A kinetic model of water adsorption, clustering and dissociation on the Fe3S4{001} surface

The interaction of water with catalyst surfaces is a common process which requires investigation. Here, we have employed density functional theory calculations to investigate the adsorption of up to ten water molecules on the {001} surface of greigite (Fe3S4), which owing to its redox properties, is of increasing interest as a catalyst, e.g. in electro-catalysis. We have systematically analyzed and characterized the modes of water adsorption on the surface, where we have considered both molecular and dissociative adsorption processes. The calculations show that molecular adsorption is the predominant state on these surfaces, from both a thermodynamic and kinetic point of view. We have explored the molecular dispersion on the surface under different coverages and found that the orientation of the molecule, and therefore the surface dipole, depends on the number of adsorbed molecules. The interactions between the water molecules become stronger with an increasing number of water molecules, following an exponential decay which tends to the interaction energy found in bulk water. We have also shown the evolution of the infra-red signals as a function of water coverage relating to the H-bond networks formed on the surface. Next we have included these results in a classical micro-kinetic model, which introduced the effects of temperature in the simulations, thus helping us to derive the water cluster size on the greigite surface as a function of the initial conditions of pressure, temperature and external potential. The kinetic model concluded that water molecules agglomerate in clusters instead of wetting the surface, which agrees with the low hydrophilicity of Fe3S4. Clusters consisting of four water molecules was shown to be the most stable cluster under a wide range of temperatures and external potential

Catalytic water dissociation by greigite Fe3S4surfaces: density functional theory study

The iron sulfide mineral greigite, Fe3S4, has shown promising capability as a hydrogenating catalyst, in particular in the reduction of carbon dioxide to produce small organic molecules under mild conditions. We employed density functional theory calculations to investigate the {001},{011} and {111} surfaces of this iron thiospinel material, as well as the production of hydrogen ad-atoms from the dissociation of water molecules on the surfaces. We systematically analysed the adsorption geometries and the electronic structure of both bare and hydroxylated surfaces. The sulfide surfaces presented a higher flexibility than the isomorphic oxide magnetite, Fe3O4, allowing perpendicular movement of the cations above or below the top atomic sulfur layer. We considered both molecular and dissociative water adsorption processes, and have shown that molecular adsorption is the predominant state on these surfaces from both a thermodynamic and kinetic point of view. We considered a second molecule of water which stabilizes the system mainly by H-bonds, although the dissociation process remains thermodynamically unfavourable. We noted, however, synergistic adsorption effects on the Fe3S4{001} owing to the presence of hydroxyl groups. We concluded that, in contrast to Fe3O4, molecular adsorption of water is clearly preferred on greigite surfaces

Selective hydrogenation of CO on Fe3S4{111}: a computational study

Fischer-Tropsch (FT) synthesis has been a recursive method to form valuable molecules from syngas. Metal surfaces have been extensively studied as a FT catalyst, among them, iron presented several phases under reaction conditions, oxide and carbides, as active sites for the FT and reverse water gas shift reaction. We present the CO reduction on an iron sulfide phase with spinel structure, Fe3S4, considering also the pathways where C-O dissociates leaving CHX species on the surface, which may feed longer aliphatic chains via FT process. We analysed the thermodynamic and kinetic availability of each step leading to O, OH species co-adsorbed on the surface as well as the formation of H2O from the hydrogenation of the alcohol group in the molecule. This detailed analysis let to energy profiles, on both active sites of the surface, and conclude that this Fe3S4 surface is high selectivity towards the formation of methanol, in full agreement with experimental results. These findings point out that the C-C bond formation on greigite takes place through an hydroxycarbene FT mechanism

Effect of strontium inclusion on the bioactivity of phosphate-based glasses

We have conducted first-principles and classical molecular dynamics simulations of various compositions of strontium-containing phosphate glasses, to understand how strontium incorporation will change the glasses’ activity when implanted into the body (bioactivity). To perform the classical simulations, we have developed a new interatomic potential, which takes account of the polarizability of the oxygen ions. The Sr-O bond length is ∼ 2.44 − 2.49Å, and the coordination number is 7.5 – 7.8. The Qn distribution and network connectivity were roughly constant for these compositions. Sr bonds to a similar number of phosphate chains as Ca does; based on our previous work [J. K. Christie et al., J. Phys. Chem. B 117, 10652 (2013)], this implies that SrO ↔ CaO substitution will barely change the dissolution rate of these glasses, and that the bioactivity will remain essentially constant. Strontium could therefore be incorporated into phosphate glass for biomedical applications

