ZrO_(2)基催化剂在天然气汽车尾气处理中的应用研究进展

Research Progress on Application of ZrO_(2) Based Catalysts in Natural Gas Vehicle Exhaust Treatment

随着汽车尾气排放法规的进一步加严和对燃油经济性要求的提高,发展适用的代用燃料发动机清洁汽车成为必然的发展趋势。压缩天然气发动机(CNG)具有较高的燃烧效率及较低的污染物排放,对比传统的汽油和柴油发动机在环境保护方面具有明显的优势,然而天然气发动机运行过程中未完全燃烧的甲烷、催化剂的低温氧化活性、抗水热抗硫中毒性能都是目前急需解决的难题。ZrO_(2)基载体具有优良的耐水和耐硫中毒性能,适用于天然气汽车尾气净化,有望克服目前催化剂易发生水和硫中毒的缺点。本文综述了用于甲烷催化氧化的锆基催化材料近年来的研究进展,阐述了稀土金属、过渡金属等元素的掺杂及制备方法对ZrO_(2)结构以及ZrO_(2)基催化剂催化性能的影响,得出以下结论:(1)四方相ZrO_(2)负载型催化剂比混合相ZrO_(2)负载型催化剂具有更好的水热稳定性。(2)Y掺杂ZrO_(2)提高了催化剂的热稳定性;CeO_(2)-ZrO_(2)混合氧化物的氧化还原行为和储氧能力取决于多种因素:包括氧化物的组成、结构、织构、相均一性和预处理等;适当的ZrO_(2)与Al_(2)O_(3)的质量比是催化剂低温氧化活性的关键;添加NiO,MgO或Co3O4可提高Pd/ZA催化剂的活性和水热稳定性。(3)不同制备方法会导致载体间相互作用的不同,进而引起织构、结构和催化活性的不同等。

With the further tightening of automobile exhaust emission regulations,improvement of fuel economy performance and stricter control of CO_(2)emission,it is an inevitable trend to develop clean vehicles with alternative fuel engines for environmental protection and energy considerations.Compressed natural gas engine(CNG)has obvious advantages in environmental protection compared with traditional gasoline and diesel engines due to its high combustion efficiency and low pollutant emission.However,the exhaust temperature of natural gas engine is low(300~550℃),and the water content(10%~15%)and S content in exhaust gas are high.At present,the biggest challenge is the oxidation of CH4at low temperature and the improvement of hydrothermal stability and S resistance of catalyst.The composition,structure and morphology of the catalyst,as well as its dynamic changes during synthesis and service,play a decisive role in its catalytic activity.The research on methane combustion catalysts mainly focuses on precious metal catalysts and non-precious metal catalysts and Palladium-supported catalyst has a low light off temperature and can maintain high catalytic activity under lean burn conditions,so it has become a research hotspot.In addition,the support has a great influence on the dispersion of active metals,the active phase state and the activity and stability of the catalyst.Therefore,the high temperature hydrothermal stability,acid-base property,oxygen storage and release capacity,oxygen migration capacity and specific surface area of the support all affect the performance of the catalyst.For this reason,many studies have tried to improve the overall performance of the catalyst by modifying the support.At present,the commonly used support is γ-Al_(2)O_(3).However,γ-Al_(2)O_(3)is unstable at high temperature and easy to be poisoned by S.In order to solve these problems,other oxide supports have been widely studied.Among them,ZrO_(2)based support has excellent water resistance and sulfur poisoning resistance,which is suitable for the purification of natural gas vehicle exhaust,and was expected to overcome the shortcomings of current catalysts prone to water and sulfur poisoning.However,ZrO_(2)has a low specific surface area and is easy to sinter at high temperatures and phase separation occurs,which leads to a decrease in catalytic activity.Therefore,some researchers have tried to improve the preparation methods to increase the specific surface area of ZrO_(2)to improve its performance.And some researchers had also improved the stability and anti-S performance of ZrO_(2)by doping rare earth and transition metal elements:rare earth and transition metal modification can change the microstructure and surface chemical composition of the support,and optimize the active species and catalytic performance of the catalyst.In this paper,the research progress of zirconium-based catalysts for methane oxidation in recent years was reviewed,the effects of rare earth metal and transition metal doping and preparation methods on the structure of ZrO_(2)and the catalytic performance of ZrO_(2)based catalysts were described.It provided theoretical guidance for the development of such catalysts:(1)The performance of ZrO_(2)supported catalyst used for the exhaust gas treatment of natural gas vehicles was related to the phase state of ZrO_(2).t-ZrO_(2)supported catalyst had better hydrothermal stability than the mixed phase ZrO_(2)supported catalyst.(2)The doping of rare earth elements or alkaline earth elements could further improve the performance of ZrO_(2)based catalysts.The doping of yttrium improved the thermal stability of the catalyst.The oxidation-reduction behavior and oxygen storage capacity of CeO_(2)-ZrO_(2)mixed oxide depend on many factors:including oxide composition,structure,texture,phase uniformity and pretreatment,and etc.The proper mass ratio of ZrO_(2)to Al_(2)O_(3)was the key to the low-temperature oxidation activity of the catalyst.The addition of NiO,MgO or Co3O4could improve the activity and hydrothermal stability of Pd/ZA catalyst.(3)Different preparation methods would lead to the difference of morphology,structure,crystal face effect and interaction between supports,which would lead to different texture,structure and catalytic activity.In recent years,the structure and morphology of zirconium-based catalysts in automotive exhaust gas catalytic reaction had been studied.In addition,ZrO_(2)and its mixed oxides with different structures and morphologies had been prepared.However,the research on zirconia with different morphologies mainly focused on ceramics,photocatalytic materials,coating materials and other fields,and the influence of morphology on the catalytic performance of the catalysts needed further research.Subsequent research might focus on the controllable synthesis of nano-ZrO_(2)materials with special structures,morphologies and exposed crystal planes,so as to control the interaction between the components and between the components and the reactants from the atomic level.It provided a new idea to solve the low temperature oxidation of CH4and improve the hydrothermal stability and S resistance of the catalyst.