Structure-activity relationships and catalytic performances of Ag- and Zn-doped Cu/MOR for methane C−H bond activation

Abstract

Efficient activation of the C-H bond in methane (CH₄) is critical for achieving methane conversion at low-temperature. In this study, based on theoretical screening using the spin density (ρ) of active O atom as the activity descriptor, density functional theory (DFT) was utilized to investigate the adsorption of CH₄ and the mechanism of the subsequent C−H bond activation on Ag- and Zn-doped [Cu₃O₃]²⁺/mordenite (MOR) surfaces, as well as Zn-doped [CuOCu]²⁺/MOR surfaces, with high ρ values indicating high catalytic activation. The results demonstrated that CH₄ was physisorbed onto active O atom, followed by further activated to yield CH₃' radicals and surface OH groups via the radical reaction mechanism. Both Ag and Zn doping reduced the activation energy and reaction energy for the cleavage of the C−H bond in CH₄, promoting the C−H activation. Further electronic structure analysis revealed the spin polarization of electronic orbitals for both the active O atom and its bonded metal atoms. After Ag doping in [Cu₃O₃]²⁺/MOR, the formation of an Ag–O bond increased the gap between the spin-up and spin-down p-band centers of O 2p (Δε_p) and its spin density, which consequently enhanced the catalytic performance of the active O atom for C−H bond activation in CH₄. Zn doping into both [Cu₃O₃]²⁺/MOR and [CuOCu]²⁺/MOR also increased Δε_p. Specifically, when the Zn 4s and Zn 4p orbitals shifted downward to appropriate low energy levels, Zn 4s and Zn 4p interacted with O 2p to induce half-metallic characteristic in active O atom. This half-metallic characteristic exceptionally promoted C−H bond activation in CH₄, which was insufficiently predicted by the activity descriptors Δε_p and ρ.