Improvement of Catalytic Activity of LaFe0.95Pd0.05O3 for Methane Oxidation under Transient Conditions

Abstract

The catalytic performance of methane oxidation catalysts, i.e., LaFe0.95Pd0.05O3, 2 wt % Pd/LaFeO3, and 2 wt % Pd/Al2O3, has been compared at 500 °C under periodic red-ox conditions. The state of palladium has been followed under operando conditions using XANES and QEXAFS at the Pd K-edge combined with mass spectrometry (MS) for product detection. Online MS data reveal that in correspondence to every change of feed composition (O2 pulses) CO2 production is enhanced over LaFe0.95Pd0.05O3 that is associated with a drop in methane concentration. This is not the case for the samples where PdO nanoparticles are deposited on the support material (Pd/LaFeO3 and Pd/Al2O3). The time-resolved QEXAFS spectra have been treated with a modulation excitation spectroscopy approach. Phase sensitive detection (PSD) enabled to highlight the subtle changes in the whiteline region. Continuous reduction−oxidation of Pd occurs in all samples at every change of feed composition. However, by this process Pd in LaFe0.95Pd0.05O3 reversibly emerges on the LaFeO3 surface under reducing conditions and enters the LaFeO3 structure under oxidizing conditions. On the contrary, Pd oscillates between the reduced and partially oxidized state in Pd/Al2O3 and Pd/LaFeO3 in which well-defined Pd nanoparticles are already available. This structural difference is responsible for the activity enhancement in correspondence of each switch and is attributed to the self-regenerative property of perovskite-type oxides.