitation method. An aqueous solution of Ce(NO3)3·6H2O,La(NO3)3·6H2O,Pr(NO3)3·6H2O and Al(NO3)3·9H2O was first mixed with ZrO(CO3)·6H2O dissolved in concentrated HNO3. The resulting mixed solution was injected into a reactor. At the same time,a buffer solution of NH3·H2O and (NH4)2CO3 in excess was added to the reactor. The pH (pH = 9),reaction temperature,and stirring speed in the reactor were carefully controlled during the process. The obtained precursors were aged at 90 °C for 4 h,then filtered and washed with distilled water until no change in pH could be detected. The precipitates were dried at 70 °C,heated at 600 °C for 3 h,and then calcined at 900 °C for 5 h. The synthesized sample CeO2-ZrO2-La2O3-PrO2-Al2O3 (mass ratio 30:10:5:5:50) was marked as CZLPA.
Pt,Pd,and Rh were loaded on CZLPA by an impregnation method using solutions of Pd(NO3)2,Pt(NO3)4,and RhCl3 as the metal precursors,respectively. After drying,Pd/CZLPA,Pt/CZLPA,and Rh/CZLPA catalyst powders were obtained. The resulting catalyst powders,Pd/CZLPA and Rh/CZLPA,Pt/CZLPA and Rh/CZLPA,and Pd/CZLPA,Pt/CZLPA,and Rh/CZLPA,were mixed in specific proportions,respectively. Then distilled water was added to each mixture,and homogeneous slurries were obtained by ball mill. The slurries were spread on a honeycomb cordierite (2.5 cm3,Corning,USA; coating amount 160 g/L). The fresh monolithic catalysts,Pd-Rh (Pd/Rh = 10/1),Pt-Rh (Pt/Rh = 10/1),and Pt-Pd-Rh (Pt/Pd/Rh = 3/7/1),were obtained after drying and calcining at 550 °C for 3 h and were labeled as Cat1f,Cat2f,and Cat3f,respectively. The amount of precious metal coating in each catalyst was 1.76 g/L. These fresh catalysts were next thermally aged at 1000 °C for 5 h in air and labeled as Cat1a,Cat2a,and Cat3a.
The textural properties of the catalysts were obtained by the N2 adsorption-desorption method on a Quantachrome SI instrument. The specific surface area,pore volume,and pore size distribution of the catalyst powders were measured at -196 °C. Prior to each measurement,the catalyst powders were degassed at 300 °C for 3 h under vacuum.
Temperature-programmed reduction in H2 (H2-TPR) was carried out in a quartz tubular micro-reactor equipped with a thermal conductivity detector. Prior to analysis,the catalyst powders (100 mg) were pre-cleaned using N2 (20 mL/min) with temperature cycling from room temperature to 450 °C,holding at this temperature for 50 min and then cooling to room temperature. Measurements were performed from room temperature to 600 °C in 5% H2-95% N2 (20 mL/min) at a heating rate of 8 °C/min.
OSC was performed in a purpose-built experimental device. Catalyst powders (200 mg) were reduced in pure H2 (40 mL/min) as the temperature was raised from room temperature to 550 °C before measurement of OSC. Then the powders were maintained at this temperature for 60 min,cooled to 200 °C,and purged with pure He (20 mL/min). The OSC was measured by injecting discrete oxygen pulses into the sample bed until no further oxygen consumption was detected by TCD.
X-ray diffraction (XRD) patterns were recorded using a Rigaku D/max-rA diffractometer with Cu Kα radiation (40 kV,25 mA,λ = 0.15406 nm). The powders were scanned within the 2θ range of 10° to 80°. The crystalline phases were identified by comparing them with reference data from the International Centre for Diffraction Data (ICDD).
The performance of the catalysts was evaluated in a multiple fixed-bed continuum flow micro-reactor with a gas mixture that simulated the exhaust from a motorcycle. First,the catalysts were pretreated under a reaction atmosphere at 550 °C for 1 h. Then activity was evaluated by cooling to the reaction temperature. The A/F characteristics were evaluated at 400 °C with an SV of 40000 h−1; the SV properties were evaluated at 400 °C and the space velocity of 20000,40000,and 60000 h−1; the temperature characteristics were evaluated at an SV of 40000 h−1. The composition of reaction gas was 0.045% C3H8,1.5% CO,0.11% NO,10% H2O,10% CO2,different levels of O2,and N2 as balance. The contents of C3H8,CO,and NO in the stimulated gas before and after reaction were analyzed using an FGA-4100 five-component analyzer (Foshan Analysis Instrument Co.,Ltd.,China),and then the conversion for each composition was calculated.
