Quantum Monte Carlo Study of the Reactions of CH with Acrolein: Major and Minor Channels

Kinetic Study of OH Radical Reactions with Cyclopentenone Derivatives

We investigated the reactions of the hydroxyl radical (OH) with cyclopentenone derivatives and cyclopentanone in a quasi-static reaction cell at 4 Torr across a 300-500 K temperature range. The OH radicals were generated using pulsed laser photolysis of hydrogen peroxide vapors, and the ketone reactants were introduced in excess. The relative concentrations of the radicals were monitored as a function of reaction time using laser-induced fluorescence. At room temperature, the reaction rate coefficients were measured to be 1.2(±0.1) × 10-11 cm3 s-1 for reaction with 2-cyclopenten-1-one (R1); 1.7(±0.2) × 10-11 cm3 s-1 for reaction with 2-methyl-2-cyclopenten-1-one (R2); and 4.4(±0. 7) × 10-12 cm3 s-1 for reaction with 3-methyl-2-cyclopenten-1-one (R3). Over the experimental temperature range, the rate coefficients can be fitted with the modified Arrhenius expressions k1(T) = 1.2 × 10-11 (T/300)0.26 exp (6.7 kJ mol-1/R {1/T - 1/300}) cm3 s-1, k2(T) = 1.7 × 10-11 (T/300)6.4 exp (27.6 kJ mol-1/R {1/T - 1/300}) cm3 s-1, k3(T) = 4.4 × 10-12 (T/300)17.8 exp (57.8 kJ mol-1/R {1/T - 1/300}) cm3 s-1. In the cases of 2-cyclopenten-1-one and 2-methyl-2-cyclopenten-1-one, the temperature dependence of the rate coefficients is similar to that calculated or measured for noncyclic conjugated ketones. We also found that the reaction with 3-methyl-2-cyclopenten-1-one was slower, with rate coefficients similar to those measured for the reaction with the saturated cyclic ketone cyclopentanone. To discuss the experimental data, we use potential energy surfaces (PES) calculated at the CCSD(T)/cc-pVTZ//M06-2X/6-311+G** level of theory. RRKM-based Master equation calculations were also performed to infer the most likely reaction products over a wide range of temperatures and pressures. We suggest that both abstraction and addition mechanisms contribute to the overall OH removal, forming radical products stabilized by resonance. We also discuss the relevance for combustion and atmospheric chemistry.

Electrocatalytic Performance of 2D Monolayer WSeTe Janus Transition Metal Dichalcogenide for Highly Efficient H2 Evolution Reaction

Nowadays, the development of clean and green energy sources is the priority interest of research due to increasing global energy demand and extensive usage of fossil fuels, which create pollutants. Hydrogen has the highest energy density by weight among all chemical fuels. For the commercial-scale production of hydrogen, water electrolysis is the best method, which requires an efficient, cost-effective, and earth-abundant electrocatalyst. Recent studies have shown that the 2D Janus transition metal dichalcogenides (JTMDs) are promising materials for use as electrocatalysts and are highly effective for electrocatalytic H2 evolution reaction (HER). Here, we report a 2D monolayer WSeTe JTMD, which is highly effective toward HER. We have studied the electronic properties of 2D monolayer WSeTe JTMD using the periodic hybrid DFT-D method, and a direct electronic band gap of 2.39 eV was obtained. We have explored the HER pathways, mechanisms, and intermediates, including various transition state (TS) structures (Volmer TS, i.e., H*-migration TS, Heyrovsky TS, and Tafel TS) using a molecular cluster model of the subject JTMD noted as W10Se9Te12. The present calculations reveal that the 2D monolayer WSeTe JTMD is a potential electrocatalyst for HER. It has the lowest energy barriers for all the TSs among other TMDs. It has been shown that the Heyrovsky energy barrier (= 8.72 kcal mol-1) in the case of the Volmer-Heyrovsky mechanism is larger than the Tafel energy barrier (= 3.27 kcal mol-1) in the Volmer-Tafel mechanism. Hence, our present study suggests that the formation of H2 is energetically more favorable via the Volmer-Tafel mechanism. This study helps to shed light on the rational design of 2D single-layer JTMD, which is highly effective toward HER, and we expect that the present work can be further extended to other JTMDs to find out the improved electrocatalytic performance.

