Mitigating methane emissions: Analyzing the microstructural evolution of Pd-based membranes under lean methane oxidation conditions

Lean methane emissions from natural gas-powered vehicles and coal mining pose significant ignition challenges, exacerbating greenhouse effects. Palladium (Pd)-based catalysts show promise for effectively utilizing lean methane, addressing both energy and environmental issues. However, the high cost and limited availability of Pd present sustainability challenges, driving extensive research efforts to minimize Pd usage while maintaining catalytic efficiency. Magnetron sputtering technology offers an innovative approach to fabricating catalytic membranes with minimal Pd consumption and high purity. Nevertheless, these membranes are sensitive to reaction environments, which can lead to morphological and structural changes. A comprehensive analysis of microstructural evolution is therefore crucial for understanding catalyst deactivation mechanisms and for developing stable, high-performance catalysts. In this study, an ultra-low loading Pd-Al2O3 membrane was prepared using magnetron sputtering. The catalytic activity results demonstrated the system's strong capability for processing lean methane. Additionally, the microstructural and phase evolution of the catalyst during lean methane oxidation was characterized using inductively coupled plasma-mass spectrometry, digital microscopy, scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The findings revealed that the Pd particle size initially decreased before increasing with reaction cycles, which was accompanied by a rise in surface oxygen species and the thickening of the Pd oxide overlayer. These factors were found to significantly influence the catalytic activity of the membranes. This study establishes a clear relationship between the catalytic performance of Pd-based membranes during methane oxidation and their microstructural changes, offering valuable insights for the development of high-stability catalysts with low Pd usage.