Low Activation Barrier Research Articles

Experimental and theoretical insights into the structure and molecular dynamics of 2,3,3',4'-tetramethoxy-trans-stilbene - a chemopreventive agent.

The methoxy analogue of a trans-stilbene compound - 2,3,3',4'-tetramethoxy-trans-stilbene - was selected to characterize its crystallographic structure, intermolecular interactions and molecular dynamics. The sample was studied using single-crystal X-ray diffraction (XRD), infrared spectroscopy (FT-IR), liquid and solid-state 1H and 13C nuclear magnetic resonance (NMR) and quasielastic neutron scattering (QENS). The compound crystallized in the orthorhombic Pbca space group. The experimental methods were supported by theoretical calculations, density functional theory (plane-wave DFT) and molecular dynamics simulations (MD) methods. Combining several experimental and simulation techniques allowed the detailed analysis of molecular reorientations and provided a consistent picture of the molecular dynamics. The internal molecular mobility of the studied compound can be associated with the reorientational dynamics of four methyl groups. Interestingly, a large diversity of the energy barriers was observed - one methyl group reoriented across low activation barriers (∼3 kJ mol-1), while three methyl groups exhibited a high activation energy (10-14 kJ mol-1) and they are characterised by very different correlation times differing by almost two orders of magnitude at room temperature. The intramolecular interactions mainly influence the activation barriers.

A theoretical study of the NbS2 monolayer as a promising anchoring material for lithium-sulfur batteries.

Developing efficient cathode materials with good conductivity to restrain the shuttle effect and promote the reaction kinetics has been a key scientific issue for the development of lithium-sulfur (Li-S) batteries. Here, by means of first-principles computations, we demonstrate that the NbS2 monolayer is a promising cathode anchoring material for Li-S batteries. It has a metallic feature that improves the conductivity of sulfur and lithium polysulfides (LiPSs) and promotes the electron transfer ability. The discharge products LiPSs are strongly anchored on the NbS2 monolayer without structural distortion, which effectively alleviates the shuttle effect. Remarkably, the reductions of sulfur to soluble LiPSs are spontaneous exothermic reactions while the subsequent reductions to insoluble LiPSs are endothermic reactions with low Gibbs free energy barriers (0.09-0.18 eV) and reaction activation barriers (0.83-0.93 eV), ensuring the rapid discharge process. In addition, the barriers of Li2S decomposition and Li atom diffusion are only 0.64 and 0.18 eV, respectively, which improve the reaction kinetics in the charging process and the cycling life of batteries. These results suggest that the NbS2 monolayer can be utilized as a promising anchoring material for Li-S batteries to achieve high reversible capacity and conversion efficiency.

Why a simple vanadate is inefficient as a catalyst in the oxidation of alkanes with H2O2 - the long-standing puzzle is solved.

In contrast to V(V) complexes with various organic ligands, a simple vanadate without any additive is inactive in neutral medium toward the oxidation of alkanes with H2O2. In this work, we discovered that the insufficient activation of H2O2 upon coordination to the simple vanadate - the commonly accepted reason for the low catalytic activity of the vanadate - cannot explain this phenomenon. Two main findings are reported here on the basis of DFT calculations. First, the generally accepted Fenton-like mechanism of the generation of the active oxidizing species (HO˙) in a vanadate/H2O2(aq)/MeCN system was revisited. A new mechanism based on the tremendous activation of the OOH ligand in the intermediate [V(OO)2(OOH)(H2O)] toward the homolytic O-O bond cleavage is not only feasible but significantly more favourable than the Fenton-like pathway. The surprisingly low activation barrier calculated for the HO˙ generation (15.4 kcal mol-1) demonstrates the efficiency of this process. The presence of easily oxidizable non-innocent OO ligands in this intermediate explains such an activation. Second, it was found that the generated HO˙ radicals may be easily captured by the V atom soon after their formation followed by the elimination of the molecular oxygen. This side reaction of the H2O2 dismutation efficiently consumes the produced HO˙ radicals decreasing their concentration in the reaction mixture and preventing the following oxidation of alkanes.