Table 1 shows the textural parameters for fresh and aged catalysts. It can be seen that the textural properties of these catalysts are similar to each other. Compared with the fresh catalysts,the surface area and pore volume of aged catalysts decreased; also the average pore radius of aged catalysts increased,indicating the formation of bigger pores caused by sintering and collapse during the aging process [ 5, 19 ]. The surface areas of the catalysts after aging are still about 70 m2/g,which is slightly higher than that of the literature [ 14, 20 ],demonstrating that the catalysts prepared in this study have excellent age resistance and are suitable for the high-temperature environment of motorcycle exhausts.
Figure 1 shows the N2 adsorption-desorption isotherms and the pore radius distribution of the catalysts. The aperture radii of the fresh catalysts were about 4-8 nm,and the shapes of the three isotherms are almost the same. They are typical of an irreversible type IV adsorption isotherm with a H2 hysteresis loop,indicating that the pores of the samples are slit and inkbottle-type mesopores. The larger the mesopores,the higher the pressure needed for capillary condensation to occur. Therefore,the extent of adsorption increases faster in the high-pressure regions,suggesting that the large pores are abundant [ 21, 22 ],which is well suited to the high-speed environment of a motorcycle exhaust. After aging,the aperture radii are about 5-12 nm and the areas of hysteresis shrink,which further suggests the collapse of small pores after aging [ 19, 23 ]. The hysteresis loops for Pd-Rh and Pt-Rh catalysts are closed at a relative pressure (p/p0) of about 0.8,which is about 23% higher than fresh catalysts. By comparison,the hysteresis loops for the Pt-Pd-Rh catalyst are closed at p/p0 ≈ 0.7,which only increased by 7.7% compared with the fresh catalyst. This result demonstrated that the extent of collapse for Pt-Pd-Rh is significantly less than that for Pd-Rh and Pt-Rh; also,the durability at high temperature is enhanced for the Pt-Pd-Rh catalyst [ 23 ].
Figure 2 displays H2-TPR profiles of the fresh and aged catalysts. All the fresh and aged catalysts feature dominant broad peaks below 400 °C,which are attributed to the reduction of Pt,Pd,and Rh,and these reduction temperatures are lower than those reported in the literature [24−27],indicating that the catalysts possess superior reductive properties. The peak area responses for the aged catalysts are suppressed relative to the fresh ones,which is due to substantial agglomeration of noble metals and shrinkage of the specific surface area and pore volume. A consequence of this is that a portion of the precious metal species are entrapped,leading to a reduction in the reduced species such that the peak area responses are decreased [ 21, 28 ]. The reduction in peak areas for the catalysts before and after aging follow the sequence Pt-Pd-Rh > Pd-Rh > Pt-Rh,indicating that Pt-Pd-Rh exhibits the best reducibility.
The OSC of the catalysts before and after aging are shown in Fig. 3. The OSCs of aged samples are lower than that of fresh ones primarily because of the sintering of the support materials and agglomeration of the precious metals [ 14, 27 ]. The OSC of Pt-Pd-Rh is the highest and that of Pt-Rh is the lowest,which may reflect the different combining capacities between the precious metals and the support materials,as well as differing synergistic effects among the noble metals [ 28 ]. This result indicated that the Pt-Pd-Rh catalyst is responsive to larger fluctuations in the A/F and the long-term high-temperature environment of the motorcycle exhaust.
XRD patterns for the catalysts before and after aging are illustrated in Fig. 4. It can be seen that all the catalysts have a typical cubic Ce0.75Zr0.25O2 phase and an Fm-3m space group. In contrast to the standard spectrum of Ce0.75Zr0.25O2,however,the characteristic diffraction peaks slightly shift to a lower 2θ degree,which is due to the doped rare earth elements,La (γLa3+= 0.106 nm) and Pr (γPr3+= 0.101 nm,γPr4+= 0.09 nm) with the larger atomic radius entering the Ce0.75Zr0.25O2 (γCe4+= 0.097 nm,γZr4+= 0.084 nm) crystal lattice [ 17, 22 ]. After aging,the diffraction peaks become sharper and more intense,and diffraction peaks for γ-Al2O3 are observed,indicating that high-temperature treatment leads to grain growth and an ordered arrangement of the cations. As shown in Fig. 4(b),the characteristic diffraction peaks for the catalysts shift slightly to higher 2θ degree after aging. This may be due to the following reasons: (1) the migration of larger radius La3+ and Pr3+/Pr4+ cations from the bulk to the surface of the crystal,and (2) the transformation of Ce3+ (γCe3+= 0.114 nm) to Ce4+ during the high-temperature calcination process,resulting in shrinkage of the cell [ 17, 22, 29, 30 ].
