Lanthanum Cobaltite Microstructure Regulation And For The Thermal Catalytic Oxidation Of Toluene

With the development of industry and the acceleration of urbanization,a large amount of organic pollutants have been discharged into the atmosphere,which exerts a serious impact on the ecological environment and human health.Volatile organic compounds?VOCs?are the major precursors of fine particle matters(PM_2.5)and photochemical ozone.The 13^th Five-Year Plan on Air Pollution Prevention and Control in Guangdong clearly points out that by 2020,the total VOCs emission will be reduced by more than 390,000 tons compared with 2015,especially in key industries.The abatement of VOCs is an urgent task for the“Blue Sky Protection Campaign”.Thermal catalytic oxidation?TCO?is regarded as one of the effective,economic and environmental-friendly way to reduce the elimination of VOCs.With the merits of flexible composition,excellent redox properties and superior thermal stability,perovskite-type oxides are considered as promising catalysts in the oxidation of VOCs,and lanthanum cobaltite?LaCoO₃?as the main active component is a kind of hot catalysts.However,their activity is still less satisfactory,compared to noble metal catalysts.The further regulation and enhancement of the catalytic activity of LaCoO₃will be beneficial for the application in VOC removal.Substitution and loading are considered to be effective methods to improve the catalytic performance of traditional catalysis.However,these methods are still rare reported in tuning the activity of TCO in LaCoO₃.Furthermore,the relevant regulation and reaction mechanism are still unclear.In the present study,the lanthanum cobaltite was applied to the toluene catalytic oxidation.The catalytic activity and restriction mechanism of LaCoO₃,which is modified by substitution,loading and surface acid treatment,were investigated by using various spectroscopy and micro-zone technology.The structure-activity relationship between physico-chemical properties and catalytic performance is also elucidated.The main conclusions of this work have been made as follows:?1?Restriction mechanism of the spin state in LaCoO3 perovskite for toluene catalytic oxidation has been illustrated.Different spin state of Co cations in LaCoO₃ NPs can be prepared by sol-gel method with calcination at 600-1000 ^oC.The proportion of high spin states increased by calcination temperature,but the catalytic activity enhanced until 700 ^oC and then gradually decreased.The toluene oxidation activity trend for LaCoO₃ exhibited a volcano shape as a function of B-site e_g filling.LCO-700 showed the best performance when their electron configuration is t⁵_2ge_g¹?intermediate spin states?.On this basis,the structure-activity relationship between the electronic structure and catalytic activity of LaCoO₃ was proved by the O p band center.When the O p band center is moderate,the interaction between the surface oxygen and toluene was gently,resulting in the best catalytic activity.?2?The distinct effect of substitution and deposition of Ag in perovskite LaCoO3on the thermally catalytic oxidation of toluene has been verified.The catalytic activity of La_1-xAg_xCoO₃ towards toluene was enhanced with Ag content.The advancement of catalytic activity was mainly ascribed to the improvement of reducibility of Co³⁺,and more surface active oxygen species.Ag was incorporated into the perovskite structure of LaCoO₃ as Ag⁺.However,the silver gradually diffused to the surface of LaCoO₃ as Ag⁰ when the silver substitution amount reached 5%.With the increase of calcination temperature from 250 to 700 ^oC,the Ag species on LaCoO₃surface varied from Ag₂O,to Ag⁰ and then both Ag⁰ and substituted Ag⁺.Ascribed to the high surface oxygen proportion,good low-temperature reducibility,and strong interaction between Ag/Ag₂O and LaCoO₃ support,supported Ag species displayed higher catalytic activity than substituted Ag⁺.?3?Reaction mechanism of toluene catalytic oxidation by Ca-substituted LaCoO3has been distinguished.A series of Ca-substituted LaCoO₃(La_1-xCa_xCoO₃,x?=?0,0.1,0.2,and 0.3)were synthesized by sol-gel method.The characterization results showed that Ca²⁺tended to replaced La³⁺at the A site due to the ionic radius of Ca²⁺?1.18??was similar to that of La³⁺?1.22??,much larger than that of Co³⁺?0.63??.Cell parameters and particles size reduced with Ca content but BET on the contrary.In addition,after the Ca substitution,the total pore volume of the sample increased from 0.024 cm³g^-1 to 0.077⁰.087 cm³g^-1,and the average pore size increased from 15.8 nm to 18.1²4.8 nm.The Ca substitution retains the perovskite-type structure of La_1-xCa_x CoO₃,but significantly enhances the catalytic activity.The increasing sequence in catalytic activity is LaCoO₃<La_0.9Ca_0.1CoO₃<La_0.7Ca_0.3CoO₃<La_0.8Ca_0.2CoO₃.In addition,the influence of surface oxygen and lattice oxygen in the catalyst on catalytic performance is significantly different,and the surface oxygen plays a major role.Furthermore,La_0.8Ca_0.2CoO₃ showed the higher catalytic activity than other cobalt-based catalysts.?4?Effect of acid treatment on toluene catalytic over La1-_xCa_xCoO3 and its related mechanism have been explored.The LaCoO₃ surface was treated by citric acid,which did not destroy the perovskite structure.The improvement of toluene catalytic oxidation by acid treatment catalysts were attributed to the increase in oxygen concentration and acid sites,and the enhancement of low-temperature reducibility.In addition,the acid treatment catalysts also reduced the occupation of active sites by water molecules and promote the formation of surface reactive oxygen species,resulting in the improvement of water-resistance.The excellent catalytic activity,stability and water resistance of acid treatment catalysts made them a high potential for industrial application.These findings not only provide scientific basis for the application of perovskite catalyst in thermal catalytic oxidation of VOCs,but also provide theoretical support for the effective emission reduction of VOCs.