α-Fe2O3表面SO2吸附及SO3催化生成的密度泛函分析

DFT study on the adsorption of SO2 and catalytic formation of SO3 on theα-Fe2O3 surface

燃煤电厂排放了大量SO2和少量SO3,SO3的产生不仅危害环境,且不利于电厂的安全运行,飞灰和锅炉壁面中的Fe2O3对SO3生成有显著催化作用,而目前对Fe2O3催化生成SO3的路径研究和机理揭示还不够深入。建立了α-Fe2O3(001)表面,利用密度泛函分析方法,对SO2和O2在α-Fe2O3(001)表面的吸附方式进行研究,得到SO2的稳定吸附构型和O2在Fe2O3表面的解离方式,利用过渡态搜索方法研究了α-Fe2O3表面催化生成SO3的反应路径和反应能垒,并对比了气相反应生成SO3的反应能垒。结果表明,SO2最稳定的吸附方式是SO2中的O原子和S原子吸附在α-Fe2O3晶体上的Fe原子上方,S原子不易在α-Fe2O3表面的晶格氧上方吸附;O2在α-Fe2O3表面的吸附能大于SO2的最大吸附能,表明O2更易在α-Fe2O3表面吸附;O2极易在表面有氧空位的Fe2O3晶体上发生解离并生成O原子,说明有氧空位存在的α-Fe2O3更易促进O2的解离和表面吸附氧的产生。SO3生成的L-H机理为气相中的SO2和O原子先在α-Fe2O3表面吸附,再结合生成SO3,该过程的反应能垒为231.65 kJ/mol;E-R机理为气相中的SO2与α-Fe2O3的表面吸附氧发生反应生成SO3,其反应能垒为24.82 kJ/mol,小于L-H机理的反应能垒,也远小于气相反应中SO3生成的反应能垒。证实Fe2O3对SO3的生成具有显著的催化作用,且E-R机理为主导的反应机理,氧空位的存在促进了O2在α-Fe2O3表面的解离,且表面吸附氧在催化过程中起关键作用。

A large amount of SO2 and a small amount of SO3 are emitted from coal-fired power plants.The generation of SO3 is not only harmful to the environment,but also dangerous to the safe operation of the power plant.Current studies show that Fe2O3 contained in the fly ash and the boiler wall have a significant catalytic effect on the SO3 formation.However,the research on the path and mechanism of SO3 catalytic formation by Fe2O3 is not deep enough.In this paper,theα-Fe2O3(001)surface was established firstly,and the adsorption configuration of SO2 and O2 onα-Fe2O3(001)surface was studied by density functional theory(DFT).The stable adsorption configuration of SO2 and the dissociation pathway of O2 on Fe2O3(001)surface were obtained.The reaction path and energy barrier of catalytic formation of SO3 onα-Fe2O3 surface were studied by using transition state search method.Then the energy barrier of catalytic formation of SO3 in gas phase reaction were compared.The results show that the most stable adsorption configuration of SO2 is that O and S atom in SO2 are adsorbed above Fe atoms onα-Fe2O3,while S atom is not easy to be adsorbed above lattice oxygen ofα-Fe2O3 crystal.The adsorption energy of O2 onα-Fe2O3 surface is greater than the maximum adsorption energy of SO2,which means that O2 is more easily adsorbed onα-Fe2O3 surface.In addition,O2 is easy to dissociate and form O atom on the defect Fe2O3 surface with oxygen vacancy,which indicates thatα-Fe2O3 with oxygen vacancy is easier to promote the dissociation of O2 and the generation of adsorbed oxygen on the surface.The L-H mechanism of SO3 formation is that SO2 and O atoms in the gas phase are adsorbed firstly on theα-Fe2O3 surface,and then combine to form SO3,and the reaction energy barrier is 231.65 kJ/mol.The E-R mechanism is that SO2 in the gas phase reacts with adsorbed oxygen onα-Fe2O3 surface to form SO3 while the reaction energy barrier is 24.82 kJ/mol,which is less than that of L-H mechanism and far less than that of SO3 formation in gas phase reaction.The above results confirm that Fe2O3 has a significant catalytic effect on the formation of SO3,and the E-R mechanism is the dominant reaction mechanism.The existence of oxygen vacancy promotes the dissociation of O2 on the surface ofα-Fe2O3,and the surface adsorbed oxygen plays an important role in the catalytic process.