The crystal size of the fresh and aged catalysts calculated according to the Scherrer equation is about 5.5-7.5 nm,which implies little change of crystal size before and after aging,indicating excellent thermal stability of the catalysts. This phenomenon may benefit from Al2O3 acting as a “diffusion barrier”,limiting the carrier materials’ contact with each other. Thus,Al2O3 inhibits agglomeration of carrier material particles and,to a certain extent,increases the thermal stability of the materials [ 14, 31 ].
Figure 5 displays the A/F for the fresh and aged catalysts,which describes the relationships between the conversions of three kinds of fomites in the simulated motorcycle exhaust at the SV of 40000 h−1 and the A/F (S value). Figure 5(a) displays the properties of fresh catalysts. When S < 1,NO undergoes full transformation for all the fresh catalysts,and the conversion for CO is 80%. The performance of Pd-Rh and Pt-Pd-Rh for conversion of C3H8 is enhanced,which is due to Pd-Rh and Pt-Pd-Rh facilitating the steam reformation reaction of C3H8 [ 17, 18, 22 ]. When S > 1,there is complete conversion of CO,but the conversion of NO decreases significantly,and the window width of NO in the case of Pd-Rh is the narrowest,owing to the conversion of NO being affected by the concentration of O2. With high concentrations of O2,the reduction of NO may be suppressed. In addition,the conversion of C3H8 over Pt-Rh and Pt-Pd-Rh declines with increase of S,while the conversion of C3H8 over Pd-Rh is still complete. This may reflect the fact that the coupling reactions of C3H8 and NO with Pt-Rh and Pt-Pd-Rh are more efficient than that with Pd-Rh. The aged catalyst properties can be seen in Fig. 5(b),in which the catalytic properties of the three pollutants declined. The main reasons for this are the collapse of keyholes,a decrease in the specific surface areas,sintering of precious metals,and a reduction in active sites during the high-temperature treatment process [ 19, 27 ]. These results are consistent with the textural properties of the aged catalysts. Figure 5(c) summarizes the window widths of the fresh and aged catalysts based on 80% conversion of the three pollutants at an SV of 40000 h−1. The window width for fresh catalysts follows the sequence Pt-Pd-Rh > Pd-Rh > Pt-Rh. Afte r aging,the window widths for the catalysts decrease to different extents,the levels of decline being in the order Pt-Rh > Pd-Rh > Pt-Pd-Rh,indicating that the Pt-Pd-Rh catalyst possesses an excellent A/F property and good resistance to aging,which agrees with the TPR and OSC findings.
Figure 6 reveals the window widths for fresh catalysts at the SV of 20000 and 60000 h−1. In conjunction with Fig. 5(a) and 5(c),it can be seen that the windows become narrow when the SV increases. When the SV increases,the residence time for exhaust gas on the catalyst becomes shorter and mass transfer is restricted so that reactants have less time to enter the catalyst channels and react. Among the three catalysts,the windows for Pt-Pd-Rh at the SV of 20000,40000,and 60000 h−1 are the widest,indicating that the Pt-Pd-Rh catalyst is well suited to the high-speed environment of motorcycle exhaust.
