Showing papers on "Enthalpy published in 2018"

Adsorption Equilibrium, Kinetics, and Thermodynamic Studies of Fluoride Adsorbed by Tetrametallic Oxide Adsorbent

Abstract: This study investigated the performance of fluoride adsorption onto a specific tetrametallic oxide adsorbent Fe–Al–Ce-Ni (FACN) and the effect of temperature on adsorption performance. The adsorption performance was determined by adsorption equilibrium, kinetics, and thermodynamic parameters. The adsorption, kinetic, and thermodynamic parameters were compared alternatively. The fluoride adsorption capacity was obtained from four different adsorption isotherm models, namely, Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D–R), and Freundlich was found to best fit model. Fluoride removal rate using adsorption (0.27 min–1) was obtained faster than reactive adsorption (0.04 min–1). Several thermodynamic parameters such as enthalpy, Gibbs free energy, entropy (ΔS > 0), and adsorption activation energy were calculated which demonstrated the feasibility and spontaneity (ΔG < 0) and exothermic nature of (ΔH < 0) the fluoride adsorption process. The adsorption process was controlled by a physical mechanis...

Adsorption of cationic dyes onto Fe@graphite core–shell magnetic nanocomposite: Equilibrium, kinetics and thermodynamics

Abstract: Magnetically separable Fe@graphite core–shell nanocomposite particles (Fe@G) were synthesized by chemical vapor deposition CVD process and characterized by XRD, HRTEM, HAADF-STEM, FTIR, Raman spectroscopy, BET and zeta potential measurements. Nanocomposite was used to adsorb two cationic dyes, Basic Yellow 28 (BY28) and Basic Red 46 (BR46), from aqueous solutions. Adsorption process was investigated under different experimental conditions of pH (3–11), initial dye concentration (10–50 mg L−1) and temperature (20–60 °C). The adsorption kinetics were examined using pseudo-first-order, pseudo-second-order and intraparticle diffusion model. The equilibrium adsorption data were analyzed by Langmuir, Freundlich and Temkin isotherm models. The results revealed that the pseudo-second-order model and Langmuir isotherm fit the kinetics and equilibrium data, respectively. In addition, various thermodynamic parameters, such as change in free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°), were also calculated. The thermodynamic analysis showed that the adsorption of BY28 and BR46 was spontaneous and endothermic.

Single and binary adsorption of sulfonamide antibiotics onto iron-modified clay: linear and nonlinear isotherms, kinetics, thermodynamics, and mechanistic studies

Abstract: Iron-modified raw kaolinite clay (Fe-MC) was synthesized by co-precipitation method, characterized, and then applied as a low-cost adsorbent to sequester sulfachloropyridazine (SCP) and sulfadimethoxine (SDM), emergent water contaminants, from aqueous media by batch equilibration at circumneutral pH. The adsorption rate was kinetically described by the pseudo-second-order model. Equilibrium monocomponent sorption data were fitted to three two-parameter linear and nonlinear isotherm models. The data were best described by Temkin and Langmuir nonlinear equations. Linearization of adsorption isotherms is demonstrated to be an unsuitable analytical tool for predicting adsorption isotherms. The Langmuir monolayer maximum adsorption capacities were 4.561 and 1.789 mg/g for SCP and SDM, respectively. The binary adsorption study showed an antagonistic adsorption process of SCP (Rq, SCP= 0.625) in the presence of SDM (Rq, SDM = 1.032). The thermodynamic parameters, namely enthalpy (ΔH), Gibbs free energy (ΔG), entropy (ΔS), Arrhenius activation energy (ΔEa), and sticking probability (S*), indicated that the processes are spontaneous, exothermic, and physical in nature. The adsorption process was attributed to hydrogen bonding and negative charge-assisted H-bonding (CAHB). Using the Langmuir isotherm, the amount of Fe-MC required for a given volume of effluent of known contaminant concentration could be predicted.

