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.