Calculation Of Enthalpy Research Articles

On the thermodynamics of hydrogen adsorption over Pt(111) in 0.05M NaOH.

The reasons for the sluggish kinetics of the hydrogen adsorption reaction in alkaline media remain a question still to be solved. This information is important to achieve a complete understanding of the mechanistic details that could lead to the production of key catalytic materials necessary for the development of a future hydrogen economy. For a better understanding of this reaction, it is important to acquire information about the thermodynamic parameters characteristic of the different steps in the reaction. Among these, the hydrogen adsorption is a key step in the process of hydrogen evolution. Although some debate still remains about the difference between adsorbed hydrogen in the underpotential deposition (UPD) region and at the overpotential deposition region, there is no doubt that understanding the former can help in the understanding of the latter. Making use of charge density measurements, we report on this paper a thermodynamic study of the hydrogen UPD process on Pt(111) in 0.05M NaOH over the range of temperatures from 283 ≤ T/K ≤ 313. The coulometric features corresponding to HUPD allow for the calculation of the hydrogen coverage and a fit to a Generalized Frumkin isotherm. From these values, different thermodynamic functions for the UPD reaction have been calculated: ΔGads, ΔSads, ΔHads, and the Pt-H bond energy. From extrapolation, a value of ΔSads ◦=-7.5±4Jmol-1K-1 was found, which is very close to 0, much lower than previously reported measurements both in acid and in alkaline solutions. Such value has an effect on the enthalpy and bond energy calculations, the latter having a decreasing tendency with pH and coverage. This tendency is completely different from the acidic systems and implies that the change in the thermodynamic functions due to the formation of the double layer and the reorganization of interfacial water has a strong influence on the process in high pH solutions.

Physical properties of chromium-doped barium titanate: Effects of chromium incorporation

This study utilizes the density functional theory to investigate the physical properties of BaTi1-xCrxO3 (x = 0, 0.005, 0.025, and 0.05) in order to analyze the doping effects of Cr. Since, doping has a substantial impact on a material's characteristics; this work aims to explore the physical properties of BaTiO3 by incorporating Cr of various concentrations to fully comprehend this system. The lattice parameters of BaTiO3 are increased with increased Cr incorporation. Besides, the formation enthalpy calculations and elastic constants confirm the thermodynamic and mechanical stability of all the doped phases. The brittle nature of undoped BaTiO3 is noticed, which gets increasingly brittle as Cr is added incrementally. Most importantly, the machinability of the pure phase is found to be slightly better than that of doped phases. The incremental doping level exhibits slightly increasing tendency of elastic anisotropy in BaTi1-xCrxO3. The semiconducting behavior of BaTiO3 is expressed as it has band gap in its electronic band structure, which supports the previous experimental and theoretical reports. Finally, the band structure and density of states reflect the decreasing affinity of band gap with increased doping.

Thermodynamic and electronic properties of ReN2 polymorphs at high pressure

High pressure synthesis of rhenium nitride pernitride ReN$_2$ with crystal structure unusual for transition metal dinitrides and high values of hardness and bulk modulus attracted significant attention to this system. We investigate the thermodynamic and electronic properties of the P2$_1$/c phase of ReN$_2$ and compare them with two other polytypes, C2/m and P4/mbm phases suggested in the literature. Our calculations of the formation enthalpy at zero temperature show that the former phase is the most stable of the three up to pressure p=170 GPa, followed by the stabilization of the P4/mbm phase at higher pressure. The theoretical prediction is confirmed by diamond anvil cell synthesis of the P4/mbm ReN$_2$ at $\approx$175 GPa. Considering the effects of finite temperature in the quasi-harmonic approximation at p=100 GPa we demonstrate that the P2$_1$/c phase has the lowest free energy of formation at least up to 1000 K. Our analysis of the pressure dependence of the electronic structure of the rhenium nitride pernitride shows a presence of two electronic topological transitions around 18 GPa, when the Fermi surface changes its topology due to an appearance of a new electron pocket at the high-symmetry Y$_2$ point of the Brillouin zone while the disruption of a neck takes place slightly off from the $\Gamma$-A line.

