Metal free hydrogenation reaction on carbon doped boron nitride fullerene: A DFT study on the kinetic issue

By the incorporation of C into (BN)₁₂ fullerene, our theoretical investigation shows that the hydrogenation reaction on carbon doped B₁₁N₁₂C cluster is both thermodynamically favored and kinetically feasible under ambient conditions. Without using the metal catalyst, the C atom can work as an activation center to dissociate H₂ molecule and provide the free H atom for further hydrogenation on the B₁₁N₁₂C fullerene, which saves the materials cost in practical applications for hydrogen storage. Moreover, the material curvature also plays an important role in reducing the activation barrier for the hydrogen dissociation on the BN fullerenes.     

Calculations of transport phenomena and reaction distribution in a polymer electrolyte membrane fuel cell

The performance of Polymer Electrolyte Membrane fuel cells depends on the design of the cell as well as the operating conditions. The design of the cell influences the complex interaction of activation effects, ohmic losses, and transport limitations, which in turn determines the local current density. Detailed models of the electrochemical reactions and transport phenomena in Polymer Electrolyte Membrane fuel cells can be used to determine the current density distribution for a given fuel cell design and operating conditions. In this work, three-dimensional, multicomponent and multiphase transport calculations are performed using a computational fluid dynamics code. The computational results for a full-scale fuel cell design show that ohmic effects due to drying of polymer electrolyte in the anode catalyst layer and membrane, and transport limitations of air and flooding in the cathode cause the current density to be a maximum near the gas channel inlets where ohmic losses and transport limitations are a minimum. Elsewhere in the cell, increased ohmic losses and transport limitations cause a decrease in current density, and the performance of the fuel cell is significantly lower than that which could be attained if the ohmic losses and transport limitations throughout the cell were the same as those near the gas channel inlets. Thus overall fuel cell design is critical in maximizing unit performance.     

Thermodynamic and kinetic properties of calcium hydride

Calcium hydride has shown great potential as a hydrogen storage material and as a thermochemical energy storage material. To date, its high operating temperature (above 800 °C) has not only hindered its opportunity for technological application but also prevented detailed determination of its thermodynamics of hydrogen sorption. In addition, calcium metal suffers from high volatility, high corrosivity from Ca (and CaH₂), slow kinetics of hydrogen sorption, and the solubility of Ca in CaH₂. In this work, a literature review of the wide-ranging thermodynamic properties of CaH₂ is provided along with a detailed experimental investigation into the thermodynamic properties of molten and solid CaH₂. The thermodynamic values of hydrogen release from both molten and solid CaH₂ were determined as ΔH_des (molten CaH₂) = 216 ± 10 kJ mol⁻¹.H₂, ΔS_des (molten CaH₂) = 177 ± 9 J K⁻¹ mol⁻¹.H_2, which equates to a 1 bar hydrogen equilibrium temperature for molten CaH₂ of 947 ± 65 °C. Similarly, in the solid-state: ΔH_des (solid CaH₂) = 172 ± 12 kJ mol⁻¹.H₂, ΔS_des (solid CaH₂) = 144 ± 10 J K⁻¹ mol⁻¹.H₂. Moreover, the activation energy of hydrogen release from CaH₂ was also calculated using DSC analysis as E_a = 203 ± 12 kJ mol⁻¹. This study provides the first thermodynamics for the Ca–H system in over 60 years, providing more accurate data on this emerging energy storage material.     

Numerical analysis of coupled transport and reaction phenomena in an anode-supported flat-tube solid oxide fuel cell

Heat and mass transfer with electrochemical reaction in an anode-supported flat-tube solid oxide fuel cell (FT-SOFC) is studied by means of three-dimensional numerical simulation. The distributions of the reaction fields in the anode-supported FT-SOFC are found to be similar to those in the planar SOFC with co-flow arrangement. However, in comparison with the latter, the concentration and activation overpotentials of the former can be reduced by additional reactant diffusion through the porous rib of the fuel channel. Parametric survey reveals that, for a fixed activation overpotential model, the output voltage can be improved by increasing the pore size of anode, while the cross-sectional geometry has smaller effect on the cell performance. Based on the results of three-dimensional simulation, we also develop a simplified numerical model of anode-supported FT-SOFC, which takes into account the concentration gradients in the thick anode of complex cross-sectional geometry. The simplified model can sufficiently predict the output voltage as well as the distributions of temperature and current density with very low computational cost. Thus, it can be used as a powerful tool for surveying wide range of anode-supported FT-SOFC design parameters.     

