High Activation Barrier Research Articles

Unraveling the reaction mechanisms for furfural electroreduction on copper.

Electrochemical routes for the valorization of biomass-derived feedstock molecules offer sustainable pathways to produce chemicals and fuels. However, the underlying reaction mechanisms for their electrochemical conversion remain elusive. In particular, the exact role of proton-electron coupled transfer and electrocatalytic hydrogenation in the reaction mechanisms for biomass electroreduction are disputed. In this work, we study the reaction mechanism underlying the electroreduction of furfural, an important biomass-derived platform chemical, combining grand-canonical (constant-potential) density functional theory-based microkinetic simulations and pH dependent experiments on Cu under acidic conditions. Our simulations indicate the second PCET step in the reaction pathway to be the rate- and selectivity-determining step for the production of the two main products of furfural electroreduction on Cu, i.e., furfuryl alcohol and 2-methyl furan, at moderate overpotentials. We further identify the source of Cu's ability to produce both products with comparable activity in their nearly equal activation energies. Furthermore, our microkinetic simulations suggest that surface hydrogenation steps play a minor role in determining the overall activity of furfural electroreduction compared to PCET steps due to the low steady-state hydrogen coverage predicted under reaction conditions, the high activation barriers for surface hydrogenation and the observed pH dependence of the reaction. As a theoretical guideline, low pH (<1.5) and moderate potential (ca. -0.5 V vs. SHE) conditions are suggested for selective 2-MF production.

Cooperative activated hopping dynamics in binary glass-forming liquids: effects of the size ratio, composition, and interparticle interactions.

Slow dynamics in supercooled and glassy liquids is an important research topic in soft matter physics. Compared to the traditionally focused one-component systems, glassy dynamics in mixture systems adds in a rich set of new complexities, which are fundamentally interesting and also relevant for many technological applications. In this paper, we apply the recently developed self-consistent cooperative hopping theory (SCCHT) to systematically investigate the effects of the size ratio, composition and interparticle interactions on the cooperative activated hopping dynamics of matrix (in larger size) and penetrant (in smaller size) particles in varied binary sphere mixture model systems, with a specific focus on ultrahigh mixture packing fractions that mimic the deeply supercooled glass transition conditions for molecular/polymeric mixture materials. Analysis shows that in these high activation barrier cases, the long-range elastic distortion associated with a matrix particle hopping over its cage confinement always generates an elastic barrier of a nonnegligible magnitude, although the ratio between the elastic barrier and local barrier contribution is sensitively dependent on all three mixture-specific system factors considered in this work. SCCHT predicts two general scenarios of penetrant-matrix cooperative activated hopping dynamics: matrix/penetrant co-hopping (regime 1) or the penetrant mean barrier hopping time shorter than that of the matrix (regime 2). Increasing the penetrant-to-matrix size ratio or the penetrant-matrix cross-attraction strength is found to universally enlarge the composition window of regime 1. Diverse dynamical properties characterising different aspects of the cooperative activated hopping process, including the penetrant and matrix transient localization lengths, penetrant and matrix hopping jump distances, different types of local and elastic activated barriers, and matrix long-time diffusivity, relaxation time and dynamic fragility are quantitatively studied against a wide range of variations over the three system factors. Of particular interest is the universal "anti-plasticization" phenomenon achievable for sufficiently strong cross-attractive interactions. The prospects this work opens for the exploration of a wide variety of polymer-based mixture materials are briefly discussed at the end.

Rh(iii)-catalyzed highly site- and regio-selective alkenyl C-H activation/annulation of 4-amino-2-quinolones with alkynes via reversible alkyne insertion.