Graphene oxide-branched polyethylenimine foams for efficient removal of toxic cations from water

Abstract: Highly porous foams based on graphene oxide functionalized with polyethylenimine are generated and used with unprecedented efficiency for adsorbing heavy metal ions. A multiscale analysis of the GO–BPEI nanocomposite provided evidence for the covalent grafting of BPEI on GO and the formation of low crystalline porous foams. The uptake experiments revealed that the GO–BPEI's adsorption of toxic cations is strongly dependent on the pH in range from 2 to 10, as a result of the different interactions at the supramolecular level between the metal ions and the GO–BPEI foam. The maximum uptake capacities for Cu(II), Cd(II) and Pb(II) are achieved at pH = 5 and exhibit values as high as 1096, 2051 and 3390 mg g−1, respectively, being ca. over 20 times greater than standard sorbents like activated carbon. The GO–BPEI composite can be easily regenerated as proven by performing adsorption cycles. Also, the thermodynamic parameters including standard Gibbs free energy (ΔGo), the enthalpy change (ΔHo) and entropy change (ΔSo) revealed the exothermic and spontaneous nature of the adsorption process.

The thermodynamics and kinetics of iodine vacancies in the hybrid perovskite methylammonium lead iodide

Abstract: The current picture of ion transport in the solar-cell absorber material CH3NH3PbI3 (MAPbI3) suffers from a disturbing lack of clarity In this study, we demonstrate that, with knowledge of the jump rate of iodine vacancies and with a defect chemical model, various experimental data reported in the literature for the ionic conductivity of MAPbI3 can be reconciled A quantitative expression for the vacancy jump rate was obtained by studying iodine tracer diffusion as a function of temperature and iodine-vacancy concentration by means of classical molecular-dynamics simulations The defect-chemical model yields acceptor concentrations in experimental samples of 1015 cm−3 and lower, and the enthalpy and entropy of anti-Frenkel disorder We also demonstrate that the generation of additional iodine vacancies can explain quantitatively the increase in the ionic conductivity under illumination Finally, the consequences for devices under bias and for grain-boundary transport are discussed

Removal of Methylene Blue from Aqueous Solution by Bone Char

Abstract: Bone char was prepared from bovine bone for the removal of methylene blue from aqueous solution. The effects of particle size, contact time, and adsorption temperature on the removal rate of methylene blue were investigated. It was found that bone char particle size had an insignificant effect. The equilibration time was found at approximately 80 min. The removal rate decreased with an increase in temperature. The intraparticle diffusion was the main rate-limiting step. The experimental data was analyzed by kinetic, isotherm, and thermodynamic equations. The results show that the pseudo-second-order kinetic model and Freundlich, Temkin, and Dubinin–Kaganer–Radushkevich isotherm models are true of the adsorption process. The spontaneous and exothermic ion-exchange adsorption process was certified by the negative values of free energy change and enthalpy change, and 13.29 kJ mol−1 of adsorption energy.

Structural, mechanical and electronic properties of (TaNbHfTiZr)C high entropy carbide under pressure: Ab initio investigation

Abstract: The structural, mechanical and electronic properties of (TaNbHfTiZr)C high entropy carbide are studied by using density functional theory in conjunction with special quasi-random structures. The proposed lattice constant difference as an empirical criterion for high entropy compounds and mixing enthalpy show the formation of (TaNbHfTiZr)C solid solution. The derived elastic stiffness constants also indicate the mechanical stability of (TaNbHfTiZr)C high entropy carbide. At zero pressure, the calculated elastic mechanics obeys the rule of mixture, whereas Vickers hardness is slightly larger than the average value of constituent binary carbides. The computed elastic parameters show that (TaNbHfTiZr)C is brittle, similar to constituent binary carbides. Under high pressure, the lattice constants decrease slightly, and mechanical properties are improved, even the brittleness-ductility transition takes place. The calculated electronic structures show that covalence in (TaNbHfTiZr)C is relatively weaker than ionic bonding. With increasing pressure, covalence in (TaNbHfTiZr)C decreases while ionicity increases. The present research will be valuable for understanding and designing of high-entropy carbides.

