Efficient Oxygen Cycle over PdO-Based Catalysts Induced by Highly Dispersed Zinc Species for Methane Combustion

Abstract

PdO-based catalysts have been the benchmark for methane combustion but they still present insufficient catalytic reactivity and stability mainly due to the difficult cycling of reactive oxygen species. In this work, PdO nanoparticles modified with highly dispersed zinc species supported on alumina (PdZn/Al₂O₃) were fabricated by an in situ liquid-phase reduction technique. Systematic characterization and DFT calculation results illustrated that zinc oxide enabled an efficient cycle of reactive oxygen species within PdO particles to facilitate methane combustion. Typically, Zn donated rich electrons to the surface lattice oxygen (O_latt) of PdO, thus generating highly reactive O_latt coordinated with both Pd and Zn (Pd–O_latt–Zn). This catalytically active structure allowed a lower energy barrier for the methane oxidation reaction on PdZn/Al₂O₃ (1.36 eV) than on Pd/Al₂O₃ (1.58 eV). The consumed active oxygen within Pd–O_latt–Zn was easily replenished by dissociating molecular oxygen, along with smooth circulation of reactive oxygen species obeying the MvK pathway. As anticipated, PdZn/Al₂O₃ with only 0.38 wt % Pd loading presented higher low-temperature reactivity and durability than Pd/Al₂O₃. In-depth mechanistic investigations revealed that reactive oxygen species were produced more smoothly over PdZn/Al₂O₃ to further dissociate CH₄ and favored the generation of more active formate and carbonate intermediates, thus contributing to its superior catalytic activity. Besides, Zn modification helped mitigate the transition of PdO to inactive Pd(OH)₂ by hindering H₂O adsorption and dissociation, resulting in enhanced water tolerance performnce. Such a strategy for establishing an efficient oxygen cycle on Pd catalysts could guide the development of highly efficient catalysts for methane combustion.