Catalytic combustion of methane over Pd/Ce–Zr oxides washcoated monolithic catalysts under oxygen lean conditions

Abstract

Pd-based monolithic catalysts were prepared using cordierites as supports and used as combustion catalysts for methane. The catalytic activity was further promoted by Ce–Zr oxides coatings. Oxides with different Ce/Zr ratios were synthesized via urea co-precipitation method and fully characterized. Catalytic performance of the monoliths was evaluated by oxygen-lean methane combustion through light-off experiments. Steep conversion curves were observed with oxygen completely depleted below 350 °C over all catalysts. Pd supported on ZrO₂ ignited the reaction at the lowest temperature while similar activities for total oxygen conversion were observed for Zr-rich monoliths. Zr-embedded oxides promoted the activity on oxygen consumption due to the interaction between Zr and Pd at the interface, dispersing and stabilizing Pd particles on the catalyst surface. The addition of Ce further promoted the stability of the catalysts. Compared with Pd/Zr, Pd/Ce1Zr2 showed little deactivation after three successive light-offs and maintained stability after 500 h time-on-stream test. Core/shell Pd particles model was proposed for the activity. Small Pd metal particles on the surface serve as an activation site for methane whereas wrapped PdO in the core stabilized by the Ce–Zr oxides acts as oxygen donor. The extent of reduction of Pd has significant effect on activity in the reaction conditions. Other impacting factors such as different oxygen concentrations and space velocities were also assessed to investigate the extent of the influence on methane catalytic combustion.