A computational study of silicate oligomerization reactions

The classical syntheses of zeolitic materials are based on silicates or alkoxysilanes in alkaline solutions, often with organic additives that act as structure-directing agents (SDA). The SDA are thought to be mainly responsible for the formation of a specific zeolite structures. However, this thesis shows that the silicate-template interactions also are important to be initial condensation reaction take place in the prenucleation stage. At this point it is also still under discussion whether growth proceeds by a monomer addition sequence or the assembly of preformed building blocks. In this thesis, the essential chemical properties of the silica condensation process such as the role of water, hydrogen bonding, counter ion effect, interaction with template have been addressed. The mechanism of the silica condensation reaction of small oligomer from dimer to pentamer in gas phase was studied in chapter 2. In this chapter, the reaction mechanism of silica condensation was investigated using DFT calculations for various structures of silicate oligomers. There are two different reaction paths for the condensation reaction: one reaction path proceeds via neutral species and the other occurs via the anionic species. An anionic mechanism that occurs in two steps. The first step is the formation of the SiO-Si linkage bond between two reactants. The second one is the removal of the water group from the intermediate to form a product. It infers that the water removal is the most difficult step in reaction pathway. Based on the calculated activation barriers of silicate formation, the anionic pathway is kinetically preferred over the neutral one. Thus, the polymerization of silicate species mainly concerns the anionic species. The ring closure reactions occur with high barriers because of loss of intramolecular hydrogen bonds. The decrease of the overall activation barrier for the formation of higher linear and branched molecules is ascribed to more favorable hydrogen bonding effects for these cases. The importance of inter- and intramolecular hydrogen bonding to the relative activation barriers of SiO-Si bond formation was found. In chapter 3, it was argued that it is essential to include explicitly water molecules in silica condensation and sol-gel chemistry computational studies. The rate limiting step is not the water removal step as in the gas phase simulation in chapter 2. The overall barrier is mostly depends on the first barrier of the SiO-Si bond formation. The activation barrier of the water removal step depends on the mechanism of water assisted internal or external proton transfer, independent from the size of oligomer. The kinetic and thermodynamic trends of formation of higher oligomers from dimer silica are found to be quite different between explicit solvent simulations and gas phase simulation. The gas phase model in chapter 2 proposes that the linear and branched higher silica structures are more favorable than the 3-ring and 4-ring. In contrast, this study in solution observed that 3-ring formation is more favorable than the formation of higher branched and ring silica oligomers. As a consequence, the 3-ring silica structure will be a dominant species during the first stage of pre-nucleation process in pure silica condensation. This is in good agreement with experimental studies of the early stage of zeolite synthesis. Water molecules are also essential to assist proton transfer and form stabilization hydrogen bond. The counter ion (Li+ and NH4+) effect to the silica condensation reaction was investigated in chapter 4. It was also shown that the activation barrier of the water removal step depends only on the water assisted, which is independent of the size of oligomer. The overall barrier mostly depends on the initial barrier of the SiO-Si bond formation. The position of cation has a strong effect on the barrier height of this step. When close to the reactive center of the dimerization reaction, Li+ does not change strongly the activation barrier, meanwhile NH4+ increase significantly the activation barrier. The relative rates of formation of the higher oligomers from dimer silica are found to be quite different between Li+ and NH4+ case. The presence of Li+ favors the linear and branched higher silica structures over the 3-ring structures. In contrast, with NH4+ in solution it has been observed that 3-ring formation is more favorable than the formation of higher branched silica oligomers. The positive charge of counter ion decreases most significantly the rates of SiO-Si bond formation in the dimer. For the first step of the dimerization reaction, the hydrogen bonding effects are more important than the electrostatic interaction. In consecutive oligomerization reaction steps, the cation has a weak effect on activation barrier. The importance of organic templates is well known for zeolite synthesis. Chapter 5 presents the catalytic role of template in the formation of the initial silicate structure. Formation of dimer, trimer and three-ring silicate oligomers has been studied in the presence of organic cation tetrapropylammonium TPA+. The interaction between TPA+ and anionic silica consist of an electrostatic part and a weak VdW part. TPA+ stabilises the transition state for Si-O bond formation. The ring closure reactions occur with comparable barriers to that for linear growth despite the unfavorable thermodynamics for ring closure reaction due to the loss of intramolecular hydrogen bonds. Chapter 6 reports an extensive investigation on the effect of the template on the relative stabilities of higher oligomers than could be considered in chapter 5. Various silicate structures from dimer to double 4-ring, related to initial stage of zeolite synthesis, have been studied in the presence of organic cations such as tetramethylammonium TMA+ and tetrapropylammonium TPA+. The results show that organic template, especially TPA+, stabilizes silicate oligomers. As we found before, the TPA+ favors the formation of linear small oligomer such as dimer and trimer. Condensation reactions in the presence of TPA+ gain more energy than with TMA+ or the alkali counter ion (e.g Na+). This is due to the favorable interaction between product and TPA+. It mainly relates to the difference in the Van-der-Waals interaction energies between template and oligomers. For both templates, the 4-ring fragment tends to be more stable than the 3-ring fragment. The unique figure of TMA+ is the higher stabilization of the 4-ring structures compared to the other oligomers. For TPA+ these differences are much less. This observation is consistent with experimental results. This thesis contributes to the understanding of the prenanoparticle oligomerization process that occurs initially in zeolite synthesis. Several factors control the chemical reactivity of silica condensation in solution at this early stage (e.g. hydrogen bonding, water rearrangement, electrostatic effect, Van-der-Waals interaction). The internal and external hydrogen bonding is crucially related the geometry of small silicate oligomers. The electrostatic interaction between that alkaline counter ion and reactive region of condensation reaction increase the activation barrier of silica condensation reaction. It has been shown that the organic template enhances the OSi-O bonding of small oligomer as well as the stabilities of higher oligomers. The Van-der- Waals interaction between the template and silicate structure is more important when the size of oligomer is larger. This implies that the organic templates not only act as the generally accepted role of structure directing agents to the nanoparticle formation but also have an important role in the earlier stage of oligomerization

