Single Ni atoms and Ni₄ clusters have similar catalytic activity for ethylene dimerization

Atomic layer deposition (ALD) of Ni on the metal–organic framework NU-1000 has been shown to generate a material that serves as a catalyst for ethylene dimerization. However, the precise nature of the active catalytic site or sites remains uncertain. Here we employ periodic density functional calculations to characterize the structure and reactivity of the deposited species. Optimized lattice constants for a sequence of structures incorporating successively more Ni₄-hydroxo clusters in the c pore of NU-1000 show good agreement with experimental trends involving multiple ALD cycles; therefore we study the catalytic cycle for this cluster in detail, and we compare it to that for a site with only a single Ni atom. We find that both the atomic Ni catalyst and the Ni₄-hydroxo cluster have higher catalytic activity in the singlet state than in the triplet state. We also find that the two catalysts have very similar activity. Thus, precise size control of the active catalytic species is not essential for ethylene dimerization in this system.