Comparative study of Pb(II) adsorption onto MIL–101 and Fe–MIL–101 from aqueous solutions

Abstract: In the present paper, the metal-organic framework–101 (MIL–101) and the iron-doped MIL–101 (Fe–MIL–101) were synthesised using the hydrothermal process. The obtained materials were characterised by means of X-ray diffraction, scanning electron microscopy, and nitrogen adsorption/desorption isotherms. The obtained Fe–MIL–101possessed MIL–101’s structure with a large specific areas (2407 m2 g–1for Fe–MIL–101and 3360 m2 g–1for MIL–101). MIL–101 and Fe–MIL–101 were used for Pb(II) adsorption from aqueous solutions. Various factors affecting adsorption, namely contact time, initial concentration, temperature, pH, and adsorbent recycling were investigated. The Arrhenius and Eyring equations were employed to calculate the kinetic parameters, viz. activation energy (Ea), enthalpy ( Δ H # ), entropy ( Δ S # ), and free energy ( Δ G # ) of the sorption process. The thermodynamic parameters, namely changes in standard Gibbs free energy ( Δ G 0 ), enthalpy ( Δ H 0 ) and entropy ( Δ S 0 ) were derived to predict the nature of the process. The equilibrium data of adsorption of Pb(II) onto MIL–101 and Fe–MIL–101 were well fitted to both Langmuir and Freundlich isotherms. The maximum monolayer adsorption capacity of Fe–MIL–101(86.20 mg·g–1) was much higher than that of MIL–101(57.96 mg g–1). It was considered that MIL–101 containing iron provided a much larger adsorption capacity and faster adsorption kinetics than MIL–101. The carboxyl group in the MIL–101 framework played a vital role for the effective Pb(II) removal from aqueous solutions, while the surface functional groups being responsible for the Pb(II) adsorption on Fe–MIL–101 were considered to be hydroxy groups that formed on the iron oxide.

Size and shape dependency of the surface energy of metallic nanoparticles: unifying the atomic and thermodynamic approaches

Abstract: Surface energy is a fundamental property of metallic nanoparticles (MeNPs), which plays a crucial role in nucleation and growth and has strong implications for the application and environmental impact of MeNPs. Surface energy (J m-2) can be size dependent, but experimental data on surface energy trends for MeNPs are inconclusive. Computational chemistry may resolve the issue, but the location and area of the surface used for scaling, which dramatically influences the outcome and interpretation, has not been properly investigated. The size dependency of the surface energy can only be determined by scaling to the thermodynamic surface of tension. To identify this, we have derived a generalized Tolman approach for non-spherical particles, which is used to analyze the thermodynamic consistency of various surface definitions. Only the physical surface, defined here, is consistent with the surface of tension. Scaling of recent computational data for faceted MeNPs to this surface yields a low size dependency of surface energy, in good agreement with the Tolman lengths corresponding to its interfacial position. We find Tolman lengths of -0.03 nm for icosahedra and -0.04 nm for cuboctahedra of gold or silver. With this result, our approach can be used to quantify the twinning energy for icosahedral nanoparticles, being ∼0.06 J per m2 twin area. To understand the unorthodox negative Tolman lengths, we have analyzed the surface energetics of the solid-gas interface of metals in relation to the liquid-vapor interface of water. Surface entropy is found to be imperative in determining the size dependence of surface free energy. At room temperature, the influence of surface entropy on surface enthalpy is much smaller for metals than for water. It explains why these two interfaces have opposite size dependencies of the surface Gibbs free energy and opposite signs of the Tolman length. For water, forming nanodroplets or nanobubbles, the Tolman length is negative (∼-0.014 nm) for the surface enthalpy, but positive (∼+0.06 ± 0.02 nm) for the surface Gibbs free energy. For MeNPs at room temperature, both entities are negative, but at high temperature, the increased surface entropy term may cause the size dependency of surface Gibbs free energy to become reversed.