A study of the transport phenomena in a wall-coated micro steam-methanol reformer

Microreactors have important advantages due to the increased heat and mass transfer resulting from the small dimensions of the system. This results in enhanced performance and efficiency. Microreformers with wall-coated or suspended catalytic layer configurations have been studied due to their lower transport resistances compared to packed-bed microreformers. In the present work, a theoretical study of the reacting flow in a wall-coated microreformer is carried out. The theoretical results for the conversions and carbon monoxide concentration of wall-coated reformers are compared with the reported experimental data. The effects of important parameters for wall-coated reformers such as the catalyst layer thickness, wall temperature, and the flow velocity of the reactants are studied. Recommendations for the design of micro wall-coated reformers are provided.     

Castor oil transesterification reaction: A kinetic study and optimization of parameters

In this paper, parameters affecting castor oil transesterification reaction were investigated. Applying four basic catalysts including NaOCH₃, NaOH, KOCH₃ and KOH the best one with maximum biodiesel yield was identified. Using Taguchi method consisting four parameters and three levels, the best experimental conditions were determined. Reaction temperature (25, 65 and 80 °C), mixing intensity (250, 400 and 600 rpm), alcohol/oil ratio (4:1, 6:1 and 8:1) and catalyst concentration (0.25, 0.35 and 0.5%) were selected as experimental parameters. It was concluded that reaction temperature and mixing intensity can be optimized. Using the optimum results, we proposed a kinetic model which resulted in establishing an equation for the beginning rate of transesterification reaction. Furthermore, applying ASTM D 976 correlation, minimum cetane number of produced biodiesel was determined as 37.1.     

Glycerol acetals, kinetic study of the reaction between glycerol and formaldehyde

The acetalization reaction between glycerol and formaldehyde using Amberlyst 47 acidic ion exchange resin was studied. These acetals can be obtained from renewable sources (bioalcohols and bioalcohol derived aldehydes) and seem to be good candidates for different applications such as oxygenated diesel additives. A preliminary kinetic study was performed in a batch stirred tank reactor studying the influence of different process parameters like temperature, feed composition and the stirring speed. A pseudo homogenous kinetic model able to explain the reaction mechanism was adjusted. Thus, the corresponding order of reaction was determined. Amberlyst 47 acidic ion exchange resin showed a fairly good behavior allowing 100% of selectivity towards acetals formation. However, the studied acetalization reaction showed high thermodynamic limitations achieving glycerol conversions around 50% using a stoichiometric feed ratio at 353 K. The product is a mixture of two isomers (1,3-Dioxan-5-ol and 1,3-dioxolane-4-methanol) and the conversion of 1,3-dioxolane-4-methanol into 1,3-Dioxan-5-ol was also observed.     

Correlated transport and optical phenomena in Ga-doped CdO films

Series of samples of lightly Ga-doped CdO thin films (3%, 6%, and 9%) have been prepared by evaporation method on glass substrate. The prepared films were characterised by X-ray diffraction (XRD), UV–VIS–NIR absorption spectroscopy, and dc-electrical measurements. The investigation shows that Ga doping widens the energygap of CdO. The optical properties were easily explained by using Tauc et al. band-to-band transitions and classical Drude theory. The electrical behaviour of the samples shows that they are degenerate semiconductors. The 6% Ga-doped CdO sample shows increase its mobility by 3.2 times, increase its conductivity by 1.5 times, increase its intrinsic bandgap, and a slight increase its transmittance relative to undoped CdO film. Explanation was given concerning these variations. From transparent conducting oxide (TCO) point of view, Ga is not sufficiently effective for CdO doping comparing to other dopants like In, Sn, Sc, and Y.     

Optimally-reduced kinetic models: reaction elimination in large-scale kinetic mechanisms

A new optimization-based approach to kinetic model reduction is presented. The reaction-elimination problem is formulated as a linear integer program which can be solved to guaranteed global optimality. This formulation ensures that the solution to the integer program is the smallest possible reduced model consistent with the user-set tolerances. The method is applied to generate optimally-reduced models for isobaric, adiabatic homogeneous combustion. The computational cost and accuracy of the reduced models are compared to those of the full mechanism. Results are shown for GRImech 3.0 and the Lawrence Livermore n-heptane combustion mechanism. The accuracy of the integer programming approach is compared to existing reaction elimination methods. The method is also applied to generate a library of reduced kinetic models for an adaptive chemistry simulation of a 2-D laminar, partially-premixed methane burner flame. Preliminary results are presented comparing the computational cost of the full GRImech 3.0 chemistry to that of the reduced model library.     

