Reaction Kinetics, Mechanisms and Catalysis最新文献

A semi-analytical study is presented on the nonlinear reaction–diffusion behavior of Langmuir–Hinshelwood (LH) kinetics under strong adsorption in heterogeneous catalytic systems. The governing model is a nonlinear reaction–diffusion equation, where nonlinearity stems from surface adsorption and intrinsic nth order kinetics. Closed-form analytical solutions for concentration profiles are developed using three distinct semi-analytical methods and verified against numerical simulations, showing excellent agreement. Limiting cases are examined to provide mechanistic insights, while parametric analysis highlights the effects of adsorption strength and kinetic parameters on concentration distribution and effectiveness factor. The proposed framework provides an efficient and accurate tool for understanding and optimizing nonlinear reaction–diffusion phenomena in heterogeneous catalysis.

This study investigates a chromium(III)-integrated silica gel (SG-Cr) synthesized using the sol–gel method with tetraethoxysilane (TEOS) and chromium(III) chloride hexahydrate. X-ray diffraction (XRD) analysis has indicated an amorphous nature of the composite. Scanning electron microscopy (SEM) analysis revealed a rough surface texture. Fourier-transform infrared spectroscopy (FTIR) results have confirmed the formation of a siloxane network and demonstrated the successful incorporation of chromium through interactions with silanol (Si–OH) groups. The SG-Cr composite was used as an adsorbent material for methylene blue (MB) and methyl orange (MO). It has shown the good performance in removing methyl orange (MO) dye from water. At 10 mg/L (MB) and 30 mg/L (MO), the removal efficiencies have reached 24.2% and 96.51% respectively. The adsorption performance was observed to be pH-dependent with the acidic conditions (pH 2) for the MO (anionic dye) removal due to electrostatic attraction between the protonated surface, whereas alkaline conditions (pH 11) has enhanced MB (cationic dye) uptake via interactions with deprotonated silanol groups. These results have shown the potential of SG-Cr suitable in the wastewater treatment, particularly for anionic contaminants in the acidic environments.

In this study, we present a simple and efficient microwave-assisted solid-phase synthesis of nitrogen-doped carbon dots (NCDs) using citric acid and 2-aminobenzimidazole as precursors. The synthesis conditions were systematically optimized, and the resulting NCDs were extensively characterized for their physicochemical and optical properties. Transmission electron microscopy revealed that the NCDs are spherical in shape with an average diameter of 5 ± 1 nm. X-ray photoelectron spectroscopy confirmed successful nitrogen incorporation into the carbon framework. The NCDs exhibited bright blue fluorescence under 320 nm UV Light, a high quantum yield of 12.6%, excellent water dispersibility, and strong photostability under varying environmental conditions. The catalytic performance of the NCDs was evaluated through NaBH₄-assisted reduction of malachite green (MG) and crystal violet (CV) dyes. In both cases, over 90% degradation was achieved within 12 min. The reaction kinetics followed a pseudo-first order model, with rate constants of 0.070 ± 0.01 min⁻¹ for MG and 0.139 ± 0.02 min⁻¹ for CV. These results highlight the potential of the synthesized NCDs as effective and eco-friendly catalysts for the rapid removal of dye pollutants in wastewater treatment applications.

The effect of the crystalline phase of manganese dioxide on aqueous-phase hydrodgenation of phenol over MnO₂-supported Ru catalysts was examined. Various Ru catalysts supported on different manganese dioxide (α-MnO₂, δ-MnO₂, and γ-MnO₂) were prepared using a wet impregnation method. The prepared catalysts were characterized using X-ray diffraction (XRD), Transmission electron microscope (TEM), hydrogen temperature-programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). Among these catalysts, Ru/δ-MnO₂ exhibited superior catalytic activity with complete phenol conversion and cyclohexanol selectivity. The characterization analysis revealed that δ-MnO₂ exhibited the highest oxygen vacancy (O_V) concentration (33%) among the studied phases. This elevated O_V content significantly promoted water molecule adsorption, thereby facilitating proton transfer across the MnO₂ surface and consequently improving hydrogenation performance. The study establishes a clear quantitative correlation between MnO₂ crystal structures and oxygen-mediated metal-support interactions, offering valuable insights for designing efficient hydrogenation catalysts applicable to both bio-oil refinement and wastewater purification processes.

The model considered here represents a cubic nonlinear reaction–diffusion process where an alosteric enzyme is being activated by its reaction product and inhibited by the influence of the substrate reactant. This model relies on the earlier seminal approach established to mimic some oscillations in mitosis and the glycolytic oscillations of adenosine triphosphate substrate (inhibitor) and adenosine diphosphate product (activator). Our aim was to examine whether the inclusion of product and substrate diffusion can cause the symmetry breaking instability under the influence of small perturbations. The perturbations are supposed to be spatially harmonic and temporally exponentially growing. The careful analysis clearly demonstrates that necessary and sufficient condition for the appearance of symmetry breaking instability and possible morphogenesis could be achieved if the value of diffusion coefficient of substrate reactant is remarkably greater than the corresponding value of product. We expect that under suitable technological conditions where the diffusion of reactants can be controllably tuned, the reaction of this type can lead to morphogenesis (inhomogeneous spatial distribution of reactants). We have proved that in the case of glycolysis under normal physiological conditions the symmetry breaking instability is not possible, which is important since the spatial distribution of pertaining activators and inhibitors should be uniform. Otherwise, their nonuniform distribution is inherent in cancer cells.