Illuminating the Role of Mo Defective 2D Monolayer MoTe2 toward Highly Efficient Electrocatalytic O2 Reduction Reaction

The fuel cell is one of the solutions to current energy problems as it comes under green and renewable energy technology. The primary limitation of a fuel cell lies in the relatively slow rate of oxygen reduction reactions (ORR) that take place on the cathode, and this is an all-important reaction. An efficient electrocatalyst provides the advancement of green energy-based fuel cell technology, and it can speed up the ORR process. The present work provides the study of non-noble metal-based electrocatalyst for ORR. We have computationally designed a 3 × 3 supercell model of metal defective (Mo-defective) MoTe2 transition metal dichalcogenide (TMD) material to study its electrocatalytic activity toward ORR. This work provides a comprehensive analysis of all reaction intermediates that play a role in ORR on the surfaces of metal-deficient MoTe2. The first-principles-based dispersion-corrected density functional theory (in short DFT-D) method was implemented to analyze the reaction-free energies (ΔG) for each ORR reaction step. The present study indicates that the ORR on the surface of metal-defective MoTe2 follows the 4e- transfer mechanism. This study suggests that the 2D Mo-defective MoTe2 TMD has the potential to be an effective ORR electrocatalyst in fuel cells.

Nanostructured Pt-Doped 2D MoSe2: An Efficient Bifunctional Electrocatalysts for both Hydrogen Evolution and Oxygen Reduction Reactions

From past two decades, two-dimensional transition metal dichalcogenides (2D TMDs) have dragged a lot of attentions towards electrocatalytic applications. Although, the edges of the 2D TMDs show excellent electrocatalytic performance,...

Raman and electrical transport properties of few-layered arsenic-doped black phosphorus

Black phosphorus (b-P) is an allotrope of phosphorus whose properties have attracted great attention. In contrast to other 2D compounds, or pristine b-P, the properties of b-P alloys have yet to be explored. In this report, we present a detailed study on the Raman spectra and on the temperature dependence of the electrical transport properties of As-doped black phosphorus (b-AsP) for an As fraction x = 0.25. The observed complex Raman spectra were interpreted with the support of Density Functional Theory (DFT) calculations since each original mode splits in three due to P-P, P-As, and As-As bonds. Field-effect transistors (FET) fabricated from few-layered b-AsP exfoliated onto Si/SiO2 substrates exhibit hole-doped like conduction with a room temperature ON/OFF current ratio of ∼103 and an intrinsic field-effect mobility approaching ∼300 cm2 V-1 s-1 at 300 K which increases up to 600 cm2 V-1 s-1 at 100 K when measured via a 4-terminal method. Remarkably, these values are comparable to, or higher, than those initially reported for pristine b-P, indicating that this level of As doping is not detrimental to its transport properties. The ON to OFF current ratio is observed to increase up to 105 at 4 K. At high gate voltages b-AsP displays metallic behavior with the resistivity decreasing with decreasing temperature and saturating below T ∼100 K, indicating a gate-induced insulator to metal transition. Similarly to pristine b-P, its transport properties reveal a high anisotropy between armchair (AC) and zig-zag (ZZ) directions. Electronic band structure computed through periodic dispersion-corrected hybrid Density Functional Theory (DFT) indicate close proximity between the Fermi level and the top of the valence band(s) thus explaining its hole doped character. Our study shows that b-AsP has potential for optoelectronics applications that benefit from its anisotropic character and the ability to tune its band gap as a function of the number of layers and As content.

Singlet-triplet gaps in diradicals obtained with diffusion quantum Monte Carlo using a Slater-Jastrow trial wavefunction with a minimum number of determinants

Diradicals are essential species in a wide range of chemical processes, whereas the computational study of their electronic structure often remains a challenge due to near-degeneracy of the frontier molecular orbitals. The fixed-node diffusion quantum Monte Carlo (FN-DMC) method is employed to calculate adiabatic energy gaps of some typical diradicals with the Slater-Jastrow trial wavefunction. The antisymmetrized part of the trial wavefunction is taken to be a linear combination of a minimum number of determinants using RB3LYP orbitals from the closed-shell singlet state or ROB3LYP orbitals from the triplet state. Our results show that using the two-determinant-Jastrow trial wavefunction is necessary to achieve reliable energy differences between closed-shell singlet states. The energy of the triplet state with MS = 1 is calculated to be lower than that with MS = 0 with FN-DMC even using trial wavefunctions with spin-pure states as their antisymmetrized parts and this difference is reduced with better orbitals. This indicates that the fixed-node error is smaller for the triplet state with MS = 1. Adiabatic energy gaps obtained from the present FN-DMC calculations are in reasonable agreement with available experimental values. Compared with results of the high level EOM-SF-CC method, energy gaps of FN-DMC with RB3LYP orbitals are slightly better than those using ROB3LYP orbitals and results of EOM-SF-CCSD. The present FN-DMC calculations using the simplest ansatz for the trial wavefunction can achieve reasonable results for these diradicals and they can readily be applied to large diradicals.

