The Catalyst Review August 2020 - 17

EXPERIMENTAL
Despite substantial research, key challenges remain in finding efficient catalysts for practical NRR applications. Most of the currently
studied catalysts show low FE and productivity of NH3 as the standard reduction potential (0 V vs. standard hydrogen electrode,
SHE) for the hydrogen evolution reaction (HER) is comparable to that for the NRR (0.057 V vs. SHE). The design of low-cost, highperformance NRR catalysts with the suppression of the HER is a vital concern. The application of high-efficiency theoretical studies
provides the opportunity to design and prepare high-performance catalysts with high catalytic activity and selectivity for NH3 and
suppression of HER, which significantly improves the FE and productivity of target NH3. Moreover, the combination of experimental
and theoretical studies should give information about the exact mechanism of the N2-to-NH3 transformation on as-prepared
catalysts, which will aid in improving their performance. Source: Liu A, Yang Y, Ren X, et al. (2020). ChemSusChem doi.org/10.1002/
cssc.202000487.
Electric-Field Mediated Chemistry: Uncovering and Exploiting the Potential of (Oriented) Electric
Fields to Exert Chemical Catalysis and Reaction Control
The interaction between external electric fields (EEF) and matter
has been the subject of numerous investigations for over 100
years. Recent studies have focused on the influence of oriented
external-electric-fields (OEEF) upon chemical reactivity. In
this perspective, the authors discuss the status of this area of
research while providing an overview of some recent advances,
as well as some pertinent challenges and outstanding problems.
External-electric fields (EEF) can influence chemical reactivity in
a significant way through their impact on field-induced ionicity
of bonds and transition states (TS). Specific "ground rules" have
emerged associated with this field of study, which include (a) An
OEEF oriented along a "bond axis" of a compound will polarize
this bond and eventually cleave it heterolytically; (b) Every
reaction possesses a "reaction axis," which is the direction in
which electron-pairing of the reactants is converted to electron
pairing of the products (Figure 1); (c) Application of an OEEF
along the "reaction axis" will lower the energy of the TS by
(i) increasing bond ionicity, as well as by (ii) mixing of chargetransfer (CT) states, which involve electron transfer from the
donor reactant to the acceptor in the direction of the OEEF; (d)
A polarizable molecule, or one having a dipole moment, will
gradually align with the direction of the OEEF.

Figure 1. MO and VB analyses of the manner by which an OEEF, oriented
along the bond-axis (Z), generates charge redistributions and dipole
moments in two homonuclear bonds, H−H and Li−Li. (a) OEEF-induced
ionicity in H2 and Li2 for positive (FZ > 0) and negative (FZ < 0) fields (±0.02
au; 1 au = 51.4 V Å−1) along the bond axis. (b) MO-based polarization
mechanism due to σ−σ* mixing in the presence of the field: the
symmetric σ orbital of Li2 without a field (center) versus the polarized ones
for FZ > 0 and FZ < 0 (on both sides). (c) VB-based polarization mechanism:
the bond-wave function (ΨH−H) without a field (left side) arises due to equal
mixing of the covalent structure (Φcov) with the degenerate ionic structures
(Φion(1), Φion(2)), producing a nonpolarized covalent bond. FZ < 0 and FZ > 0
(right side) exert selective stabilization of the ionic structure that opposes
the field's polarity. The selective covalent−ionic mixing thereby leads to
polarized bond-wave functions.

An OEEF along the "reaction axis," which is the direction
whereby electronic reorganization converts reactants' to
products, will accelerate reactions, control regioselectivity,
induce spin-state selectivity, and elicit mechanistic crossovers. Flipping the direction of the OEEF will, on the other hand, lead to
inhibition. Orienting the OEEF off the reaction axis enables control over stereoselectivity, enantioselectivity, and product selectivity.
For polar/polarizable reactants, the OEEF itself will act as tweezers, orienting the reactants and driving their reaction. OEEFs also
affect bond-dissociation energies and dissociation modes (covalent vs. ionic), as well as alteration of molecular geometries and
supramolecular aggregation. The use of designed local electric fields using salt additives or polar solvent environments has also been
the subject of intense investigation.
The "key" to gaining access to this powerful synthetic OEEF toolbox lies in microscopic control over the alignment between the
molecule and the applied field. However, although several proofs-of-principle have been obtained with the help of an experimental
STM setup, a truly scalable and widely applicable technique to harness OEEFs in practice has not yet been developed. Nevertheless,
alternative approaches, most of which exploit the (scalable) tweezing effect, are currently being investigated. These investigations
include the use of pulsed electric fields in flow-reactors, as well as alternative sources of electric fields, e.g., laser-induced oscillating
fields, plasmonic modes in nanocavities, triboelectricity, and piezo-electric fields. Source: Shaik S, Danovich D, Joy J, et al. (2020).
J.Am.Chem.Soc. https://dx.doi.org/10.1021/jacs.0c05128.

The Catalyst Review 										

	

August 2020

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http://doi.org/10.1002/cssc.202000487 http://doi.org/10.1002/cssc.202000487 https://dx.doi.org/10.1021/jacs.0c05128
The Catalyst Review August 2020

Table of Contents for the Digital Edition of The Catalyst Review August 2020