Thermodynamic and kinetic model of reforming coke-oven gas with steam

The experiments of reforming the methane of coke-oven gas with steam were performed. The effects of the thermodynamic factors, such as the H₂O/CH₄ ratio, the conversion temperature (T) of methane and the reaction time (t), on the methane conversion rate have been investigated. The experimental results show that the H₂O/CH₄ ratio within the range of 1.1–1.3 and the temperature 1223–1273 K are the reasonable thermodynamic conditions for methane conversion. A methane conversion of more than 95% can be achieved when the H₂O/CH₄ ratio is 1.2, the conversion temperature is above 1223 K and the conversion time is up to 15 s respectively. In additional, kinetic data of different reaction conditions were measured, and a dynamic model of methane conversion was proposed and verified. All results demonstrated that the results of the dynamic models agree well with the experiments, of which the deviation is less than 1.5%.     

Density functional theory study of the hydrogen evolution reaction in haeckelite boron nitride quantum dots

To satisfy arising energy needs and to handle the forthcoming worldwide climate transformation, the major research attention has been drawn to environmentally friendly, renewable and abundant energy resources. Hydrogen plays an ideal and significant role is such resources, due to its non-carbon based energy and production through clean energy. In this work, we have explored catalytic activity of a newly predicted haeckelite boron nitride quantum dot (haeck-BNQD), constructed from the infinite BN sheet, for its utilization in hydrogen production. Density functional theory calculations are employed to investigate geometry optimization, electronic and adsorption mechanism of haeck-BNQD using Gaussian16 package, employing the hybrid B3LYP and wB97XD functionals, along with 6–31G(d,p) basis set. A number of physical quantities such as HOMO/LUMO energies, density of states, hydrogen atom adsorption energies, Mulliken populations, Gibbs free energy, work functions, overpotentials, etc., have been computed and analysed in the context of the catalytic performance of haeck-BNQD for the hydrogen-evolution reaction (HER). Based on our calculations, we predict that the best catalytic performance will be obtained for H adsorption on top of the squares or the octagons of haeck-BNQD. We hope that our prediction of most active catalytic sites on haeck-BNQD for HER will be put to test in future experiments.     

Numerical study on micro-reformer performance and local transport phenomena of the plate methanol steam micro-reformer

The objective of this work is to investigate the transport phenomena and performance of a plate steam methanol micro-reformer. Micro channels of various height and width ratios are numerically analyzed to understand their effects on the reactant gas transport characteristics and micro-reformer performance. In addition, influences of Reynolds number and geometric size of micro channel on methanol conversion of micro-reformer and gas transport phenomena are also explored. The predicted results demonstrated that better performance is noted for a micro channel reformer with lower aspect-ratio micro channel. This is due to the larger the chemical reaction surface area for a lower aspect-ratio channel reformer. It is also found that the methanol conversion decreases with increasing Reynolds number Re. The results also indicate that the smaller micro channel size experiences a better methanol conversion. This is due to the fact that a smaller micro channel has a much more uniform temperature distribution, which in turn, fuel utilization efficiency is improved for a smaller micro channel reformer.     

Mass transport phenomena in direct methanol fuel cells

Clean and highly efficient energy production has long been sought to solve energy and environmental problems. Fuel cells, which convert the chemical energies stored in fuel directly into electrical energy, are expected to be a key enabling technology for this century. This article is concerned with one of the most advanced fuel cells – direct methanol fuel cells (DMFCs). We present a comprehensive review of the state-of-the-art studies of mass transport of different species, including the reactants (methanol, oxygen and water) and the products (water and carbon dioxide) in DMFCs. Rather than elaborating on the details of the previous numerical modeling and simulation, the article emphasizes: i) the critical mass-transport issues that need to be addressed so that the performance and operating stability of DMFCs can be upgraded, ii) the basic mechanisms that control the mass-transport behaviors of reactants and products in this type of fuel cell, and iii) the previous experimental and numerical findings regarding the correlation between the mass transport of each species and cell performance.     