Calculation of the vaporization enthalpies of alkylaromatic hydrocarbons as a function of temperature from their molecular structure

Vaporization enthalpy is one of the most important parameters for the description of vapor-liquid equilibrium in pure substances and mixtures. However, the experimental data on the vaporization enthalpy in the wide temperature range are rarely available. Here we tested the validity of a new approach for the estimation of the temperature dependence of the vaporization enthalpy of alkylaromatic compounds, which is based on a correlation between the heat capacity change on transition from a liquid to an ideal gas and vaporization enthalpy at 298.15 K. The combination of this approach and previously developed scheme for the calculation of the vaporization enthalpies of alkylarenes at 298.15 K is a way to estimate vaporization enthalpy at an arbitrary temperature from the molecular structure. 314 vaporization enthalpies of 156 alkylaromatic compounds between 240 and 650 K were calculated. The calculated magnitudes were compared with the available literature data and the values derived by widely used Chickos et al. scheme. In most cases, an agreement with the literature data was within the limits of experimental error (RMS = 1.9 kJ·mol−1). The performance of Chickos et al. scheme was comparable when the temperature range of adjustment was less than 100 K, but decreased in the wider adjustment intervals and for the larger molecules.

Application of super cells for quantum-mechanical calculations of the mixing enthalpy of the BCC-phase of the Fe-V system for the basic state

In this work, the super-cells were used for quantum mechanical calculations of the mixing enthalpia of the BCC phase of the Fe-V system for the ground state. The values of total energy were calculated using 16 -th and 54- atomic super-cells for both clean components and alloys. The mixing enthalpy (ΔH) for the BCC phase was calculated on four 16- and 54- atomic super-cells in the vicinity of pure components, on the basis of which the dependence of the concentration ΔH for BCC alloys in the ferromagnetic state of the Fe-V system of the ground state was built.

Research on the composition of UV curing modified compounds for 3D printing based on HyperChem simulation

The precursor conversion method provides the possibility of 3D printing ceramic materials, and the resin system with polysilazane as the precursor is expected to prepare high-performance ceramic materials for aviation. In this paper, the UV curing reaction system of polysilazane for 3D printing is taken as the research object, and hyperchem8.0 software is used as the research means. The model construction, chemical bond energy calculation, reaction enthalpy calculation and other modules in the software are used to simulate and predict the reaction process and mechanism of UV curing, which provides a theoretical basis for the selection and optimization of the subsequent UV curing reaction system for 3D printing.

Considerations and complications with "first principles" dynamic modelling of industrial compressors

Readily available processing hardware and "off-the-shelf" (OTS) simulation software has made "high fidelity" first principles models of both steady and transient states, for both axial and centrifugal industrial compressors, relatively easy to construct. These high-fidelity models are finding their way into "real-time. digital twin" performance monitors, front-end engineering design, and post-design – pre-construction compressor performance evaluation. The compressor models are useful for reliably demonstrating the compressor and – to some degree, based on the complexity of the model – process response to various operating conditions. Once the model is constructed, it is trivial to run a "what-if" analysis of compressor performance to answer questions related to (a) recommendations or validation of the recycle/vent valve size and actuation speed, (b) general piping layout and sizing around the compressor, (c) and hot gas bypass requirements, to name a few. This paper takes a practical approach in discussing the compressor and process parameters necessary for building these dynamic "high-fidelity" industrial-compressor models. We identify compressor inputs and compressor responses that are faithfully modeled by first-principle equations available in the simulation software and those that typically require a compromise between an "ab initio" and data-fitting approximation. We discuss the simulation's tendency to overstate pressure excursions during surge events and understate the compressor operation in the "stonewall" region. We also discuss using the simulator software's compressor-stage enthalpy calculations to predict and quantify the compressor train reverse rotation. We use our broad experience and understanding of the compressor operation and simulation and our experience with the AVEVA™ Dynamic-Simulation "OTS" simulation software as the basis for this discussion.

Design of Novel Phosphopantetheine Adenylyltransferase Inhibitors: A Potential New Approach to Tackle Mycobacterium tuberculosis.