AN EFFICIENT ALGORITHM FORMINING HIGH UTILITY ASSOCIATION RULES FROM LATTICE

In business, most of companies focus on growing their profits. Besides considering profit from each product, they also focus on the relationship among products in order to support effective decision making, gain more profits and attract their customers, e.g. shelf arrangement, product displays, or product marketing, etc. Some high utility association rules have been proposed, however, they consume much memory and require long time processing. This paper proposes LHAR (Lattice-based for mining High utility Association Rules) algorithm to mine high utility association rules based on a lattice of high utility itemsets. The LHAR algorithm aims to generates high utility association rules during the process of building lattice of high utility itemsets, and thus it needs less memory and runtim

A pedestrian path-planning model in accordance with obstacle's danger with reinforcement learning

Most microscopic pedestrian navigation models use the concept of "forces" applied to the pedestrian agents to replicate the navigation environment. While the approach could provide believable results in regular situations, it does not always resemble natural pedestrian navigation behaviour in many typical settings. In our research, we proposed a novel approach using reinforcement learning for simulation of pedestrian agent path planning and collision avoidance problem. The primary focus of this approach is using human perception of the environment and danger awareness of interferences. The implementation of our model has shown that the path planned by the agent shares many similarities with a human pedestrian in several aspects such as following common walking conventions and human behaviours

Note: A new truncation correction for the configurational temperature extends its applicability to interaction potentials with a discontinuous force

Supramolecular & Biomaterials Chemistr

Linear functional state bounding for perturbed time-delay systems and its application

This paper considers time-delay systems with bounded disturbances. We study a new problem of finding an upper bound of an absolute value function of any given linear functional of the state vector starting from the origin of the system. Based on the Lyapunov-Krasovskii method combining with the recent Wirtinger-based integral inequality that has just been proposed by Seuret & Gouaisbaut (2013. Wirtinger-based integral inequality: application to time-delay systems. Automatica, 49, 2860-2866), sufficient conditions for the existence of an upper bound of the function are derived. The obtained results are shown to be more effective than those adapted from the existing works on reachable set bounding. Furthermore, the obtained results are applied to refine existing ellipsoidal bounds of the reachable sets. The effectiveness of the obtained results is illustrated by two numerical examples

Introducing an agricultural app to rice farmers: A pilot study in Can Tho, Vietnam