3,4-Fused 2-quinolone frameworks are important structural motifs found in natural products and biologically active compounds. Intermolecular alkenyl C-H activation/annulation of 4-amino-2-quinolone substrates with alkynes is one of the most efficient methods for accessing such structural motifs. However, this is a formidable challenge because 4-amino-2-quinolones have two cleavable C-H bonds: an alkenyl C-H bond at the C3-position and an aromatic C-H bond at the C5-position. Herein, we report the Rh(iii)-catalyzed highly site-selective alkenyl C-H functionalization of 4-amino-2-quinolones to afford 3,4-fused 2-quinolones. This method has a wide substrate scope, including unsymmetrical internal alkynes, with complete regioselectivity. Several control experiments using an isolated key intermediate analog suggested that the annulation reaction proceeds via reversible alkyne insertion involving a binuclear Rh complex although alkyne insertion is generally recognized as an irreversible process due to the high activation barrier of the reverse process.

Metal-Free C–H Difluoromethylation of Imidazoles with the Ruppert–Prakash Reagent

The reaction of trimethyl(trifluoromethyl)silane-tetrabutylammonium difluorotriphenylsilicate (CF3SiMe3-TBAT) with a series of imidazoles gives products of the formal difluorocarbene insertion into the C-H bond at the C-2 position (i.e., C-difluoromethylation). According to NMR spectra, the corresponding 2-(trimethylsilyl)difluoromethyl-substituted derivatives are likely formed as the intermediates in the reaction, and then, they slowly convert to 2-difluoromethyl-substituted imidazoles. Quantum chemical calculations of two plausible reaction mechanisms indicate that it proceeds through the intermediate imidazolide anion stabilized through the interaction with solvent molecules and counterions. In the first proposed mechanism, the anion reacts with difluorocarbene without an activation barrier, and then, the CF2 moiety of the adduct attacks the CF3SiMe3 molecule. After the elimination of the CF3 anion, 2-(trimethylsilyl)difluromethyl-substituted imidazole is formed. Another possible reaction pathway includes silylation of imidazolide anion at the N-3 atom, followed by the barrierless addition of difluorocarbene at the C-2 atom and then by 1,3-shift of the SiMe3 group from N-3 to the carbon atom of the CF2 moiety. Both proposed mechanisms do not include steps with high activation barriers.

How a Chromium Tricarbonyl Complex Catalyzes the [3 + 2] Cycloaddition Reaction of N-Substituted Phenylnitrones with Styrene: A Molecular Electron Density Theory Analysis

The reaction mechanisms of [3 + 2] cycloaddition (32CA) between two N-substituted phenylnitrones (NPPN and NtBPN) and styrene (STY) in the presence of a chromium tricarbonyl complex (Cr(CO)3) have been studied within the framework of molecular electron density theory at the MPWB1K/6-311G(d,p) level. Activation energy analysis reveals that these 32CA reactions take place with high activation barriers due to their non-polar character and with the exo/ortho approach as a favorable reaction path. The activation energy of TS1 series (NPPN) is about 5–6 kcal/mol lower than that of the TS2 series (NtBPN), due to the steric hindrance of tert-butyl at the TS2 structures. Coordination of chromium tricarbonyl to each reactant to form the corresponding complex slightly increases the nucleophilic and electrophilic character of NPPN:Cr, NtBPN:Cr, and STY:Cr, which can be considered a strong nucleophile and electrophile able to participate in a polar 32CA reaction. This polar character in the presence of the chromium tricarbonyl complex was verified by the higher (up to 0.20e) values of global electron density transfer for Cr(CO)3-coordinated transition states. Electron localization function analysis along the exo/ortho reaction path reveals the non-concerted mechanism in the formation of a new bond with the initial appearance of a C–C single bond followed by the O–C one.