Tuberculosis (TB) has been a challenging disease worldwide, especially for the neglected poor populations. Presently, there are approximately 2 billion people infected with TB worldwide and 10 million people in the world fell ill with active TB, leading to 1.5 million deaths. The classic treatment is extensive and the drug- and multi-drug resistance of Mycobacterium tuberculosis has been a threat to the efficacy of the drugs currently used. Therefore, the rational design of new anti-TB candidates is urgently needed. With the aim of contributing to face this challenge, 78 compounds have been proposed based on SBDD (Structure-Based Drug Design) strategies applied to target the M. tuberculosis phosphopantetheine adenylyltransferase (MtPPAT) enzyme. Ligand-Based Drug Design (LBDD) strategies were also used for establishing Structure-Activity Relationships (SAR) and for optimizing the structures. MtPPAT is important for the biosynthesis of coenzyme A (CoA) and it has been studied recently toward the discovery of new inhibitors. After docking simulations and enthalpy calculations, the interaction of selected compounds with MtPPAT was found to be energetically favorable. The most promising compounds were then synthesized and submitted to anti-M. tuberculosis and MtPPAT inhibition assays. One of the compounds synthesized (MCP163), showed the highest activity in both of these assays.

Thermodynamics of hydrate systems using a uniform reference state

AbstractFormation of natural gas hydrates during processing and transport of natural gas has historically been a significant motivator for hydrate research. The last three decades have also seen the focus increasingly shifting towards CH4 hydrates as a potential energy source. And in the context of climate changes, the impact of hydrate‐related processes is coming more to the forefront as well. This interest is not only limited to leakage fluxes of CH4 from natural gas hydrates but also flux from conventional hydrocarbon systems entering the seafloor at temperature and pressure allowing for hydrate formation. Alternative ways to treat formally overdetermined hydrate systems is an important focus in this work. The most common method used for assessment of hydrate phase transitions involves several fitted parameters to calculate the free energy difference between liquid water and empty hydrate. This technique calls for an empirical fitting of fundamental thermodynamic properties. Numerical codes based on this method limit the models to hydrate formation only from free gas and liquid water. This is at least true for all commercial and academic codes that were examined prior to this work. This work addresses the advantages in using residual thermodynamics for all phases, including hydrates. In addition to making it possible to handle many alternative hydrate routes leading to hydrate formation or dissociation, the presented method also opens a way to calculate a variety of needed thermodynamic properties (e.g., enthalpies of pure components and mixtures) in a simple and consistent way. This approach will be illustrated through calculations of various hydrate phase transitions, examples of free energy calculations for comparison of phase stability, and calculation of enthalpies of hydrate formation. Calculated enthalpies are compared with experimental data as well as results derived from applying the Clapeyron equation. Mechanisms for conversions of in situ CH4 hydrate to facilitate safe CO2 storage are also discussed. A very simple Clapeyron‐based scheme for calculation of enthalpies for hydrate phase transitions is also proposed.

Method to identify the phase structures of high entropy alloys with modified lattice distortion enthalpy

Severe lattice distortion is one of important characteristics of high entropy alloys (HEAs) and exercises considerable influence over phase structures, and atomic volume V is thought to be the key factor for lattice distortion. In the present work, the calculation of V was modified by taking the stacking density effects into consideration, and the calculation of lattice distortion enthalpy (∆HLD) caused by elastic lattice distortion for HEAs was optimized accordingly. With this method, two phase structure formation rules were proposed: valence electron concentration(VEC)−∆HLD rule for crystal structure of solid solution and chemical enthalpy(∆Hchem)−∆HLD rule for phase composition. The prediction effect of these two rules were verified by analyzing 100 existing HEAs, and the intrinsic effect of lattice elastic distortion on the phase formation of HEAs were discussed. The results of the research would be helpful in the composition design of HEAs.

