Controlled Synthesis Of Ironbased Perovskite Catalysts For Toluene Catalytic Oxidation Mechanism Study

Nowadays,environmental problems are becoming more and more prominent,among which the emission of organic volatile compounds have become one of the main sources of air pollution,and it is urgent to develop an efficient treatment strategy.In this regard,catalytic oxidation,as a low-temperature and high-efficient treatment method,shows many advantages such as low operating temperature,better safety,and low energy consumption,and is gradually gaining attention among enterprises.The development of catalyst plays a crucial role in its application,and the reaction activity could be affected by its physic-chemical properties.At present,noble metal materials are commonly used as catalyst in industry,but the high price and easy poisoning properties prevent them from being used on a large scale and put a lot of cost pressure on enterprises.Therefore,catalysts based on transitional metal oxide have attracted great interest in recent years.Due to the low price and the redox ability from the multi-valent state of active elements,transition metal oxides are gradually applied in many fields.Therefore,catalytic oxidation based on transition group metal materials has been investigated detailedly.In this work,through regulating and controlling the defects,we aim to achieve better catalytic performance over transitional metal oxide catalysts.Taking toluene and carbon monoxide as a modeling compound,the mechanism of promoting the catalytic performance over defective transition metal catalysts were investigated.The regulation of cationic defects was achieved through tuning the ratio of A-site cation in perovskite oxide catalysts.More oxygen vacancies were confirmed over the stoichiometric sample,which is beneficial to oxygen mobility and low-temperature reactivity.According to a series of characterizations,the average oxidation state of B-site cation was enhanced significantly,which is conducive to the electron transfer between the surface and reactants.Based on the above work,a new strategy to modulate the surface A/B element ratio was developed.Surface etching of A-site cation was applied to remove the lanthanum atom.More Fe-O termination was exposed over the surface and it could serve more active sites for the catalytic reaction.Furthermore,the weak Fe-O bond may boost oxygen mobility and enhance the catalytic performance correspondingly.The catalytic performance tests show that the sample after 5 h etching possesses the best performance with 90%conversion at 277.3℃.To further improve the reactivity of perovskite oxide materials,double perovskite oxides La₂Ni Fe O₆ with nanorod structure were synthesized by a one-step hydrothermal method and compared with simple perovskite and double perovskite synthesized by sol-gel method.Based on a series of characterization results,more oxygen vacancies,higher specific surface area,and better low-temperature performance were obtained with the composition change and the morphology of the nanorod.The experiment results showed that the double perovskite oxide with nanorod morphology exhibited optimal activity,and it could achieve 90%conversion of toluene at 243°C,which was about90°C lower than that of the simple perovskite La Fe O₃ synthesized by sol-gel method.Based on previous work,the properties of studied perovskite oxide catalysts were tuned through coupling multiple strategies to enhance the catalytic performance for toluene oxidation.According to a series of characterizations,the specific surface area decreases with the deficiency of A-site elements but is accompanied by higher B-site cation valence and more surface oxygen vacancies correspondingly,which may contribute to performance enhancement.The sample,L_1.85NFO,achieved 90%conversion of toluene at 253°C,which is a significant performance improvement over previous work.To exploit more possibilities of the studied iron-based perovskites,a small amount of cobalt was introduced into the lattice of L_0.90FO.A controlled composite oxide model with surface exsolved particles was constructed by inducing active element migration via exsolution.Through multiple characterization methods,exsolved samples possess a higher specific surface area accompanied by more exposed active sites and surface defects.Furthermore,the strong interaction between exsolved particles and the matrix was also confirmed.It’s worth noting that all the exsolved samples exhibit excellent performance for toluene oxidation and 90%conversion could be achieved at 237℃.