Search Results (723)

The emission of hydrogen cyanide (HCN) from formaldehyde (CH₂O) during ammonia-assisted selective catalytic reduction (NH₃-SCR) remains a critical challenge for aftertreatment of bio-hybrid fuel combustion exhaust. The mechanistic details of HCN formation are still poorly understood, especially on widely deployed commercial catalysts like Cu-SSZ-13. In this work, we employed density functional theory calculations in combination with the Energetic Span Model to elucidate HCN formation pathways from CH₂O in the presence of NO₂ and H₂O over Cu-SSZ-13. The results revealed the HCN formation pathway with intermediate methylene imine as the dominant one under typical reaction conditions. These findings resonate very well with reports of hexamethylenetetramine (HMT) formation during NH₃-SCR with CH₂O, for which methylene imine is a critical intermediate. Turnover frequency (TOF) estimations highlighted the strong influence of NO₂ and H₂O: higher NO₂ concentrations promoted CO selectivity and suppressed HCN by oxidizing CH₂O to HCOOH, while lower H₂O enhanced HCN formation. These findings establish a detailed mechanistic framework for HCN emission on Cu-SSZ-13 and suggest that controlling NO₂/NO_x ratios and water content can mitigate HCN formation during NH₃-SCR. Full article

The crystal molecular structure of the co-crystal of boric acid (BA) with triphenylphosphine oxide (TPO) 1:2, H₃BO₃∙2Ph₃PO, was determined by single-crystal X-ray diffraction. In the crystal, pairs of the boric acid molecules are connected to each other by hydrogen bonds, and each acid molecule forms two hydrogen bonds with the oxygen atoms of two triphenylphosphine oxide molecules. The crystal packing of the new complex was compared with the crystal packings of the known 1:1 complex of boric acid and triphenylphosphine oxide, and boric acid and triphenylphosphine oxide themselves. Also, the energetic preferences and densities of the co-formers (BA and TPO) were compared for their pure forms and co-crystals. It is shown that more tight crystal packing is not necessarily accompanied by higher stability. Full article

Surface segregation in metal alloys critically determines their electrocatalytic performance, yet how chemisorbed oxygen alters segregation behavior under reaction conditions remains poorly understood. Using density functional theory, we quantify the segregation energies on the (111) surface of PdM (M = Co, Ru, Pt) alloys with chemisorbed atomic oxygen. In vacuum, all three alloying elements exhibit positive segregation energies (0.28 eV for Co, 0.40 eV for Ru, and 0.04 eV for Pt) on the topmost layer, indicating that surface segregation is energetically unfavorable. Upon oxygen adsorption, however, this trend reverses for Co and Ru: their segregation energies shift by −0.18 eV and −0.33 eV, respectively, driving these atoms strongly toward the surface. In contrast, Pt shows only a marginal shift of 0.03 eV, retaining its preference for the bulk. Further analysis of oxygen adsorption and the associated electronic structure reveals that the strength of surface–adsorbate binding governs these segregation trends under reactive conditions. The present work offers a theoretical foundation for the rational design of Pd-based alloy catalysts for applications such as the hydrogen evolution reaction. Full article

The synthesis of n-bromobutane from n-butanol is a classic undergraduate organic chemistry experiment, primarily intended to illustrate the bimolecular nucleophilic substitution (S_N2) mechanism. However, this experiment is commonly plagued by low yields and the formation of byproducts (e.g., n-butene and di-n-butyl ether), which confuse students. To reveal the molecular origin of these competitive pathways, this study employs density functional theory (DFT) calculations to systematically investigate the reaction mechanism under acid catalysis. Four potential reaction pathways were explored: S_N2 substitution, E2 elimination, intermolecular etherification, and a high-energy E2 pathway. The computational results indicate that the S_N2 pathway to n-bromobutane is kinetically and thermodynamically favorable due to its low energy barrier. In contrast, the E2 elimination pathway possesses a higher energy barrier (18.8 kcal/mol vs. 13.5 kcal/mol for S_N2), explaining why elevated temperatures favor the formation of n-butene. Moreover, the etherification pathway was found to be the most energetically demanding, consistent with the trace amounts of di-n-butyl ether observed experimentally. These findings provide a quantitative molecular-level rationale for the strict temperature control and standardized reagent addition sequences in the laboratory protocol. By visualizing the potential energy surfaces, this computational approach bridges the gap between theoretical mechanism and practical operation, offering a valuable pedagogical tool for enhancing student understanding. Full article

