Interdiffusion and the vacancy wind effect in Ni–Pt and Co–Pt systems

Researches on thermophysical properties of CO₂ are needed because of its connection to global warming. The amount of CO₂ in the atmosphere is much affected by that of CO₂ contained in the ocean. The diffusion coefficient of CO₂ in high pressure seawater is an important parameter for its environmental estimation. In the present study an apparatus for the measurement of the mutual diffusion coefficient of high pressure liquid has been constructed based on the principle of the Taylor dispersion method. The mutual diffusion coefficients of the CO₂- water have been measured under high pressure. The main part of the apparatus consists a closed­circuit channel, a concentration detecting system and a pressure control system. A straight tube of 2104 mm was used as the diffusion tube. In order to cancel disturbance due to injection of the solute, the concentration change was observed optically at two positions along the diffusion tube.

Diffusion in the Ag-Cd system was investigated at 600°C with vapor-solid and solid-solid diffusion couples in the α-phase for the determination of intrinsic diffusion coefficients, atomic mobilities and interdiffus ion coefficients. Intrinsic diffusion coefficients, DCd and DAg, determined experimentally, were compared with those predicted from the models of Darken, Manning, and Dayananda with the aid of tracer diffusion and thermodynamic data. The experimental coefficients for various alpha alloys were found to be higher than those evaluated from each of the three models. On the other hand, the experimental values of the ratio, DCd/DAg were found to be in good agreement with the models of Manning and Dayananda where the vacancy wind effects on intrinsic diffusion are taken into account. Darken’s relations are considered inadequate to interrelate tracer and intrinsic diffusion coefficients in the Ag-Cd system, since vacancy wind effects are found to be appreciable in this system.

The oxidation of CO with oxygen over (0.25–6.4)% CuO/CeO2 catalysts in excess H2 is studied. CO conversion increases and the temperature range of the reaction decreases by 100 K as the CuO content is raised. The maximal CO conversion, 98.5%, is achieved on 6.4% CuO/CeO2 at 150°C. At T > 150°C, the CO conversion decreases as a result of the deactivation of part of the active sites because of the dissociative adsorption of hydrogen. CO is efficiently adsorbed on the oxidized catalyst to form CO-Cu+ carbonyls on Cu2O clusters and is oxidized by the oxygen of these clusters, whereas it is neither adsorbed nor oxidized on Cu0 of the reduced catalysts. The activity of the catalysts is recovered after the dissociative adsorption of O2 on Cu0 at T ∼ 150°C. The activation energies of CO, CO2, and H2O desorption are estimated, and the activation energy of CO adsorption yielding CO-Cu+ carbonyls is calculated in the framework of the Langmuir-Hinshelwood model.

Diffusion coefficients of CO2 molecule and bicarbonate ion in hemoglobin solution measured by fluorescence technique.

Correlation between estimated diffusion coefficients from different types of diffusion couples in multicomponent system

Up to now there existed no theoretical calculations of the mixing enthalpies for the group-B alloy systems using the famous Miedema's theory or by the theoretical methods of the first principle. Therefore such calculations for the 10 group–IIB-IIIB binary alloy systems are performed for the first time by a subregular model. The results show that the agreement between the experimental data and calculations is pretty good and could be accepted both from the theoretical viewpoint and from the experimental one. It also can be concluded from the results that the subregular model could be used for calculating the mixing enthalpies for the group-B alloys, at least for the IIB-IIIB alloy systems. The positive mixing enthalpies for all of the 10 alloy systems are discussed.

Study of the pyrolysis of biomass using thermo-gravimetric analysis (TGA) and concentration measurements of the evolved species

Here, we used a combined molecular dynamics/active learning (AL) approach to create machine learning models that can predict the diffusion coefficient of epichlorohydrin and chloropropene carbonate, the reactant and product of a common CO<sub>2</sub> cycloaddition reaction, in metal–organic frameworks (MOFs). Nanoporous MOFs are effective catalysts for the cycloaddition of CO<sub>2</sub> to epoxides. The diffusion rates within nanoporous catalysts can control the rate of reaction as the reactants and products must diffuse to the active sites within the MOF and then out of the nanoporous material for reusability. However, the diffusion process is routinely ignored when searching for new materials in catalytic applications. Here we verified improvement during the AL process by consistently tracking metrics on the same groups of MOFs to ensure consistency. Metal identity was found to have little impact on diffusion rates, while structural features like pore limiting diameter act as a threshold where a minimum value is needed for high diffusion rates. We identified the MOFs with the highest epichlorohydrin and chloropropene carbonate diffusion coefficients which can be used for further studies of reaction energetics.

