Theoretical insights into C-H bond activation of methane by transition metal clusters: the role of anharmonic effects

Facile Low-Temperature synthesis of novel carbon nitrides for efficient conversion of carbon dioxide into Value-Added chemicals

The interest in using carbon nitrides (CN) for CO2 conversion has stimulated extensive research on CN synthesis. Herein, we report the synthesis of two novel CN materials using low-cost commercially available precursors at low temperatures in a short duration of time. Two CN materials, one derived from 5-amino tetrazole (named 4NZ-CN) and the other derived from 3, 5-diamino-1, 2, 4-triazole (named 3NZ-CN) precursors, are prepared by refluxing these precursors for 2 h at 100 °C. 4NZ-CN and 3NZ-CN catalysts show higher surface areas (55.80 and 52.00 m2 g-1) and more basic sites (10.05 and 5.65 mmol g-1) than the conventional graphitic carbon nitride (g-C3N4) derived from melamine, for which the corresponding values are 9.20 m2 g-1 and 0.62 mmol g-1, respectively. In addition, both CN exhibit a 3-fold higher catalytic activity for CO2 cycloaddition to epoxides than g-C3N4. The structure-activity relationship was ascertained using a combination of experimental and computational studies, and a catalytic mechanism was proposed. This work provides a facile strategy for the synthesis of novel CN materials at relatively low temperatures, and the developed catalysts show remarkable performance in the conversion of CO2 to value-added chemicals.

Rashba and Dresselhaus effects in doped methylammonium lead halide perovskite MAPbI3

Inorganic-organic lead halide perovskites, particularly methylammonium lead halide (MAPbI3) perovskite, have been regarded as promising materials for optoelectronics and spintronics. However, the practical applications of these perovskites are limited by lead toxicity and instability under air and pressure. This study investigates the substitution of Pb with Sn and Ge in cubic MAPbI3 perovskite. The properties of the resulting hybrid perovskites are compared using state-of-the-art first-principles-based methodologies, viz., density functional theory (DFT) with generalized gradient approximation (PBE) and hybrid functional (HSE06), in conjunction with spin-orbit coupling (SOC). Here, we mainly study the Rashba-Dresselhaus (RD) effect, which arises due to two major mechanisms: (i) the breaking of inversion symmetry (static and dynamic) and (ii) SOC, originating from the presence of heavy elements. We find significant spin-splitting effects in the conduction band minimum and valence band maximum for hybrid perovskites. To gain a deeper understanding of the observed spin-splitting, the spin textures are analyzed, and Rashba coefficients are calculated. We find that the Dresselhaus effect comes into play in substituted hybrid structures in addition to the usual Rashba effect observed in the pristine compound. Additionally, we observe that the strength of Rashba spin-splitting is substantially tuned by the application of uniaxial strain (±5%). Moreover, certain hybrid perovskites exhibit mechanical stability and ductility, making them potential candidates in perovskite-based optoelectronics and spintronics applications.

O-Vacancy Mediated Partially Inverted Ferrospinels for Enhanced Activity in the Sulfuric Acid Decomposition for Hydrogen Production

Understanding the characterization of a tailored Co3O4 spinel with Fe3+ doping poses a challenge due to the surface state complexity in bifunctional catalysts with higher cation diversity. Doping with secondary metal results in a double spinel structure (a hybrid of normal and inverted spinels). This enhances the catalytic properties by generating more active oxygen vacancies. The cobalt-rich (FeCo2O4) hybrid spinel and iron-rich (CoFe2O4) inverted spinel are synthesized using a wet impregnation method, supported over oxidized SiC (SiC-Pretrt) for an improved metal-support interaction. FeCo2O4 on pretreated SiC exhibits the highest catalytic activity (90% conversion at 1173 K) and stability (over 100 h) in sulfuric acid decomposition of the iodine-sulfur process for hydrogen production. This improved performance is attributed to the high electronegativity of Co3+, oxygen vacancies, and strong metal-support interaction. The high electronegativity of Co3+ weakens the S-O bond in M-S-O, enhancing the catalytic activity of the spinels. These results are further corroborated by detailed characterization and density functional theory calculations.

The nature of stability and adsorption interactions of binary Au-Li clusters with bridge adsorption structures

Earlier findings have confirmed that CO molecules have propensities to adsorb on low-coordinated gold atoms (top sites) of Au-based clusters, which can be treated by the Blyholder model wherein the σ donation and π-back donation take place. Here, the structural features and stability of (AuLi)_n (n = 1-9) clusters were first analyzed using the GA-DFT method. The new adsorption modes, vibration frequencies and electronic interactions for Au-Li clusters with CO were investigated in detail. More excitingly, we found that CO prefers to adsorb on the bridge sites of the Au-Li clusters rather than on the top sites, which are much lower in energies than the top adsorptions, and the C-O stretching frequencies are also red-shifted. AIMD simulations show that the adsorption structures still have good thermal stability at 500 K. The density of states reveals that the electronic structures of Au-Li clusters have excellent stability for the bridge adsorptions of CO molecules. The ETS-NOCV analysis and NPA charges show that the direction of charge flow is from Au-Li clusters → CO. Our study provides an idea to elucidate the new adsorption mechanism on Au-Li clusters and the connection between the geometries and reaction properties.

Relative Populations and IR Spectra of Cu38 Cluster at Finite Temperature Based on DFT and Statistical Thermodynamics Calculations

The relative populations of Cu₃₈ isomers depend to a great extent on the temperature. Density functional theory and nanothermodynamics can be combined to compute the geometrical optimization of isomers and their spectroscopic properties in an approximate manner. In this article, we investigate entropy-driven isomer distributions of Cu₃₈ clusters and the effect of temperature on their IR spectra. An extensive, systematic global search is performed on the potential and free energy surfaces of Cu₃₈ using a two-stage strategy to identify the lowest-energy structure and its low-energy neighbors. The effects of temperature on the populations and IR spectra are considered via Boltzmann factors. The computed IR spectrum of each isomer is multiplied by its corresponding Boltzmann weight at finite temperature. Then, they are summed together to produce a final temperature-dependent, Boltzmann-weighted spectrum. Our results show that the disordered structure dominates at high temperatures and the overall Boltzmann-weighted spectrum is composed of a mixture of spectra from several individual isomers.

Theoretical Prediction of Structures, Vibrational Circular Dichroism, and Infrared Spectra of Chiral Be4B8 Cluster at Different Temperatures

Lowest-energy structures, the distribution of isomers, and their molecular properties depend significantly on geometry and temperature. Total energy computations using DFT methodology are typically carried out at a temperature of zero K; thereby, entropic contributions to the total energy are neglected, even though functional materials work at finite temperatures. In the present study, the probability of the occurrence of one particular Be4B8 isomer at temperature T is estimated by employing Gibbs free energy computed within the framework of quantum statistical mechanics and nanothermodynamics. To identify a list of all possible low-energy chiral and achiral structures, an exhaustive and efficient exploration of the potential/free energy surfaces is carried out using a multi-level multistep global genetic algorithm search coupled with DFT. In addition, we discuss the energetic ordering of structures computed at the DFT level against single-point energy calculations at the CCSD(T) level of theory. The total VCD/IR spectra as a function of temperature are computed using each isomer's probability of occurrence in a Boltzmann-weighted superposition of each isomer's spectrum. Additionally, we present chemical bonding analysis using the adaptive natural density partitioning method in the chiral putative global minimum. The transition state structures and the enantiomer-enantiomer and enantiomer-achiral activation energies as a function of temperature evidence that a change from an endergonic to an exergonic type of reaction occurs at a temperature of 739 K.