甲烷燃烧贵金属催化剂研究新进展

New Research Progress on Precious Metal Catalysts for Methane Combustion

采用替代性清洁燃料和安装排放后处理装置是目前控制和削减内燃机汽车尾气污染物排放的最有效手段。天然气被认为是极具替代潜力的清洁燃料,然而其中未燃烧的甲烷会造成严重的温室效应。甲烷具有稳定的正四面体结构、C-H键难于被活化断裂,因此成为天然气车的排放控制焦点和研究热点。以贵金属为活性组分的贵金属催化剂显示出比非贵金属催化剂更强的甲烷催化氧化活性,耐水、耐硫和耐高温性能,是实现甲烷充分燃烧的主流催化材料。最新出台的轻型汽车污染物排放限值及测量方法(中国第六阶段)更严格地限制了尾气中污染物的排放,为新一代天然气车用高效尾气净化催化剂的研发提供借鉴和参考,本文重点围绕催化活性提升和耐水、耐硫性能增强等,综述了近10年来国内外在甲烷燃烧贵金属催化剂方面的最新研究进展,并提出负载在掺杂了过渡金属或稀土元素的三维有序大孔或介孔材料上的Pd基双金属和多金属催化剂将成为未来尾气净化催化剂活性、稳定性提升的主要方向。

Automotive vehicles not only facilitate human life and promote industrial development,but also bring serious environmental pollution problems,such as acid rain,photochemical smog,ozone layer destruction,haze and greenhouse effect and other extreme weather phenomena.The use of alternative clean fuels and the installation of aftertreatment devices are currently the most effective means of controlling and reducing exhaust emissions from internal combustion engine vehicles.As a high-quality,high-efficiency,green and environmentally friendly low-carbon energy,natural gas can form a benign complement to the development of renewable energy,and it is the only bridge connecting traditional fossil energy and renewable energy.Due to its abundant reserves,low price,low pollutant emissions,and good safety,natural gas has also become the most potential alternative clean fuel in the transportation field.Nowadays,compressed and liquefied natural gas(CNG and LNG)has brought huge environmental and economic benefits to more than 28.54 million vehicles worldwide,from heavy-duty buses and trucks to light buses.China’s natural gas vehicle technology is relatively mature,and it has become the country with the largest number of natural gas vehicles(NGV)in the world for several consecutive years.Under the multiple driving forces of energy structure optimization,environmental pollution control and climate change constraints,NGV also have a high development potential in the future.Although natural gas is considered the cleanest hydrocarbon fuel,unburned methane in the exhaust from NGV is the focus and difficulty of emission pollution control.Methane is more difficult to be oxidized than other unconverted hydrocarbons derived from gasoline.This is due to the highly stable C-H bond(450 kJ·mol^(-1))in the nonpolar CH_(4) molecule,which makes it difficult to adsorb on the catalyst surface.As a result,a higher reaction temperature is required to activate CH_(4) molecules(the theoretical ignition temperature of CH_(4) is 538 ℃).However,the exhaust temperature of NGV is usually lower than 500 ℃,which makes it challenging to completely oxidize methane at low temperature.In addition to the temperature barrier,the presence of H_(2)O and toxic sulfur compounds in the exhaust composition may hinder the oxidation of low-concentration methane(500×10^(-6)~1500×10^(-6))in the exhaust gas of NGV.One possible solution is catalytic combustion,where methane is oxidized to carbon dioxide on a catalytically active solid surface at a relatively low reaction temperature.Catalyst is the core of the catalytic combustion reaction of methane and the key to the exhaust gas purification device of NGV.Currently widely reported methane combustion catalysts for NGV mainly include precious metal catalysts and non-precious metal catalysts.Thanks to the special d-orbital structure of precious metals and the moderately strong metal surface for adsorbing reactants,precious metals as catalytic active component showed stronger methane catalytic oxidation activity,higher water,sulfur and high temperature resistance than non-precious metal catalysts,and are the mainstream catalytic materials to achieve full combustion of methane.The latest vehicle emission limits and measurement methods(CHINA VI)has more stringently restricted the emission of pollutants in the exhaust gas,to provide a reference for the development of new generation high-efficiency exhaust gas purification catalysts for NGV,the latest research progress on the Pd,Pt,Rh,Au,Ru,Ag,Os and Ir eight kinds of precious metal catalysts for methane combustion in the past 10 years at home and abroad was reviewed in this paper.It was found that the Pd-based catalyst was still the most efficient precious metal catalyst for methane combustion,but its activity was strongly inhibited by the water and sulfide in the reaction atmosphere.In order to improve the hydrothermal stability of Pdbased catalysts and improve the combustion behavior of methane in the presence of sulfides,researchers have tried to achieve this by modifying or replacing the carrier,improving the preparation method,and constructing a bimetallic system.Research on the support effect of Pd-based catalysts includes hydrophobic modification of the support,doping of alkaline earth metals and/or transition metals or even non-metal elements into the support,and use those materials with high oxygen surface mobility and mesoporous or macroporous materials as support;the research on preparation methods includes the improvement of existing synthesis methods and the development of new preparation techniques;the research on bimetals mainly revolves around the construction of Pd-Pt bimetallic catalysts,because Pt usually has a strong ability to resist water and sulfur poisoning.Rh is more expensive than Pd,so it is usually used in industrial catalytic reactions with higher economic benefits.In addition,it also shows a strong ability to reduce NOx.As for the other five precious metal catalysts,their methane catalytic activity is usually quite poor.In short,with people’s increasing attention to environmental issues and the gradual tightening of exhaust emission standards,we proposed that Pd-based bimetallic and multi-metal catalysts supported on three-dimensional ordered macroporous or mesoporous materials doped with transition metals or rare earth elements will become the main direction for the activity and stability improvement of exhaust gas purification catalysts in the future.