Figure 3. (a) Gibbs free energy changes for the
N2 adsorbed (*N2 →
*N2H), (b) Gibbs free energy changes for the first
protonate step (*N2 → *N2H), (c) Gibbs
free energy changes for the last protonate step (*NH2 →
*NH3), (d)-(g) Features importance of the GBR model
precited for Gibbs free energy changes of ΔG*N2 (end
on), ΔG*N2 (side on), ΔG*N2→*N2H, and
ΔG*NH2→*NH3, respectively, where Ne,
ra, θd, rTM, χ,
Ei, rd, m, EA are the
outmost electron number, the average bond length between TM atom and the
coordinated C and N atom, d electron count, bond length between TM
atoms, electronegativity, first ionization energy, radus of TM atoms,
mass of TM atoms, electron affinity, the values of the parameters are
summed in Table S8 and Table S9 .
In brief, several potential candidates are selected to show excellent
electrocatalytic performance due to their proper N2capture energy small Gibbs free energy changes of the first and last
hydrogenate steps. It is worth noting that the relatively unimportant
structural properties, such as rTM, ra,
and rd, play the predominant role in the
N2 capture hydrogenation process even though the
structural variety is entirely removed. In fact, those factors do not
contradict the proposed atomic diversity effect since the objects of
rd are the same. Hypothetically, we think the distance
between TM atoms may play an essential role during the hydrogenation
process since both ra and rd are highly
related to the rTM, which should be further
investigated.
To accurately obtain certain steps determining the NRR process, the
whole Gibbs free energy changes are systematically investigated for the
selected homonuclear systems. The complete reaction contains six
proton-couple electron transfer steps (N2 +
3H2 → 2NH3), making several possible
intermediates (*N2H,
*N2H2,
*N2H3,
*N2H4, *N, *NH, *NH2,
*NH3) existed under the electrocatalytic process. The
difference in the N2 configurations leads to various
reaction pathways for the eNRR. To obtain detail information of the NRR
on TM2@sitex (x = 1, 2, 3, 4), the
traditional distal, alternative, and enzymatic reaction pathways are
selected for limiting potential searching. Besides, we expand a new
reaction pathway for the end-on configuration systems for the confirmed
’capture-charge distribution-recapture-charge redistribution’ mechanism
and name it Distal-1, as depicted in Figure 4 (a) . For the
distal and alternative mechanisms, we do not have much discussion on
them since a single TM atom participated predominately in the
electrocatalytic NRR process, which does not efficiently reflect the
synergistic effect for DAC. Among the screened systems,
Ni2@site4 and
Re2@site4 follow the distal and
alternative reaction pathway, and the limiting potentials are -2.69
V/-2.31 V and -0.61 V/-1.47 V, respectively. For the highlighted
distal-1 and enzymatic reaction pathway, the synergistic effect between
dual TM atoms varies with each other. But both reaction pathways enlarge
the charge distribution asymmetry under the ’capture-charge
distribution-recapture-charge redistribution’ mechanism, thus
accelerating the hydrogenation process. Among the rest systems,
Mn2@site1 and
Co2@site2 follow the Distal-1 reaction
pathway, while Ru2@site1,
W2@site2,
Os2@site3,
V2@site4,
Cr2@site4, and
W2@site4 go through the enzymatic
reaction pathway. Briefly, Co2@site2,
V2@site4, and
Cr2@site4 are selected as examples for
analysis. For the Co2@site2 system,
Gibbs free energy change is -0.32 eV after N2 capture,
within N-N bond length elongated to 1.15 Å; continuously, the
*N2 to *N2H need an energy input of 0.44
eV, within the N-N bond increase to 1.25
Å and the second TM atom
participated to bond with the proximal N atom. Then, the distal N atom
keeps being attacked by the second proton-electron pair to form
*N2H2 with an energy input of 0.08 eV,
and the N-N bond expands into 1.33 Å, demonstrating the further
activation of the N2 molecule. After the third proton
hydrogenation, the first NH3 is formed and desorbed away
from the adsorbed system, leaving a bald N atom adsorbed on the active
site, and the energy change of this process is -0.87 eV. Subsequently,
the proton-electron pair continues to attack the isolate N atom until
the second NH3 form, within energy changes of -0.30 eV,
-0.70 eV, and +0.35 eV for *N → *NH, *NH → *NH2, and
*NH2 → *NH3, respectively. Therefore,
among the six electron-participated intermediates, the maximum values of
the input energy are 0.44 eV during the first protonation process; thus,
the *N2 to *N2H is the potential
determining step for the Mn2@site1system via end-on configuration, and the limiting potential is -0.44 V,
as depicted in Figure 4(b) and Figure S8 .
