Ti2N nitride MXene evokes the Mars-van Krevelen mechanism to achieve high selectivity for nitrogen reduction reaction

We address the low selectivity problem faced by the electrochemical nitrogen (N2) reduction reaction (NRR) to ammonia (NH3) by exploiting the Mars-van Krevelen (MvK) mechanism on two-dimensional (2D) Ti2N nitride MXene. NRR technology is a viable alternative to reducing the energy and greenhouse gas emission footprint from NH3 production. Most NRR catalysts operate by using an associative or dissociative mechanism, during which the NRR competes with the hydrogen evolution reaction (HER), resulting in low selectivity. The MvK mechanism reduces this competition by eliminating the adsorption and dissociation processes at the sites for NH3 synthesis. We show that the new class of 2D materials, nitride MXenes, evoke the MvK mechanism to achieve the highest Faradaic efficiency (FE) towards NH3 reported for any pristine transition metal-based catalyst—19.85% with a yield of 11.33 μg/cm2/hr at an applied potential of − 250 mV versus RHE. These results can be expanded to a broad class of systems evoking the MvK mechanism and constitute the foundation of NRR technology based on MXenes.

A comparative analysis of the mechanisms of ammonia synthesis on various catalysts using density functional theory

In this review, we present the recent progress in ammonia synthesis research using density functional theory (DFT) calculations on various industrial catalysts, metal nitrides and nano-cluster-supported catalysts. The mechanism of ammonia synthesis on the industrial Fe catalyst is generally accepted to be a dissociative mechanism . We have recently found, using DFT techniques, that on Co 3 Mo 3 N (111) surfaces, an associative mechanism in the synthesis of ammonia can offer a new low-energy pathway that was previously unknown. In particular, we have shown that metal nitrides that are also known to have high activity for ammonia synthesis can readily form nitrogen vacancies which can activate dinitrogen, thereby promoting the associative mechanism. These fundamental studies suggest that a promising route to the discovery of low-temperature ammonia synthesis catalysts will be to identify systems that proceed via the associative mechanism, which is closer to the nitrogen-fixation mechanism occurring in nitrogenases.

Unimolecular, bimolecular and intramolecular hydrolysis mechanisms of 4-nitrophenyl β-D-glucopyranoside

1,2-<i>trans</i>-Glycosides hydrolyze through different mechanisms at different pH values, but systematic studies are lacking. Here we report the pH-rate constant profile for the hydrolysis of<i> </i>4-nitrophenyl β-D-glucoside. An inverse kinetic isotope effect of <i>k</i>(H₃O⁺)/<i>k</i>(D₃O)⁺ = 0.65 in the acidic region indicates that the mechanism requires the formation of the conjugate acid of the substrate for the reaction to proceed, with heterolytic cleavage of the glycosidic C-O bond. Reactions in the pH-independent region exhibit general catalysis with a single proton in flight, a normal solvent isotope effect of <i>k</i>_H/<i>k</i>_D = 1.5, and when extrapolated to zero buffer concentration show a small solvent isotope effect <i>k</i>(H₂O)/<i>k</i>(D₂O) = 1.1, consistent with water attack through a dissociative mechanism. In the basic region, solvolysis in ¹⁸O-labelled water and H₂O/MeOH mixtures allowed detection of bimolecular hydrolysis and neighboring group participation, with a minor contribution of nucleophilic aromatic substitution. Under mildly basic conditions, a bimolecular concerted mechanism is implicated through an inverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.5 and a strongly negative entropy of activation (D<i>S</i>^‡ = –13.6 cal mol^–1 K^–1). Finally, at high pH, an inverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.6 indicates that the formation of 1,2-anhydrosugar is the rate determining step.<br>

Unimolecular, bimolecular and intramolecular hydrolysis mechanisms of 4-nitrophenyl β-D-glucopyranoside

1,2-<i>trans</i>-Glycosides hydrolyze through different mechanisms at different pH values, but systematic studies are lacking. Here we report the pH-rate constant profile for the hydrolysis of<i> </i>4-nitrophenyl β-D-glucoside. An inverse kinetic isotope effect of <i>k</i>(H₃O⁺)/<i>k</i>(D₃O)⁺ = 0.65 in the acidic region indicates that the mechanism requires the formation of the conjugate acid of the substrate for the reaction to proceed, with heterolytic cleavage of the glycosidic C-O bond. Reactions in the pH-independent region exhibit general catalysis with a single proton in flight, a normal solvent isotope effect of <i>k</i>_H/<i>k</i>_D = 1.5, and when extrapolated to zero buffer concentration show a small solvent isotope effect <i>k</i>(H₂O)/<i>k</i>(D₂O) = 1.1, consistent with water attack through a dissociative mechanism. In the basic region, solvolysis in ¹⁸O-labelled water and H₂O/MeOH mixtures allowed detection of bimolecular hydrolysis and neighboring group participation, with a minor contribution of nucleophilic aromatic substitution. Under mildly basic conditions, a bimolecular concerted mechanism is implicated through an inverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.5 and a strongly negative entropy of activation (D<i>S</i>^‡ = –13.6 cal mol^–1 K^–1). Finally, at high pH, an inverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.6 indicates that the formation of 1,2-anhydrosugar is the rate determining step.<br>

Unimolecular, bimolecular and intramolecular hydrolysis mechanisms of 4-nitrophenyl β-D-glucopyranoside

