Influence of the synthesis method on the structure of Pd-substituted perovskite catalysts for methane oxidation

Flame spray synthesis (FSS), ultrasonic spray combustion (USC) and solution combustion synthesis (SCS) were selected for the preparation of Pd-substituted perovskite-type oxides and were compared with the citric acid method (CM). La(Fe,Pd)O3 and La(Mn,Pd)O3 catalysts were characterized using N2-adsorption (BET method), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The structure of Pd was characterized using X-ray absorption near edge structure (XANES) spectroscopy: Pd in octahedral coordination can be clearly distinguished from PdO-like species from the shape of the whiteline. Except for SCS samples, the BET surface area of all samples varied between 12 and 39 m2/g. In the case of LaFeO3±δ, only CM and USC methods provided Pd in octahedral coordination, whereas a mixed Pd2+/Pd0 oxidation state was obtained from FSS and SCS methods. On the contrary, Pd supported on LaMnO3±δ was obtained irrespective of the synthesis method. As a consequence, the peculiar structure of Pd obtained from each synthesis method influenced the catalytic activity for methane oxidation. The characterization and catalytic data confirm that the coordination of Pd depends on perovskite composition and suggest that it is also dependent on the synthesis method.