10 research outputs found
Interface Engineering of Earth-Abundant Transition Metals Using Boron Nitride for Selective Electroreduction of CO<sub>2</sub>
Two-dimensional atomically
thin hexagonal boron nitride (h-BN) monolayers have attracted considerable
research interest. Given the tremendous progress in the synthesis
of h-BN monolayers on transition metals and their potential as electrocatalysts,
we investigate the electrocatalytic activities of h-BN/Ni, h-BN/Co,
and h-BN/Cu interfaces for CO<sub>2</sub> reduction by the first-principles
density functional theory. We find that with the h-BN monolayer on
the metal, electrons transfer from the metal to the interface and
accumulate under the B atoms. By calculating the binding energies
of three key intermediates (H, HCOO, and COOH) for hydrogen evolution
and CO<sub>2</sub> reduction, we find that H binding on the metal
can be significantly weakened by the h-BN monolayer, preventing the
hydrogen evolution reaction (HER). However, the binding strength of
HCOO is strong on both the metal and h-BN/metal, especially for Ni
and Co, promoting the CO<sub>2</sub> reduction channel. On the basis
of the free-energy diagrams, we predict that h-BN/Ni and h-BN/Co will
have very good electrocatalytic activities for CO<sub>2</sub> reduction
to HCOOH, while the competitive HER channel is filtered out by the
surface h-BN monolayer. Our study opens a new way for selective electroreduction
of CO<sub>2</sub> via the interface engineering of the h-BN/metal
system
Metallic Hydrogen in Atomically Precise Gold Nanoclusters
Hydrogen–metal interaction
is the foundation of many technologies
and processes, but how hydrogen behaves in atomically precise gold
nanoclusters remains unknown even though they have been used in hydrogenation
catalysis and water splitting. Herein, we investigate how hydrogen
interacts with [Au<sub>25</sub>(SR)<sub>18</sub>]<sup><i>q</i></sup> clusters and mono-atom-doped bimetallic [M<sub>1</sub>Au<sub>24</sub>(SR)<sub>18</sub>]<sup><i>q</i></sup> clusters
(M = Pt, Pd, Ag, Cu, Hg, or Cd) from first principles. We find that
hydrogen behaves as a metal in these clusters and contributes its
1s electron to the superatomic free-electron count. This opposite
behavior compared to that of the hydride in Cu and Ag clusters allows
the small hydrogen to interstitially dope the gold clusters and tune
their superatomic electronic structure. The doping energetics shows
that when an eight-electron superatom is formed after H doping, the
binding energy of H is much stronger, while binding of H with an already
eight-electron superatom is much weaker. Indeed, frontier orbitals
and the HOMO–LUMO gaps of [Au<sub>25</sub>H<sub>1</sub>(SR)<sub>18</sub>]<sup>0</sup>, [Au<sub>25</sub>H<sub>2</sub>(SR)<sub>18</sub>]<sup>+</sup>, [PtAu<sub>24</sub>H<sub>2</sub>(SR)<sub>18</sub>]<sup>0</sup>, [PdAu<sub>24</sub>H<sub>2</sub>(SR)<sub>18</sub>]<sup>0</sup>, [AgAu<sub>24</sub>HÂ(SR)<sub>18</sub>]<sup>0</sup>, and [CuAu<sub>24</sub>HÂ(SR)<sub>18</sub>]<sup>0</sup> all have very similar features,
because they are all eight-electron superatoms. By calculating the
Gibbs free energies of hydrogen adsorption, we predict that PtAu<sub>24</sub>(SR)<sub>18</sub>, PdAu<sub>24</sub>(SR)<sub>18</sub>, and
center-doped CuAu<sub>24</sub>(SR)<sub>18</sub> can be good electrocatalysts
for the hydrogen evolution reaction
Voltage-Driven Molecular Catalysis: A Promising Approach to Electrosynthesis
The combination of electrocatalysis and molecular catalysis
is
an increasingly popular approach to designing catalysts for electrosynthetic
processes. We recently found that the electrostatic potential drop
across the double layer contributes to the driving force for electron
transfer between a dissolved reactant and a molecular catalyst immobilized
directly on the electrode surface. The applied electrode potential
can increase the oxidizing (or reducing) ability of a surface-bound
molecular catalyst, thus making it suitable for charge-transfer processes,
which it normally would not be able to catalyze. In this article,
