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-Ndicy/hNCNC SSC features the coordinated Pd by two pyridinic N and two pyrrolic N (PdN 2 py N 2 pr ). Without using dicyandiamide, the obtained Pd1/hNCNC SSC features the coordinated Pd by pyridinic N and C (PdN py C4−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 PdN 2 py N 2 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 PdN 2 py N 2 pr moiety presents a favorable desorption of ethylene. Consequently, the Pd1-Ndicy/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.