Cobalt-induced highly-electroactive Li2S heterostructured cathode for Li-S batteries

Lithium sulfide (Li2S) is a promising cathode material with a high theoretical capacity (1166 mA h g−1) that can be paired with nonlithium-metal anodes, which can eliminate the safety issue related with lithium anode. Nevertheless, its poor electronic conductivity and low Li ion diffusion lead to the high activation barrier of Li2S and sluggish kinetic conversion to polysulfides, hindering its commercialization. Herein, Li2S particles coated by Co nanomaterial-decorated porous carbon shells (Li2S/Co@C) are catalytically synthesized in-situ as the Li2S-Co heterostructures to enhance Li2S reactivity and its kinetic conversion via a carbothermic reduction. This Li2S/Co@C shows an ultra-low activation potential of 3.12 V, smaller by 0.74 V compared with commercial Li2S. Significantly, it presents an initial reversible capacity of 1006 mA h g−1 and maintains a high reversible capacity of 335 mA h g−1 at 0.1 C (1 C = 1166 mA g−1) after 500 cycles. An outstanding rate capacity is also achieved with a reversible capacity of 148 mA h g−1 at 3 C. More importantly, in-/ex-situ characterizations underscore that Co nanomaterials can serve as an Li2S-Co heterostructure catalyst to enhance the reactivity of Li2S, lithium polysulfides, and sulfur, thereby achieving high performance in Li-S batteries.

3,3]‐Sigmatropic Rearrangements of Naphthyl 1‐Propargyl Ethers: para‐Propargylation and Catalytic Asymmetric Dearomatization

AbstractThe para‐Claisen rearrangement of aryl 1‐propargyl ethers involves two‐step [3,3]‐sigmatropic rearrangements and dearomatization process, which has high activation barriers and is of challenge. Here we discovered thermal para‐Claisen rearrangement of naphthyl 1‐propargyl ethers, and it enabled the formation of formal para‐C−H propargylation products upon rearomatization. Chirality transfer occurred if optically active propargyl ethers were employed, leading to the construction of aryl/propargyl‐containing stereogenic centers. Moreover, catalytic asymmetric dearomatization of naphthyl 1‐propargyl ethers with different substitution at para‐position gave access to benzocyclohexenones bearing all‐carbon quaternary stereocenters. The reaction was accelerated by a chiral N,N′‐dioxide/Co(OTf)2 complex catalyst to achieve high yields (up to 98 %) and high enantioselectivities (up to 93 % ee). The DFT calculations and experimental results provided important clues to clarify the para‐Claisen rearrangement process as well as the chiral induction and remote delivery.

3,3]-Sigmatropic Rearrangements of Naphthyl 1-Propargyl Ethers: para-Propargylation and Catalytic Asymmetric Dearomatization.

The para-Claisen rearrangement of aryl 1-propargyl ethers involves two-step [3,3]-sigmatropic rearrangements and dearomatization process, which has high activation barriers and is of challenge. Here we discovered thermal para-Claisen rearrangement of naphthyl 1-propargyl ethers, and it enabled the formation of formal para-C-H propargylation products upon rearomatization. Chirality transfer occurred if optically active propargyl ethers were employed, leading to the construction of aryl/propargyl-containing stereogenic centers. Moreover, catalytic asymmetric dearomatization of naphthyl 1-propargyl ethers with different substitution at para-position gave access to benzocyclohexenones bearing all-carbon quaternary stereocenters. The reaction was accelerated by a chiral N,N'-dioxide/Co(OTf)2 complex catalyst to achieve high yields (up to 98 %) and high enantioselectivities (up to 93 % ee). The DFT calculations and experimental results provided important clues to clarify the para-Claisen rearrangement process as well as the chiral induction and remote delivery.

Metal-oxide nanocomposite catalyst simultaneously boosts the oxygen reduction reactivity and chemical stability of solid oxide fuel cell cathode

