Revealing and characterizing the
catalytic sites, along with elucidating
a convenient activity descriptor, can provide essential guidance in
determining efficient electrocatalytic catalysts for the CO2 reduction reaction (CO2RR). In this work, the mechanism
of CO2 reduction to methane (CH4) on 23 transition
metal-coordinated nitrogen-doped carbon M–N4–C
single-atom catalysts (SACs) was studied by density functional theory
calculations, a step forward to revealing the effects of the axial
O atom (M–N4O–C) on their catalytic activity.
The electrocatalytic reduction activity of CO2 over M–N4–C SACs is strongly dependent on the outmost d-shell
electron numbers and electronegativity of the selected metals. The
introduction of the axial O atom changes the coordination structure
of the central metal atoms, which not only improves the stability
of M–N4O–C SACs (especially electrochemical
stability) but also affects the adsorption strength of intermediate
species and then improves or reduces the catalytic activity, which
depends on the intrinsic properties of the metal atoms. More importantly,
by considering the comprehensive effects of the number of outmost
d-shell electrons, the electronegativity, coordinate numbers, and
bonding length of the central metal atom and the nearest neighbor
atom, a descriptor (φ) based on the intrinsic properties of
materials was developed to correlate the catalytic activity. The volcano-shaped
relationships between the φ and limiting potentials were well
established. In particular, five SACs (Mn–N4–C,
Cr–N4–C, Os–N4O–C,
Ru–N4O–C, and Rh–N4O–C)
close to the summit of the volcano were screened. Based on this descriptor,
the catalyst activity can be predicted directly from the characteristics
of the material instead of the expensive calculation of adsorption
energies. This work is of great significance for understanding the
mechanism of electrocatalytic CO2RR and the design of efficient
and stable electrocatalysts.