Superhard carbon-rich C–N compounds hidden in compression of the mixture of carbon black and tetracyanoethylene

The recent successful synthesis of Pnnm-CN structure inspired researchers to explore new superhard 3D C–N compounds via a similar method, that is, high-pressure and high-temperature experiments involving carbon and nitrogen elements. Naturally, in some past high-pressure experiments involving C–N precursors, whether there are undiscovered C–N compounds in the product is worthy of being re-examined. This work proposed four novel carbon-rich C–N compounds, namely, r-C17N4, sc-C4N, sc-C17N4–I, and sc-C17N4-II, which possibly coexist with the unidentified simple cubic carbon phase in the shock compression products of the carbon black and tetracyanoethylene mixture. Most of the X-ray diffraction peaks from the experiment could be understood by the proposed C–N compounds. Furthermore, the calculations of formation enthalpies using experimental materials as precursors show that these four C–N compounds are more easily synthesized than the carbon structures previously used to explain the unknown phase. These phenomena indicate that C–N compounds might be synthesized in this experiment, not just the new carbon phase. The current study may provide a new idea for the experimental research of C–N compounds and inspire further research to reevaluate previous experimental findings.

Effect of co-alloying elements on the structural stability, elastic, ductility and thermodynamic properties of TiAl intermetallic compound

The considerable lack of room temperature ductility and toughness is widely recognized as an important limitation for the commercialization of titanium aluminides in industrial engineering. To meet this target, we optimize a new polycrystalline alloy based on Gamma-titanium aluminide co-alloying with Mo–Cr, Mo–Nb and Mo–Re atom pairs that simultaneously satisfy the physical criteria of workability using the density functional theory within the framework of the generalized gradient approximation. The formation enthalpy calculations showed that the Ti–Ti sites remain the preferential sites of Mo–Cr and Mo–Nb atomic defects, while the Mo–Re elements tend to occupy Ti–Al sites. The elastic properties systematically suggest that TiAl–Mo–Nb system is further resistant to tensile deformation along c-axis and a-axis (b-axis). In addition, the co-alloying Mo–Re atom pair exhibit improved mechanical performance compared to the other systems. These trends are confirmed by analyzing the electronic structure. Using quasi-harmonic approximation, thermodynamic properties were also predicted by including the description of the temperature. The results of co-alloying with molybdenum and rhenium atoms confirm effectively their usability in turbine blades and engine-valves made of Gamma-TiAl alloys.

Hydrate Phase Transition Kinetic Modeling for Nature and Industry–Where Are We and Where Do We Go?

Hydrate problems in industry have historically motivated modeling of hydrates and hydrate phase transition dynamics, and much knowledge has been gained during the last fifty years of research. The interest in natural gas hydrate as energy source is increasing rapidly. Parallel to this, there is also a high focus on fluxes of methane from the oceans. A limited portion of the fluxes of methane comes directly from natural gas hydrates but a much larger portion of the fluxes involves hydrate mounds as a dynamic seal that slows down leakage fluxes. In this work we review some of the historical trends in kinetic modeling of hydrate formation and discussion. We also discuss a possible future development over to classical thermodynamics and residual thermodynamics as a platform for all phases, including water phases. This opens up for consistent thermodynamics in which Gibbs free energy for all phases are comparable in terms of stability, and also consistent calculation of enthalpies and entropies. Examples are used to demonstrate various stability limits and how various routes to hydrate formation lead to different hydrates. A reworked Classical Nucleation Theory (CNT) is utilized to illustrate that nucleation of hydrate is, as expected from physics, a nano-scale process in time and space. Induction times, or time for onset of massive growth, on the other hand, are frequently delayed by hydrate film transport barriers that slow down contact between gas and liquid water. It is actually demonstrated that the reworked CNT model is able to predict experimental induction times.

Simulation of Heat Flow Curves of NC‐Based Propellants – Part 1: Determination of Reaction Enthalpies and Other Characteristics of the Reactions of NC and Stabilizer DPA Using Quantum Mechanical Methods

AbstractThe heat flow curves of nitrocellulose (NC)‐based propellants are now an important assessment quantity for their quality and safety in storage and service. The measurable heat flow is the summarized effect of the rates of all the reaction heats of the reactions going on in the material. To get a detailed assessment instead the only global one requires knowing the heat flow parts caused by NC and by stabilizer reactions. For this, in turn, one needs to find the reaction enthalpies of the essential NC decomposition reactions and the stabilizer reactions at the temperature of the measurement of the heat flow. These data are not available in the literature. To get meaningful data it was necessary to use quantum mechanical calculations, here so‐called DFT (density functional theory) methods, to screen the possible reactions by transition states and the energetic situation between educts and products. For the calculation of the reaction enthalpies and the transition states, the reactants were embedded in a polarizable continuum, which simulates the NC environment for the reactants in NC and for the stabilizer reactions. In addition, to calculate bond dissociation enthalpies of NC and of nitroglycerine a type of shell model was used called ONIOM (‘Our own N‐layered Integrated molecular Orbital and molecular Mechanics’ model). The results from the determinations of reaction enthalpies are used in the second part of the publication.