Conductive perovskite oxides have received much attention as promising candidates for solid oxide fuel cell (SOFC) cathodes. However, they show chemical instability due to the segregation of A-site cations on the surface at high temperatures, which causes their performance to degrade. Moreover, the high activation barrier to oxygen reduction reactions makes these materials more difficult to use as electrodes at lower temperatures (less than 700 °C). Herein, by combining two general techniques: nanoparticle infiltration and atomic layer deposition (ALD), we significantly improve both the durability and reactivity of a state-of-the-art perovskite oxide, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF). Ag nanocatalysts are dispersed onto an LSCF cathode via wet infiltration, after which they are covered with a thin ZrO2 layer via ALD to prevent the agglomeration of nanocatalysts and the segregation of Sr ions. Accordingly, the Ag/ZrO2 nanocomposite-deposited LSCF cathode shows the electrode resistance of 0.085 Ωcm2 at 650 °C over 200 h, which is, to the best of our knowledge, the near-record level among all nanocatalyst-infiltrated LSCF cathode to date. Our result suggests a new research direction to fully utilize nanocatalysts and perovskite oxides in SOFCs, which can be a shortcut to their commercialization.

Theoretical and Experimental Insights of Benzimidazole Catalyzed by the Epoxy-Acrylic Acid Reaction.

This study focuses on the experimental and molecular-level investigation of epoxy acrylate formation. Epoxy acrylate vinyl ester resin was prepared by a reaction of diglycidyl ether of bisphenol-A-based epoxy resin and acrylic acid, using benzimidazole as a catalyst. It was confirmed that benzimidazole can effectively catalyze this reaction. FTIR analysis of the product revealed a simple addition esterification reaction between the epoxide group and carboxylic group of acrylic acid excluding the side reactions (e.g., etherification). DFT computational studies were performed to theoretically explore the insights of reaction mechanisms. The calculations revealed that the benzimidazole-catalyzed reaction dominates the uncatalyzed reaction. A comparison of calculated activation energies showed that concerted mechanisms are less significant in such reactions owing to their high activation barriers.

Cyclo[18]carbon Formation from C18Br6 and C18(CO)6 Precursors.

Although cyclo[18]carbon has been isolated experimentally from two precursors, C18Br6 and C18(CO)6, no reaction mechanisms have yet been explored. Herein, we provide insight into the mechanism behind debromination and decarbonylation. Both neutral precursors demonstrate high activation barriers of ∼2.3 eV, while the application of an electric field can lower the barriers by 0.1-0.2 eV. The barrier energy of the anion-radicals is found to be significantly lower for C18Br6 compared to C18(CO)6, confirming a considerably higher yield of cylco[18]carbon when the C18Br6 precursor is used. Elongation of the C-Br bond in the anion-radical confirms its predissociation condition. Natural bonding orbital analysis shows that the stability of C-Br and C-CO bonds in the anion-radicals is lower compared to their neutral species, indicating a possible higher yield. The applied analysis provides crucial details regarding the reaction yield of cyclo[18]carbon and can serve as a general scheme for tuning reaction conditions for other organic precursors.

Electrochemically Upcycling Waste Nitrogen into Ammonia in a Membrane-Free Alkaline Electrolyzer

