Complex Solid State Reaction Research Articles

Artificial neural networks in kinetic analysis of glass crystallization: The case of complex nucleation-growth mechanisms

The selected artificial neural networks were trained and tested to determine the kinetics of theoretically simulated signals for two overlapping independent nucleation-growth processes. Whereas the hybrid convolutional neural network did not perform well, the multilayer perceptron (MLP) showed great potential for the kinetic analysis of complex solid-state reactions and transformation mechanisms. In particular, the MLP architecture exhibited remarkable robustness with respect to the scatter in kinetic data as well as the ability to accurately deal with practically fully overlapping kinetic peaks. When trained on a full spectrum of double-process overlaps, the MLP architecture returned very precise estimates of the kinetic parameters during the testing phase despite the limited data sample used for some of the training. This level of accuracy was observed in the case of both overlapping processes being roughly similarly sized, and for the dominant process in the cases of the two processes being largely disproportionate in magnitude.

Ultramarine pigments

Abstract Ultramarine pigments are representatives of the inorganic blue pigments. They are characterized by intensive shades of blue and relatively weak violet and pink color tones. The composition of the pigments can be summarized by the formula Na7Al6Si6O24S3. The color of ultramarine is generated by incorporation of free polysulfide radicals in the structure of a sodalite-derived crystal lattice. Ultramarine pigments are synthesized using a series of complex solid-state reactions. The pigments are mainly used in plastics, paints and powder coatings and printing inks, but also in paper and paper coatings.

Analysis of Solid-State Reaction Mechanisms with Two-Dimensional Fourier Transform Infrared Correlation Spectroscopy.

The utility of two-dimensional generalized correlation spectroscopy (2D-COS) for tracking complex solid-state reactions is demonstrated using infrared spectra acquired during a photochemically induced decomposition reaction. Eleven different thin films, consisting of six monometallic and five bimetallic 2-ethylhexanoate complexes, were tracked as a function of photolysis time. Overlapping peaks in the infrared fingerprint region are readily discriminated using 2D-COS, enabling individual vibrational components to be used to distinguish whether carboxylate ligands are free/ionic or bound in a chelating, bridging, or monodentate fashion. This classification enables the decomposition mechanism to be tracked for all 11 samples, revealing that ligands bound in monodentate and bridging fashions are first converted to chelates before being lost as volatile products for all samples. The magnitude of the measured first-order rate constants for loss of chelated ligands is found to correlate linearly to the asymmetric stretching frequency of monodentate ligands but exhibits a V shape when plotted against the electronegativity of the metal center. We propose that loss of chelated ligands proceeds via C-O scission for highly electronegative transition metals but M-O scission for transition metals with low electronegativity. These results establish 2D-COS as a powerful tool to deconvolute and correlate individual components, enabling mechanistic analysis of complex chemical reactions.

A thermal-gradient approach to variable-temperature measurements resolved in space.

Temperature is a ubiquitous environmental variable used to explore materials structure, properties and reactivity. This article reports a new paradigm for variable-temperature measurements that varies the temperature continuously across a sample such that temperature is measured as a function of sample position and not time. The gradient approach offers advantages over conventional variable-temperature studies, in which temperature is scanned during a series measurement, in that it improves the efficiency with which a series of temperatures can be probed and it allows the sample evolution at multiple temperatures to be measured in parallel to resolve kinetic and thermodynamic effects. Applied to treat samples at a continuum of tem-peratures prior to measurements at ambient temperature, the gradient approach enables parametric studies of recovered systems, eliminating temperature-dependent structural and chemical variations to simplify interpretation of the data. The implementation of spatially resolved variable-temperature measurements presented here is based on a gradient-heater design that uses a 3D-printed ceramic template to guide the variable pitch of the wire in a resistively heated wire-wound heater element. The configuration of the gradient heater was refined on the basis of thermal modelling. Applications of the gradient heater to quantify thermal-expansion behaviour, to map metastable polymorphs recovered to ambient temperature, and to monitor the time- and temperature-dependent phase evolution in a complex solid-state reaction are demonstrated.