Figure 7 shows the conversions for C3H8,CO,and NO at different temperatures for fresh and aged catalysts in simulated motorcycle exhaust at an SV of 40000 h−1. The T50 (conversion of 50% for specific reaction temperature),T90 (conversion of 90% for specific reaction temperature),and ΔT (T90 - T50) are listed in Table 2. The conversions for C3H8,CO,and NO rise as the temperature increases,rapidly approaching complete conversion at high temperature. As a result of the high conversion of the saturated hydrocarbon over Pt,the T50 and T90 values for C3H8 for the Pt-Rh catalyst are the lowest among that of the three catalysts [ 24, 32 ]. After aging,owing to the sintering of the support materials and agglomeration of precious metals,the surface activity sites decline and the catalytic properties decrease [ 32, 33 ]. Consequently,the T50,T90,and ΔT of the catalysts are higher,especially for the Pt-Rh catalyst. The T50 and T90 for CO and NO are relatively lower,which can be ascribed to the higher content of CO in the motorcycle exhaust compared with car fueled by gasoline. Once the CO ignites,the exhaust temperature rapidly increases,resulting in rapid and complete conversion of CO and NO. Moreover,the difference of T50 between CO and NO is not clear,demonstrating a high degree of coupling between the two molecules. Considering the T50,T90 and ΔT data for the three pollutants before and after catalyst aging,the performance data for Pt-Pd-Rh is the most remarkable,particularly the results for the textural properties and the H2-TPR and OSC data.
A new composite oxide,CeO2-ZrO2-La2O3-PrO2-Al2O3,was synthesized by a co-precipitation method,and the supported monolith catalysts,Pd-Rh,Pt-Rh,and Pt-Pd-Rh,were obtained through an impregnation route. Of the three catalysts investigated,the Pt-Pd-Rh catalyst showed the best redox properties and the most effective oxygen storage capacity. Furthermore,the A/F,SV,and temperature characteristics for the Pt-Pd-Rh catalyst were the most superior. It is concluded that the new composite material is suitable as a motorcycle exhaust catalyst and can meet future environmental emission standards.
2010年我国的摩托车年产量为2500万辆,占世界总年产量的40%以上; 摩托车保有量为1亿辆,占世界总保有量的50%以上[ 1 ]. 中国已成为世界上最大的摩托车生产大国和使用大国.据统计,2011年我国摩托车尾气中的碳氢化合物(HC)、CO和氮氧化合物(NOx)的年排放量已达748万吨[ 2 ];此外,与汽油车和柴油车相比,摩托车尾气中PM2.5的尺度仅为0.04μm,危害大,因此摩托车尾气净化程度的提高已刻不容缓[ 3, 4 ].2008年7月1日我国开始全面实施摩托车第III阶段排放标准[ 5 ],未来摩托车第IV阶段排放标准正在起草过程中,将更为严格[ 6, 7 ].此外,与汽车相比,未来的摩托车即使使用电子喷射技术控制空燃比,依然面临着高温、高空速且空燃比波动较大等难题[ 8 ].因此,摩托车尾气要满足未来排放标准,必须提高催化剂的性能.
摩托车尾气净化催化剂的载体材料一般由Ce-Zr储氧材料和Al2O3组成.这是因为CeO2与ZrO2形成的固溶体具有良好的储氧性能和可促进水-气转化反应等优点,但抗老化和耐久性能较差[ 9, 10, 11 ],而Al2O3具有高比表面积、大孔体积和高抗老化等优异性能[ 12, 13 ].Morikawa等[ 14 ]研究发现,用Al2O3改性的Ce-Zr稀土储氧材料兼有储氧材料和Al2O3的共同优点,同时Al2O3的加入起到了“扩散障碍”的作用,抑制了Ce-Zr材料和贵金属的烧结团聚.文献[ 15, 16, 17 ]报道,在含铈氧化物中掺杂其它过渡金属和稀土元素可改变其体相结构,从而改善其在低温条件下氧的传输和高温热稳定性.摩托车尾气净化催化剂的活性组分主要有Pt,Pd和Rh,通常分为单Pd型、Pd-Rh型、Pt-Rh型以及Pt-Pd-Rh型催化剂[ 18 ].
为了将储氧材料(OSM)与Al2O3的优异性能结合起来,并能通过稀土元素的掺杂进一步提高载体材料的性能,本文采用共沉淀法制备了新型复合氧化物CeO2-ZrO2-La2O3-PrO2-Al2O3,以其作为载体制备了Pd-Rh,Pt-Rh和Pt-Pd-Rh型整体式催化剂,并结合摩托车尾气排放特征考察了老化前后催化剂的空燃比特性、空速特性和温度特性.
