Mechanistic Understanding of Cr Poisoning on La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O_(3-δ) (LSCF)

摘要

The effects of Cr poisoning on the catalytic activity ofLa_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O_(3-δ) (LSCF) and the poisoning mechanism wasinvestigated. AC impedance spectroscopy and temperatureprogrammed isotopic exchange (TPX) were consistent in provingpoisoning mainly degraded the surface catalytic activity as apassive layer evolved on the surface or near-surface region. Theformation of small particles was clearly observed in scanningelectron microscopy when LSCF was heat treated in the presenceof Crofer22APU. Low angle X-ray diffraction identified thesewere SrCrO_4. As a result of Sr diffusion to form SrCrO_4 particles,it was found that a Sr-deficient phase was formed and a phasetransition from rhombohedral to cubic perovskite occurred in thenear surface region. While previous Cr poisoning studies on LSCFmostly emphasized the effects of SrCrO_4 formation, the Srdeficient surface layer also significantly degrades the catalyticactivity of LSCF due to reduction of oxygen vacancies and surfaceCo/Fe concentration.