Incinerability of PFOA and HFPO-DA: Mechanisms, kinetics, and thermal stability ranking

Thermal treatment is currently the only full-scale technology for the destruction of per- and polyfluoroalkyl substances (PFAS) in wastes. However, concerns exist about the emission of incomplete combustion products from incinerators. To address critical knowledge gaps around the incinerability of PFAS, highest-level coupled cluster theory was applied to elucidate the primary thermal destruction pathways, mechanisms, and kinetics for perfluorooctanoic acid (PFOA) and hexafluoropropylene oxide dimer acid (HFPO-DA, sometimes referred to as “GenX”). These PFAS were then ranked based on U.S. EPA's Thermal Stability Index, which ranges from Class 1 for the most thermally stable chemicals to Class 7 for the most thermally labile chemicals. Initial decomposition of both carboxylic acids proceeds through HF elimination at the α-carbon, but for PFOA surprisingly shifts towards C-COOH bond dissociation in the presence of ammonia due to ammonium stabilization of the variational transition state. Branching substantially reduces the required incineration temperatures, while chain length has no impact on perfluoroalkyl carboxylic acid (PFCA) decomposition rates. Bimolecular reactions with secondary species such as counterions and radicals have lower activation barriers than unimolecular decomposition, but their second-order kinetics depend on concentration and thus waste stream composition, which varies in practice. Simulating radical concentrations under typical incineration conditions, the determined free energies of activation suggest an incinerability ranking for linear PFOA in Class 3, for branched PFOA in Class 4, and for HFPO-DA as well as both PFOA and HFPO-DA ammonium salts in Class 5. Collectively, the temperatures needed to destroy 99.99% of PFOA, HFPO-DA, and other PFCAs in 2 seconds gas residence time are well below those of other commonly incinerated organic compounds such as chlorobenzene (Class 1). Ultimately, sufficient supply of radical donors is critical to ensure safe, efficient, and complete destruction of PFAS during thermal treatment.

Highly efficient hydrogen evolution from formic acid using B, N co-doped carbon-supported Pd nanoparticles

A facile novel B, N co-doped carbon was fabricated by high-temperature pyrolysis of a mixture of ethylenediamine tetraacetic acid disodium salt and boric acid, on which Pd nanoparticles were loaded to synthesize highly efficient Pd/BNC catalyst for dehydrogenation of formic acid. The type and content of N are regulated by the doping of B elements in carbon materials. The high content of N and B doping provides more anchor sites for Pd and strengthens the interaction between the support and Pd nanoparticles, thus forming highly dispersed Pd nanoparticles and regulating the electronic property of Pd. Therefore, the obtained Pd/BNC catalyst displayed enhanced performance (TOF = 2760 h−1) and a lower activation barrier (31.84 kJ/mol). Furthermore, the catalyst also exhibited wonderful cycling stability for hydrogen evolution from formic acid.

Microscopic Diffusion of Atomic Hydrogen and Water in HER Catalyst MoS2 Revealed by Neutron Scattering.

The design of novel and abundant catalytic materials for electrolysis is crucial for reaching carbon neutrality of the global energy system. A deliberate approach to catalyst design requires both theoretical and experimental knowledge not only of the target reactions but also of the supplementary mechanisms affecting the catalytic activity. In this study, we focus on the interplay of hydrogen mobility and reactivity in the hydrogen evolution reaction catalyst MoS2. We have studied the diffusion of atomic hydrogen and water by means of neutron and X-ray photoelectron spectroscopies combined with classical molecular dynamics simulations. The observed interaction of water with single-crystal MoS2 shows the possibility of intercalation within volume defects, where it can access edge sites of the material. Our surface studies also demonstrate that atomic hydrogen can be inserted into MoS2, where it then occupies various adsorption sites, possibly favoring defect vicinities. The motion of H atoms parallel to the layers of MoS2 is fast with D ≈ 1 × 10-9 m2/s at room temperature and exhibits Brownian diffusion behavior with little dependence on temperature, i.e., with a very low diffusion activation barrier.

