Acetylene Semihydrogenation over the Flexible Heptazine-Based g-C3N4 Monolayer with Embedded Single Atoms (SA = Cu, Ag, Au, Ni, Pd, Pt)

Abstract

Graphitic carbon nitride (g-C₃N₄) is a highly effective support for single-atom catalysts in semihydrogenation of alkynes, an important industrial reaction. However, the understanding of the synergistic effects between the transition metal single atoms (TM-SAs) and the support and reaction mechanisms remains limited. In this work, the mechanism of acetylene semihydrogenation over transition metal (TM) single-atom embedded in heptazine-based g-C₃N₄ monolayer catalysts (TM₁/H-g-C₃N₄, TM = Cu, Ag, Au, Ni, Pd, and Pt) is studied using density functional theory (DFT) calculations and microkinetic simulations. The results reveal that the frustrated Lewis pairs (FLPs) formed between TMs and the heptazine N of H-g-C₃N₄ exhibit high activity in promoting the heterolytic cleavage of H₂. The hydrogenation of the key intermediate C₂H₃*(M) benefits from a synergistic interaction between the H-g-C₃N₄ support and the TM, where the slight distortion of the flexible H-g-C₃N₄ monolayer facilitates the direct addition of H* from the nitrogen vacancy to C₂H₃*(M), thus lowering the energy barrier. By comparing the energetic span (δE) and selectivity parameter (ΔE_s), it is found that Ag₁/H-g-C₃N₄ exhibits the best selectivity for acetylene semihydrogenation among the catalysts studied. Microkinetic simulations further show that Ag₁/H-g-C₃N₄ can achieve a high C₂H₄ yield and selectivity even under mild reaction conditions (503 K and 1 bar). These findings provide valuable insights into the synergistic catalytic effects between transition metal single atoms and supports, offering a theoretical foundation for the design of two-dimensional single-atom catalysts.