CO₂ adsorption on subnanometric metal clusters is highly sensitive to the computational protocol used to describe the potential energy surface, particularly when several low-lying geometries and spin states are accessible. In this work, CO₂ adsorption on Cu₄ and Sc₄ clusters was investigated using density functional theory (DFT) to evaluate how the choice of functional/basis-set protocol, spin multiplicity, initial geometry, and vibrational stability affects the predicted adsorption behavior. Four representative computational protocols (TPSSh, r²SCAN-3c, PBE-D4/def2-TZVP, and PBE0-SDD) were assessed for isolated clusters and cluster–CO₂ complexes. The lowest harmonic vibrational frequency, ω_min, was used as a diagnostic criterion to distinguish true minima from unstable or weakly defined stationary points. Selected cases were also cross-checked using the ORCA and Gaussian quantum-chemistry packages to assess whether comparable computational settings yielded consistent stationary-point character. The results show that Cu₄ generally exhibits weak CO₂ binding, whereas Sc₄ displays stronger but more protocol-dependent adsorption, consistent with its higher structural flexibility and more pronounced Lewis-acid character. Low-frequency and imaginary modes were found in several optimized structures, indicating that adsorption energies should not be interpreted without prior vibrational validation. The comparison also shows that variations in functional/basis-set treatment and spin multiplicity can alter both the optimized geometry and the predicted adsorption strength. Therefore, CO₂ adsorption on small metal clusters should be discussed using combined structural, vibrational, and energetic criteria rather than electronic adsorption energies alone. Overall, this study provides a protocol-oriented framework for evaluating the reliability of DFT predictions in CO₂ adsorption on Cu₄ and Sc₄ clusters. Full article

Objectives: This study aimed to elucidate the multi-target therapeutic mechanisms of Camellia sinensis phytochemicals in periodontitis using an integrative multi-scale molecular modeling strategy. Methods: An integrated in silico strategy was employed, incorporating network-based pharmacological analysis, protein interaction network evaluation, molecular docking assessment, density functional theory (DFT) computations, molecular dynamics (MD) trajectory analysis, MM/PBSA-derived binding energy estimation, and residue-level energetic contribution profiling. Overlapping targets between C. sinensis and periodontitis-associated genes were identified, followed by topological screening to determine crucial hub proteins. The most promising target was subjected to detailed structural and energetic evaluation. Results: Intersection analysis identified 23 common targets, with AKT1, myeloperoxidase (MPO), MMP2, MMP3, MMP9, STAT1, IL2, BCL2, ESR1, and SERPINE1 emerging as central hubs. Functional enrichment highlighted AGE–RAGE and JAK–STAT signaling pathways and extracellular matrix remodeling processes. Docking revealed MPO as the most favorable core target. Gallate-containing catechins, particularly (−)-gallocatechin gallate (−9.63 kcal/mol) and gallocatechin 3-O-gallate (−9.52 kcal/mol), exhibited more favorable binding affinities than the standard inhibitor 4-ABAH (−6.02 kcal/mol). DFT analysis demonstrated moderate HOMO–LUMO gaps (4.31–4.78 eV) and favorable dipole moments supporting electronic stability and reactivity. MD simulations confirmed stable complex formation over 100 ns, with persistent hydrogen bonding and consistent ligand retention. MM/PBSA calculations further validated a favorable binding of (−)-gallocatechin gallate (−27.66 ± 7.53 kcal/mol) and gallocatechin 3-O-gallate (−26.09 ± 8.96 kcal/mol), comparable to or exceeding 4-ABAH (−25.88 ± 4.44 kcal/mol). Conclusions: C. sinensis phytochemicals, particularly gallate-containing catechins, exhibit stable, energetically favorable interactions with MPO, supporting their potential as competitive inhibitors that modulate oxidative stress and inflammatory pathways in periodontitis. Full article