Ammonia (NH3) is a primary form of reactive nitrogen (Nr) which is an essential nutrient for all lives on the earth. Over the past century, the anthropological N2-fixing process in the industry (i.e., the Haber-Bosch process) has contributed to a significant portion of NH3 production, and has also led to the continued accumulation of Nr in our ecosystems that has caused alarming and profound damages to the ecosystems as well as human welfare, as the rate of Nr generation is not balanced by the natural nitrification-denitrification process for its elimination. For example, leakage of excessive nitrate (NO3 −) into the water bodies caused by overfertilization of crop fields and discharged streams from food processing facilities has led to the formation of “dead zones” in coastal areas created by eutrophication. Denitrification in nature is also accompanied by the considerable generation of nitrous oxide (N2O) which possesses century-long stability and a 300-time greater potential for greenhouse effect than CO2. Therefore, restoring the balance between the generation and elimination of Nr is a challenging but urgent task faced by our human beings today.Sustainable solutions to this human-induced matter have been actively pursued in recent years either by converting Nr to harmless N2 (i.e., denitrification), or by enhancing the effective circulation and utilization of Nr in the cycle of the nitrogen element. For instance, the electrochemical reduction of nitrate (NO3RR) is a promising approach as it can eliminate Nr without the need for additional oxidant/reductant. Selective NO3RR toward N2 is highly desirable but remains difficult to achieve due to the high activation barrier for linking two N atoms on typical catalyst surfaces, and thus the coexistence of competing pathways toward other N-containing products. Alternatively, if other forms of Nr in waste resources can be converted to NH3 in an electrochemical device, this process will not only alleviate the environmental impacts of those Nr, but also simultaneously produce NH3 that could substantially decrease the NH3 demand from the Haber-Bosch process, reduce the use of fossil-derived H2 in NH3 production, and decelerate the accumulation of Nr.In this work, we seek to develop an electricity-driven process that can convert various forms of Nr in real waste resources into manageable NH3 products. The key component is a membrane-free alkaline electrolyzer (MFAEL) which transforms Nr into NH3 as the sole N-containing product. With the inexpensive and robust MFAEL system, we achieved an ampere-level partial current density towards NH3 production. By properly choosing the conditions of NH3 collection from MFAEL, continuous production of pure NH3-based chemicals can be realized without the need for additional separation procedures. Techno-economic analysis (TEA) suggests the potential economic feasibility of the waste-to-NH3 process by coupling electrodialysis (ED) for Nr concentration and the MFAEL process for Nr conversion, offering an all-sustainable route for upcycling waste N into the highest-demanded N-based chemical product, so that the growing trend of Nr buildup can be largely decelerated and reversed.

A Mechanistic Study of Methanol Steam Reforming on Ni2P Catalyst

Methanol steam reforming (MSR) is a promising technology for on-board hydrogen production in fuel cell applications. Although traditional Cu-based catalysts demonstrate high catalytic activity and selectivity towards CO2 relative to CO, which is produced via methanol decomposition, they suffer from poor thermal stability and rapid coke formation. Nickel phosphides have been widely investigated in recent years for many different catalytic reactions owing to their remarkable activity and selectivity, as well as their low cost. In this work, we present a mechanistic study of methanol decomposition and MSR pathways on Ni2P using density functional theory (DFT) calculations. DFT-predicted enthalpic barriers indicate that MSR may compete with methanol decomposition on Ni2P, in contrast to other transition metals (e.g., Pt, Pd, and Co) which primarily decompose methanol into CO. The formaldehyde intermediate (CH2O*) can react with co-adsorbed hydroxyl (OH*) from water dissociation to produce H2COOH* which then undergoes subsequent dehydrogenation steps to produce CO2 via H2COOH*→ HCOOH* → HCOO* → CO2. We also examined the conversion of CO into CO2 via the water–gas shift (WGS) reaction, but we ruled out this pathway because it exhibits high activation barriers on Ni2P. These findings suggest that Ni2P is a promising new catalyst for MSR.

Antioxidant activity of caffeic acid: thermodynamic and kinetic aspects on the oxidative degradation pathway

Caffeic acid is a phenolic secondary metabolite from plants, which is known for its antioxidant properties. The effective mitigation of methanol-induced oxidative stress by caffeic acid depends on the direct radical scavenging as well as the formation of new metabolites via oxidative degradation. Herein, thermodynamic and kinetic aspects of the oxidative degradation pathway of caffeic acid in the presence of radical CH3O• and its isomer, •CH2OH are discussed for the first time, employing density functional theory (DFT). The direct radical scavenging activity of caffeic acid against these radicals is verified via hydrogen atom transfer (HAT) and radical adduct formation (RAF) mechanisms. HAT is predicted to be more feasible than RAF mechanism as per the computed data. Additionally, energetic details of the proposed oxidative degradation pathway of radical adduct intermediates toward the formation of a cyclic metabolite is analyzed. Kinetic studies indicated a significant tunneling contribution to the H abstraction pathways having high activation barriers. Further, our results imply that the newly formed metabolites exhibit comparable antioxidant activity with that of caffeic acid.

