Figure 5 . (a) Charge density difference of the represented
N2 captured Mn2@site1and V2@site4 systems, the yellow and
green area stand for the charge depletion and accumulation, the
isosurface value is 0.05 Å3; the dark magenta and red
ball represent the Mn and V atoms. (b) the values of charges transfer
between the adsorbed N2 molecule and the substrate for
the Mn2@site1,
Co2@site2,
Ru2site1,
W2@site2,
Os2@site3,
V2@site4, and
Cr2@site4; (c)-(j) pDOS and COHP of N-N
bond for the selected N2 adsorbed
TM2@sitex (x = 1, 2, 3, 4) systems,
including Mn1@site1,
Co2@site1,
Ru2@site2,
W2@site2,
Os2@site3,
V2@site4, and
Cr2@site4, respectively.
Generally, several moieties, such as the adsorbed spices, active dual TM
sites, neighboring atoms bonded to the TM site, and the substrate
without TM and TM-bonded atoms, are common in most eNRR studies. But in
our systems, the TM sites are divided into the fundamental moiety of
intermediate spices (moiety 1) and the substrate without TM and
TM-bonded atoms (moiety 6). Four new moieties are implemented in our
systems, including moiety 2, moiety 3, moiety 4, and moiety 5. As
depicted in Figure 6(a) , to ensure the various roles of TM1 and
TM2 and the adjacent atoms affected by each TM atom. For the
Mn2@site1 system, all the intermediate
species gain electrons from the substrate except for the last species
(*NH3). Meanwhile, both TM atoms lose electrons after
adsorbing the N2 molecule except for the TM2 atom in
*N2 intermediate, leading to electrons transfer into the
adsorbed spices, similar to most studies in the eNRR field. However, we
note that moiety 4 and moiety 5 gain electrons, where the extra
electrons may come from the substrate or the TM atoms, but considering
the substrate gains electrons in 3 to 7 protonation process, moiety 4
and moiety 5 gain electrons from the TM atoms in 3 to 7 protonation
steps reasonably. In contrast, the 1 to 3 steps remain uncertain.
Besides, it is notable that each TM atom and its adjacent atoms show
differential charge transfer for the whole hydrogenate steps, mainly
from the fully heteronuclear of two homonuclear TM atoms. For a similar
end-on configuration Co2@site2 system,
the charge distribution changes significantly; moiety 1 obtains
electrons for the first six proton steps and loses electrons for the
last step. For Moiety 4 and 5, slight charge transformation in moiety 4
while huge electron transferred into moiety 5, demonstrate each TM atom
act in various roles. Meanwhile, all moiety 2, moiety 3, and moiety 6 as
donors donate electrons to moiety 1, moiety 4, and moiety 5, causing the
TM atom to act as the electron donor and carriers among the eNRR process
for the Co2@site2 system. Charge
transformation between various moieties becomes more complex for the
side-on configuration N2 adsorbed systems. Taking
V2@site4 and
Cr2@site4 as examples, both moiety 2 and
moiety 3 lose electrons for the whole interaction. Meanwhile, moiety 6
loses electron less than TM1 and TM2 slightly. In contrast, moiety 4 and
moiety 5 obtain most of the electrons compared with moiety 1, suggesting
that TM-bonded atoms obtain electrons from both TM atoms and moiety 6,
while the electron transfer into moiety1 only originates from the TM
atoms. Therefore, both TM atoms act as two roles during the
electrocatalytic process; that is, both the TM atoms lose electron and
donated them to the adjacent atoms (either adsorbed species or TM-bonded
atom in the substrate), which is inversed to most of the eNRR studies,
where the TM atoms act as charge carrier among most studies. The primary
origin may be due to the additional TM atom participating in the eNRR
process and offering more electrons. Those may explain why heteronuclear
DAC is more effective than homonuclear DAC for the eNRR.