Partial Thermal Condensation Mediated Synthesis of High-Density Nickel Single Atom Sites on Carbon Nitride for Selective Photooxidation of Methane into Methanol

Abstract

Direct selective transformation of greenhouse methane (CH₄) to liquid oxygenates (methanol) can substitute energy-intensive two-step (reforming/Fischer-Tropsch) synthesis while creating environmental benefits. The development of inexpensive, selective, and robust catalysts that enable room temperature conversion will decide the future of this technology. Single-atom catalysts (SACs) with isolated active centers embedded in support have displayed significant promises in catalysis to drive challenging reactions. Herein, high-density Ni single atoms are developed and stabilized on carbon nitride (NiCN) via thermal condensation of preorganized Ni-coordinated melem units. The physicochemical characterization of NiCN with various analytical techniques including HAADF-STEM and X-ray absorption fine structure (XAFS) validate the successful formation of Ni single atoms coordinated to the heptazine-constituted CN network. The presence of uniform catalytic sites improved visible absorption and carrier separation in densely populated NiCN SAC resulting in 100% selective photoconversion of (CH₄) to methanol using H₂O₂ as an oxidant. The superior catalytic activity can be attributed to the generation of high oxidation (Ni^III═O) sites and selective C─H bond cleavage to generate •CH₃ radicals on Ni centers, which can combine with •OH radicals to generate CH₃OH.

Keywords: Ni single atom catalysts; carbon nitride; heterogeneous catalysts; methane oxidation; photocatalysis.