The_Catalyst_Review_August_2023 - 18

reacts spontaneously with the dissolved CO2 to form carbamic acid, which quickly equilibrates to give ammonium carbamate. Although
it is not clear which of these species is responsible for the protonation of the strained alkene, the strained cyclohexene is ultimately
protonated in the rate-determining step to give a carbenium ion, which quickly undergoes the C− O bond formation. The newly
formed urethane can be converted into other valuable products, such as C1
In summary, the authors have provided mild and convenient conditions to contra-thermodynamically create urethanes from CO2
building blocks. Notably, the byproduct of these reactions
is cyclohexanol, which reacts exergonically to eliminate water and regenerate the cyclohexene upon exposure to catalytic amounts of
strong acid.
and a
variety of primary and secondary aliphatic amines in good yields. Albeit in a stepwise fashion, light, and cyclohexenes can be combined
to drive a process that would traditionally be accomplished via high-energy and toxic reagents. Schoch TD and Weaver JD (2023). J. Am.
Chem. Soc., https://doi.org/10.1021/jacs.3c04837
Promoting Oxygen Reduction Reaction on Carbon-based Materials by Selective Hydrogen Bonding
The electrochemical oxygen reduction reaction (ORR) is
fundamental for many energy conversion and storage devices.
However, the sluggish reaction kinetics and low limiting potential
(UL) remains a significant bottleneck for efficient ORR activity.
From the perspective of the ORR mechanism on a single active
site, the adsorption-free energies of two key intermediates (*OOH
and *OH) are linked by a linear scaling relationship because these
intermediates are bonded to the active site via the same oxygen
atom (Figure 1a, left side). However, adding a hydrogen bond
(H-bond) acceptor to form a selective noncovalent hydrogen bond
interaction with *OOH, would lead to the exclusive stabilization of
*OOH and breaking of the original scaling (Figure 1a, right side).
Herein, the authors describe a design strategy to plant a hydroxyl
group as the hydrogen bond acceptor in order to selectively
bond with the ORR intermediate of *OOH via a hydrogen
bond interaction while not affecting the ORR intermediate of
*OH. Arrangement of planted *OH sites and ORR active site is
dependent on the catalyst itself, where the spatial configuration
could be planar, arched, or vertical.
Figure 2. Optimized geometric configurations and Gibbs free energy
profiles of the ORR process on CoN4
-(OH)TiN4[Gr] (a,b), CoN4
TiN4[CNC] (c, d) and CoN4-(OH)TiN4[Gr-Gr] (e, f) structures.
-(OH)
Figure 1. a) Schematic illustration of H-bond mediation
mechanism. The difference in spatial extent enables the
generation of a hydrogen bond between the terminal H atom
of *OOH while avoiding the interaction with *OH. The dotted
bond represents hydrogen bond. b) Schematic diagram of
distance-dependent hydrogen bond formation for three spatial
configurations: planar, arched, and vertical. d1
, d2
and d3
stand
for the distances between the active site and the H-bond
acceptor site for the three systems.
Carbon-based single-atom catalysts are capable of incorporating
additional moieties to enrich the catalytic activity. For this reason,
these workers chose graphene and carbon nanocone (CNC) as
the catalyst substrate to simultaneously anchor the ORR active
site and the planted site of the H-bond acceptor. In addition,
first-principles calculations indicate that single metal atoms of
titanium (Ti), vanadium (V), or scandium (Sc) coordinated by four
nitrogen atoms can bind with *OH strongly. Therefore, TiN4
was
selected as a demonstration of the *OH planted site due to its high
metallicity and the capability to bind *OH firmly to form (OH)TiN4
The ORR active site is TMN4 (TM=Co, Rh, Ir), with *OOH formation
.
potentially being the rate determining step.
As shown in Figure 2a,b, the UL for CoN4
-(OH)TiN4
[Gr] is
1.06 V contributed by the potential determining step (PDS) of
*OOH formation, which is 0.42 V higher than the UL of CoN4TiN4[Gr],
suggesting that the leverage of the H-bond greatly
optimizes the ORR activity. In these two cases, the free energy
changes of the *OH desorption are nearly the same, indicating
that the introduction of H-bond acceptor barely affects the
*OH intermediate, confirming the selective H-bond mediation
18
The Catalyst Review
August 2023

https://www.doi.org/10.1021/jacs.3c04837

The_Catalyst_Review_August_2023

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