A Mechanistic Study of Asymmetric Transfer Hydrogenation of Imines on a Chiral Phosphoric Acid Derived Indium Metal-Organic Framework

A density functional theory (DFT) study is reported to examine the asymmetric transfer hydrogenation (ATH) of imines catalyzed by an indium metal-organic framework (In-MOF) derived from a chiral phosphoric acid (CPA). It is revealed that the imine and reducing agent (i.e., thiazoline) are simultaneously adsorbed on the CPA through H-bonding to form an intermediate, subsequently, a proton is transferred from thiazoline to imine. The transition state TS-R and TS-S are stabilized on the CPA via H-bonding. Compared to the TS-S, the TS-R has shorter H-bonding distances and longer C-H···π distances, it is more stable and experiences less steric hindrance. Consequently, the TS-R exhibits a lower activation barrier affording to the (R)-enantiomer within 68.1% ee in toluene. Imines with substituted groups such as -NO2, -F, and -OCH3 are used to investigate the substitution effects on the ATH. In the presence of an electron-withdrawing group like -NO2, the electrophilicity of imine is enhanced and the activation barrier is decreased. The non-covalent interactions and activation-strain model (ASM) analysis reveal that the structural distortions and the differential noncovalent interactions of TSs in a rigid In-MOF provide the inherent driving force for enantioselectivity. For -OCH3 substituted imine, the TS-S has the strongest steric hindrance, leading to the highest enantioselectivity. When the solvent is changed from toluene to dichloromethane, acetonitrile, and dimethylsulfoxide with increasing polarity, the activation energies of transition state increase whereas their difference decreases. This implies the reaction is slowed down and the enantioselectivity becomes lower in a solvent of smaller polarity. Among the four solvents, toluene turns out to be the best for the ATH. The calculated results in this study are in fairly good agreement with experimental observations. This study provides a mechanistic understanding of the reaction mechanism, as well as substitution and solvent effects on the activity and enantioselectivity of the ATH. The microscopic insights are useful for the development of new chiral MOFs toward important asymmetric reactions.

Importance of amine in carbon dioxide conversion to methanol catalyzed by Ru-PNP complex

The theoretical insight with DFT calculations was carried out to explore the mechanism of CO2 conversion to methanol. The tandem catalysis with the Ru-Macho-BH complex and ethylenediamine (EDA) was investigated. Three possible routes, Route A, B, and C, were realized for the process. Route A (δE > 60 kcal mol−1) consists of three steps, formic acid formation, formamide formation, and methanol formation (from alkanolamine). Route B (δE = 35.07 kcal mol−1) contains four steps, formic acid formation, formamide formation, formaldehyde synthesis, and methanol formation (from formaldehyde). Route C (δE = 39.14 kcal mol−1), a non-amine route, involves three steps, formic acid formation, formaldehyde dehydration, and methanol formation. Route B has the lowest energetic span and, thus, we propose it as a favorite route for CO2 conversion to methanol. Although Route C has a lower activation barrier, it has a larger δE. The exogenic reaction of the formamide formation step in Route B can affect lower energy requirements in further methanol formation. In the absence of EDA, CO2 hydrogenation to formic acid cannot produce, because the formate form is trapped by the catalyst. Thus, it can explain why CO2 conversion to methanol is impossible without the presence of amines.

Substituent‐Controlled Energetics and Barriers of Mechanochromic Spiropyran‐Functionalized Poly(ɛ‐caprolactone)

AbstractIn a joint theoretical and experimental study, it is shown that the onset of the mechanically‐induced spiropyran (SP) to merocyanine (MC) isomerization can be controlled by both the regiochemistry and the substitution pattern of SP. Four SP‐based bifunctional initiators with consistently varied polymer chain anchor point and substituent are used to synthesize poly‐ε‐caprolactone (PCL). Theoretical calculations (1S and 3S COGEF methods) and in‐situ visible light absorption measurements of films during uniaxial stress–strain experiments consistently show varying activation barriers of the force‐induced ring‐opening reaction of SP to give MC. SPs with PCL chains attached in ortho‐position to the pyran oxygen isomerize at lower stress than their para‐analogs. NO2‐substituted SP mechanophores exhibit a lower activation barrier compared with H‐substituted ones, but only if the NO2 substituent is located in para‐position relative to the O at the pyran half. These results are consistent with theoretical loading rate‐dependent rupture forces required to break the CO bond of SP and may guide mechanophore design.