Plantix is an agricultural app developed by a private company based in Germany which offers a diagnosis and advice for more than 30 crops. It has great potential as a new form of extension service complementing a traditional face-to-face extension service. The CGIAR Plant Health Initiative seeks to introduce the app as part of a package of innovations available for integrated pest and disease management to facilitate behavioral change among farmers. Plantix has been widely used in India but has not yet become very common in Vietnam. The aim of this pilot study was to test the usability of Plantix app for progressive rural rice growers in Vinh Thanh District, Can Tho City, Vietnam. A group of 15 farmers (5 woman and 11 men) participated in the training on use of the app and provided feedback after a two-week trial. The results show that the farmers seem to have some trust issues with the app’s diagnosis, as it reportedly failed to identify or distinguish early signs of certain pests and diseases on young rice. Nevertheless, the farmers showed interest in using Plantix app and considered it a useful tool once its early detection capacity is improved. The participants were experienced large-scaled rice farmers who were already quite familiar with the symptoms of regular pests and diseases. They agreed with the knowledge provided by Plantix. The farmers appeared to be conscious of the recommended dosage of chemical pesticides as written on product labels, however, the current local practice still involves overusing. This suggests that additional measures might be needed along with the introduction and dissemination of Plantix for a stronger impact on farmers’ behaviors. The app can be very useful for farmers when they start growing new crops, when there are new pests and diseases, and when they have difficulties distinguishing one disease from others with similar symptoms. Some of the farmers suggested the need for information about new generation pesticides. They also expressed the need for recommendations of specific “top-ranked” pesticide products, to help them navigate the large number of products and brands currently on the market. Considering the gender division of labor and decision-making, the results show that men are usually responsible for pest and disease management, but women are also involved in decision-making to some extent. It will be interesting to observe how women’s improved knowledge on pest and disease management obtained through Plantix will lead to change in household decisions on pesticide use. The participating farmers use various sources such as TV, Internet, extension workers, input suppliers and peer farmers as means of accessing information and exchanging knowledge. Male farmers tend to learn new agricultural technologies through social networks within their villages and the Internet more often, while the women do that through the Internet and TV. However, both groups trusted their own experience the most. Therefore, introducing the community’s key farmers to new technologies though demonstration and success stories could work as a strategy to ignite behavior change among peer farmers. In addition, communication via the Internet and TV could quickly reach a large number of farmers. Involving input suppliers and extension workers in the introduction of the Plantix app could also be helpful. While this pilot study focused on large-scaled farmers, there are smallholder farmers from poor households in some remote areas in the Mekong Delta. Their needs and knowledge may be significantly different from those of progressive farmers. Introducing the app to different types of farmers can help us understand the diverse needs and priorities of farmers in the process of digitalization

Positron annihilation lifetime spectroscopy at a superconducting electron accelerator

The Helmholtz-Zentrum Dresden-Rossendorf operates a superconducting linear accelerator for electrons with energies up to 35 MeV and average beam currents up to 1.6 mA. The electron beam is employed for production of several secondary beams including X-rays from bremsstrahlung production, neutrons, and positrons. The secondary positron beam after moderation feeds the Monoenergetic Positron Source (MePS) where positron annihilation lifetime (PALS) and positron annihilation Doppler-broadening experiments in materials science are performed in parallel. The adjustable repetition rate of the continuous-wave electron beams allows matching of the pulse separation to the positron lifetime in the sample under study. The energy of the positron beam can be set between 0.5 keV and 20 keV to perform depth resolved defect spectroscopy and porosity studies especially for thin films

Positron annihilation analysis of nanopores and growth mechanism of oblique angle evaporated TiO2 and SiO2 thin films and multilayers

The nano-porosity embedded into the tilted and separated nanocolumns characteristic of the microstructure of evaporated thin films at oblique angles has been critically assessed by various variants of the positron annihilation spectroscopy. This technique represents a powerful tool for the analysis of porosity, defects and internal interfaces of materials, and has been applied to different as-deposited SiO and TiO thin films as well as SiO/TiO multilayers prepared by electron beam evaporation at 70° and 85° zenithal angles. It is shown that, under same deposition conditions, the concentration of internal nano-pores in SiO is higher than in TiO nanocolumns, while the situation is closer to this latter in TiO/SiO multilayers. These features have been compared with the predictions of a Monte Carlo simulation of the film growth and explained by considering the influence of the chemical composition on the growth mechanism and, ultimately, on the structure of the films

Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment

This paper describes an analysis of the angular distribution of W->enu and W->munu decays, using data from pp collisions at sqrt(s) = 7 TeV recorded with the ATLAS detector at the LHC in 2010, corresponding to an integrated luminosity of about 35 pb^-1. Using the decay lepton transverse momentum and the missing transverse energy, the W decay angular distribution projected onto the transverse plane is obtained and analysed in terms of helicity fractions f0, fL and fR over two ranges of W transverse momentum (ptw): 35 < ptw < 50 GeV and ptw > 50 GeV. Good agreement is found with theoretical predictions. For ptw > 50 GeV, the values of f0 and fL-fR, averaged over charge and lepton flavour, are measured to be : f0 = 0.127 +/- 0.030 +/- 0.108 and fL-fR = 0.252 +/- 0.017 +/- 0.030, where the first uncertainties are statistical, and the second include all systematic effects.Comment: 19 pages plus author list (34 pages total), 9 figures, 11 tables, revised author list, matches European Journal of Physics C versio