Synthesis of a Highly Stable Pd@CeO<sub>2</sub> Catalyst for Methane Combustion with the Synergistic Effect of Urea and Citric Acid

Making use of synergy between urea and citric acid, a core–shell Pd@CeO<sub>2</sub> catalyst with spherical morphology was facilely synthesized by a hydrothermal method. The formation mechanism of the core–shell structure in the presence of citric acid and hydrogen peroxide was studied. Results showed that the Pd@CeO<sub>2</sub> catalyst exhibited high catalytic activity in methane oxidation. Pd nanoparticles were well stabilized by CeO<sub>2</sub> shell encapsulation, resulting in high stability of the catalyst. A high CH<sub>4</sub> conversion of 99% was retained after 50 h on-stream reaction at 500 °C. Additionally, many tiny pores on the CeO<sub>2</sub> shell surface were beneficial for the full contact between reactants and active components. Pd nanoparticles were highly dispersed inside the shell, improving the utilization efficiency of active components. The results also demonstrated that the Pd species in the catalyst existed in the form of oxidation state, mainly in PdO (ca. 66.6%), which played an essential part in methane combustion.