The Onset of Dehydrogenation in Solid Ammonia Borane: An Ab Initio Metadynamics Study.

The discovery of effective hydrogen storage materials is fundamental for the progress of a clean energy economy. Ammonia borane (H3 BNH3 , AB) has attracted great interest as a promising candidate but the reaction path that leads from its solid phase to hydrogen release is not yet fully understood. To address the need for insights in the atomistic details of such a complex solid state process, in this work we use ab-initio molecular dynamics and metadynamics to study the early stages of AB dehydrogenation. We show that the formation of ammonia diborane (H3 NBH2 (μ-H)BH3 ) leads to the release of NH4 + , which in turn triggers an autocatalytic H2 production cycle. Our calculations provide a model for how complex solid state reactions can be theoretically investigated and rely upon the presence of multiple ammonia borane molecules, as substantiated by standard quantum-mechanical simulations on a cluster.

The Onset of Dehydrogenation in Solid Ammonia Borane: An Ab Initio Metadynamics Study

AbstractThe discovery of effective hydrogen storage materials is fundamental for the progress of a clean energy economy. Ammonia borane (H3BNH3, AB) has attracted great interest as a promising candidate but the reaction path that leads from its solid phase to hydrogen release is not yet fully understood. To address the need for insights in the atomistic details of such a complex solid state process, in this work we use ab‐initio molecular dynamics and metadynamics to study the early stages of AB dehydrogenation. We show that the formation of ammonia diborane (H3NBH2(μ‐H)BH3) leads to the release of NH4+, which in turn triggers an autocatalytic H2 production cycle. Our calculations provide a model for how complex solid state reactions can be theoretically investigated and rely upon the presence of multiple ammonia borane molecules, as substantiated by standard quantum‐mechanical simulations on a cluster.

Electronic Structure and Band Gap of Fullerenes on Tungsten Surfaces: Transition from a Semiconductor to a Metal Triggered by Annealing.

The understanding and control of molecule-metal interfaces is critical to the performance of molecular electronics and photovoltaics devices. We present a study of the interface between C60 and W, which is a carbide-forming transition metal. The complex solid-state reaction at the interface can be exploited to adjust the electronic properties of the molecule layer. Scanning tunneling microscopy/spectroscopy measurements demonstrate the progression of this reaction from wide band gap (>2.5 eV) to metallic molecular surface during annealing from 300 to 800 K. Differential conduction maps with 104 scanning tunneling spectra are used to quantify the transition in the density of states and the reduction of the band gap during annealing with nanometer spatial resolution. The electronic transition is spatially homogeneous, and the surface band gap can therefore be adjusted by a targeted annealing step. The modified molecules, which we call nanospheres, are quite resistant to ripening and coalescence, unlike any other metallic nanoparticle of the same size. Densely packed C60 and isolated C60 molecules show the same transition in electronic structure, which confirms that the transformation is controlled by the reaction at the C60-W interface. Density functional theory calculations are used to develop possible reaction pathways in agreement with experimentally observed electronic structure modulation. Control of the band gap by the choice of annealing temperature is a unique route to tailoring molecular-layer electronic properties.

Kinetics and safety analysis of sulfide mineral self-heating

Thermal methods of analysis such as DSC and TGA provide a powerful methodology for the study of solid reactions. This paper proposes an improved thermal analysis methodology for thermal stability and safety investigation of complex solid-state reactions. The proposed methodology is based on differential iso-conversional approach and involves peak separation and individual peak analysis for kinetic analysis and safety prediction. The proposed thermal analysis method was coupled with Mineral Libration Analysis (MLA) to investigate self-heating of sulfide mineral ores. The influence of sample’s mineralogy on thermal degradation was examined and discussed. The information gained from the advanced kinetic analysis of DSC/TGA measurements were up-scaled for TMR and SADT determination. The described thermal analysis method provides not only an understanding of sulfide mineral self-heating, but also aids the design of effective mitigation measures for their adverse environmental and safety effects.

