Mathematical Modeling and Simulation of Nonlinear Process in Enzyme Kinetics

A deep and analytical understanding of the enzyme kinetics has attracted a great attention of scientists from biology, medicine, chemistry, and pharmacy. Mathematical models of enzyme kinetics offer several advances for this deep and analytical understanding due to their in compensable potential in predicting kinetic processes and anticipating appropriate interventions when required. This chapter concerns mathematical modeling analysis and simulation of enzyme kinetics. Experimental data and available knowledge on enzyme mechanics are used in constituting a mathematical model. The models are either in the form of linear or nonlinear ordinary differential equations or partial differential equations. These equations are composed of kinetic parameters such as kinetic rate constants, initial rates, and concentrations of enzymes. The nonlinear nature of enzymatic reactions and a large number of parameters have caused major issues with regard to efficient simulation of those reactions. In this work, an enzymatic system that includes Michaelis-Menten and Ping Pong kinetics is modeled in the form of differential equations. These equations are solved numerically in which the system parameters are estimated. The numerical results are compared with the results from an existing work in literature

In situ atomic-scale oscillation sublimation of magnesium under CO₂ conditions.

Understanding the interactive role between Mg and CO₂ is crucial for many technological applications, including CO₂ storage, melting protection, corrosion resistance, and ceramic welding. Here we report observations of rapid oscillation sublimation of Mg at room temperature in the presence of both CO₂ gas and electron irradiation using environmental transmission electron microscopy. The sublimation is mainly related to phase transformation of amorphous MgCO₃. Differing from the direct formation of gas-state MgCO₃, which attributes to the sublimation of pure Mg under a mild electron beam dose, a unique oscillation process is detected during the process of Mg sublimation under a harsh electron beam dose. The main reason stems from the first-order reaction of a reversible decomposition-formation of amorphous MgCO₃. These atomic-level results provide some interesting insights into the interactive role between Mg and CO₂ under electron beam irradiation

Facile Preparation of Silver Halide Nanoparticles as Visible Light Photocatalysts

In this study, highly efficient silver halide (AgX)-based photocatalysts were successfully fabricated using a facile and template-free direct-precipitation method. AgX nanoparticles, which included silver chloride (AgCl), silver bromide (AgBr) and silver iodide (AgI), were synthesized using different potassium halides and silver acetate as reactive sources. The size distribution of the AgX nanoparticles was determined by the reaction time and ratio of the reagents, which were monitored by UV-vis spectra. The as-prepared AgX nanoparticles exhibited different photocatalytic properties. This shows the differences for the photodegradation of methyl orange and Congo red dyes. In addition, the AgCl nanoparticle-based photocatalyst exhibited the best photocatalytic property among all three types of AgX nanoparticles that are discussed in this study. Therefore, it is a good candidate for removing organic pollutants

Heterojunction-composited architecture for Li–O2 batteries with low overpotential and long-term cyclability.

The crucial issue among lithium−oxygen batteries (LOBs) lies in the development of highly efficient catalysts to improve their large discharge−charge polarization, poor rate capability, and short cycle life. Herein, a composite of three-dimensional honeycomb graphene-supported a Mo/Mo2C heterojunction has been synthesized and can be utilized as a self-supported LOB cathode directly. The LOBs based on the Mo/Mo2C heterojunction composite cathode show a low overpotential of 0.52 V, a high discharge capacity of about 12016 mAh g−1 at 100 mA g−1, and a long-term cyclability (about 360 cycles) under a restricted capacity of 1000 mAh g−1 at 100 mA g−1, which exceeds the features of the majority of Mo-based catalysts for LOBs reported so far. Based on both experimental tests and density functional calculations, it is confirmed that the outstanding electrochemical performance is closely associated with a hierarchical porous structure for convenient oxygen/electrolyte diffusion, a large number of activity sites (interfaces/defects) for high capacity, and a high conductivity with metallic bonds for good rate capability. The method can be extended to prepare other metal based heterojunctions

A Co-Doped MnO2 catalyst for Li-CO2 batteries with low overpotential and ultrahigh cyclability.

Li-CO2 batteries can not only capture CO2 to solve the greenhouse effect but also serve as next-generation energy storage devices on the merits of economical, environmentally-friendly, and sustainable aspects. However, these batteries are suffering from two main drawbacks: high overpotential and poor cyclability, severely postponing the acceleration of their applications. Herein, a new Co-doped alpha-MnO2 nanowire catalyst is prepared for rechargeable Li-CO2 batteries, which exhibits a high capacity (8160 mA h g−1 at a current density of 100 mA g−1), a low overpotential (≈0.73 V), and an ultrahigh cyclability (over 500 cycles at a current density of 100 mA g−1), exceeding those of Li‐CO2 batteries reported so far. The reaction mechanisms are interpreted depending on in situ experimental observations in combination with density functional theory calculations. The outstanding electrochemical properties are mostly associated with a high conductivity, a large fraction of hierarchical channels, and a unique Co interstitial doping, which might be of benefit for the diffusion of CO2, the reversibility of Li2CO3 products, and the prohibition of side reactions between electrolyte and electrode. These results shed light on both CO2 fixation and new Li-CO2 batteries for energy storage

In-situ atomic-scale phase transformation of Mg under hydrogen conditions.