Thermodynamics and Kinetics of the Cathode-Electrolyte Interface in All-Solid-State Li-S Batteries.

Lithium–sulfur batteries (LSBs) are among the most promising energy storage technologies due to the low cost and high abundance of S. However, the issue of polysulfide shuttling with its corresponding capacity fading is a major impediment to its commercialization. Replacing traditional liquid electrolytes with solid-state electrolytes (SEs) is a potential solution. Here, we present a comprehensive study of the thermodynamics and kinetics of the cathode–electrolyte interface in all-solid-state LSBs using density functional theory based calculations and a machine learning interatomic potential. We find that among the major solid electrolyte chemistries (oxides, sulfides, nitrides, and halides), sulfide SEs are generally predicted to be the most stable against the S8 cathode, while the other SE chemistries are predicted to be highly electrochemically unstable. If the use of other SE chemistries is desired for other reasons, several binary and ternary sulfides (e.g., LiAlS2, Sc2S3, Y2S3) are predicted to be excellent buffer layers. Finally, an accurate moment tensor potential to study the S8|β-Li3PS4 interface was developed using an active learning approach. Molecular dynamics (MD) simulations of large interface models (>1000s atoms) revealed that the most stable Li3PS4(100) surface tends to form interfaces with S8 with 2D channels and lower activation barriers for Li diffusion. These results provide critical new insights into the cathode–electrolyte interface design for next-generation all-solid-state LSBs.

Theoretical study of δ-5 boron monolayer as an anode material for Li- and non-Li-ion batteries

We have studied the electrochemical performance of the δ-5 boron monolayer as an anode material for alkali metal (AM) and alkaline earth metal (AEM) ion batteries using density-functional theory simulations. The electronic properties, adsorption, diffusion rate, and storage behavior of various metal atoms (M) in the δ-5 boron monolayer are explored. Our study shows that the δ-5 boron monolayer possesses high electrical conductivity and a low activation barrier for electron and metal-ion transit (0.493–1.117 eV), indicating a fast charge/discharge rate. Furthermore, the theoretical capacities of the δ-5 boron monolayer for Li, Na, and K are found to be greater than those of commercial graphite. The average open-circuit voltage for AM and AEM is reasonably low and in the range of 0.34–1.30 V. Our results show that δ-5 boron monolayer could be a promising anode material in lithium-ion and non-lithium-ion rechargeable batteries.Graphical abstract

Thermal stability of energetic 6,8-dinitrotriazolo[1,5-a]pyridines: Interplay of thermal analysis and quantitative quantum chemical calculations

Understanding the thermal stability of energetic materials is of great importance for storage and safety issues. In the present work, we report on the thermolysis kinetics and mechanism of the two fused energetic heterocycles, viz., 6,8-dinitrotriazolo[1,5-a]pyridine (1) and 2-amino-6,8-dinitro[1,2,4]triazolo[1,5-a]pyridine (2). The experimental kinetics obtained with the aid of advanced thermal analysis techniques was complemented by mechanistic insights from high-level quantum chemical calculations. More specifically, the simultaneous thermal analysis (STA) revealed that both compounds evaporate under atmospheric pressure. To shift the evaporation to higher temperatures and to study the sole decomposition kinetics, we employed the differential scanning calorimetry (DSC) under elevated external pressure (2.0 MPa). The analysis of DSC data was performed using the Kissinger, isoconversional, and model-fitting approaches. The thermolysis of 1 occurs in the melt and is described by the kinetic scheme comprised of two independent non-integer order autocatalytic reactions with the kinetic parameters Ea1=124.2±0.5 kJ mol−1, log(A1/s−1)=9.34±0.04 and Ea2=80.8±0.7 kJ mol−1, log(A1/s−1)= 4.86±0.07. At the same time, the obtained kinetic parameters for 2 are too large to be realistic, which is a consequence of the overlap of melting and decomposition processes near the melting point. To get a deeper insight into the decomposition mechanism, the experimental data were complemented by theoretical thermolysis pathways with the activation barriers calculated using the domain-based local pair natural orbital (DLPNO) modifications of coupled cluster techniques. The same primary decomposition channels, viz., the nitro-nitrite isomerization and CNO2 bond cleavage turned out to compete in the thermolysis of 1 and 2. Due to a lower activation barrier (∼240 kJ mol−1), the former reaction path dominates at lower temperatures, whereas in the experimental temperature ranges (>500 K) the CNO2 bond cleavage with a higher preexponential factor is the fastest elementary reaction (the activation barrier ∼280 kJ mol−1). The obtained experimental and theoretical Arrhenius parameters exhibit a kinetic compensation effect, that is, the rate constant values in solid, melt, and gas phases are close to each other.