In this study, a series of La-α-MnO₂ catalysts with different La doping amounts were prepared by hydrothermal method. It was found that the introduction of La could reduce the crystallinity of α-MnO₂ and increase the specific surface area of the catalyst. Furthermore, La doping can significantly weaken the Mn–O bond, increase the surface Mn³⁺ and O_ads content, and enhance the oxygen mobility and low-temperature reducibility. The catalytic results indicated that La doping could significantly improve the catalytic oxidation performance of toluene, and 1%La-α-MnO₂ catalyst exhibited the best catalytic activity for toluene oxidation. The toluene conversion rate reaches 90% when the reaction temperature is only 215 °C. In addition, 1%La-α-MnO₂ catalyst presented good stability and repeatability during 50 h durability test under water vapor.

The development of Pt-based catalysts with high catalytic activity and stability is the key to improving the electrooxidation performance of dimethyl ether (DME). Increasing the number of active sites in the catalyst can mitigate performance degradation caused by reduced Pt loading. In this study, we propose a core–shell structured composite support where tin dioxide (SnO₂) is encapsulated within carbon nanotubes (CNTs) to form CNTs@SnO₂, followed by Pt deposition via a solvothermal method. This unique architecture prevents SnO₂ from covering Pt deposition sites on the outer CNTs surface. The encapsulated SnO₂ modifies the CNTs surface, providing abundant OH_ads groups that facilitate the removal of poisoning CO_ads intermediates and stabilize Pt nanoparticles. The Pt/CNTs@SnO₂ exhibites a high electrochemical surface area (ESA, 89.85 m² g⁻¹) and mass activity (MA, 312.33 mA mg_Pt⁻¹). The accelerated potential cycling tests (APCT) show that the ESA of the Pt/CNTs@SnO₂ catalyst is only attenuated by 26.88% after 5000 cycles. These results demonstrate that Pt/CNTs@SnO₂ can maximize Pt utilization and co-catalytic effects, leading to superior catalytic activity and stability for DME electrooxidation compared to conventional Pt/CNTs, Pt/CNTs+SnO₂ and Pt/SnO₂-CNTs catalysts. This work provides a promising strategy for designing high-performance Pt-based catalysts for fuel cell applications.

Nine novel non-metallocene titanium-based catalysts (Z1–Z9) were synthesized and evaluated for ethylene polymerization to produce polyethylene wax (PE-WAX). The polymerization conditions were systematically optimized, and the structure–activity relationships of the catalysts were elucidated. Structural differences among the catalysts significantly influenced their performance: alkoxy catalysts (Z1–Z3), featuring aliphatic ligands, exhibited lower activity compared to aryloxy catalysts (Z4–Z9) with aromatic ligands. Specifically, ortho-substituted aryloxy catalysts Z7 and Z9 demonstrated the highest activity due to optimized steric and electronic effects from their substituent positions. A clear correlation was observed between polymer molecular weight and catalyst burial volume (V_Bur), where larger V_Bur values hindered ethylene monomer insertion, promoting chain transfer and resulting in shorter polymer chains. With the highest-activity Z7 catalyst, density functional theory calculations clarified the active center morphology, identifying two distinct Ti-alkyl intermediates existing stably simultaneously. Based on these findings, a mechanistic model for the formation of PE-WAX was proposed, wherein the synergistic effect of dual active centers lowered the chain-transfer energy barrier, enhanced catalytic activity, and allowed the controlled synthesis of low-molecular-weight PE-WAX with high crystallinity and a narrow molecular weight distribution.

In this study, a silicon-bridged diphosphine/CrCl₃(C₄H₈O)₃/modified methylaluminoxane catalytic system (PNSiP/CrCl₃(THF)₃/MMAO) was constructed, to explore the possibility of producing high-carbon olefins via ethylene/light α-olefins co-oligomerization. The choice of α-olefin monomer, ethylene pressure, and reaction temperature played crucial roles in determining the distribution of products. In presence of ethylene and 1-hexene, the primary high-carbon products are C₁₀ olefins. In contrast, when ethylene and 1-octene are present, the main high-carbon products are C₁₂ olefins. Increasing temperature and reducing pressure can promote the oligomerization of α-olefins with ethylene, thereby significantly improving the selectivity of high-carbon products. Detailed identities of C₁₀-C₁₄ olefins, assigned by gas chromatographic and mass spectrometric, strongly supported a mechanism involves five-, seven- and nine-membered metallacyclic intermediates composed by ethylene and α-olefins units. Mechanistic analysis of C₁₂ and C₁₄ isomers revealed that co-trimerization and co-tetramerization reactions occurred concurrently during the reaction process. Furthermore, the concentration of specific carbon-number olefins, along with the reaction conditions, influenced the relative probability of the two pathways.

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