Impact of Pd single-site coordination structure on catalytic performance for semihydrogenation of acetylene

Semihydrogenation of trace acetylene in an ethylene gas stream is a vital step for the industrial production of polyethylene, in which Pd single-site catalysts (SSCs) have great potential. Herein, two Pd SSCs with different coordination structures are prepared on hierarchical nitrogen-doped carbon nanocages (hNCNC) by regulating the nitrogen species with or without using dicyandiamide. With using dicyandiamide, the obtained Pd₁-N_dicy/hNCNC SSC features the coordinated Pd by two pyridinic N and two pyrrolic N ( ${\text{PdN}}_{\text{2}}^{\text{py}}{\text{N}}_{\text{2}}^{\text{pr}}$). Without using dicyandiamide, the obtained Pd₁/hNCNC SSC features the coordinated Pd by pyridinic N and C ( ${\text{PdN}}_x^{\text{py}}{\text{C}}_{\text{4}-x}$, x = 1–4). The former exhibits an 18-fold increase in catalytic activity compared to the latter. Theoretical results reveal the abundant unoccupied orbital states above the Fermi level of ${\text{PdN}}_{\text{2}}^{\text{py}}{\text{N}}_{\text{2}}^{\text{pr}}$ moiety, which can facilitate the activation of substrate molecules and dynamics of acetylene hydrogenation as supported by the combined theoretical and experimental results. In addition, the ${\text{PdN}}_{\text{2}}^{\text{py}}{\text{N}}_{\text{2}}^{\text{pr}}$ moiety presents a favorable desorption of ethylene. Consequently, the Pd₁-N_dicy/hNCNC SSC exhibits high C₂H₂ conversion (99%) and C₂H₄ selectivity (87%) at 160 °C. This study demonstrates the impact of Pd single-site coordination structure on catalytic performance, which is significant for the rational design of advanced Pd SSCs on carbon-based supports.