采用共沉淀法制备CeO2-ZrO2-La2O3-PrO2-Al2O3复合氧化物.按一定化学计量比称取Ce(NO3)3·6H2O,ZrO(CO3)·6H2O,La(NO3)3·6H2O,Pr(NO3)3·6H2O和Al(NO3)3·9H2O,其中ZrO(CO3)·6H2O用HNO3溶解后加入其余硝酸盐的溶液配成浓度为10%的盐溶液.以NH3·H2O和(NH4)2CO3的混合溶液作沉淀剂进行滴定并保持pH=9,所得沉淀经90oC陈化4h,过滤,洗涤后,于70°C干燥,600°C预分解3h,900°C焙烧5h,得到质量比为30:10:5:5:50的CeO2-ZrO2-La2O3-PrO2-Al2O3载体材料,记为CZLPA.
以CZLPA为载体,Pd(NO3)2,Pt(NO3)4和RhCl3为贵金属前驱体,采用等体积浸渍法制备催化剂.首先将Pd(NO3)2,Pt(NO3)4和RhCl3分别浸渍到CZLPA上,经干燥后得到Pd/CZLPA,Pt/CZLPA和Rh/CZLPA三种催化剂粉末.然后分别将Pd/CZLPA和Rh/CZLPA催化剂粉末,Pt/CZLPA和Rh/CZLPA催化剂粉末,Pd/CZLPA,Pt/CZLPA和Rh/CZLPA催化剂粉末加入一定量蒸馏水进行球磨混合,制成浆液,取堇青石蜂窝基体(美国Corning公司)小样(2.5cm3)进行涂覆,涂覆量为160g/L,经空气吹扫,干燥,再于550oC焙烧3h,得到新鲜的整体式Pd-Rh(Pd/Rh=10/1)型、Pt-Rh(Pt/Rh=10/1)型和Pt-Pd-Rh(Pt/Pd/Rh=3/7/1)型催化剂,分别记为Cat1f,Cat2f和Cat3f.其中,贵金属的涂覆量均为1.76g/L.
将制得的新鲜整体式催化剂置于高温炉中,在空气气氛下1000°C热老化处理5h,得到老化的Pd-Rh型、Pt-Rh型和Pt-Pd-Rh型催化剂,分别记为Cat1a,Cat2a和Cat3a.
催化剂织构性能测定在美国康塔公司SI型比表面测定仪上进行.首先将催化剂粉末样品于300°C下抽真空预处理3h,然后以N2为吸附质,在-196°C下进行N2吸附-脱附实验,计算样品的比表面积、孔体积和平均孔径.
H2程序升温还原(H2-TPR)测定在自组装微型流动反应装置上进行,催化剂粉末样品用量为100mg.样品先在N2气流(20mL/min)中加热至450°C,并保持50min,然后降至室温,切换到5%H2-95%N2混合气(20mL/min),再以8°C/min的升温速率升到600°C,TCD检测耗氢量.
储氧性能(OSC)测定在自组装的实验装置上进行,催化剂粉末样品用量200mg.样品首先在40mL/min的H2气流下升温至550°C,并保持60min进行活化,再切换为He(20mL/min),自然降温至200°C,脉冲注入O2,TCD检测.
X射线衍射(XRD)测定在日本理学电机D/max-rA型X射线衍射仪上进行,采用CuKα(λ=0.15406nm)作为衍射源,管电压40kV,管电流25mA,扫描范围2θ = 10°-80°,晶相与国际衍射数据中心(ICDD)相比较来辨别.
催化剂的评价在实验室自制的多路固定床连续流动微型反应器中进行,各路气体分别用质量流量计控制进入混合器.反应前,催化剂在反应气氛围下于550°C预处理1h,然后降至反应温度进行活性评价.其中,空燃比特性于温度为400°C,空速为40000h-1条件下测定;空速特性于温度为400°C,空速为20000,40000和60000h-1条件下测定;温度特性在空速为40000h-1条件下降温测定.反应气为摩托车尾气模拟气,组成为0.045% C3H8,1.5% CO,0.11% NO,10% H2O,10% CO2和不同含量的O2,以N2为平衡气.采用FGA-4100型汽车排气分析仪(佛分环保仪器检测设备制造有限公司)分析反应前后尾气中各组分的含量,计算各组分的转化率.
表1为新鲜和老化催化剂的织构参数.由表可见,三种催化剂的织构性能相近.与新鲜催化剂相比,老化后催化剂的比表面积和孔体积下降,平均孔径增大,表明老化后催化剂中的小孔因内聚力较大,烧结坍塌融聚到一起而形成大孔[ 5, 19 ].老化后催化剂的比表面积仍有70m2/g左右,略高于文献值[ 14, 20 ],说明以新型复合氧化物载体材料CZLPA负载的催化剂具有较好的抗高温老化能力,适合摩托车尾气的高温环境.