The stability of the competing orthorhombic Pnma and monoclinic P2₁/c phases of Praseodymium pentaphosphate (PrP₅O₁₄) have been studied using density functional theory (DFT). At 0 K, the Pnma structure is found to be preferred over the P2₁/c one with the enthalpy change with pressure of both phases highlighting a shift in phase preference from Pnma to P2₁/c at ∼2.5 GPa. Independently of the predicted high-pressure structural phase transition at 0 K, our computed elastic properties and phonon dispersion bands as a function of pressure indicate a phonon instability at ∼4.5 GPa due to the appearance of imaginary frequencies, followed by a dynamical instability at 8.5 GPa due to the violation of the Born criteria on the Pnma structure of PrP₅O₁₄. These results eliminate the orthorhombic structure as a possible high-pressure candidate for the monoclinic P2₁/c polymorph. Furthermore, the relative stability of the orthorhombic and monoclinic polymorphs has been evaluated at ambient pressure and as a function of temperature by means of vibrational free-energy calculations. The results indicate a free-energy crossing at 42 K, with the Pnma phase being energetically favored from 0 K to 42 K, after which the P2₁/c phase becomes preferred. These results demonstrate why PrP₅O₁₄ can only be obtained at ambient pressure in the monoclicnic P2₁/c polymorph, different to other rare earth pentaphosphates. Full article

Sulfur is a well-known catalyst poison, particularly for catalysts based on transition metals. Herein, we studied the adsorption of sulfur species on small nanoparticles (~1 nm in size) of the face centered cubic (fcc) transition metals (Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au) using density functional theory (DFT) modeling. At low sulfur coverage (one S atom per nanoparticle), sulfur preferentially occupies the surface hollow sites of the nanoparticles. At higher coverage, however, the subsurface diffusion of S in Ni, Pd, and Ag nanoparticles becomes energetically favorable with low activation energies. Among the considered metals, sulfur binds most strongly to Rh and Ir, and most weakly to Ag and Au. The present results shed light on the facility of S-poisoning on such metal nanoparticles, either by surface blocking or by underlying sulfurization of the metal. Full article

Hydrogen trapping at carbide/matrix interfaces is important for improving the resistance of steels to hydrogen embrittlement. In this work, the segregation behavior of hydrogen at the coherent α-Fe/V₄C₃ interface was investigated by first-principles calculations. Representative hydrogen sites were considered systematically, including interstitial sites in the near-interface region, interfacial sites, and carbon-vacancy sites in V₄C₃. All of the sites examined are energetically favorable for hydrogen trapping, but the carbon vacancy inside V₄C₃ exhibits the strongest trapping tendency. Charge density, Bader charge, and density-of-states analyses indicate that hydrogen at this site gains more electrons and forms stronger interactions with neighboring V atoms, leading to enhanced stability. The behavior of H₂ at the internal carbon vacancy was also evaluated. After structural relaxation, the H₂ molecule dissociated into two separate H atoms, indicating that hydrogen is more stably trapped in atomic rather than molecular form. These findings reveal the crucial role of carbon vacancies in regulating hydrogen trapping at the α-Fe/V₄C₃ interface and provide atomic-scale insight into the hydrogen trapping mechanism of vanadium carbide precipitates in steels. Full article