Direct Calorimetric Observation of the Rigid Amorphous Fraction in a Semiconducting Polymer

Abstract: The performance of polymeric semiconductors is profoundly affected by the thermodynamic state of its crystalline and amorphous fractions and how they affect the optoelectronic properties While intense research has been conducted on the crystalline features, fundamental understanding of the amorphous fraction(s) is still lacking Here, we employ fast scanning calorimetry to provide insights on the glass transition of the archetypal conjugated polymer poly(3-hexylthiophene) (P3HT) According to the conceptual definition of the glass transition temperature (Tg), that is, the temperature marking the crossover from the melt in metastable equilibrium to the nonequilibrium glass, an enthalpy relaxation should be observed by calorimetry when the glass is aged below Tg Thus, we are able to identify the enthalpy relaxations of mobile and rigid amorphous fractions (MAF and RAF, respectively) of P3HT and to determine their respective Tg Our work moreover highlights that the RAF should be included in structural mod

Thermodynamic Hydricities of Biomimetic Organic Hydride Donors

Abstract: Thermodynamic hydricities (ΔGH–) in acetonitrile and dimethyl sulfoxide have been calculated and experimentally measured for several metal-free hydride donors: NADH analogs (BNAH, CN-BNAH, Me-MNAH, HEH), methylene tetrahydromethanopterin analogs (BIMH, CAFH), acridine derivatives (Ph-AcrH, Me2N-AcrH, T-AcrH, 4OH, 2OH, 3NH), and a triarylmethane derivative (6OH). The calculated hydricity values, obtained using density functional theory, showed a reasonably good match (within 3 kcal/mol) with the experimental values, obtained using “potential pKa” and “hydride-transfer” methods. The hydride donor abilities of model compounds were in the 48.7–85.8 kcal/mol (acetonitrile) and 46.9–84.1 kcal/mol (DMSO) range, making them comparable to previously studied first-row transition metal hydride complexes. To evaluate the relevance of entropic contribution to the overall hydricity, Gibbs free energy differences (ΔGH–) obtained in this work were compared with the enthalpy (ΔHH–) values obtained by others. The results i...

Novel semi-interpenetrating network structural phase change composites with high phase change enthalpy

Abstract: High phase change enthalpy, controllable temperature, and stable shape can expand the application of phase change materials (PCMs) in energy storage. In this study, a series of novel form-stable PCMs with high phase change enthalpy (169–195 J/g) and controllable temperature (45.3–61.4°C) were prepared. The PCMs exhibited a semi-interpenetrating polymer network (semi-IPN) structure resulting from the combination of polyethylene glycol (PEG) and a three-dimensional (3-D) network gel. The gel itself featured an inherent phase change characteristic and a 3-D network structure. Thus, it improved the phase transition enthalpy of the materials and facilitated the formation of a semi-IPN that endowed the materials with excellent form-stable properties. In addition, the latent heat of the composites (169–195 J/g) is much higher than most of the previously reported composites using PEG as phase change component (68–132 J/g). © 2017 American Institute of Chemical Engineers AIChE J, 64: 688–696, 2018

Entropy Determination of Single-Phase High Entropy Alloys with Different Crystal Structures over a Wide Temperature Range.