Simultaneous determination of sorption, heat of sorption, diffusion coefficient and glass transition depression in polymer–CO 2 systems

The interfacial tension (IFT) and diffusion coefficient of CO 2 in a hydrocarbon in a carbonated water (CW) environment were estimated for pressures between 10-100 bar and at temperatures of 25 C, 35 C, and 45 C using axisymmetric pendant drop shape analysis (ADSA). We developed a dynamic numerical model accounting for the changes in properties at the CW-hydrocarbon interface owing to the swelling of the hydrocarbon drop (HD). This approach eliminates an error of about 36% arising from the assumption of a static boundary/volume in conventional methods. The results indicated that the IFT between CW and the hydrocarbon is a function of the CO 2 phase density and solubility. At low pressures (<60 bar, 25 C), IFT is directly proportional to the pressure, while at pressures larger than 60 bar, it is inversely proportional to the pressure. The physical behaviour of the CW-hydrocarbon system is driven mainly by a balance between CO 2 solubility and IFT, resulting in a constant CO 2 saturation. Despite the decrease in CO 2 solubility with temperature, the diffusion coefficient of CO 2 increases. The reduction in IFT with temperature counteracts the decrease in CO 2 solubility, leading to a higher diffusion coefficient as temperature increases. It is shown here that the CWhydrocarbon system induces more swelling and lower viscosity compared to similar systems but with pure CO 2 . As such, CW's potential for enhanced oil recovery (EOR) is realised.

Electrochemical determination of the diffusion coefficient of cations into Chevrel phase-based electrochemical transfer junction by potential step chronoamperometry and impedance spectroscopy

An experimental approach for measuring carbon dioxide diffusion coefficient in water and oil under supercritical conditions

We perform dynamic Monte Carlo simulations to understand the kinetics of CO oxidation on Pt surface. Our simulations are performed on a model for CO electro-oxidation, where oxygen-containing species (adsorbed OH) formed on the Pt surface reacts with adsorbed CO by a Langmuir–Hinshelwood mechanism to form CO2. A systematic study of the model parameters shows that we obtain better electrocatalytic activity by increasing the surface diffusion coefficient of CO (kD) or the rate of reaction between adsorbed CO and adsorbed OH. We also show that if the initial state of adsorbed CO is a closely packed structure, increasing the surface coverage can result in negative voltammetry peak shifts because of the formation of weakly adsorbed states of CO. We have also modeled bulk CO voltammetry by introducing a CO adsorption rate, k4. We show that near-zero currents are obtained for the potentials (E) where the rate of OH adsorption, k1(E), is much smaller than k4. We also note that kD plays no significant role in CO oxidation when kD≪k4. Our results also show that bulk CO voltammetry peaks shift to more positive potentials compared to the stripping, as observed in experiments. Our potential step simulations show no difference in bulk and stripping current transients in the potential regions where k1(E)≫k4, whereas they vary significantly when k4≅k1(E).

Electrochemical studies on diffusion and redox behavior of various metal ions in some molten glasses

The general formulation of the classical kinetic theory, which is needed to predict transport properties of gases in situations where the hydrodynamic equations are valid, is reviewed. A rigid convex model of tetrahedral symmetry is used to predict the Senftleben-Beenakker effect of a static magnetic field on the thermal conductivity and viscosity of pure CH/sub 4/, CD/sub 4/ and CF/sub 4/. The parameters of the model are optimized and are found to assume physically reasonable values. The calculations agree with experiment to a degree comparable to that of similar work on diatomic molecules. A generalized scattering cross section, ..gamma.., is defined which can be evaluated exactly for the limiting cases of a spherical soft potential and rigid ovaloids. For a general soft nonspherical interaction of the Kihara type, a suitable approximation for the momentum dependence is made with the following attributes: ..gamma.. reduces to the form for soft sphere and rigid ovaloid in the limits and the resulting matrix elements of the collision operator can be written in terms of the familiar ..cap omega..* integrals. This formulation is used to investigate thermal diffusion in binary isotopic mixtures of CO. Calculations are made in an 80/sup 0/K to 300/sup 0/K range which includes the inversion temperatures for all mixtures studied. Thermal conductivity and diffusion coefficients of CO are also calculated. The parameters of the model can be adjusted to account for the major features of the experimental data. The physical significance of the parameters is discussed. (auth)