This paper presents the results of numerical calculation of steady-state magnetorheological fluid flow in the gap of an eccentric seal subjected to an external radial magnetic field. A coupled problem combining magnetic field analysis and laminar viscoplastic flow with Bingham rheology is solved to obtain pressure and velocity distributions within the seal gap, from which the hydrodynamic reaction forces of the fluid film and the rotor friction torque are determined. A parametric study was conducted in the ranges of rotor angular velocity ω = 100–400 rad/s, relative eccentricity ε = 0–0.9, and magnetic flux density B₀ = 0–0.5 T at the pressure differential Δp = 2 atm. Analysis of the results shows that increasing the magnetic flux density from 0 to 0.5 T leads to an increase in the seal reaction force from 12 N to 642 N and the friction torque from 0.35 N·m to 11.23 N·m. The most intensive growth of both characteristics is observed in the range B₀ = 0–0.3 T, beyond which saturation occurs as the MRF yield stress reaches its plateau value. An optimal control range of B₀ = 0.1–0.2 T was determined, ensuring maximum seal energetic efficiency as quantified by the load capacity-to-friction torque ratio, which is maximized at 70 N/(N·m). Based on the obtained results, the consequences of using magnetorheological seals on the performance of the rotor system are discussed, including the analysis of the sealing effect on rotor-dynamic stability. Within the proposed optimal range, it is shown that an increase in magnetic flux density leads to a sign reversal of the horizontal reaction F₂, while the monotonic growth of the ratio |F₂|/F₁ indicates an intensification of cross-coupling and a corresponding reduction in the rotordynamic stability margin at higher values of B₀. Full article

Understanding the adsorption mechanisms of anions onto functional monomers is crucial for various applications in environmental remediation and chemical separation. In this study, we investigated the interactions of ReO₄⁻, Cl⁻, SO₄²⁻, and F⁻ with several organic functional monomers featuring aliphatic chains, cyclic saturated or unsaturated rings, and NH/NH₂ functional groups through density functional theory calculations. Employing a rigorous multilevel optimization strategy, we explored the geometric and energetic features of their complexes, focusing on hydrogen bonding and anion–heterocycle interactions. Our results highlight the strong affinity of ReO₄⁻, Cl⁻, SO₄²⁻ for amine-type functional monomers, while also revealing the distinct interaction patterns of F⁻ with aromatic rings containing nitrogen. This comprehensive analysis elucidates diverse binding mechanisms, providing insights into designing effective adsorbents for selective anion capture. Full article

High-value metabolites, such as antibiotics and enzymes, are primarily produced using filamentous fungi. However, their morphological complexity increases broth viscosity during biomass growth, hindering industrial scale-up by impairing both power input and mass transfer. The interaction between biomass growth, rheology, power input, and oxygen transfer is first addressed here by evaluating mycelial rheology and determining the volumetric mass transfer coefficient (k_La) (dynamic method) and oxygen uptake rate (respirometry) across different operating conditions. These confirmed that the mycelial broth’s pseudoplastic behavior significantly influences volumetric power input and k_La correlations. However, specific power input analysis revealed that operating at higher stirring rates (800 rpm) at higher cell-density cultures is 28.17% more energetically efficient than at low speeds (500 rpm). Furthermore, the oxygen supply-to-demand ratio, calculated via Excel model-fitting, allowed for the estimation of “metabolic power input” which represents the required energy to fit oxygen demand. Results also reveal that at 3.67 ± 0.34 g L⁻¹ of biomass effectively channel up to 51% of total energy toward aerobic metabolism, compared to only 17–30% for 0.73 ± 0.01 g L⁻¹ of biomass. These findings show that volumetric power inputs around 4 kW m⁻³ improve oxygen transfer efficiency, even at relatively high biomass concentrations. Full article