Integrating vacancy engineering and energy-level adapted coupling of electrocatalyst for enhancement of carbon dioxide conversion

CO2 activation capability and conductivity of electrocatalysts are two crucial factors to address the high activation barrier and slow kinetics of CO2 reduction reaction (CO2RR). Still, substantial promotion of both factors remains challenging. Herein, we report sulfur-deficient tin disulfide covered nitrogen-doped hollow carbon spheres (SnS2−x/NHCS) as electrocatalyst for CO2RR. Combining experimental analysis with calculations, we demonstrate that the S vacancies and energy-level adapted coupling can decrease the reaction barrier, increase the CO2 adsorption capacity and affinity with the intermediate, and promote the conductivity, delivering a current density of 35.3 mA cm−2 at − 1.2 V vs. reversible hydrogen electrode (RHE) and a Faradaic efficiency for formate > 80% in a large potential range from − 0.8 to − 1.2 V (vs. RHE). This work unravels the relationship of the vacancy engineering/hybrid coupling of SnS2−x/NHCS with its CO2RR performance, and proposes a convenient route to boost the CO2 activation capability and conductivity of electrocatalysts simultaneously.

First-principles investigation of interaction between the atomic oxygen species and carbon nanostructures

• The following trends in the value of adsorption energies of triplet oxygen atoms on the carbon nanostructures were identified: diamond-like structures (sp 3 ) < graphene-like structures (sp 2 ) < fullerenes & nanotubes (sp 2 , pentagons, high curvature). • For the graphene flake and fullerene structures there are common precursors for carbon desorption in the form of CO/CO 2 molecules: (1) ether-ether-epoxy precursor and (2) epoxy-ether-epoxy. • Etching of sp 3 hybridized carbon structures can proceed faster than etching of sp 2 structures. • For graphene structure, unzipping process is more favorable than reduction process at lower temperatures. In this work we perform a systematic theoretical study of oxidation process of different carbon nanomaterials by atomic oxygen. In order to understand the atomistic mechanism of interaction between atomic oxygen and carbon nanostructures and to explain experimental results on their oxygen treatment, first-principles calculations were performed. Influence of spin state of the atomic oxygen on its interaction with different carbon nanostructures (finite size graphene flake, carbon nanotube, fullerene, diamondoid) was investigated. We found that the structure and stability of the adsorption complexes are determined by the oxygen spin state and depend on the carbon nanostructure morphology and hybridization. The atomistic mechanisms of chemical transformations driven by high local oxygen coverage, such as CO/CO 2 desorption and graphene unzipping, were investigated. Calculated barriers of CO 2 desorption allow to explain experimental data on thermal reduction of the oxidized carbon structures. In case of graphene, competition between channels resulting in CO 2 desorption and plane unzipping was observed. We found that for the temperatures ~400 K and lower unzipping process is favorable, since CO 2 desorption precursor formation proceeds through higher activation barrier (1.3 eV) and requires temperatures above 450 K.

Oxygen-vacancy-induced charge localization and atomic site activation in ultrathin Bi4O5Br2 nanotubes for boosted CO2 photoreduction

Solar CO2 reduction into value-added carbon fuels emerges as a sustainable and significant approach for CO2 conversion. However, poor photoabsorption, sluggish dynamics of multi-electrons transfer and high CO2 activation barrier substantially hamper CO2 photoreduction efficiency. Herein, freestanding ultrathin Bi4O5Br2 nanotubes with abundant oxygen vacancies were initially fabricated for optimizing these crucial processes. The ultrathin-shelled topologies of open tubular scaffolds and abundant surface oxygen vacancies endow the Bi4O5Br2 nanotubes with extended photo-response region and boosted charge separation. Furthermore, the presence of oxygen vacancies alters charge density distribution and affords abundant localized electrons on the catalysts’ surface. These not only reinforce covalent interactions, electron exchange and transfer between CO2 and oxygen vacancies, but also lower the CO2 reaction energy barriers and stabilize the rate-limiting COOH* intermediate. As a result, the OV-rich Bi4O5Br2 ultrathin nanotubes exhibit an outstanding CO production rate of 19.56 µmol g−1h−1 without using any cocatalyst or sacrificial agent, approximately 11.2 times higher than the Bi4O5Br2 nanoplates counterpart. This work provides insights into the future design of ultrathin hollow scaffolds for solar fuel photosynthesis and other applications.

