A general strategy for preparing pyrrolic-N4 type single-atom catalysts via pre-located isolated atoms

Single-atom catalysts (SACs) have been applied in many fields due to their superior catalytic performance. Because of the unique properties of the single-atom-site, using the single atoms as catalysts to synthesize SACs is promising. In this work, we have successfully achieved Co1 SAC using Pt1 atoms as catalysts. More importantly, this synthesis strategy can be extended to achieve Fe and Ni SACs as well. X-ray absorption spectroscopy (XAS) results demonstrate that the achieved Fe, Co, and Ni SACs are in a M1-pyrrolic N4 (M= Fe, Co, and Ni) structure. Density functional theory (DFT) studies show that the Co(Cp)2 dissociation is enhanced by Pt1 atoms, thus leading to the formation of Co1 atoms instead of nanoparticles. These SACs are also evaluated under hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), and the nature of active sites under HER are unveiled by the operando XAS studies. These new findings extend the application fields of SACs to catalytic fabrication methodology, which is promising for the rational design of advanced SACs.


Note:
The Pt L3 edge XANES result of Pt1/NCNS is distinct from the Pt foil, it shows a very intense high white line (WL) at higher photon energy compared to that of Pt foil, indicating Pt is at a high oxidization state with a significant increase in 5d hole counts compared to Pt metal (Supplementary Fig. 3a). It is established that the area under the WL curve is proportional to the total unoccupied state of Pt 5d orbitals which also can reflect the oxidation state of Pt1 atoms 1 . After quantitative analysis of the WL area, we found that the average oxidation state of Pt1 atom is around +2.4, indicating the Pt could bond with two N atoms (Supplementary Fig. 3a

Note:
As the Co1Pt1/NCNS exhibits the best catalytic activity, we have taken it as a typical example for the further theoretical understanding of the interaction of Co1 and Pt1 in the HER. 1) Considering the Co1Pt1/NCNS is in a single-layered structure, we have adjusted the distances between Co1 and Pt1 single atoms to investigate the different catalytic activity of Co1 single atoms in HER (Supplementary Figs. 22a to e). However, the hydrogen adsorption free energies (ΔGH) at T = 298.15K and P = 1atm are almost the same, despite different distances between Co1 and Pt1 single atoms, indicating the interaction of Pt1 and Co1 atoms is less pronounced in the single-layered structure. 2) According to the literature, the interaction between two single atoms in the doublelayered structure could promote catalytic performance 4,5 . We further consider the bilayer structure in our system and found the interactions between Co1 and Pt1 in each layer are strongly enhanced, with the bond distance of Co-Pt decreased to 2.91 Å. The ΔGH of Co1 is -0.13 eV on the double-layered model, which is much lower than that in the single-layer structure (0.23 eV) (Supplementary Fig. 22f).

Supplementary Fig. 23
Top and side view of the calculated charge distribution of single (a) and double (b)-layered Co1Pt1/NCNS. Yellow and cyan iso-surface represent electron accumulation and electron depletion, respectively. The white, blue, brown, orange, and silver spheres represent H, N, C, Co, and Pt, respectively.

Note:
We have compared the charge distributions of single/double layered Co1Pt1/NCNS (Supplementary Figs. 23a and b). As shown in Supplementary Fig. 23, the charge polarization on double-layered Co1Pt1/NCNS shows obvious differences between single and double-layered models. Differing from the single-layered structure, the electron accumulation is strongly enhanced by the interlayer on the double-layered Co1Pt1/NCNS 6 . As a result, the adsorption of protons is facilitated (Supplementary Fig.  22f). Therefore, the Co1Pt1/NCNS shows better performance than the Pt1/NCNS in HER could be likely due to the extra contribution of Co1 pyrrolic-N4 sites in both single/double-layered structures.
Supplementary Fig. 24 The durability test of Co1Pt1/NCNS without IR correction at the sweep voltage between -0.1 and 0.4 V at a scan rate of 0.1 V s -1 in 0.5 M H2SO4.

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The OER occurs at alkaline conditions via the following steps: The durability test of Ni 1 Pt 1 /NCNS at a constant current density of 10 mA/cm 2 .

Note:
Although it has been reported in the literature that the pyridinic-N4 type non-noble metal SACs exhibited high catalytic performance in OER, the investigation of pyrrolic-N4 type SACs is rarely studied 7 . Note that the axial O2 molecules are not considered in the theoretical calculation because of the O2 evolution process. According to the literature, the d-band center near the Fermi level is linked with the adsorption of reactants 8 . As shown in Fig. 6a and Supplementary Table 4, the d-band center of metal atom in M1Pt1/NCNS increases from Ni to Fe, indicating there would be much stronger adsorption of oxygen species on Fe1 atom than that on Ni1 and Co1 atoms.
Based on previous work 9 , we theoretically investigated the OER mechanism on M1pyrrolic N4 (M = Co, Fe, and Ni) catalysts by considering the adsorption of *OH, *O, *OOH, and *OO intermediates (Supplementary Fig. 25), with the Gibbs free energy diagram calculated at T = 298.15K and P = 1atm shown in Supplementary Fig. 26a Fig. 26c). More importantly, the Ni1Pt1/NCNS exhibits significant stability, without showing an obvious activity decrease after 10 h at a current density of 10 mA/cm 2 in durability testing ( Supplementary Fig. 26d).