Developing
efficient catalysts for nitrogen fixation is becoming
increasingly important but is still challenging due to the lack of
robust design criteria for tackling the activity and selectivity problems,
especially for electrochemical nitrogen reduction reaction (NRR).
Herein, by means of large-scale density functional theory (DFT) computations,
we reported a descriptor-based design principle to explore the large
composition space of two-dimensional (2D) biatom catalysts (BACs),
namely, metal dimers supported on 2D expanded phthalocyanine (M2-Pc or MM′-Pc), toward the NRR at the acid conditions.
We sampled both homonuclear (M2-Pc) and heteronuclear (MM′-Pc)
BACs and constructed the activity map of BACs by using N2H* adsorption energy as the activity descriptor, which reduces the
number of promising catalyst candidates from over 900 to less than
100. This strategy allowed us to readily identify 3 homonuclear and
28 heteronuclear BACs, which could break the metal-based activity
benchmark toward the efficient NRR. Particularly, using the free energy
difference of H* and N2H* as a selectivity descriptor,
we screened out five systems, including Ti2-Pc, V2-Pc, TiV-Pc, VCr-Pc, and VTa-Pc, which exhibit a strong capability
of suppressing the competitive hydrogen evolution reaction (HER) with
favorable limiting potential of −0.75, −0.39, −0.74,
−0.85, and −0.47 V, respectively. This work not only
broadens the possibility of discovering more efficient BACs toward
N2 fixation but also provides a feasible strategy for rational
design of NRR electrocatalysts and helps pave the way to fast screening
and design of efficient BACs for the NRR and other electrochemical
reactions.
An all-atom force field for a class of the room temperature ionic liquids of the 1-alkyl-3-methylimidazolium
cation family was developed. The model is based on the AMBER force field with modifications on several
parameters. The refinements include three aspects. (1) The force coefficients of the bond and angle parameters
were adjusted to fit the vibrational frequency data, from both experiment and ab initio calculations. (2) The
parameters for two types of torsions, which are absent in the original AMBER, were obtained by fitting the
torsion energy profiles depending on dihedral angles. (3) The results of the minimum interaction energies
and geometries for several ion pairs, calculated from ab initio and the force field, respectively, are compared.
Then, the van der Waals (VDW) diameter of a type of hydrogen atom (H5) is adjusted. To validate the force
field, we performed molecular dynamics (MD) simulations for five RTILs. The predicted densities are in
better agreement than those reported from other simulations. The space distribution functions (SDFs) obtained
from MD are visualized to depict the microscopic structures of these liquids. The internal energy components
and the self-diffusion constants are also discussed.
On-site production
of hydrogen peroxide (H2O2) using electrochemical
methods could be more efficient than the
current industrial process. However, due to the existence of scaling
relations for the adsorption of reaction intermediates, there is a
long established trade-off between the activity and selectivity of
the catalysts, as the enhancement of catalytic activity is typically
accompanied by a four-electron O2 reduction reaction (ORR),
leading to the reduced selectivity for the H2O2 production. Herein, by means of density functional theory (DFT)
computations, we reported the feasibility of several classes of important
and representative experimentally achievable single-atom catalysts
(SACs) toward two-electron ORR, paying attention to their stability,
selectivity, and activity at the acidic medium. Starting from 210
two-dimensional (2D) SACs, we demonstrated that 31 SACs have the potential
to break the metal-based scaling relations and simultaneously achieve
high activity and selectivity toward H2O2 production
and screened out 7 SACs with higher activity than the PtHg4 in acidic media. Especially, a noble metal-free SAC, namely, a single
Zn atom centered phthalocyanine (Zn@Pc-N4), has a remarkable
activity improvement with a small overpotential of 0.15 V. Moreover,
using multivariable analysis and machine-learning techniques, we provided
a comprehensive understanding of the underlying origin of the selectivity
and activity of SACs and unveiled the intrinsic correlations between
structure and catalytic performance. This work may pave a way to the
design and discovery of more promising materials for H2O2 production.
1-Butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) is one of the promising room-temperature ionic liquids. To test the refined force field for [bmim][BF4] proposed in our previously work (J. Phys. Chem. B, 2004, 108, 12978-12989), thermodynamic properties of mixtures of [bmim][BF4](1)+ acetonitrile (2) are presented by using molecular dynamics over the whole concentration range. The calculated densities are in good agreement with the experimental data with deviations less than 2%, indicating the force field is applicable to the mixtures. In addition, the diffusion constants, viscosities, heats of vaporization, cohesive energy densities and excess properties of the mixtures are reported. The microscopic structures are discussed in detail, corresponding to the thermodynamic properties.
To achieve efficient ammonia synthesis via electrochemical nitrogen reduction reaction (NRR), a qualified catalyst should have both high specific activity and large active surface area. However, integrating these two merits into one single material remains a big challenge due to the difficulty in balancing multiple reaction intermediates. Here, it is demonstrated that the boron‐analogues of MXenes, namely “MBenes”, could cope with the challenge and achieve the high activity and large reaction region simultaneously toward NRR. Using extensive density functional theory computations and taking 16 MBenes as representatives, it is identified that seven MBenes (CrB, MoB, WB, Mo2B, V3B4, CrMnB2, and CrFeB2) not only have intrinsic basal plane activity for NRR with limiting potentials ranging from −0.22 to −0.82 V, but also possess superior capability of suppressing the competitive hydrogen evolution reaction. Particularly, different from the MXenes whose surface oxidation may block the active sites, once oxidized, these MBenes can catalyze NRR via the self‐activating process, reducing O*/OH* into H2O* under reaction conditions, and favoring the N2 electroreduction. As a result, the exceptional activity and selectivity, high active area (≈1019 m−2), and antioxidation nature render these MBenes as pH‐universal catalysts for NH3 production without introducing any dopants and defects.
The onion-ring structure is validated in the Pd-Pt bimetallic clusters of total atom numbers 147 and 309 through the Monte Carlo method by using the second-moment approximation of the tight-binding (TB-SMA) potentials, which is conceived in predicting the possible structures of the bimetallic clusters by He et al. [J. Am. Chem. Soc. 2003, 125, 11034] and Hwang et al. [J. Am. Chem. Soc. 2005, 127, 11140]. In the onion-ring structure, Pd atoms and Pt atoms occupy alternate layers of the clusters. The formation of the onion-ring structure can be associated with the fact that the single Pt impurity is favorable to stay in the subsurface layer and the central part of bimetallic clusters.
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