图1为新鲜和老化催化剂的N2吸附-脱附等温线和孔径分布.可以看出,新鲜催化剂的孔径集中在4-8nm,各催化剂样品的吸脱附曲线均表现出典型的Ⅳ型等温线和H2型滞后环,说明样品的孔为狭缝型和墨水瓶型的介孔.介孔越大,毛细管凝聚发生的压力越高,所以高压区吸附量上升较快,说明大孔较多[ 21, 22 ],适合摩托车尾气的高空速环境.老化后,孔径集中在5-12nm,向大孔方向移动,而且回滞环面积有不同程度的收缩,这进一步说明老化后小孔坍塌,孔径变大[ 19, 23 ].同时,Pd-Rh型和Pt-Rh型催化剂的回滞环均在相对压力(p/p0)为0.8左右开始闭合,比新鲜催化剂增大了23%,而Pt-Pd-Rh型催化剂的回滞环在p/p0=0.7左右开始闭合,比新鲜催化剂仅增大了7.7%,说明Pt-Pd-Rh型催化剂小孔坍塌程度较小,抗高温老化能力更优[ 23 ].
图2为新鲜和老化催化剂的H2-TPR谱.由图可见,新鲜催化剂和老化催化剂的还原峰温均在400°C以下,归属为贵金属Pd,Pt和Rh的还原峰[ 24, 25, 26, 27 ],且相比于文献[ 24, 25, 26, 27 ]中以Ce基材料为载体的贵金属催化剂,还原峰温向低温偏移,说明本文制备的催化剂具有较好的还原性能.与新鲜催化剂相比,老化后样品的还原峰面积有所下降.这是由于贵金属在载体表面发生团聚、烧结,加之载体材料的比表面积和孔体积下降,使得一部分贵金属物种被包裹掩埋,造成可还原物种量减少所致[ 21, 28 ].老化前后三种催化剂的还原峰面积大小为Pt-Pd-Rh > Pd-Rh > Pt-Rh,可见Pt-Pd-Rh型催化剂还原性能较好.
图3为新鲜和老化催化剂的储氧量.由图可见,与新鲜催化剂相比,老化后三种催化剂的储氧量均有所下降,主要是由于载体材料的烧结和贵金属的团聚所致[ 14, 27 ].其中,老化前后Pt-Pd-Rh型催化剂的储氧量最高,Pt-Rh型催化剂的最低.这可能是各贵金属元素与载体材料的结合能力和贵金属之间的协同作用不同的结果[ 28 ].由此可见本文制备的Pt-Pd-Rh型催化剂适合于空燃比波动较大且长期高温的摩托车尾气环境.
图4是新鲜和老化催化剂的XRD谱. 可以看出,新鲜和老化催化剂均为典型的立方结构Ce0.75Zr0.25O2相,空间群为Fm-3m.相对于Fm-3m结构的Ce0.75Zr0.25O2标准谱,谱峰向小角度偏移,这可能是载体材料中掺杂的原子半径较大的稀土元素La(γLa3+=0.106nm)和Pr(γPr3+=0.101nm,γPr4+=0.09nm)进入到Ce0.75Zr0.25O2晶格(γCe4+=0.097nm,γZr4+=0.084nm)的结果[ 17, 22 ].老化后,&l t;/ span>各催化剂的衍射峰变得强而尖锐,并检测到γ-Al2O3的衍射峰,表明高温焙烧导致晶粒长大,晶型趋于完整.另外,老化后三种催化剂的衍射峰向大角度偏移,这是由于半径较大的La3+,Pr3+/Pr4+由体相向表面迁移,同时高温焙烧后Ce3+(γCe3+=0.114nm)向Ce4+转化,造成晶胞收缩[ 17, 22, 29, 30 ].
根据Scherrer公式计算得老化前后催化剂平均晶粒尺寸均在5.5−7.5nm,即老化后催化剂的平均晶粒尺寸变化不大,说明催化剂的热稳定性能较好. 这可能得益于Al2O3在材料中充当了“扩散障碍”的分散物质,限制了载体材料之间的相互接触,从而在一定程度上抑制了载体材料颗粒的团聚长大,增加了材料的热稳定性[ 14, 31 ].
