Directional Construction of the Highly Stable Active-Site Ensembles at Sub-2 nm to Enhance Catalytic Activity and Selectivity.

Precise control over the size, species, and breakthrough of the activity-selectivity trade-off are great challenges for sub-nano non-noble metal catalysts. Here, for the first time, a "multiheteroatom induced SMSI + in situ P activation" strategy that enables high stability and effective construction of sub-2 nm metal sites for optimizing selective hydrogenation performance is developed. It is synthesized the smallest metal phosphide clusters (<2 nm) including from unary to ternary non-noble metal systems, accompanied by unprecedented thermal stability. In the proof-of-concept demonstration, further modulation of size and species results in the creation of a sub-2 nm site platform, directionally achieving single atom (Ni 1 ), Ni 1 +metal cluster (Ni 1 +Ni n ), or novel Ni 1 +metal phosphide cluster synergistic sites (Ni 1 +Ni 2 P n ), respectively. Based on thorough structure and mechanism investigation, it is found the Ni 1 +Ni 2 P n site is motivated to achieve electronic structure self-optimizing through synergistic SMSI and site coupling effect. Therefore, it speeds up the substrate adsorption-desorption kinetics in semihydrogenation of alkyne and achieves superior catalytic activity that is 56 times higher than the Ni 1 site under mild conditions. Compared to traditional active sites, this may represent the highly effective integration of atom utilization, thermal stability, and favorable site requirements for chemisorption properties and reactivities of substrates.