Configurational analysis of uranium-doped thorium dioxide

While thorium dioxide is already used industrially in high temperature applications, more insight is needed about the behaviour of the material as part of a mixed-oxide (MOX) nuclear fuel, incorporating uranium. We have developed a new interatomic potential model, commensurate with a prominent existing UO2 potential, to conduct configurational analyses of uranium-doped ThO2 supercells. Using the GULP and Site Occupancy Disorder (SOD) computational codes, we have analysed the distribution of low concentrations of uranium in the bulk material, but have not observed the formation of uranium clusters or a single dominant configuration

Теоретико-методологічні засади адаптивного інноваційного розвитку

Подано визначення змісту управління адаптивним інноваційним розвитком, заснованого на використанні його здібностей до трансформації з урахуванням особливостей зовнішнього та внутрішнього середовища суб’єкта сукупності дій, необхідних для здійснення впливу на процеси в усіх сферах управління, що забезпечує інноваційну, організаційно-управлінську, технічну, фінансову та кадрову стійкість. Ключові слова: інновації, інноваційний розвиток, адаптація, адаптивність, механізм, система, процес.Представлено определение содержания управления адаптивным инновационным развитием, основанным на использовании его способностей к трансформации с учетом особенностей внешней и внутренней среды субъекта совокупности действий, необходимых для осуществления влияния на процессы во всех областях управления, обеспечивающих инновационную, организационно-управленческую, техническую, финансовую и кадровую устойчивость. Ключевые слова: инновации, инновационное развитие, адаптация, адаптивность, механизм, система, процесс.The paper presents the definition of management of adaptive innovation-based development that is based on the use of its ability to transform in view of external and internal environment of the subject of actions necessary for making influence on the processes in all areas of management, providing innovative, organizational, administrative, technical, financial and personnel stability of the production company. Keywords: innovation, innovation-based development, adaptation, adaptability, mechanism, system, process

Density functional theory calculations of the hydrazine decomposition mechanism on the planar and stepped Cu(111) surfaces

We have investigated the adsorption of hydrazine (N2H4) and its reactivity on terraces and steps of Cu(111) surfaces by first-principles calculations in order to gain insight into the hydrazine decomposition mechanism. We have investigated different possibilities for the N–N and N–H bond cleavage for any intermediate states by analysing the reaction and barrier energies of each elementary step. We have found that hydrazine dehydrogenation via N–H bond scission is neither energetically nor kinetically favourable on the flat and stepped surfaces, but hydrazine prefers to form NH2via N–N bond decoupling on the Cu(111) with an activation energy below 1 eV. The NH2 molecule reacts fairly easily with co-adsorbed NH2 to form NH3 as well as with N2Hx (x = 1–4) by abstracting hydrogen to produce NH3 and N2 molecules on both the flat and stepped surfaces. We also found that all intermediates except NNH prefer N–N bond breaking as the most likely dissociation pathway, where the amide and imide intermediates produced can be hydrogenated to form NH3 in the presence of hydrogen. NNH is the only intermediate, which prefers to dissociate via a highly exothermic N–H bond breaking process to produce an N2 molecule after overcoming a small barrier energy. We also studied the production of H2 by recombination of hydrogen ad-atoms which, considering the activation energies, is particularly favoured under conditions of moderate temperatures. Our results agree well with experiments suggesting that N2H4 adsorbs dissociatively on copper above ∼300 K leading to N2, NH3 and H2. In general, the lower coordination of the steps is found to lead to higher reactivity than on the flat Cu(111) surface. Furthermore, the calculations show that the influence of step edge atoms is very different for the intra- and intermolecular dehydrogenation mechanisms. They also increase the barrier of N–N decoupling of all the existing species in the reaction

The effect of water on the binding of glycosaminoglycan saccharides to hydroxyapatite surfaces: A molecular dynamics study

Classical molecular dynamics (MD) simulations have been employed to study the interaction of the saccharides glucuronic acid (GlcA) and N-acetylgalactosamine (GalNAc) with the (0001) and (011̄0) surfaces of the mineral hydroxyapatite (HAP). GlcA and GalNAc are the two constituent monosaccharides of the glycosaminoglycan chondroitin sulfate, which is commonly found in bone and cartilage and has been implicated in the modulation of the hydroxyapatite biomineralization process. MD simulations of the mineral surfaces and the saccharides in the presence of solvent water allowed the calculation of the adsorption energies of the saccharides on the HAP surfaces. The calculations show that GalNAc interacts with HAP principally through the sulfate and the carbonyl of acetyl amine groups, whereas the GlcA interacts primarily through the carboxylate functional groups. The mode and strength of the interaction depends on the orientation of the saccharide with respect to the surface and the level of disruption of the layer of water competing with the saccharide for adsorption sites on the HAP surface, suggesting that chondroitin 4-sulfate binds to the layer of solvent water rather than to HAP