1. Introduction
Ammonia (NH3) plays an essential role in human society
for fertilizer, industry chemistry, dyes, hydrogen carriers, etc.1, 2 Currently, the primary industrial method to
produce ammonia is the century-wide Haber-Bosch
process3, 4, where the massive energy is required to
break the triple bond of nitrogen (N2), leading to
nearly 2% energy cost annually. Meanwhile, this process releases 1%
CO2 of the whole emission every year, accelerating the
greenhouse effect and climate warming5, 6. As an
alternative strategy,
electrocatalytic nitrogen
reduction reaction (eNRR) for NH3 synthesis has
attracted more attention for its multichannel energy input and
CO2-free emmosion7, 8.
Nowadays, various electrocatalysts have been investigated for eNRR,
including bulk catalysts, two-dimensional catalysts, atomic cluster
catalysts, single-atom catalysts, dual-atom catalysts, MOF-based
catalysts, and three-atom cluster catalysts. For example, the PdCu
nanoparticle9, MXenes10,
Fe3 cluster11,
W/g-CN12,
Mo@C9N413,
NiCo@GDY14 and so on15, 16, 17, 18.
Among them, dual atom catalyst (DAC) shows excellent electrocatalytic
performance for eNRR since the synergistic catalytic process among two
active sites 19, 20. Detailly, the DAC can be divided
into two categories, the homonuclear 21, 22, 23 and
heteronuclear ones24, 25. For the homonuclear DAC, the
two active sites show the same properties, while the same characters of
the dimers make the activity of DAC very limited26.
Compared to homonuclear DAC, heteronuclear DAC with various properties
in both electronic and structural characters, leading the active sites
not only play different role in catalytic process, but also break the
scaling relationship for eNRR, which effectively enable highly activity
and selectivity during the eNRR process19, 27. For
example, He et al, demonstrated that B site and Si site take different
roles in electrocatalytic eNRR process and put forward
‘capture-backdonation-recapture’ mechanism for metal-free dual
atom-based catalyst19. Hu et al. verified that the
scaling relationship is broken through adding additional atom in
M/C3N system, leading to significantly decreased in
limiting potential in
M1M2/C3N
system28. Nevertheless, for the widely studied
transitional metal (TM) based heteronuclear dual atom catalyst, the
underlying mechanism is still unclear. On one hand, the roles of each TM
atoms could not sensitively detect and efficiently differentiate during
the eNRR process due to high activity and low distinction of them. On
the other hand, heteronuclear induced atomic diversity could enlarge
this dilemma and pollute the intrinsic electronic influence during the
underlying electronic mechanism finding process, thus leading people
could not make deep understanding to the intrinsic electronic determined
mechanism finding exactly. Therefore, how to remove the heteronuclear
atomic influence and enhance the intrinsic properties’ sensitivity for
investigating the underlying mechanism is extremely urgent for TM-based
dual atom catalyst.
To solve this dilemma, an efficient strategy is to enlarge the intrinsic
properties’ variety to minimize the diversity of two TM atoms. For
example, turning dual heteronuclear TM atoms into single active atoms
and spontaneously removes this influence, while this strategy violates
the purpose of DAC’s investigation, such as the synergistic effect among
two active sites. It should be noted that maintaining dual TM atoms
homogeneity29, 30 could remove the atomic diversity
effect as well, but it makes the intrinsic properties can’t be detected
efficiently at the same time due to the same intrinsic properties they
owned. Comparatively, it is impossible to answer how to remove the
heteronuclear induced atomic diversity for heteronuclear by uniting
their atomic size properties. However, it is easy to make the intrinsic
properties different when atomic properties remain the same for
homonuclear dual TM atom catalysts. Therefore, changing the strategy to
keep structural properties the same but make the intrinsic properties
vary could be an efficient method for sensitively detecting and
differentiating the roles of each TM atom in the catalytic process. In
fact, this phenomenon is noticed in some experiments and is also
initially investigated for organic synthesis31 and
CO2RR32. Meantime, the complicated
coordinating environment of the substrate (especially for the porous
materials) further guarantees the feasibility of this proposal33, 34. However, a general and systematic
investigation of the intrinsic mechanism is rare, especially since this
strategy can remove the atomic diversity effect. Hence, by considering
the different configuration properties as proved in the experiment, it
is evident to study the same atomic properties but intrinsic properties’
variety homonuclear DAC and dig out the intrinsic mechanisms for dual TM
atoms catalyst.
Combining the advantage of homonuclear DAC’s atomic similarity and the
configuration difference induced heteronuclear-like DAC, as well as the
high sensitivity of N2 molecule, we designed 23 kinds of
dual transition metals doping on four asymmetric defective
C3N (total of 92 systems) to investigate the homonuclear
induced atomic size-constant DAC for electrocatalytic NRR by density
functional theory. After ascertaining the stability of the doped
systems, the homonuclear twain TM atom catalyst systems exhibit
differential intrinsic properties identified by analyzing the intrinsic
properties like Bader charge analysis, magnetic analysis, and d-band
center analysis, demonstrating the feasibility of our proposal. Then, a
novel and extended ’capture-charge distribution-recapture-charge
redistribution’ mechanism was developed by performing the
N2 capture, activation, and hydrogenation and analyzing
the capture capacity charge distribution as well as intrinsic electronic
properties combined with machine learning. Finally, a total 5 candidates
are selected as high active, high selective, and high stable
electrocatalysts for NRR. Our work not only makes a deep understanding
of the catalytic behavior of NRR but also develops a new method for
designing heteronuclear-like catalysts.