we report the initial application of voltage-driven molecular catalysis
to electroorganic synthesis. The metal-free, purely organic molecular
catalyst (TEMPO) attached to a carbon electrode showed potential-dependent
activity for the oxidation of toluene, which does not occur if TEMPO
is used as a homogeneous catalyst. Surface-attached TEMPO also shows
significant catalytic activity toward benzyl alcohol oxidation even
at pH 7. The products of toluene and benzyl alcohol oxidations were
identified by nuclear magnetic resonance, Fourier transform infrared,
and ultraviolet–visible spectroscopy to evaluate the reaction
yield and selectivity. The effect of the applied electrode potential
on these catalytic processes was elucidated by density functional
theory calculations
Kinetics and Mechanism of Methanol Conversion over Anatase Titania Nanoshapes
The
kinetics and mechanism of methanol dehydration, redox, and
oxidative coupling were investigated at 300 °C under dilute oxygen
concentration over anatase TiO<sub>2</sub> nanoplates and truncated-bipyramidal
nanocrystals in order to understand the surface structure effect of
TiO<sub>2</sub>. The two TiO<sub>2</sub> nanoshapes displayed both
(001) and (101) facets, with a higher fraction of the (001) facet
exposed on the nanoplates, while truncated-bipyramidal nanocrystals
were dominated by the (101) facet. A kinetic study using in situ titration
with ammonia shows that the active sites for methanol dehydration
are acidic and nonequivalent in comparison to redox and oxidative
coupling. In situ FTIR spectroscopy reveals that adsorbed methoxy
is the dominant surface species for all reactions, while the observed
methanol dimer is found to be a spectator species through isotopic
methanol exchange, supporting the dissociative mechanism for methanol
dehydration via surface methoxy over TiO<sub>2</sub> surfaces. Density
functional theory calculations show that the formation of dimethyl
ether involves the C–H bond dissociation of an adsorbed methoxy,
followed by coupling with another surface methoxy on the 5-fold-coordinated
Ti cations on the (101) surface, similar to the mechanism reported
on the (001) surface. Kinetic isotope effects are observed for dimethyl
ether, formaldehyde, and methyl formate in the presence of deuterated
methanol (CD<sub>3</sub>OH and CD<sub>3</sub>OD), confirming that
the cleavage of the C–H bond is the rate-limiting step for
the formation of these products. A comparison between estimated kinetic
parameters for methanol dehydration over various TiO<sub>2</sub> nanocrystals
suggests that (001) has a higher dehydration reactivity in comparison
to (101), but the surface density of active sites could be limited
by the presence of residual fluorine atoms originating from the synthesis.
The (001) surface of TiO<sub>2</sub> is also more active than the
(101) surface in redox and oxidative coupling of methanol, which is
due to the reactive surface oxygen on (001) in comparison to the (101)
surface
Fluorescence of Hydroxyphenyl-Substituted “Click” Triazoles
The
structural and optical properties of hydroxyphenyl-substituted-1,2,3-triazole
molecules (“click” triazoles) are described. “Click”
triazoles are prepared from the copperÂ(I)-catalyzed azide–alkyne
cycloaddition reactions. The alkyne-derived C4 substituent of a “click”
triazole engages in electronic conjugation more effectively with the
triazolyl core than the azide-derived N1 substituent. Furthermore,
triazolyl group exerts a stronger electron-withdrawing effect on the
N1 than the C4 substituent. Therefore, the placement of an electron-donating
group at either C4 or N1 position and the presence or the absence
of an intramolecular hydrogen bond (HB) have profound influences on
the optical properties of these compounds. The reported “click”
triazoles have fluorescence quantum yields in the range of 0.1–0.3
and large apparent Stokes shifts (8000–13 000 cm<sup>–1</sup>) in all tested solvents. Deprotonation of “click”
triazoles with a C4 hydroxyphenyl group increases their Stokes shifts;
