Controlled Synthesis Of Highly Active Nanosized LaFe-based Perovskites And Mechanism Study For Their Catalytic Removing Small Molecule Pollution Gases

Many compositions of perovskite mixed oxides have beendemonstrated as high potential catalysts, especially as environmentalcatalysts. Among these perovskites, LaFe-based perovskite structuresattract much attention due to their great versatility, excellenthigh-temperature thermal and hydrothermal stability as well as interestingactivities, in addition to the low cost of the constituting elements, even ifFe shows very limited redox ability. A series of LaFe-based perovskites(LaFe_1-x-yCu_xPdyO_3-δ) were prepared through conventional citric acidroute, and were then fully characterized using traditional techniques aswell as advanced oxygen isotopic exchange and equilibration techniques.It was shown that the reducibility and oxygen desorption were stronglyaltered by the properties of the substituting cation, even if iron remainshardly reducible up to1000℃. In addition, large differences in oxygenmobility were measured by oxygen isotopic exchange: Largely higheroxygen mobility was achieved over the Cu-containing sample, while Pd substitution strongly inhibited the oxygen mobility of LaFe structure.These observations correlated well with the different catalyticperformances in low-temperature CO oxidation over these materials.Indeed, Cu-containing materials presented the highest catalytic activities,while Pd-substituted structure showed a low-temperature activity similaras the LaFe parent material. CO oxidation is usually considered as asuprafacial reaction, where only adsorbed gas-phase species are involved.Nevertheless, the participation of the surface oxygen （in pure ferritestructure） or even the bulk oxygen （in Cu-substituted material） from thesolid to this reaction was strongly confirmed by oxygen mobilitymeasurements, suggesting a redox-type oxidation mechanism overLaFe-based perovskites.These perovskites were further investigated for the selective catalyticreduction （SCR） of NO by C₃H₆in the presence of O₂. The adsorbedspecies and surface reactions were detected for mechanistic study bymeans of NO-or C₃H₆/O₂-TPD （temperature-programmed desorption） aswell as in-situ DRIFTS （diffuse refectance Fourier transformspectroscopy）, in order to discriminate the effects of copper or palladiumpartial substitution. With respect to LaFeO₃, Cu²⁺incorporation improvedSCR performance, due to its promotion to the generation of active surfacespecies C_xH_yO_z. The excellent low-temperature SCR performance overLaFe0.94Pd0.06O3was mainly attributed to the strongly declined occupation of the active sites by the inactive ionic nitrates, as well as a rapid reactionbetween adNOx（nitrites/nitrates） and C_xH_yO_zspecies. A mechanism washerein proposed with the generation of adNOxand C_xH_yO_zsurfacespecies and then the further formation of some organo nitrogencompounds （ONCs）/CN/NCO as important intermediates. Moreover,the acceleration of two aspects （the formation of inactive ionic nitrate andthe direct oxidation of C₃H₆by O₂） contributed to a negative effect of O₂excess for NO reduction, while Pd substitution signifcantly increased theO₂tolerance ability.Being a successful extension of the benzyl alcohol route, a simplenon-aqueous method for the preparation of nanocrystalline LaFeO₃wasdeveloped. The obtained sample was then fully characterized and testedfor CO or CH4oxidation reaction. The morphology was investigated bymeans of TEM and SEM, and temperature programmed oxygen isotopicexchange technique was also used to measure the oxygen mobilitycapacity. With respect to citric-acid-route LaFeO₃, thebenzyl-alcohol-route material （both calcined at500℃for6h） presentedrelatively individual and quasi-spherical nanosized particles, with muchhigher specific surface area (43.6m²g^-1vs.23.1m2g^-1) as well as lowercrystal size （17.6nm vs.21.5nm）. Much better redox property andstrongly enhanced surface/bulk oxygen mobility were also achieved,resulting in the improved catalytic oxidation performances （255℃vs. 280℃for T50%in CO oxidation,410℃vs.432℃for T50%in CH₄oxidation）. Due to the participation of surface/subsurface oxygens fromthe solid, redox surface oxidation mechanism was herein assigned to COoxidation over LaFeO₃, while CH₄oxidation followed an intrafacialmechanism, involving the intense participation of bulk oxygens.