Selective Reduction of CO2 on Ti2C(OH)2 MXene through Spontaneous Crossing of Transition States.

Direct reduction of gas-phase CO2 to renewable fuels and chemical feedstock without any external energy source or rare-metal catalyst is one of the foremost challenges. Here, using density functional theory and ab initio molecular dynamics (AIMD) simulations, we predict Ti2C(OH)2 MXene as an efficient electron-coupled proton donor exhibiting simultaneously high reactivity and selectivity for CO2 reduction reaction (CRR) by yielding valuable chemicals, formate, and formic acid. This is caused by CO2 spontaneously crossing the activation barrier involved in the formation of multiple intermediates. Metallic Ti2C(OH)2 contains easily donatable protons on the surface and high-energy electrons near the Fermi level that leads to its high reactivity. High selectivity arises from low activation barrier for CRR as predicted by proposed mechanistic interpretations. Furthermore, H vacancies generated during the product formation can be replenished by exposure to moisture, ensuring the uninterrupted formation of the products. Our study provides a single-step solution for CRR to valuable chemicals without necessitating the expensive electrochemical or low-efficiency photochemical cells and hence is of immense interest for recycling the carbon.

Highly Efficient ZIF-67-Derived PtCo Alloy–CN Interface for Low-Temperature Aqueous-Phase Fischer–Tropsch Synthesis

Designing new materials for selective Fischer–Tropsch synthesis (FTS) is an elegant way to enhance local feedstock utilization like biomass and waste. In this approach, we have designed a thermally and chemically stable bimetallic PtCo/NC hybrid nanocomposite catalyst derived from a zeolitic imidazolate framework (ZIF-67, which contains cobalt as a metal center) through carbonization for low-temperature (413–473 K) aqueous-phase Fischer–Tropsch synthesis (AFTS). The selectivity of the desired range of hydrocarbons is adjusted using a highly dispersed PtCo bimetallic alloy, which facilitates extraordinary reduction of a metal oxide to active species by the synergic effect under the AFTS reaction conditions. The ZIF-derived catalyst tested in this study exhibited the highest activity to date for very low temperatures (433 K) in aqueous-phase Fischer–Tropsch synthesis with CO conversion rates between 0.61 and 1.20 molCO·molCo–1·h–1. Insights of the remarkable catalyst activity were examined by in situ X-ray photoelectron spectroscopy (XPS) studies corroborated by density functional theory (DFT) calculation. The bimetallic Co3Pt (111) surface was found to be highly active for the C–C coupling reaction between surface-adsorbed C and CO, forming a CCO intermediate with a very low activation barrier (Ea = 0.37 eV), in comparison to the C–C coupling activation barrier obtained over the Co (111) surface (Ea = 0.87 eV). This unique approach and observations create a new path for developing next-generation advanced catalyst systems and processes for selective low-temperature FTS.