while the opposite (or quenching) occurs to the triazoles with an
N1 hydroxyphenyl substituent. For the triazoles that contain intramolecular
HBs, neither experimental nor computational results support a model
of excited state intramolecular proton transfer (ESIPT). Rather, the
excited state internal (or intramolecular) charge transfer (ICT) mechanism
is more suitable to explain the fluorescence properties of the hydroxyphenyl-substituted “click”
triazoles; specifically, the large Stokes shifts of these compounds
Fluorescence of Hydroxyphenyl-Substituted “Click” Triazoles
The
structural and optical properties of hydroxyphenyl-substituted-1,2,3-triazole
molecules (“click” triazoles) are described. “Click”
triazoles are prepared from the copperÂ(I)-catalyzed azide–alkyne
cycloaddition reactions. The alkyne-derived C4 substituent of a “click”
triazole engages in electronic conjugation more effectively with the
triazolyl core than the azide-derived N1 substituent. Furthermore,
triazolyl group exerts a stronger electron-withdrawing effect on the
N1 than the C4 substituent. Therefore, the placement of an electron-donating
group at either C4 or N1 position and the presence or the absence
of an intramolecular hydrogen bond (HB) have profound influences on
the optical properties of these compounds. The reported “click”
triazoles have fluorescence quantum yields in the range of 0.1–0.3
and large apparent Stokes shifts (8000–13 000 cm<sup>–1</sup>) in all tested solvents. Deprotonation of “click”
triazoles with a C4 hydroxyphenyl group increases their Stokes shifts;
while the opposite (or quenching) occurs to the triazoles with an
N1 hydroxyphenyl substituent. For the triazoles that contain intramolecular
HBs, neither experimental nor computational results support a model
of excited state intramolecular proton transfer (ESIPT). Rather, the
excited state internal (or intramolecular) charge transfer (ICT) mechanism
is more suitable to explain the fluorescence properties of the hydroxyphenyl-substituted “click”
triazoles; specifically, the large Stokes shifts of these compounds
Fluorescence of Hydroxyphenyl-Substituted “Click” Triazoles
The
structural and optical properties of hydroxyphenyl-substituted-1,2,3-triazole
molecules (“click” triazoles) are described. “Click”
triazoles are prepared from the copperÂ(I)-catalyzed azide–alkyne
cycloaddition reactions. The alkyne-derived C4 substituent of a “click”
triazole engages in electronic conjugation more effectively with the
triazolyl core than the azide-derived N1 substituent. Furthermore,
triazolyl group exerts a stronger electron-withdrawing effect on the
N1 than the C4 substituent. Therefore, the placement of an electron-donating
group at either C4 or N1 position and the presence or the absence
of an intramolecular hydrogen bond (HB) have profound influences on
the optical properties of these compounds. The reported “click”
triazoles have fluorescence quantum yields in the range of 0.1–0.3
and large apparent Stokes shifts (8000–13 000 cm<sup>–1</sup>) in all tested solvents. Deprotonation of “click”
triazoles with a C4 hydroxyphenyl group increases their Stokes shifts;
while the opposite (or quenching) occurs to the triazoles with an
N1 hydroxyphenyl substituent. For the triazoles that contain intramolecular
HBs, neither experimental nor computational results support a model
of excited state intramolecular proton transfer (ESIPT). Rather, the
excited state internal (or intramolecular) charge transfer (ICT) mechanism
is more suitable to explain the fluorescence properties of the hydroxyphenyl-substituted “click”
triazoles; specifically, the large Stokes shifts of these compounds
Fluorescence of Hydroxyphenyl-Substituted “Click” Triazoles
The
structural and optical properties of hydroxyphenyl-substituted-1,2,3-triazole
molecules (“click” triazoles) are described. “Click”
triazoles are prepared from the copperÂ(I)-catalyzed azide–alkyne
cycloaddition reactions. The alkyne-derived C4 substituent of a “click”
triazole engages in electronic conjugation more effectively with the
