Effect of oxygen species and catalyst structure on the performance of methane activation over Pd-Pt catalysts

This paper investigates C-H bond activation in methane over monometallic Pd, Pt and bimetallic Pd-Pt catalysts via a differential reactor, chemisorption system, HAADF-STEM, TPR and XPS methods. The results show that catalytic performances are strongly related to oxygen intermediates and particle structures. Variation of oxygen coverage could affect the rate-determining step of methane oxidation because Pd and Pt active sites are not both highly active in the same oxidative environment. When oxygen pressure decreases, lattice oxygen would transfer to the particle surface to increase the number of surface oxygen species. In addition, some large bimetallic particles have been observed to have core-shell structures after oxygen pulses. Pt species, due to their high free energy, migrate to the inner part of the bimetallic particles, forming Pt-Pd core-shell structural particles. Element distribution results show that some of the Pt atoms are dispersed on the support surface, forming small isolated clusters or single atoms. Pd species are mainly concentrated in large particles. The TPR and XPS results investigate surface oxygen and lattice oxygen and compare their variation with the Pd/Pt loading ratio. As Pd loading increases, the amount of surface oxygen increases significantly, which in turn improves the catalytic performance.