Copper Catalysts in Semihydrogenation of Acetylene: From Single Atoms to Nanoparticles

Metal particle size plays essential roles in metal-catalyzed heterogeneous reactions. Reducing the particle size from a few nanometers down to atomically dispersed single atoms alters both the morphology and electronic property of the metal enormously, thus greatly modulating its catalytic performance. Detailed investigation of the particle size effect as well as taking account of a metal single-atom catalyst (SAC), the frontier in heterogeneous catalysis, can provide a deeper insight into structure–activity relations and facilitate the rational design of advanced metal catalysts. However, such studies have been rarely reported. Herein, Cu single atoms and nanoparticles with different sizes of about 3.4, 7.3, and 9.3 nm were synthesized on an alumina support using atomic layer deposition. Comprehensive microscopic and spectroscopic characterization shows that the Cu single atoms remain very stable and have 1+ valence state after reduction at 300 °C in hydrogen. In semihydrogenation of acetylene in excess of ethylene, we show that a decrease in the Cu particle size reduces the activity considerably but gradually improves both the ethylene selectivity and durability. In particular, the Cu SAC exhibits the highest ethylene selectivity of 91% at the complete conversion along with excellent long-term stability for at least 40 h, in sharp contrast with the rapid deactivation on Cu nanoparticle catalysts. In situ thermogravimetry measurements further reveal that coke formation on Cu1 SAC is significantly suppressed by up to ∼89% compared to that on the 9.3 nm Cu nanoparticle catalysts. In brief, our findings demonstrate that SACs can be promising candidates for selective hydrogenation reactions in terms of high selectivity and high coking resistance.