Surface/Interface Regulation Promoted Catalytic Oxidation Over Perovskite Oxides

In recent years,with the rapid development of China’s social economy and the continuous advancement of industrialization,urbanization and modernization,the associated air pollution has attracted increasing attention.Especially in the Yangtze River Delta and other economically developed regions and industrial areas,serious air pollution(The days of heavy pollution account for more than 95%of the year)has greatly affected people’s life.Therefore,it is urgent to efficiently control air pollutants emission.Nowadays,end-pipe treatment is one of the most widely applied pollutants control technologies.In this regard,catalytic purification plays a crucial role in the control of pollution emission from stationary and mobile sources.Platinum-group metal catalysts,as main commercial catalysts for environmental catalysis,have some practical drawbacks such as low reserves,high cost,and sintering deactivation at high temperature,which severely limits their long-term and large-scale application.Perovskite oxides have been widely studied in these years as promising noble metal alternatives because of their low cost,high abundance,and high thermal stability.However,the low specific surface area and surface reactivity of perovskite oxides have been greatly restricting their development and further commercialization.The morphology and surface properties of the catalyst can greatly influence their catalytic performance.In recent years,researchers have developed a series of ordered mesoporous/macroporous and nano-array perovskite oxides through morphology construction,which effectively increased the specific surface area of the catalyst,promoted the exposure of the active site,and improved mass transfer of the reactants;On the other hand,the surface catalytic properties of perovskite catalysts were enhanced by A/B site substitution/doping,surface nonstoichiometry and surface defect engineering strategy.Both of above design and fabrication concepts could efficiently improve catalytic oxidation performance of perovskite catalysts.In addition,further multidisciplinary literature investigation suggests that surface/interface engineering can modulate geometric structure and electronic properties of surface active sites on catalyst surface,which promotes the interaction of reactant with catalyst surface,simultaneously enhances redox properties,oxygen exchange ability and oxygen activation capacity,thus effectively facilitating the catalytic oxidation process.On the basis of above ideas,we thus combine morphology control strategy and surface/interface activation strategy to develop advanced interface catalysts and surface modification catalysts with ordered micro/nano structure to achieve efficient removal of pollutants.Meanwhile,comprehensive structure-activity relationship between perovskite surface/interface properties(oxygen vacancy,Lewis acid site,electron transfer between interface)and catalytic performance would be elucidated,which could provide a rational strategy for the design and fabrication of high-activity and high-stability perovskite catalysts.The main research contents of this doctoral dissertation are as follows:1.Efficient charge transfers between perovskite oxide based composite materials can greatly influence the overall catalytic performance.However,interface effect,the associated surface lattice oxygen activation mechanism and surface catalytic reaction mechanism in composite oxides are less understood.Herein,we constructed hetero-epitaxial interface of ZnO/La0.8Sr0.2CoO3(ZnO/LSCO)via DFT modeling and experimentally synthesized ZnO/LSCO interface catalysts with nano-array structure by a combination of hydrothermal method and wash-coating method,systematacially investigating the promotion mechanism of interface effect.Theoretical and experimental investigation results suggest that interfacical structure,which acts as "electron regulator",could effectively regulate the electron transfer between Zn 3d-O 2p hybrid orbital in ZnO and Co eg orbital in LSCO,which promotes the rapid generation and refill of oxygen vacancy with unpaired electron(Vo·),thus enhancing the activity&mobility of surface lattice oxygen and facilitaing CH4 oxidation.Besides,we expanded the application of perovskite catalyst in photothermal oxidation of CH4 and investigated solar-driven interfacial effect.It is revealed that the synergetic drive of photothermal and photoelectric effects further facilitates reversible charge transfer,which leads to higher lattice oxygen activity and thus better solardriven CH4 oxidation activity than thermal catalytic system.The interfacial structure not only results in good thermal catalytic performance over LSCO catalysts,but also has a strong response to photothermal catalysis,which is helpful for achieveing efficient environmental catalysis of stationary sources by utilizing solar photothermal effect in the future.2.The insights on the primary active oxygen specie and its relation with oxygen vacancy is essential for the design of highly efficient low-temperature oxidation catalysts.DFT calculations were firstly employed to predict the potential effect of oxygen vacancy on O2 activation.It revealed that the engineering of Vo facilitates reactants(O2 and CO)interaction and enables lower energy for ratedetermining step of CO oxidation on perovskite surface.Based on this,we developed a novel oxygen vacancy enriched ordered macroporous La0.8Sr0.2CoO3(Vo-OM LSCO)monolithic catalysts by employing in-situ solution assembly and liquid-phase reduction.Comprehensive theoretical and experimental investigation revealed that the construction of ordered macropore structure enabled the higher degree of accessible reactive surface for effective mass transport through favorable gas diffusion by one hand;and on the other hand,molecule O2 is more favorably adsorbed and dissociated on the engineered oxygen vacancies via one electron transfer process to form monatomic oxygen ions(O-).Meanwhile,the oxygen vacancy will switch the reaction pathway of CO oxidation from E-R mechanism to L-H mechanism with lower energy barrier.With effective mass transport,a richness of O-and thermodynamically favorable reaction pathway,the modulated perovskite monolithic catalysts exhibit superior catalytic activity for CO oxidation.In addition,we revealed that monoatomic oxygen ions(O-)is the primary oxygen active specie for perovskite oxide for the first time.3.In-depth understanding of surface properties-activity relationship could provide a fundamental guidance for the design of highly efficient perovskite-based catalysts.Here,oxalic acid,as the selective etchant and reductant,was employed to exfoliate surface Sr layer from La0.8Sr0.2CoO3(LSCO),simultaneously inducing the reduction of Co3+to Co2+.As a result,surface B-site Co2+/Co3+active Lewis acid sites and oxygen vacancies are both populated on the perovskite surface.We have both theoretically and experimentally investigated the synergistic effects of the Lewis acid sites and oxygen vacancies in perovskite oxides on methane combustion.It suggested that the population of electron-deficient Lewis acid sites and single-electron trapped Vo-enables a localized electron cloud shift,which regulates surface electronic and coordination structures of catalyst surface.This unique electronic effect promotes back-donating interaction between CH4 and LSCO surface,and enables high surface reactivity(excellent oxygen activation capacity,lattice oxygen mobility and reducibility),thus benefiting CH4 adsorption and activation.4.The construction of macroporous structure of catalysts provides a rational strategy for soot efficient oxidation,but one of the main handicaps is how to robustly integrate the well-defined macroporous oxidation catalysts on scalable DPF to realize efficient soot purification in diesel engine exhaust.Meanwhile,it has great practical significance to further promote soot oxidation over perovskite catalysts without the use of noble metal.Here,we started with the formulation optimization of perovskite integrated on DPF in terms of elemental compositions(La0.8K0.2CoO3)and then fabricated ordered m acroporous La0.8K0.2CoO3 catalysts based DPF(OM LKCO)by facile in-situ solution selfassembly method.It is noted that the thickness of macroporous LKCO skeleton on DPF can be precisely controlled by varying synthesis conditions.Furthermore,KNO3 decoration on OM LKCO surprisingly decreases soot oxidation temperature,with soot conversion of 85%at 450℃.This is due to that(1)the construction of macroporous framework improved contact efficiency of soot and catalyst,simultaneously enabled more active oxygen species and better reducibility;(2)KNO3 decoration significantly increased amount of surface active species.In addition,we developed a novel dropwise addition method to achieve precise control of soot loading weight on CDPF.