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Unveiling the Active Structure of Single Nickel Atom Catalysis: Critical Roles of Charge Capacity and Hydrogen Bonding
journal contribution
posted on 2020-03-11, 18:41 authored by Xunhua Zhao, Yuanyue LiuA single nickel atom embedded in
graphene is one of the most representative
single-atom catalysts, and it has a high activity and selectivity
for electrochemical CO2 reduction (CO2R) to
CO. However, the catalytic origin, especially the coordination structure
of Ni, remains highly puzzling, as previous density functional theory
(DFT) calculations showed that all the possible structures should
be inactive and/or nonselective. Here, using ab initio molecular dynamics (AIMD) and a “slow-growth” sampling
approach to evaluate the reaction kinetic barriers, we show that the
charge capacity (of the site) and hydrogen bonding (with the intermediates),
which were neglected/oversimplified in previous DFT calculations,
play crucial roles, and including their effects can resolve the catalytic
origin. Particularly, a high charge capacity allows the catalytic
site to carry more charges than required for the electrochemical step,
lowering the electrochemical barrier, and hydrogen bonding promotes
the reaction that produces polar intermediates by stabilizing the
intermediates and facilitating the H transfer from water, explaining
the high selectivity for CO2R over the hydrogen evolution
reaction. Consequently, we find that a hybrid coordination environment
(with one nitrogen and three carbon atoms) for the Ni-atom is most
active and selective for CO2R. Our work not only explains
a long-standing puzzle for an important catalyst but also highlights
the crucial roles of charge capacity and hydrogen bonding, which can
help elucidate the mechanisms of other heterogeneous electrocatalysts
in aqueous solution and enable more effective catalyst design.