1,2-<i>trans</i>-Glycosides hydrolyze through different mechanisms at different pH values, but systematic studies are lacking. Here we report the pH-rate constant profile for the hydrolysis of<i> </i>4-nitrophenyl β-D-glucoside. An inverse kinetic isotope effect of <i>k</i>(H₃O⁺)/<i>k</i>(D₃O⁺ = 0.65 in the acidic region indicates that the mechanism requires the formation of the conjugate acid of the substrate for the reaction to proceed, with heterolytic cleavage of the glycosidic C-O bond. Reactions in the pH-independent region exhibit general catalysis with a single proton in flight, a normal solvent isotope effect of <i>k</i>_H/<i>k</i>_D = 1.5, and when extrapolated to zero buffer concentration show a small solvent isotope effect <i>k</i>(H₂O)/<i>k</i>(D₂O) = 1.1, consistent with water attack through a dissociative mechanism. In the basic region, solvolysis in ¹⁸O-labelled water and H₂O/MeOH mixtures allowed detection of bimolecular hydrolysis and neighboring group participation, with a minor contribution of nucleophilic aromatic substitution. Under mildly basic conditions, a bimolecular concerted mechanism is implicated through an inverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.5 and a strongly negative entropy of activation (D<i>S</i>^‡ = –13.6 cal mol^–1 K^–1). Finally, at high pH, an inverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.6 indicates that the formation of 1,2-anhydrosugar is the rate determining step.<br>

Ruthenium Complex Hydride Catalysts as a Platform for Ammonia Synthesis

Mild-condition ammonia synthesis from N2 and H2 is a long-sought-after scientific goal and a practical need, especially for the intensively pursued “Green Ammonia” production using renewable H2. Under this context, there have been growing interests in the development of new catalysts for effectively catalyzing N2+H2 to NH3. Particular attention has been given to Ru-based catalysts because they are well known to be more active at lower temperatures and pressures than non-noble-metal based catalysts. Here, we demonstrate that a series of Ru complex hydrides An[RuHm], where A is alkali or alkaline earth metal, n= 2, 3 or 4 and m = 6 or 7, exhibit universal and high catalytic activities that far exceed the benchmark Ru metal catalysts under mild conditions. Detailed investigations on the ternary Ru complex hydride catalytic system disclose that the kinetic behaviors depend strongly on the identity of alkali or alkaline earth metal cations. In clear contrast to the closed packed Ru metal catalyst, the unique configuration and synergized scenario of the Ru complex hydride center prefer a non-dissociative mechanism for N2 activation and hydrogenation, which provides a new platform for the design and development of efficient NH3 synthesis catalysts.

Ruthenium Complex Hydride Catalysts as a Platform for Ammonia Synthesis

Mild-condition ammonia synthesis from N2 and H2 is a long-sought-after scientific goal and a practical need, especially for the intensively pursued “Green Ammonia” production using renewable H2. Under this context, there have been growing interests in the development of new catalysts for effectively catalyzing N2+H2 to NH3. Particular attention has been given to Ru-based catalysts because they are well known to be more active at lower temperatures and pressures than non-noble-metal based catalysts. Here, we demonstrate that a series of Ru complex hydrides An[RuHm], where A is alkali or alkaline earth metal, n= 2, 3 or 4 and m = 6 or 7, exhibit universal and high catalytic activities that far exceed the benchmark Ru metal catalysts under mild conditions. Detailed investigations on the ternary Ru complex hydride catalytic system disclose that the kinetic behaviors depend strongly on the identity of alkali or alkaline earth metal cations. In clear contrast to the closed packed Ru metal catalyst, the unique configuration and synergized scenario of the Ru complex hydride center prefer a non-dissociative mechanism for N2 activation and hydrogenation, which provides a new platform for the design and development of efficient NH3 synthesis catalysts.

pH-Rate Profile for Hydrolysis of 4-Nitrophenyl β-D-Glucopyranoside: Unimolecular, Bimolecular and Intramolecular Cleavage Mechanisms

1,2-<i>trans</i>-Glycosides hydrolyze through different mechanisms at different pH values, but systematic studies are lacking. Here we report the pH-rate constant profile for the hydrolysis of<i> </i>4-nitrophenyl β-D-glucoside. An inverse kinetic isotope effect of <i>k</i>(H₃O⁺)/<i>k</i>(D₃O⁺ = 0.65 in the acidic region indicates that the mechanism requires the formation of the conjugate acid of the substrate for the reaction to proceed, with heterolytic cleavage of the glycosidic C-O bond. Reactions in the pH-independent region exhibit general catalysis with a single proton in flight, a normal solvent isotope effect of <i>k</i>_H/<i>k</i>_D = 1.5, and when extrapolated to zero buffer concentration show a small solvent isotope effect <i>k</i>(H₂O)/<i>k</i>(D₂O) = 1.1, consistent with water attack through a dissociative mechanism. In the basic region, solvolysis in ¹⁸O-labelled water and H₂O/MeOH mixtures allowed detection of bimolecular hydrolysis and neighboring group participation, with a minor contribution of nucleophilic aromatic substitution. Under mildly basic conditions, a bimolecular concerted mechanism is implicated through an inverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.5 and a strongly negative entropy of activation (D<i>S</i>^‡ = –13.6 cal mol^–1 K^–1). Finally, at high pH, an inverse solvent isotope effect of <i>k</i>(HO^–)/<i>k</i>(DO^–) = 0.6 indicates that the formation of 1,2-anhydrosugar is the rate determining step.<br>