The origin for hydrocarbons fast transport and photoswitching permeation behavior in grafted laminar CdTe membranes

Thin laminar CdTe nanoflake membranes grafted by oleic acid were investigated for sorption characteristics and permeation of permanent gases and hydrocarbons depending on the process temperature. A huge influence of temperature was revealed, enabling enhancement of the ideal selectivity for butane-methane separation from 28 to over 100 with the change of process conditions from 25 °C to 10 °C. The separation factor attains 26 in mixed-gas CH4/C4H10 separation experiments. Both sorption enthalpy and activation energy of diffusion were evaluated for penetrants, exhibiting the role of low activation barriers in fast transport of condensable hydrocarbons. The experimental results were proved by a semi-empirical calculation, illustrating swelling of grafted layer with heavy hydrocarbons. The effect was proved experimentally with in situ diffraction experiment. The low activation energy for the transport of condensable hydrocarbons, strong temperature dependence of permeance, and huge extinction coefficient of CdTe nanoflakes enable modulating the membrane permeance with laser irradiation, demonstrated with permeance photoswitching.

Theoretical Study of the Activation Reaction of a Zr+/P-Based Frustrated Lewis Pair with Carbon Dioxide.

The combination reactions of carbon dioxide with a Zr+/P-based frustrated Lewis pair (FLP) were computationally explored within the density functional theory framework [B3LYP-D3(BJ)/def2-TZVP]. Results showed that these reactions are exothermic, associated with relatively low activation barriers, and proceed concertedly involving Zr+-O and P-C chemical bond formations. Theoretical analysis revealed that the shorter the Zr+···P bond length of the Zr+/P-based FLP, the shorter the stretching O-C bond length of CO2 upon reaction, the larger the ∠OCO bending angle of CO2, the smaller the deformation energy of CO2, the lower the barrier height, and the greater the reactivity between the Zr+/P-based FLP and CO2. According to the energy decomposition analysis-natural orbitals for chemical valence, the bonding natures of their associated transition states are determined by the singlet-singlet interaction (donor-acceptor interaction), not the triplet-triplet interaction (electron-sharing interaction). Moreover, the bonding characteristics between Zr+/P-based FLPs and CO2 are established predominantly by the lone pair orbital(P) → the empty p-π* orbital (CO2) interaction, not the empty d-orbital(Zr+) ← the filled p-π orbital (CO2) interaction. With the use of the activation strain model, theoretical examinations showed that the reactivity trend of such combination reactions is mainly attributed to the deformation energies of the deformed reactants. The relationship between deformed geometrical structures and related activation energies is in good agreement with Hammond's postulate.

Oxidation of methane and ethylene over Al incorporated N-doped graphene: A comparative mechanistic DFT study

It is generally recognized that developing effective methods for selective oxidation of hydrocarbons to generate more useful chemicals is a major challenge for the chemical industry. In the present study, density functional theory calculations are conducted to examine the catalytic partial oxidation of methane (CH4) and ethylene (C2H4) by nitrous oxide (N2O) over Al-incorporated porphyrin-like N-doped graphene (AlN4-Gr). Adsorption energies for the most stable configurations of CH4, C2H4, and N2O molecules over the AlN4-Gr catalyst are determined to be −0.25, −0.64, and −0.40 eV, respectively. According to our findings, N2O can be efficiently split into N2 and Oads species with a negligible activation energy on the AlN4-Gr surface. Meanwhile, CH4 and C2H4 molecules compete for reaction with the activated oxygen atom (Oads) that stays on the surface. The energy barriers for partial methane oxidation through the CH4 + Oads → CH3° + HOads and CH3° + HOads → CH3OH reaction steps are 0.16 eV and 0.27 eV, respectively. Furthermore, the produced CH3OH may be overoxidized by Oads to give formaldehyde and water molecules by overcoming a relatively low activation barrier. The activation barriers for C2H4 epoxidation are small and comparable to those for CH4 oxidation, implying that AlN4-Gr is highly active for both reactions. The high energy barrier for the 1,2-hydrogen shift in the OCH2CH2 intermediate, on the other hand, makes the production of acetaldehyde impossible under normal conditions. According to the population analysis, the AlN4-Gr serves as a strong electron donor to aid in the charge transfer between the Al atom and the Oads moiety, which is necessary for the activation of CH4 and C2H4. The findings of the present study may pave the way for a better understanding of the catalytic oxidation the CH4 and C2H4, as well as for the development of highly efficient noble-metal free catalysts for these reactions.