Metal-functionalized boron nitride nanostructures represent promising platforms for lightweight solid-state hydrogen storage. In this work, we perform a comprehensive density functional theory (DFT) investigation of pristine and metal-decorated B₄N₄ quantum dots (M = Li, Ti) to evaluate their structural stability, adsorption energetics, and near-ambient storage performance. Pristine B₄N₄ is highly stable but interacts weakly with H₂ (E_ads ≈ −0.12 eV), leading to negligible uptake under operating conditions. Li decoration moderately enhances adsorption through charge-induced polarization (E_ads ≈ −0.15 eV) but offers limited stabilization beyond the first few molecules. In contrast, Ti decoration fundamentally reshapes the interaction landscape, strengthening electrostatic, polarization, and dispersion contributions and enabling significantly stronger yet reversible H₂ binding (E_ads ≈ −0.36 eV). Sequential adsorption calculations predict maximum theoretical capacities of 14, 18, and 20 H₂ molecules for pristine, Li-, and Ti-decorated systems, respectively. Grand canonical thermodynamics show that Ti–B₄N₄ retains nearly its full loading at 30 bar and 298 K, while pristine and Li-decorated clusters store only negligible amounts. Under desorption conditions (3 bar, 373 K), Ti–B₄N₄ releases most of its stored hydrogen, yielding an exceptional reversible capacity of 15.1 wt%. Energy decomposition analysis attributes this performance to cooperative electrostatic, polarization, and dispersion enhancements. Ti–B₄N₄ emerges as a highly promising theoretical candidate, warranting future experimental validation. Full article

First-principles calculations based on density functional theory (DFT) are a powerful tool in data-oriented materials research. The choice of approximation for the exchange-correlation functional is crucial, as it strongly affects the accuracy of DFT calculations. This study compares the performance capabilities of three approximations on the energetics, mechanical and electronic properties, and crystal structure of NaAlP₂O₇, which is an insulator with a wide band gap that suppresses its electronic conductivity. Two of these approximations are based on Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation (GGA) and the other on the strongly constrained and appropriately normed (SCAN) meta-GGA. We explore these materials as a contribution to the development of new solid electrolytes (SEs) for sodium-ion batteries (NIBs), which have the potential to mitigate challenges related to lifecycle, safety, and low ionic conductivity. The performance of these batteries largely emanates from the extraordinary demand for high-performing energy storage technologies. This study revealed that PBEsol accurately predicted lattice parameters that closely aligned with experimental values. However, r2SCAN provided the most reliable predictions of the structural and electronic properties of the NaAlP₂O₇ solid electrolyte compared to PBE and PBEsol. Findings demonstrated that the material is structurally, mechanically, electronically, and thermodynamically stable, but exhibits vibrational instability, which may scatter ions and reduce ionic conductivity due to the presence of imaginary frequencies. Our results highlight the importance of selecting appropriate functionals for solid electrolyte DFT computations. The r2SCAN functional appears to be a promising choice for calculating NaAlP₂O₇ properties. Full article

Designing sustainable pedestrian infrastructure in hyper-arid cultural landscapes requires balancing visitor experience, heritage protection, and environmental constraints. This study develops a statistically grounded model for planning sustainable walking trails in Al-Ula, Saudi Arabia, using multi-spectral remote sensing data integrated with expert-based evaluation. A GIS-based Multi-Criteria Decision-Making (MCDM) framework was applied to assess topographic slope, vegetation cover (NDVI), built-up density (NDBI), Land Surface Temperature (LST), and solar exposure. Indicator weights were validated through a three-round Delphi survey involving fifteen experts. The results indicate strong consensus among experts, identifying LST (21%) and slope (20%) as the most influential determinants of trail suitability in desert environments. These findings highlight the critical role of thermal stress in shaping safe and sustainable pedestrian mobility in hot climates. The optimized 44.5 km trail network, classified into three difficulty levels, improves energetic efficiency by reducing caloric expenditure by 24% compared to conventional routing. In addition, the proposed network has the potential to reduce carbon emissions associated with heritage-related travel by approximately 75% through modal shift from vehicles to walking. The framework provides a practical decision-support tool for planners seeking to develop low-carbon, climate-responsive tourism infrastructure aligned with the objectives of Saudi Arabia’s Vision 2030. Full article