triazolyl core than the azide-derived N1 substituent. Furthermore,
triazolyl group exerts a stronger electron-withdrawing effect on the
N1 than the C4 substituent. Therefore, the placement of an electron-donating
group at either C4 or N1 position and the presence or the absence
of an intramolecular hydrogen bond (HB) have profound influences on
the optical properties of these compounds. The reported “click”
triazoles have fluorescence quantum yields in the range of 0.1–0.3
and large apparent Stokes shifts (8000–13 000 cm<sup>–1</sup>) in all tested solvents. Deprotonation of “click”
triazoles with a C4 hydroxyphenyl group increases their Stokes shifts;
while the opposite (or quenching) occurs to the triazoles with an
N1 hydroxyphenyl substituent. For the triazoles that contain intramolecular
HBs, neither experimental nor computational results support a model
of excited state intramolecular proton transfer (ESIPT). Rather, the
excited state internal (or intramolecular) charge transfer (ICT) mechanism
is more suitable to explain the fluorescence properties of the hydroxyphenyl-substituted “click”
triazoles; specifically, the large Stokes shifts of these compounds
Fluorescence of Hydroxyphenyl-Substituted “Click” Triazoles
The
structural and optical properties of hydroxyphenyl-substituted-1,2,3-triazole
molecules (“click” triazoles) are described. “Click”
triazoles are prepared from the copperÂ(I)-catalyzed azide–alkyne
cycloaddition reactions. The alkyne-derived C4 substituent of a “click”
triazole engages in electronic conjugation more effectively with the
triazolyl core than the azide-derived N1 substituent. Furthermore,
triazolyl group exerts a stronger electron-withdrawing effect on the
N1 than the C4 substituent. Therefore, the placement of an electron-donating
group at either C4 or N1 position and the presence or the absence
of an intramolecular hydrogen bond (HB) have profound influences on
the optical properties of these compounds. The reported “click”
triazoles have fluorescence quantum yields in the range of 0.1–0.3
and large apparent Stokes shifts (8000–13 000 cm<sup>–1</sup>) in all tested solvents. Deprotonation of “click”
triazoles with a C4 hydroxyphenyl group increases their Stokes shifts;
while the opposite (or quenching) occurs to the triazoles with an
N1 hydroxyphenyl substituent. For the triazoles that contain intramolecular
HBs, neither experimental nor computational results support a model
of excited state intramolecular proton transfer (ESIPT). Rather, the
excited state internal (or intramolecular) charge transfer (ICT) mechanism
is more suitable to explain the fluorescence properties of the hydroxyphenyl-substituted “click”
triazoles; specifically, the large Stokes shifts of these compounds
Atomically Precise Bimetallic Au<sub>19</sub>Cu<sub>30</sub> Nanocluster with an Icosidodecahedral Cu<sub>30</sub> Shell and an Alkynyl–Cu Interface
Bimetallic
nanoclusters <b>Au</b><sub><b>19</b></sub><b>Cu</b><sub><b>30</b></sub> with chemical composition
of [Au<sub>19</sub>Cu<sub>30</sub>(Cî—ĽCR)<sub>22</sub>(Ph<sub>3</sub>P)<sub>6</sub>Cl<sub>2</sub>]Â(NO<sub>3</sub>)<sub>3</sub> (where
RCî—ĽC is from 3-ethynylthiophene (H<sub>3</sub>C<sub>4</sub>S-3î—¸Cî—ĽCH) or ethynylbenzene (PhCî—ĽCH)) has been
synthesized. Single X-ray structural analysis reveals that <b>Au</b><sub><b>19</b></sub><b>Cu</b><sub><b>30</b></sub> has a multishelled core structure of Au@Au<sub>12</sub>@Cu<sub>30</sub>@Au<sub>6</sub>, comprising a centered icosahedral Au<sub>13</sub> (Au@Au<sub>12</sub>) surrounded by an icosidodecahedral Cu<sub>30</sub> shell and an outmost shell of a chairlike hexagonal Au<sub>6</sub>. The alkynyl carbon is bound to the hollow sites on the Au<sub>19</sub>Cu<sub>30</sub> nanocluster surface, which is a novel interfacial
binding mode in alkynyl-protected alloy nanoclusters. The Cu<sub>30</sub> icosidodecahedron is unprecedented and <b>Au</b><sub><b>19</b></sub><b>Cu</b><sub><b>30</b></sub> represents
the first alkynyl-protected Au–Cu alloy nanocluster