Realizing efficient C-N coupling via electrochemical co-reduction of CO2 and NO3- on AuPd nanoalloy to form urea: Key C-N coupling intermediates

Electrochemical C-N coupling of carbon dioxide and oxynitride under ambient environment is an emerging approach which promisingly enables sustainable production of valuable industrial chemicals, such as amines and their derivatives. However, the understanding of the C-N coupling is still in its infancy. Herein, we reported highly efficient electrochemical co-reduction of carbon dioxide and nitrate to form urea catalyzed by AuPd nanoalloy. Faradic efficiency and urea formation rate achieved 15.6% and 204.2 μg·mg−1·h−1, respectively, in a gas-tight H-type cell under ambient environment. The DFT studies on the C-N coupling mechanism showed that hydroxylamine was the most likely C-N coupling N-intermediate among many nitrogen-containing intermediates and that *NH2OH and *CO, rather than *NH2 and *CO, were confirmed to realize the C-N coupling to form urea through a one-step synergistic coupling which is a thermodynamically spontaneous process with a low activation barrier. This work not only provides new insights into the C-N coupling of oxynitride and carbon dioxide under ambient environment, but also may pave a way for promoting sustainable production of C-N coupling products.

Enhanced electrocatalytic performance of Ni-based phosphides via dual regulation of Co-introducing and graphene-support for hydrogen evolution

Ni-based phosphides are a kind of promising electrocatalyst for the hydrogen evolution reaction (HER). However, the catalytic activity of phosphides still needs further improvement. Herein, with the incorporation of Co, highly dispersed spherical NiCoP supported on graphene nanosheets (NiCoP/GR) as HER catalyst was successfully prepared by the simple impregnation and phosphorization method. The highly-dispersed morphology and the synergetic interaction between NiCo bimetal endow the as-prepared NiCoP/GR with high catalytic performance for HER. When the current density is 10 mA cm−2 in 0.5 M H2SO4, the as-prepared NiCoP/GR exhibited a low overpotential of 195 mV, and the Tafel slope was only 66.4 mV dec−1. Furthermore, the density functional theory (DFT) calculations reveal that the low activation barrier of NiCoP/GR can be ascribed to the adjustment of electronic structure. As a result, the HER catalytic performance of NiCoP/GR is significantly enhanced by electronic structure adjustment. This work provides a new approach for introducing multiple metal elements and graphene support material with unique properties to boost the catalytic performance of transition metal phosphide catalysts.

Three-Dimensional Quantitative Structure and Activity Relationship of Flavones on Their Hypochlorite Scavenging Capacity.

Flavonoids, a class of polyphenolic substances widely present in the plant realm, are considered as ideal hypochlorite scavengers. However, to our knowledge, little study has focused on the structure-activity relationship between flavonoids and hypochlorite scavenging capacity. Herein, we report for the first time the three-dimensional quantitative structure and activity relationship (3D-QSAR) combined with comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA). Four models derived from CoMFA and CoMSIA with different combinations of descriptors were built and compared; the CoMFA model, which included both steric and electrostatic fields, showed great potential (R2 = 0.989; Q2 = 0.818) in predictive quality according to both internal and external validation criteria. Additionally, the average local ionization energy (ALIE), electrostatic potential (ESP), and orbital weighted dual descriptor (OWDD) were determined to identify the key structural moiety for scavenging capacity of flavonoids against hypochlorite. The computational results indicated that hypochlorous acid (HClO) serves as an electrophile undergoing electrophilic addition to the C6 carbon, which has the highest negative charge density, which are influenced by the functional groups on the flavones. The DFT calculated mechanism revealed the catalytic role of water of mono- and di-chlorination reactions, characterized by low activation barriers, and the involvement of neutral, instead of high-energy carbocation, intermediates.