On the State of Pd in Perovskite-Type Oxidation Catalysts of Composition A(B,Pd)O3±δ (A = La, Y; B = Mn, Fe, Co)

Incorporation of palladium within the perovskite-type lattice is a valuable strategy to prevent sintering phenomena that reduce the performance of automotive catalysts. However, occupation of B-site coordination sites by Pd can strongly depend on the nature of the perovskite-type oxide and care must be taken for structural interpretation. Pd-based La(B,Pd)O-3 +/-delta (B = Mn, Fe, Co) perovskite-type oxides and Y(Fe,Pd)O-3 +/-delta have been prepared by the amorphous citrate method by introducing the Pd salt to the precursors solution with the aim to substitute the B-site. The state of Pd was thoroughly characterized using X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction with hydrogen. Pd incorporation was achieved for La(Fe,Pd)O-3 +/-delta and La(Co,Pd)O-3 +/-delta that showed the XANES white line of Pd in octahedral coordination. On the contrary, La(Mn,Pd)O-3 +/-delta showed XANES features and reduction behavior typical of segregated PdO particles. The state of Pd in Y(Fe,Pd)O-3 +/-delta appeared to be dependent on the calcination temperature. Calcination at 700 degrees C produced cationic palladium with strong interaction with the YFeO3 structure but likely in a different coordination environment than in LaFeO3. Calcination at 800 degrees C provided improved crystallinity and forced Pd to form a PdO phase. The markedly different states of Pd are interpreted on the base of ionic radii arguments. The smaller size of Mn cations does not favor occupation of octahedral sites by the larger Pd3+. The observed states adopted by Pd strongly influenced methane oxidation activity. The highest catalytic activity was found for the samples where palladium was predominantly dispersed as supported PdO particles. Lowest catalytic activity was measured for samples containing octahedral Pd, thus confirming that Pd incorporation in the perovskite-type lattice may not be favorable for methane oxidation.