Development of Students’ Conceptual Understanding through STEAM Project Integration in Thermochemistry

This study aims to develop students' conceptual understanding by integrating the Science, Technology, Engineering, Art, and Mathematics (STEAM) project in thermochemistry. The qualitative research was employed with interviews, reflective journals, observations, students’ worksheets, and conceptual understanding tests on 40 students in senior high school. The STEAM project was implemented through a project-based learning model to develop students' understanding of exothermic and endothermic, combustion reactions, and enthalpy concepts. The STEAM project being developed is a steamship using three fuels: methanol, ethanol, and palm oil. The results showed that the student's understanding of enthalpy calculation had developed. Otherwise, the concept of exothermic, endothermic, and combustion reaction, especially related to applying the concept in daily life, is still undeveloped. Thus, some of the students experienced misconceptions. Therefore, the integration of contextual learning such as STEAM needs to be continuously applied to provide students with opportunities to develop their conceptual understanding and application in daily life.

Pd/Cu(220) interdiffusion under compressive stress

This work investigates the effect of the external compressive pressure on the interdiffsuion between Pd and Cu. By analyzing ESCA atomic depth profiling without/with the external compressive pressure, two important findings were observed: (1) Pd/Cu(220) interdiffusion was found to be promoted by the external compressive pressure. (2) Under external compressive pressure, Pd atoms diffuse across the interdiffusion interface into the Cu(220) side more than the Cu atoms across the interdiffusion interface into the Pd side. Combining Matano approach and the obtained ESCA atomic profiles, the interdiffusion diffusivities and the activation enthalpy (H) of Pd/Cu(220) interdiffusion can be calculated. The activation enthalpy (0.694 eV) of Pd/Cu(220) interdiffusion is reduced to 0.625 eV and 0.604 eV with applying external pressures of 125 MPa and 250 MPa, respectively. The present calculation of the activation enthalpy implies that the applying compressive stress results in negative changes of the activation enthalpy (δH) and the free energy of activation (δG) for the Pd/Cu(220) interdiffusion. With Gibbs equation, the negative changes of δG and δH can further deduce the relation of the entropy change (δS) and the activation of enthalpy (δH) for the Pd/Cu(220) interdiffusion system under the external compressive stress, which are (1) the change of the entropy has to be positive (δS > 0) or (2) the change of the entropy (δS) has to be smaller than the term of δH/T., i.e., δS< δH/T.

Imidacloprid (I) in several aqueous co-solvent mixtures: Solubility, solvent effect, solvation thermodynamics and enthalpy–entropy compensation

