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.