4.1 Modulating the N coordination
Controlling the coordination environment of metal sites to enable the
electronic modulation of active center is crucial for constructing
high-efficiency catalysts and exploring the reaction mechanism of
ECR.[83-85] In particular, the coordination
mechanism of single atoms with different N numbers and types needs to be
clarified.
In view of above analysis, Jiang’s group developed a general host-guest
co-protection strategy to synthesize Ni single atom catalysts
(NiSA-Nx -C) with different
N-coordination numbers by introducing polypyrrole (PPy) into the
metal-organic bimetallic framework (PPy@MgNi-MOF-74) (Figure
5 a).[86] Mg2+ ions create
separation space for Ni atoms, and PPy as N source prevents the
aggregation of separated Ni atoms. The experimental results indicate
that the N coordination number of single-atom Ni sites for target
catalyst exhibits an unexpected effect on the ECR performance. The
NiSA-N2-C structure with the lowest N
coordination number demonstrates higher FECO (98%)
(Figure 5b) and TOF value (1622 h−1) (Figure 5c),
significantly superior to the ECR activity of
NiSA-N3-C and
NiSA-N4-C. The theoretical calculation
reveals that NiSA-N2-C structure
displays lower free energy of forming COOH* and CO* intermediates
(Figure 5d), compared with NiSA-N3-C and
NiSA-N4-C.
Cheng et al. prepared atomically dispersed Ni-based catalysts with
different N/C coordination numbers (named
Ni@Nx Cy ) by regulating
pyrolysis temperature (Figure
5e).[87] The fitting results of extended X-ray
absorption fine structure (EXAFS)
testify that when the pyrolysis temperature increases from 800 °C to
1100 °C, the N coordination number of
Ni@Nx Cycatalyst decreases from 4 to 1, while the C coordination number
increases from 0 to 3 (Figure 5f). From the activity evaluation of
Ni@Nx Cy samples, it was
found that
Ni@N2C2-1000
catalyst with the 2 N and 2 C atoms coordinated Ni SA displayed the
optimal ECR activity with the highest 98.7% FEco at
−0.7 V. DFT calculations indicate
that the Ni-N2C2 active site is
conducive to the generation of more unfilled Ni 3d orbitals to present
the lowest free energy barrier (~0.80 eV) for
rate-determining step (Figure 5g). Furthermore, the lower differences in
the limiting potentials of ECR and HER for
Ni-N2C2 active site is well-consistent
with the optimal ECR selectivity of
Ni@Nx Cy -1000 (Figure 5h).
This work reveals the interior relationship between intrinsic activity
and coordination environment over M-N-C catalyst, and provides a
feasible method for improving catalytic activity.
In addition, as described in Section 3.2 , Sun conducted an
in-depth analysis of the Ni and N coordination structures —
Ni@N3 (pyrrole) as
the active center.[65] The DFT result demonstrates
that Ni@N3 (pyrrole), as the main active center of ECR,
benefits for the formation of *COOH intermediates and desorption of *CO.
The free energy of Ni@N3 (pyrrole), lower than that of
Ni@N4, promotes the conversion and selectivity of
CO2.