This contribution dealt with the saturated solubility, solvation behavior, and dissolution and transfer belongings of imidacloprid in some aqueous co-solvent mixtures of n-propanol, acetone, ethanol and methanol by means of experiment and mathematical correlations. Solubility experiments were conducted via a commonly used technique, namely shake-flask method, under 101.2 kPa from 283.15 to 323.15 K. The maxima of the obtained imidacloprid solubility was recorded in neat n-propanol/acetone/ethanol/methanol at temperature T = 323.15 K, which corresponded to 227.7 × 10−5, 2851 × 10−5, 237.9 × 10−5 and 409.9 × 10−5 (mole fraction), respectively; and the minimum, in single water (0.6792 × 10−5 at 283.15 K). The Hansen solubility parameters were used here to illustrate the imidacloprid solubility behavior. Solubility modeling through Jouyban–Acree–van’t Hoff, mixture response surface (MRS), Jouyban–Acree and modified Wilson models achieved the relative average deviations of no higher than 3.01%. As well, the extended Hildebrand solubility approach analyzed the saturated solubility of imidacloprid at 298.15 K attaining the maximum of relative average deviation (RAD) of 1.16%. The key contributions of solution descriptors to the imidacloprid solubility were observed as dipolarity-polarizability and solubility parameter studied through molecular interactions of solvent-solvent and solute-solvent species to the imidacloprid solubility in terms of the linear solvation energy relationships. A quantitatively investigation upon local mole fractions was executed by utilizing the powerful tool of inverse Kirkwood–Buff integrals through considering several properties of solutions. The local mole fractions of n-propanol/acetone/ethanol/methanol were higher than that of the bulk ones in n-propanol/acetone/ethanol/methanol-rich and middle proportions, representing preferential solvation of the imidacloprid by co-solvents (n-propanol/acetone/ethanol/methanol). It is speculated within these regions imidacloprid acts as a Lewis acid with the molecules of co-solvents. Thermodynamic calculation of dissolution properties and enthalpy–entropy compensation study revealed an entropy-driven mechanism and endothermic process of imidacloprid dissolved in blended solvents. Transfer property analysis designated more advantageous solubilization ability over the intermediate and rich-co-solvent contents.

Influence of heating temperature and holding time on the formation sequence of iron aluminides at the interface of Fe/Al coatings

Fe/Al coatings designed with different compositions were fabricated by cold spray. The effects of heat treatment on the formation of iron aluminides, such as FeAl, Fe3Al and Fe2Al5, at the Fe/Al interface were investigated. The formation sequence of iron aluminides was strongly influenced by heating temperature and the holding time. Enthalpy calculation method was used to explain the formation sequence of iron aluminides, as a supplement to Fe/Al binary phase diagram. By combining these two methods, the process of diffusion and interfacial reactions can be clarified. As a result, the heat treatment schedule can be optimized to regulate the formation sequence of iron aluminides formed at the interface, which will help inhibit the brittleness and improve the mechanical properties of Fe/Al coatings.

Investigation of structural, electronic and elastic properties of matlockite-type SrFI under high pressure: An Ab-initio study

In this study, Matlockite (PbFCl) type SrFI compound with a two-dimensional layered structure has been studied using density functional theory (DFT) and generalized gradient approximation (GGA) under high hydrostatic pressure. Calculations were made using the Siesta Method. In ambient conditions, matlockite type structures crystallize in tetragonal structure with space group P4/nmm. In nature, these structures are known as alkaline earth fluoro-halides. When gradually increasing pressures were applied to this structure, the phase transition was observed to the orthorhombic structure with space group Pmmn. In addition to these studies, some physical properties of SrFI, such as lattice parameters, bulk modulus, the derivative of the bulk modulus, and volume values were also calculated. On the other hand, total energy and enthalpy calculations were made to obtain the phase transition pressure values that are in good agreement with the experimental results. According to these calculations, phase transition pressure was predicted at about 44.8 GPa. Besides, how electronic and elastic properties of SrFI compound change with applied pressure were also examined. In the electronic structure calculations, about 5.09 eV band gap value was obtained for P4/nmm phase at 0 GPa pressure. Under increasing pressure, the 0.16 eV band gap value was calculated for the Pmmn phase of SrFI. According to these results, the stable phases obtained of the SrFI compound have semiconductor characters since the both phases have a conceivable band gap. In addition, elastic constants were calculated and two structures of SrFI were found to be mechanically stable.

Nonlinear enthalpy transformation for transient convective phase change in Smoothed Particle Hydrodynamics (SPH)

A three-dimensional model is presented for the prediction of solidification behavior using a nonlinear transformation of the enthalpy equation in a Smoothed Particle Hydrodynamics (SPH) discretization. The effect of phase change in the form of release and absorption of latent heat is implemented implicitly as variable source terms in the enthalpy calculation. The developed model is validated against various experimental, analytical, and numerical results from the literature. Results confirm accuracy and robustness of the new procedure. Finally, the SPH model is applied to a study of suspension plasma spraying (SPS) by predicting the impact and solidification behavior of molten ceramic droplets on a substrate.