Silicalite-1 defect-trapped PdO clusters with long-term super-stability for catalytic oxidation of methane in wet conditions

Abstract

Development of a stable catalyst resistant to steam-induced sintering (10 vol%−15 vol% H₂O, 450–550 °C) is challenging for catalytic combustion/oxidation of methane. We use the silicalite-1's internal defects to trap PdO clusters to construct a stable catalyst (PdO/silicalite-1). Silicalite-1 with sufficient amounts of silanol defects and ion exchange at alkaline condition are critical for this defect-trapping strategy. PdO/silicalite-1 shows long-term super-stability with respect to the temperatures and steam concentrations. The conversion remained stable over 200 h at 500 °C (10 vol% H₂O) and declined slightly during 380 h at 550 °C (10 vol% H₂O). PdO/silicalite-1's catalytic activity deteriorated slightly after hydrothermal aging at 750 °C or thermal aging at 850 °C. Electron microscopy reveals two growth modes for PdO clusters within silicalite-1 (PdO_in) and on the external surface (PdO_surf). PdO_surf clusters agglomerate into PdO particles of 5–15 nm directly, but PdO_in clusters first fuse into strip-shaped PdO_in clusters, then migrate outwards and further agglomerate into PdO nanoparticles.