For the enzymatic mechanism under the cooperation effect between two TM
atoms in atomic diversity free DAC, as depicted in Figure 4(a) ,
the H+ + e- pair initially
protonates the distal-like adsorbed N atom as confirmed former, then the
second H+ + e- pair points out to
protonate the other N atom. Gradually, the additional
H+ + e- pairs from the solution
continue to hydrogenate the protonated N atoms until all the
NH3 desorption. As shown in Figure 4 for
V2@site4 and
Cr2@site4 systems, N2 is
spontaneously captured by the active site with side-on configuration
within adsorption energy changes of -0.83 eV and -0.49 eV. Meantime, the
N-N bonds increase to 1.22 Å and 1.21 Å, respectively. Then the
initially activated N2 is attacked by the first
H+ + e- pair, forming *N-*NH spices,
where the Gibbs free energy climbs to -0.55 eV and -0.19 eV, and the N-N
bonds increase to 1.33 Å and 1.30 Å. The second H+ + e- pair
successfully turns to hydrogenate the other side N atom, generating
*NH-*NH with the Gibbs free energy changes of 0.91 eV and 0.23 eV.
Consequently, the third and fourth H+ +
e- pair continue to attack the formed *NH-*NH to
generate *NH-*NH2 and
*NH2-*NH2, causing the breakdown of the
N-N triple bond. At the same time, the changes of Gibbs free energy are
0.18 eV and -0.58 eV, meaning that a 0.18 eV energy input is enough for
the N-N bond breaking for V2@site4.
Excitingly, the N-N broken energy of the
Cr2@site4 system is -0.58 eV, suggesting
the N-N bond’s spontaneous breaking. Finally, for the fifth and sixth
protonation, the H+ + e- pair keeps
attacking both isolated *NH2, causing the formation of
*NH3. For this process, the Gibbs free energy climb step
by step for both V2@site4 and
Cr2@site4 systems, where the energy
inputs are 0.19 eV and 0.05 eV for fifth step and 0.18 eV and 0.19 eV
for sixth step, respectively. For the desorption of
*NH3, the energy changes of this process usually do not
conclude for limiting potential finding since any H+ +
e- pair is participating. Therefore, among the six
H+ + e- pair protonation processes,
the max Gibbs free energy changes of
V2@site4 and
Cr2@site4 systems are 0.28 eV and 0.29
eV for Gibbs energy changes of *N-*N → *N-*NH in both
V2@site4 and
Cr2@site4 systems, thus the limiting
potential of them are -0.28 V and 0.29 V vs CHE (Computational Hydrogen
Electrode71) , respectively. Similarly, the Gibbs free
energy is -0.36 V, -0.25V, and -0.48 V for the rest of
Ru2@site1,
W2@site2, and
Os2@site3 systems, respectively.
It is noted that for all the selected candidates, the
Mn2@site1 and
Co2@site2 exhibit end-on configuration
while Ru2@site1,
W2@site2,
Os2@site3,
V2@site4, and
Cr2@site4 show side-on configuration.
But for the hydrogenation process, especially for the
*N2 to *N2H, *N2H to
*N2H2, *N-*N to *N-*NH, *N-*NH to
*NH-*NH, *NH-*NH to *NH-*NH2, *NH-*NH2to *NH2-*NH2,
*N2H2 to *N, *N to *NH, and *NH to
*NH2 steps, nearly all of them adsorbed on both TM
active sites (see in Figure S9 ), thus lower the activation
energy during the protonation process. For the
Ni2@site4,
W2@site4, and
Re2@site4 systems, the isolated active
site participated in the initial N-N activation and hydrogenation
process reaction steps. In contrast, the second TM site does not
participate in it significantly, thus leading to their hard
hydrogenation of them. Hence, dual TM atoms must take part in the eNRR
process to enhance the cooperation effect between dual atoms. At the
same time, the adsorption behavior, activation behavior, and desorption
behavior are significantly changed since the heteronuclear of dual TM
exhibits differential properties in heteronuclear DAC, thus breaking the
limitation of homonuclear DAC.