Mathematical modeling of transport phenomena in porous SOFC anodes

In the present study, a mathematical model describing the transport of multi-component species inside porous SOFC anodes is developed. The model considers the reaction zone layer as a distinct volume rather than a mere mathematical surface (boundary condition) as treated in the existing models. The reaction zone layer is a relatively thin layer in the vicinity of electrolyte where electrochemical H₂ oxidation takes place to produce electrons and water vapor. The model also incorporates the effect of Knudsen diffusion in the porous electrode and reaction zone layers. Simulations are performed using multi-component ethanol reformate fuel to predict the distribution of multi-component species in the electrode and reaction zone layers at different loads (current densities). In addition, the effect of shift reaction on the concentration overpotential is examined. Moreover, the effect of treating reaction zone layer as a discrete volume is investigated.     

Modeling of transport phenomena in hybrid laser-MIG keyhole welding

Mathematical models and associated numerical techniques have been developed to investigate the complicated transport phenomena in spot hybrid laser-MIG keyhole welding. A continuum formulation is used to handle solid phase, liquid phase, and the mushy zone during the melting and solidification processes. The volume of fluid (VOF) method is employed to handle free surfaces, and the enthalpy method is used for latent heat. Dynamics of weld pool fluid flow, energy transfer in keyhole plasma and weld pool, and interactions between droplets and weld pool are calculated as a function of time. The effect of droplet size on mixing and solidification is investigated. It is found that weld pool dynamics, cooling rate, and final weld bead geometry are strongly affected by the impingement process of the droplets in hybrid laser-MIG welding. Also, compositional homogeneity of the weld pool is determined by the competition between the rate of mixing and the rate of solidification.     

Thermodynamic and kinetic factors effecting hydrogen absorption on metal hydrides

The work of Guvendiren et al. on the effects of additives on mechanical milling and hydrogenation of Magnesium Powder which is published in this journal [Guvendiren M, Bayboru E, Ozturk T. International Journal of Hydrogen Energy 2004; 29: 491–496] shows excellent experimental work which agrees with previous published work. However they did not explain the right phenomenon which is undergoing during hydrogen absorption on magnesium hydride's system. In this communication it is the objective to distinguish between thermodynamic and kinetics factor effecting hydrogen absorption. It will be shown that the phase rule of thermodynamics will determine the variation of Pressure–composition isotherms at constant temperature during hydrogen absorption in Magnesium or Titanium. This is because the Pressure–composition isotherms at constant temperature is different for a single-phase (e.g. beta-Titanium) than two phases (e.g. delta and epsilon phases). Thus the data of Guvendiren et al. published in this journal (Fig. 5) can be explained by this effect.     

Numerical study of reactant gas transport phenomena and cell performance of proton exchange membrane fuel cells

A two-dimensional numerical model has been established to investigate the performance of the PEM fuel cells. Parameters used in the analysis include the porosity and thickness of the gas diffuser layer (GDL). Results show that increasing the porosity of gas diffusion layer causes the increasing of mass transfer of fuel and air and results in a higher reaction rate. Therefore, a better performance of the fuel cell and more fuel consumption rate are observed. It is also demonstrated that the performance of the fuel cell increases with a decrease in the thickness of gas diffusion layer. The effects of liquid water condensation and flow directions of fuel and air are also considered in this analysis. Predicted results show that the performance of the PEM fuel cell without consideration of liquid water effect is always higher than that with consideration of liquid water effect. In addition, the performance of fuel cell with co-flow pattern of fuel and air is larger than that with counter flow.     

Three-dimensional simulation of heat and mass transport phenomena in planar SOFCs

High temperature Solid Oxide Fuel Cells (SOFCs) represent a promising and efficient technology for electrochemical conversion of chemical energy of a fuel into electrical energy. The future development of such technology depends on the availability of detailed and efficient multi-dimensional modeling tools. In this paper, a new three-dimensional finite element algorithm, based on a detailed mathematical model for fuel cells and on the fully explicit Artificial Compressibility (AC) Characteristic Based Split (CBS) scheme, is employed for the effective and efficient modeling of heat and mass transport phenomena coupled with electrochemical reactions in SOFC. The thermal field in the fuel cell is analyzed and the influence of the operating temperature on the fuel cell overall performance is investigated. The three-dimensional results obtained in this work are also compared to the results carried out by employing the two-dimensional version of the present scheme. The results are validated against experimental data available in the literature.