A‐Site Substitution Motivating the Phase Transformation of LaNiO3 Perovskite for High‐Performance Artificial N2 Fixation

AbstractElectrocatalytic N2 reduction reaction (NRR) to NH3 under ambient conditions is an attractive strategy compared to the traditional Haber‐Bosch process. However, there still remains a great challenge due to the high activation barrier of N2 and consequent sluggish kinetics. Herein, A‐site substitution of LaNiO3 perovskites by Sr‐doping is proposed which can lead to phase transformation and to the formation of oxygen vacancies, significantly improving the NRR performance of La1‐xSrxNiO3 (LSNO). The appropriate oxygen vacancy concentration can be modulated by changing the stoichiometric ratio of La and Sr elements. LSNO‐III (x=0.5) delivers the highest ammonia production rate of 55.9 μg h−1 mg−1cat. at −0.7 V and Faraday efficiency of 23.17 % at −0.5 V versus the reversible hydrogen electrode in 0.1 M Na2SO4. Density functional theory (DFT) calculations further demonstrate that introduction of oxygen vacancies decreases the distance of the d‐band center to the Fermi energy level, which not only facilitates adsorption and activation of N2 molecules and subsequent hydrogenation reactions, but also further optimizes reaction pathways in NRR.

Ordered Ag Nanoneedle Arrays with Enhanced Electrocatalytic CO2 Reduction via Structure-Induced Inhibition of Hydrogen Evolution.

Silver is an attractive catalyst for converting CO2 into CO. However, the high CO2 activation barrier and the hydrogen evolution side reaction seriously limit its practical application and industrial perspective. Here, an ordered Ag nanoneedle array (Ag-NNAs) was prepared by template-assisted vacuum thermal-evaporation for CO2 electroreduction into CO. The nanoneedle array structure induces a strong local electric field at the tips, which not only reduces the activation barrier for CO2 electroreduction but also increases the energy barrier for the hydrogen evolution reaction (HER). Moreover, the array structure endows a high surface hydrophobicity, which can regulate the adsorption of water molecules at the interface and thus dynamically inhibit the competitive HER. As a result, the optimal Ag-NNAs exhibits 91.4% Faradaic efficiency (FE) of CO for over 700 min at -1.0 V vs RHE. This work provides a new concept for the application of nanoneedle array structures in electrocatalytic CO2 reduction reactions.

Mechanistic aspects of the Diels-Alder reaction between (E)-N-benzylidene-2,2-difluoro-1-phenylethenamine and 2-vinyl pyridine: A molecular electron density theory study

A MEDT study is carried out on dehydrofluorination of benzyltrifluoromethylketimine (BFK) to (E)-N-benzylidene-2,2-difluoro-1-phenylethenamine (BFPA), and the Diels-Alder (DA) reaction of BFPA with 2-vinylpyridine (2VP), which has been performed experimentally by Fu and co-workers. PES analysis indicated that in the dehydrofluorination reaction, the deprotonation of BFK is the RDS in the presence of 2,2,6,6-tetramethylpiperidine (TMP), and the presence of 2,2,6,6-tetramethylpiperidinium ion generated in the deprotonation step, is essential for defluorination. In consistent with the experimental work, it was found that the formation of cis ortho disubstituted cycloadduct from the DA reaction is more favorable kinetically. The high activation barriers rationalize the performing of reactions in harsh conditions. The ELF analysis suggested that the DA reaction takes place through a two-stage one-step mechanism via the coupling of pseudoradical centers. IGM analysis confirmed the endo stereoselectivity predominance of the reaction. Finally, it was found that the BF3 Lewis acid makes the reaction unfavorable.