Abstract: We determined the entropy of high entropy alloys by investigating single-crystalline nickel and five high entropy alloys: two fcc-alloys, two bcc-alloys and one hcp-alloy Since the configurational entropy of these single-phase alloys differs from alloys using a base element, it is important to quantify the entropy Using differential scanning calorimetry, cp-measurements are carried out from −170 °C to the materials’ solidus temperatures TS From these experiments, we determined the thermal entropy and compared it to the configurational entropy for each of the studied alloys We applied the rule of mixture to predict molar heat capacities of the alloys at room temperature, which were in good agreement with the Dulong-Petit law The molar heat capacity of the studied alloys was about three times the universal gas constant, hence the thermal entropy was the major contribution to total entropy The configurational entropy, due to the chemical composition and number of components, contributes less on the absolute scale Thermal entropy has approximately equal values for all alloys tested by DSC, while the crystal structure shows a small effect in their order Finally, the contributions of entropy and enthalpy to the Gibbs free energy was calculated and examined and it was found that the stabilization of the solid solution phase in high entropy alloys was mostly caused by increased configurational entropy

Solvation energies of the proton in methanol revisited and temperature effects

Abstract: We report in this work the absolute solvation enthalpies and the absolute solvation free energies of the proton in methanol at temperatures ranging from 20 to 340 K and an extrapolation to a desired temperature. To achieve this, we thoroughly investigated the structures of neutral methanol clusters (MeOH)n=2–10 and those of the protonated methanol decamer H+(MeOH)n=10 at the M06-2X/6-31++g(d,p) level of theory. As a result, we noted that up to the octamer, the population of the neutral methanol clusters is constituted by cyclic isomers. For nonamers and decamers, both cyclic and branched cyclic isomers contribute to the population of the clusters. Moreover, folded or distorted cyclic isomers are the most favored at low temperatures, while higher temperatures favored the flat cyclic isomers for n = 7–9. For the methanol decamer, a branched cyclic isomer is found to be the most favored at low temperatures. Elsewhere, the infrared spectra of all the investigated structures are provided and compared against experiment. The binding energy of neutral methanol is calculated at the X/6-31++g(d,p) levels of theory, where X represents the DFT functionals M062X, APFD, MN15, ωB97XD and M08HX. It is observed that these functionals provide results in good agreement with the experimental vaporization enthalpy. However, the APFD functional shows the best performance followed by the other functionals in the order of M062X, MN15 and ωB97XD. Furthermore, the calculated solvation energies of the proton in methanol at these various levels of theory and at MP2/6-31++g(d,p) show that the ωB97XD functional shows the best performance in evaluating the solvation enthalpy and the solvation free energy of the proton in methanol and the calculated values are respectively −1140.5 kJ mol−1 and −1100.7 kJ mol−1 at room temperature. Elsewhere, we noted that the absolute solvation enthalpy of the proton in methanol is less affected by a change in temperature. However, the absolute solvation free energy of the proton in methanol remains constant only at temperatures lower than 180 K. For higher temperatures, the absolute solvation free energy of the proton in methanol increases as a linear function of the temperature and can be approximated by ΔGm(H+,T) = 0.200T − 1161.4.

Heat Capacity and Thermodynamic Property of Lithium Pentaborate Pentahydrate

Abstract: In order to recover cesium tetraborate pentahydrate (Cs2O·2B2O3·5H2O) from the high concentration cesium-containing salt lake brines and geothermal water resources, the molar heat capacity of Cs2O·2B2O3·5H2O has been measured with a precision calorimeter at the temperature from 303 to 349 K. It was found that there is no phase transition and thermal anomalies. The molar heat capacity of cesium tetraborate pentahydrate is fitted as Cp,m (J·mol−1·K−1) = 593.85705 + 48.0694[T − (Tmax + Tmin)/2]/(Tmax − Tmin)/2] + 24.86395[(T − (Tmax + Tmin)/2)/(Tmax − Tmin)/2]2 + 0.53077[(T − (Tmax + Tmin)/2)/(Tmax − Tmin)/2]3, and the relevant thermodynamic functions of enthalpy, entropy, and Gibbs free energy of cesium tetraborate pentahydrate are also obtained at intervals of 2 K from 303 to 349 K.