3.4 Origin of high catalytic activity on TM2@sitex (x = 1, 2, 3, 4)
The intrinsic electronic properties of the atomic diversity free dual TM atom catalyst, including charge density difference, Bader analysis, projected density of state (pDOS), and COHP, are systematically investigated for the dual TM atom cooperative eNRR process. We primarily focus on charge distribution for the selected candidates. Typically, the charge density difference of end-on and side-on configurations of the screened seven systems is presented by Mn2@site1 and V2@site4. The charge density difference of the representative Mn2@site1 and V2@site4 meets the results analyzed previously, guaranteeing the accuracy of our analysis. Quantitively, charges back to the captured N2 from the substrate are insured, as shown in Figure 5 (b), meeting the analyzed ’backdonation’ process. Next, the pDOS of the N2adsorbed systems and the COHP of the N2 molecule are explored to determine adsorption and activation behavior. As plotted inFigure 5 (b)-(g), the pDOS of the d orbital for each TM atom (colored dark and blue) are shown for comparison; meantime, the orbital of adsorbed N2 is also considered for analyzing the hybridization between d orbital of TM atom and the orbital of N2. As it can be seen, for all the selected systems, TM1 and TM2 exhibit observable variety d orbital of the pDOS, further revealing the intrinsic properties’ variety induced by coordinate environment change. Besides, the electron orbital of N2 is highly hybridized with the d orbital of dual TM atoms. For the Mn2@site1 system, the Mn1 atom is the main contributor near the Fermi level, where the d orbital of Mn1 acts with the 3σ of the N2 molecule; the primary reason is that N2 adsorbed on the Mn1 site, which is suitable for charge transfer. For the deep occupied orbital, the d orbital of Mn1 and Mn2 significantly contribute to N2 activation gradually. In contrast, the d orbital of Mn2 becomes a deeper occupied state since it is far from the Fermi level. Those results demonstrated that both Mn atoms gain electrons in the Mn2@site1system, and the d orbital of Mn2 gains more electrons from the substrate. Meantime, both Mn atoms take up the unoccupied state above the fermi level, indicating that both Mn atoms lose electrons as well, which meet the analyzed ”donate-back donate”19mechanism. In addition, the d orbital electron of the Mn1 atom overlaps with N2 more than Mn2 atoms, where the main reason is that the N2 molecule bonds with Mn1, but it can’t be ignored that the Mn2 offers an extra force to attract the N2 molecule, thus explain why both Mn atom bond with N2 in the hydrogenation process.
To better explore the activation of the N2 molecule, COHP analysis is employed to illustrate the bond nature of adsorbed N2; the injected electrons from the Mn2@site1 lower the occupied 3σ orbital of N2, leading the antibonding state occurred near the fermi energy, thus weakening the N-N triple bond, and further confirmed by integrated COHP (-ICOHP) (-ICOHP = -18.40). Similarly, For the Co2@site2 via end-on configuration, the interaction between N2 and Co atoms are weaker than Mn2@site1 systems, which mainly originate from fewer electron transfer among them and the little activation of N-N molecule, as confirmed by the Bader analysis (nearly 0.36 |e|) and ICOHP (-20.80). Still, both Co atoms contribute to the activation of N2 than the Mn atom, which can be seen from the pDOS hybridization. For the side-on configuration, including Ru2@site1, W2site2, Os2@site3, V2@site4, and Cr2@site4 system, the interaction between N2 and TM active sites is more significant than the end-on configuration. Quantitively, the side-on configuration’s charge transfer between N2 and substrate range from 0.66 |e| to 0.96 |e| is larger than that of the end-on configuration (range from 0.36 |e| to 0.44 |e|), as shown in figure 5(b) . What’s more, the orbital hybridization between N2 and TM atoms is not limited to the 3σ orbital of N2 but expands to 2π and 2σ* orbitals of N2 since the co-adsorption of N2 for both TM atoms. This led to the full activation of the N2 molecule, and it is also observed in COHP analysis, where the ICOHP values increased to -18.15, -16.18, -19.08, -10.99, and -17.99 for Ru2@site1, W2@site2, Os2site3, V2@site4, and Cr2@site4, respectively. The main reason is the cooperation of the two TM atoms, which offer an intrinsic force to pull the triple bond of N-N. Still, the cooperation effect is also observed in the hydrogenation process for the end-on configuration, thus lowering the Gibbs free energy barriers under the protonation process. Therefore, for both side-on and end-on configurations, though the side-on configuration takes more advantages than the end-on ones for the N2 activation and the *N2 to *N2H hydrogenation, the cooperation during the protonated spices is also an important factor for whole eNRR, which should be comprehensively studied in the future investigations. Hence, we explore the intermediate spices to illustrate the intrinsic mechanism further. As an excellent descriptor in the eNRR field, the charge transformation has been widely employed to dig out intrinsic mechanical properties for the intermediate steps 72, 73.