Magnesium hydrogenation issue poses a serious obstacle to designing strong and reliable structural materials, as well as offering a safe alternative for hydrogen applications. Understanding phase transformation of magnesium under hydrogen gas plays an essential role in developing high performance structural materials and hydrogen storage materials. Herein, we report in-situ atomic-scale observations of phase transformation of Mg and Mg-1wt.%Pd alloy under hydrogen conditions in an aberration-corrected environmental transmission electron microscopy. Compare with magnesium hydrogenation reaction, magnesium oxidation reaction predominately occurs at room temperature even under pure hydrogen gas (99.9%). In comparison, magnesium hydrogenation is readily detected in the interface between Mg and Mg6Pd, due to catalytic role of Mg6Pd. Note that the nanoscale MgH2 compound transfers into MgO spontaneously, and the interface strain remarkably varies during phase transformation. These atomic-level observations and calculations provide fundamental knowledge to elucidate the issue of magnesium hydrogenation

Achieving high strength and ductility in magnesium alloys via densely hierarchical double contraction nanotwins.

Light-weight magnesium alloys with high strength are especially desirable for the applications in transportation, aerospace, electronic components, and implants owing to their high stiffness, abundant raw materials, and environmental friendliness. Unfortunately, conventional strengthening methods mainly involve the formation of internal defects, in which particles and grain boundaries prohibit dislocation motion as well as compromise ductility invariably. Herein, we report a novel strategy for simultaneously achieving high specific yield strength (∼160 kN m kg-1) and good elongation (∼23.6%) in a duplex magnesium alloy containing 8 wt % lithium at room temperature, based on the introduction of densely hierarchical {1011}-{101 1} double contraction nanotwins (DCTWs) and full-coherent hexagonal close-packed (hcp) particles in twin boundaries by ultrahigh pressure technique. These hierarchical nanoscaled DCTWs with stable interface characteristics not only bestow a large fraction of twin interface but also form interlaced continuous grids, hindering possible dislocation motions. Meanwhile, orderly aggregated particles offer supplemental pinning effect for overcoming latent softening roles of twin interface movement and detwinning process. The processes lead to a concomitant but unusual situation where double contraction twinning strengthens rather than weakens magnesium alloys. Those cutting-edge results provide underlying insights toward designing alternative and more innovative hcp-type structural materials with superior mechanical properties

Coherent interface strengthening of ultrahigh pressure heat-treated Mg-Li-Y alloys.

Achieving good strength-ductility of Mg alloys has always been a crucial issue for the widespread applications of Mg-based structural materials. Herein, an unexpected double-stage strengthening phenomenon was discovered in Mg-8Li-1Y(wt.%) alloys through high pressure (6 GPa) heat treatments over a range of 700-1300°C. Attractively, the yield strength values are improved remarkably without losing their ductility. The low temperature strengthening mechanism is mainly driven by the formation of large-volume nanoscale contraction twins. In contrast, the high-temperature strengthening reason is ascribed to the presence of densely nano-sized stacking faults. Both coherent interfaces contribute effectively to high mechanical strength without any tradeoff in ductility

High-strong-ductile magnesium alloys by interactions of nanoscale quasi-long period stacking order unit with twin.

Magnesium alloys with high strength in combination of good ductility are especially desirable for applications in transportation, aerospace and bio-implants owing to their high stiffness, abundant raw materials, and environmental friendliness. However, the majority of traditional strengthening approaches including grain refining and precipitate strengthening can usually prohibit dislocation movement at the expense of ductility invariably. Herein, we report an effective strategy for simultaneously enhancing yield strength (205 MPa, 2.41 times) and elongation (23%, 1.54 times) in a Mg-0.2Zn-0.6Y (at.%) alloy at room temperature, based on the formation of a nanosized quasi-long period stacking order unit (QLPSO)-twin structure by ultrahigh-pressure treatment followed by annealing. The formation reason and strong-ductile mechanism of the unique QLPSO-twin structure have been clarified by transmission electron microscopy observations and molecule dynamics simulations. The improved strength is mainly associated with the presence of nanosized QLPSO and the modified <86.3o QLPSO-twin boundary (TB) interface, effectively pinning dislocation movement. Comparatively, the enhanced ductility is related to the <3.7o QLPSO-TB interface and micro-kinks of nanoscale QLPSO, providing some paths for plastic deformation. This strategy on the QLPSO-twin structure might provide an alternative perspective for designing innovative hexagonal close-packed structural materials with superior mechanical properties

Self-reductive synthesis of MXene/Na0.55Mn1.4Ti0.6O4 hybrids for high-performance symmetric lithium ion batteries.

Increasing environmental problems and energy challenges have created an urgent demand for the development of green and efficient energy-storage systems. The search for new materials that could improve the performance of Li-ion batteries (LIBs) is one of today's most challenging tasks. Herein, a stable symmetric LIB based on the bipolar material-MXene/Na0.55Mn1.4Ti0.6O4 was developed. This bipolar hybrid material showed a typical MXene-type layered structure with high conductivity, containing two electrochemically active redox couples, namely, Mn4+/Mn3+ (3.06 V) and Mn2+/Mn (0.25 V). This MXene/Na0.55Mn2O4-based symmetric full cell exhibited the highest energy density of 393.4 W h kg−1 among all symmetric full cells reported so far, wherein it is bestowed with a high average voltage of 2.81 V and a reversible capacity of 140 mA h g−1 at a current density of 100 mA g−1. In addition, it offers a capacity retention of 79.4% after 200 cycles at a current density of 500 mA g−1. This symmetric lithium ion full battery will stimulate further research on new LIBs using the same active materials with improved safety, lower costs and a long life-span