Intercalation of first row transition metals inside covalent-organic frameworks (COFs): a strategy to fine tune the electronic properties of porous crystalline materials

Covalent-organic frameworks (COFs) have emerged as an important class of nano-porous crystalline materials with many potential applications. Here we present an strategy to control their electronic properties.

Rotational dynamics of the organic bridging linkers in metal–organic frameworks and their substituent effects on the rotational energy barrier

Organic bridging linkers or ligands play an important role in gas and fuel storage, CO₂ capture, and controlling the radical polymerization reactions in metal–organic frameworks (MOFs) nanochannels. The rotation of the linkers causes the expansion of the pore size and pore volume in MOFs. To understand the rotational behavior of organic linkers in MOFs and the substituent effects of the linkers, we investigated the equilibrium structure, stability, potential energy curves (PECs), and rotational energy barriers of the organic bridging linkers of a series of MOF model systems imposing three constrained imaginary planes. Both the dispersion-uncorrected and dispersion-corrected density functional theory (DFT and DFT-D i.e. B3LYP and B3LYP-D3) methods with the correlation consistent double-ζ quality basis sets have been applied to study the model MOF systems [Cu₄(X)(Y)₆(NH₃)₄] (where X = organic bridging linker, and Y = HCO₂). The present study found that the structural parameters and rotational energy barrier of the model MOF containing 1,4-benzendicarboxylate (BDC) linker are in accord with previous experiments. This study reveals that rotational barriers significantly differ depending on the substituents of organic linkers, and the linker dynamical rotation provides information about the framework flexibility with various potential applications in porous materials science. Changing the linkers in the MOFs could be helpful for designing various new kinds of flexible MOFs which will have many important applications in gas storage and separation, catalysis, polymerization, sensing, etc.

Organic bridging linkers or ligands play an important role in gas and fuel storage, CO₂ capture, and controlling the radical polymerization reactions in metal–organic framework (MOF) nanochannels.

Low-temperature Synthesis of Heterostructures of Transition Metal Dichalcogenide Alloys (WxMo1-xS2) and Graphene with Superior Catalytic Performance for Hydrogen Evolution

Large-area (∼cm²) films of vertical heterostructures formed by alternating graphene and transition-metal dichalcogenide (TMD) alloys are obtained by wet chemical routes followed by a thermal treatment at low temperature. In particular, we synthesized stacked graphene and W_xMo_1-xS₂ alloy phases that were used as hydrogen evolution catalysts. We observed a Tafel slope of 38.7 mV dec^-1 and 96 mV onset potential (at current density of 10 mA cm^-2) when the heterostructure alloy was annealed at 300 °C. These results indicate that heterostructures formed by graphene and W_0.4Mo_0.6S₂ alloys are far more efficient than WS₂ and MoS₂ by at least a factor of 2, and they are superior compared to other reported TMD systems. This strategy offers a cheap and low temperature synthesis alternative able to replace Pt in the hydrogen evolution reaction (HER). Furthermore, the catalytic activity of the alloy is stable over time, i.e., the catalytic activity does not experience a significant change even after 1000 cycles. Using density functional theory calculations, we found that this enhanced hydrogen evolution in the W_xMo_1-xS₂ alloys is mainly due to the lower energy barrier created by a favorable overlap of the d-orbitals from the transition metals and the s-orbitals of H₂; with the lowest energy barrier occurring for the W_0.4Mo_0.6S₂ alloy. Thus, it is now possible to further improve the performance of the "inert" TMD basal plane via metal alloying, in addition to the previously reported strategies such as creation of point defects, vacancies and edges. The synthesis of graphene/W_0.4Mo_0.6S₂ produced at relatively low temperatures is scalable and could be used as an effective low cost Pt-free catalyst.