Interface Engineering of Earth-Abundant Transition Metals Using Boron Nitride for Selective Electroreduction of CO₂

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 CO₂ 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 CO₂ 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 CO₂ 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 CO₂ 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 CO₂ 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 [Au₂₅(SR)₁₈]^<i>q</i> clusters and mono-atom-doped bimetallic [M₁Au₂₄(SR)₁₈]^<i>q</i> 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 [Au₂₅H₁(SR)₁₈]⁰, [Au₂₅H₂(SR)₁₈]⁺, [PtAu₂₄H₂(SR)₁₈]⁰, [PdAu₂₄H₂(SR)₁₈]⁰, [AgAu₂₄H­(SR)₁₈]⁰, and [CuAu₂₄H­(SR)₁₈]⁰ all have very similar features, because they are all eight-electron superatoms. By calculating the Gibbs free energies of hydrogen adsorption, we predict that PtAu₂₄(SR)₁₈, PdAu₂₄(SR)₁₈, and center-doped CuAu₂₄(SR)₁₈ 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 TiO₂ nanoplates and truncated-bipyramidal nanocrystals in order to understand the surface structure effect of TiO₂. The two TiO₂ 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 TiO₂ 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 (CD₃OH and CD₃OD), 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 TiO₂ 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 TiO₂ 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 copper­(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^–1) 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 copper­(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^–1) 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 copper­(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^–1) 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 copper­(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^–1) 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 copper­(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^–1) 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 Au₁₉Cu₃₀ Nanocluster with an Icosidodecahedral Cu₃₀ Shell and an Alkynyl–Cu Interface

Bimetallic nanoclusters <b>Au</b>_<b>19</b><b>Cu</b>_<b>30</b> with chemical composition of [Au₁₉Cu₃₀(CCR)₂₂(Ph₃P)₆Cl₂]­(NO₃)₃ (where RCC is from 3-ethynylthiophene (H₃C₄S-3CCH) or ethynylbenzene (PhCCH)) has been synthesized. Single X-ray structural analysis reveals that <b>Au</b>_<b>19</b><b>Cu</b>_<b>30</b> has a multishelled core structure of Au@Au₁₂@Cu₃₀@Au₆, comprising a centered icosahedral Au₁₃ (Au@Au₁₂) surrounded by an icosidodecahedral Cu₃₀ shell and an outmost shell of a chairlike hexagonal Au₆. The alkynyl carbon is bound to the hollow sites on the Au₁₉Cu₃₀ nanocluster surface, which is a novel interfacial binding mode in alkynyl-protected alloy nanoclusters. The Cu₃₀ icosidodecahedron is unprecedented and <b>Au</b>_<b>19</b><b>Cu</b>_<b>30</b> represents the first alkynyl-protected Au–Cu alloy nanocluster