Catalytic activity and sulfur tolerance for Mn- substituted La_(0.75)Sr_(0.25)CrO_(3±δ) in gas containing H_2S

摘要

Lanthanum chromites substituted by transition metal are potentially applied as anode catalysts in solid oxide fuel cell fed with fuel gas containing H_2S. In order to understand the effect of composition on catalytic activity and sulfur tolerance of anode catalysts, La_(0.75)Sr_(0.25)Cr_(1-x)Mn_xO_(3±δ) (noted as LSCM, x = 0.2, 0.5, 0.8) series were synthesized and characterized by XRD, XPS and H_2-TPR. The results demonstrate that LSCM55 with moderate Mn-substitution content has the highest activity for H_2S, which is attributed to considerable reducibility of Mn~(4+) determined by H_2-TPR. XRD patterns reveal that as-synthesized samples with different Mn-substitution contents have different sulfur tolerance. Sr 3d_(5/2), Cr 2p_(3/2), Mn 2p_(3/2) regions of XPS for samples with different Mn-substitution contents imply that dominance of Mn~(4+), responsible for the catalytic activity, is the consequence of the interactions between cations at A-site and B-site. On the other hand, evolution of Mn in H_2S atmosphere indicates Mn~(3+) in Mn~(4+) -O-Mn~(3+) clusters contributes greatly to the sulfur tolerance of LSCM. By analyzing consistency in the contents of lattice oxygen, S species (mainly sulfate) and Mn~(3+), a hypothetical mechanism of LSCM catalysts involving catalysis and sulfur tolerance was proposed.