图5是新鲜和老化的催化剂在40000h-1空速条件下的空燃比特性,即C3H8,CO和NO转化率与空燃比S的关系曲线.对于新鲜催化剂,在富燃区(S<1),三种催化剂均能使NO达到完全转化,并使得CO转化率达到80%以上,但Pd-Rh型和Pt-Pd-Rh型更有利于C3H8的转化,这可能是由于富燃时Pd-Rh型和Pt-Pd-Rh型催化剂上更有利发生C3H8的蒸汽重整反应[ 17, 18, 22 ].在贫燃区(S>1),随着S值的增大 ,三种催化剂均能催化氧化CO达到完全转化,而NO的转化率则急剧下降,并且Pd-Rh型催化剂的NO窗口明显窄于其它两类催化剂. 这是由于NO受O2浓度的影响,还原反应受到限制.同时,Pt-Rh和Pt-Pd-Rh型催化剂上C3H8的转化率随着S值的增大有所下降,而Pd-Rh型催化剂上的C3H8仍然可以达到完全转化,这可能是由于Pt-Rh和Pt-Pd-Rh型催化剂上C3H8和NO的偶联反应更为明显.与新鲜催化剂比较,老化后的催化剂对三种污染物的转化能力均下降.主要原因是高温老化过程中,小孔坍塌,比表面积下降,贵金属烧结,活性位点减少,催化反应受限[ 19, 27 ].这与老化后催化剂的织构性能下降相一致.图5(c)汇总了三种催化剂在40000h-1空速下老化前后空燃比窗口的宽度. 由图可见,新鲜催化剂的空燃比窗口宽度顺序为Pt-Pd-Rh>Pd-Rh>Pt-Rh;老化后,三种催化剂空燃比窗口宽度均有所下降,下降程度顺序为Pt-Rh>Pd-Rh>Pt-Pd-Rh.这表明Pt-Pd-Rh催化剂的空燃比性能及抗老化性能优异. 这与TPR和OSC结果相一致.
图6为新鲜催化剂在20000和60000h-1空速下的空燃比窗口.结合图5(a)和(c)可见,随着空速的增大,催化剂空燃比窗口不同程度变窄.这是由于空速增大,气体在催化剂上停留时间变短,传质受到限制,甚至反应物来不及进入催化剂孔道进行反应所致.可以发现,Pt-Pd-Rh型催化剂在20000,40000和60000h-1空速下的空燃比窗口最宽,空速特性最好,更适合于摩托车尾气的高空速环境.
图7是新鲜和老化催化剂在40000h-1空速下模拟摩托车尾气中C3H8,CO和NO转化率随温度的变化曲线. 三种污染物对应的起燃温度T50(转化率达50%时的温度)、完全转化温度T90(转化率达90%时的温度)和ΔT(T90与T50的差值)列于表2.由图7可知,对于新鲜和老化催化剂,随着温度的升高,C3H8,CO和NO的转化率升高,并可以迅速达到完全转化.其中,Pt-Rh型催化剂对C3H8的起燃温度和完全转化温度最低,这是因为贵金属Pt对饱和烃的转化较好[ 24, 32 ].老化后,各催化剂的T50,T90和ΔT都有所升高,而以Pt-Rh型催化剂最为明显.这主要是由于老化后,载体材料坍塌烧结,贵金属团聚,造成表面活性位数量下降,因而催化剂活性下降[ 32, 33 ].CO与NO的T50和T90都相对较低,这是因为摩托车尾气中CO含量相对汽车较高,一经起燃,CO反应热导致尾气温度骤升,所以很快就达到完全转化.此外,CO和NO的T50相差较小,说明CO和NO二者耦合较好.综合老化前后三种污染物的T50,T90和ΔT,以Pt-Pd-Rh型催化剂的性能最优,与表征结果一致.
采用共沉淀法制备了新型复合氧化物材料CeO2- ZrO2-La2O3-PrO2-Al2O3,并以此为载体制备了Pd-Rh,Pt-Rh和Pt-Pd-Rh型催化剂.三种催化剂中,Pt-Pd-Rh型催化剂在老化前后还原性能和储氧量最优,空燃比特性、空速特性及温度特性最优,适合于未来摩托车尾气排放标准催化剂的需求.
2014, Vol. 35 Issue (9): 1482-1491
PDF (692 KB)
2014, Vol. 35 Issue (9): 1482-1491
PDF (692 KB)