Kinetic Analysis of Complex Solid-State Reactions. A New Deconvolution Procedure

The kinetic analysis of complex solid-state reactions that involve simultaneous overlapping processes is challenging. A method that involves the deconvolution of the individual processes from the overall differential kinetic curves obtained under linear heating rate conditions, followed by the kinetic analysis of the discrete processes using combined kinetic analysis, is proposed. Different conventional mathematical fitting functions have been tested for deconvolution, paying special attention to the shape analysis of the kinetic curves. It has been shown that many conventional mathematical curves such as the Gaussian and Lorentzian ones fit kinetic curves inaccurately and the subsequent kinetic analysis yields incorrect kinetic parameters. Alternatively, other fitting functions such as the Fraser-Suzuki one properly fit the kinetic curves independently of the kinetic model followed by the reaction and their kinetic parameters, and moreover, the subsequent kinetic analysis yields the correct kinetic parameters. The method has been tested with the kinetic analysis of complex processes, both simulated and experimental.

Advanced kinetics for calorimetric techniques and thermal stability screening of sulfide minerals

Thermal methods of analysis such as differential scanning calorimetry (DSC) provide a powerful methodology for the study of solid reactions. This paper proposes an improved thermal analysis methodology for thermal stability investigation of complex solid-state reactions. The proposed methodology is based on differential iso-conversional approach and involves peak separation, individual peak analysis and combination of isothermal/non-isothermal DSC measurements for kinetic analysis and prediction. The proposed thermal analysis, which coupled with Mineral Libration Analyzer (MLA) technique was employed to investigate thermal behavior of sulfide mineral oxidation. The importance of various experimental variables such as particle size, heating rate and atmosphere were investigated and discussed. The information gained from such an advanced thermal analysis method is useful for scale-up processes with potential of significant savings in plant operations, as well as in mitigating adverse environmental and safety issues arising from handling and storage of sulfide minerals.

Pharmaceutical microcalorimetry: recent advances in the study of solid state materials

This paper discusses the theory and experimental application of microcalorimetry to the study of solid state reactions of pharmaceutical importance. The practical example selected is that of formulated products containing benzoyl peroxide – a treatment for acne and athlete’s foot. A newly developed procedure for data analysis is outlined and preliminary results from chemometric-based analysis of complex solid state reaction schemes is presented. Finally, the microcalorimetric requirements for such stability studies are contrasted with the newly emerging multi-channel “chip calorimeters” that operate in the nano range (watts, material, concentration) with high throughput potential.

Application of the Arrhenius equation to solid state kinetics: can this be justified?

Application of the Arrhenius equation to the kinetics of solid state reactions has been criticised on several grounds. One of the main theoretical objections has been that the energy distribution amongst the immobilised constituents of a crystalline reactant is not represented by the Maxwell–Boltzmann equation. The detailed band structure of a solid may, however, include interface energy levels (analogous to impurity levels in semiconductors) that participate in the crucial bond redistribution step. Occupancy of such levels would be determined by energy distribution functions based on Fermi–Dirac statistics for electrons, and Bose–Einstein statistics for phonons. For the highest energies, necessary for reaction, both distributions approximate to an exponential energy term, thereby countering the above objection to the application of the Arrhenius equation to reactions of solids. The existence of such interface levels, with a limited range of energies, would also allow for the variation of apparent activation energy with extent of reaction, and also with temperature, reported for many complex solid state reactions.

ChemInform Abstract: Round Robin Test on the Kinetic Evaluation of a Complex Solid State Reaction from 13 European Laboratories. Part 1. Kinetic TG‐Analysis.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Kinetic analysis of complex reactions

TG and DSC techniques have been extensively used to study complex solid state reactions. For complex reactions (constituting of all exothermic or all endothermic one step first order individual reactions), it has been shown that the results of TG and DSC instruments may not be identical. This is because the TG instrument is incapable of correctly recording the true effective reaction rate of complex reactions (if the reaction rate is not proportional to the total amount of reactants). This may happen when the reaction rates of the individual reactions in the composite reaction mixture are significantly different. In this communication it has also been suggested that the Friedman analysis of obtaining activation energy (E) may be inapplicable for complex or composite reaction due to the fact that, there may be no unique effective constant conversion at various heating rates.