Extended first-principles thermochemistry for the oxidation of titanium tetrachloride

A detailed first-principles investigation of the gas-phase precursor chemistry of titanium tetrachloride (TiCl4) in an O2 environment is used to identify the thermodynamically most stable oxidation products. Candidate species are systematically proposed based on twelve manually defined base moieties in combination with possible functional groups attached to each moiety. The ground state geometry and vibrational frequencies for each candidate species are calculated using density functional theory at the B97-1/6-311+G(d,p) level of theory. A set of 2; 328 unique candidate species are found to be physically reasonable. Their thermochemical data are calculated by applying statistical thermodynamics. Standard enthalpies of formation are estimated, if unknown, by using a set of error-cancelling balanced reactions. An equilibrium composition analysis of a mixture of TiCl4/O2 (50 mol%) at 3 bar is performed to identify the thermodynamically stable products. At low temperatures, below approximately 700 K, trimer species are dominant. This is followed by a mid-temperature range of 700 to 1975 K where Ti2OCl6 is the most abundant species, before its thermodynamic stability decreases. Between 1200 and 1825 K TiCl4 is the most stable monomer. At temperatures above 1975 K TiOCl2 becomes the dominant species. This species has been measured experimentally. A structural analysis is used to suggest further potentially stable higher polymers and defines a starting point to investigate the mechanisms leading to the formation of titanium dioxide (TiO2) particles

Numerical simulation and parametric sensitivity study of titanium dioxide particles synthesised in a stagnation flame

A detailed population balance model is used to simulate titanium dioxide nanoparticles synthesised in a stagnation flame from titanium tetraisopropoxide (TTIP) precursor. A two-step simulation methodology is employed to apply the detailed particle model as a post-process to flame profiles obtained from a fully coupled simulation with detailed gas-phase chemistry, flow dynamics and a simple particle model. The detailed particle model tracks the size and coordinates of each primary in an aggregate, and is able to resolve the particle morphology, permitting direct comparison with experimental measurements through simulated TEM-style images. New sintering parameters, informed by molecular dynamics simulations in the literature, are introduced into the model to account for the sintering behaviour of sub-10 nm particles. Simulated primary and aggregate particle size distributions were in excellent agreement with experimental measurements. A parametric sensitivity study found particle morphology to be sensitive to the sintering parameters, demonstrating the need to apply careful consideration to the sintering behaviour of nano-sized particles in modelling studies. The final particle morphology was not found to be sensitive to other model parameters

A systematic method to estimate and validate enthalpies of formation using error-cancelling balanced reactions

The Combustion Institute This paper presents an automated framework that uses overlapping subsets of reference data to systematically derive an informed estimate of the standard enthalpy of formation of chemical species and assess the consistency of the reference data. The theory of error-cancelling balanced reactions (EBRs) is used to calculate estimates of the standard enthalpy of formation. Individual EBRs are identified using linear programming. The first part of the framework recursively identifies multiple EBRs for specified target species. A distribution of estimates can then be determined for each species from which an informed estimate of the enthalpy is derived. The second part of the framework iteratively isolates inconsistent reference data and improves the prediction accuracy by excluding such data. The application of the framework is demonstrated for test cases from organic and inorganic chemistry, including transition metal complexes. Its application to a set of 920 carbon, hydrogen and oxygen containing species resulted in a rapid decrease of the mean absolute error for estimates of the enthalpy of formation of each species due to the identification and exclusion of inconsistent reference data. Its application to titanium-containing species identified that the available reference values of TiOCl and TiO(OH) 2 are inconsistent and need further attention. Revised values are calculated for both species. A comparison with popular high-level quantum chemistry methods shows that the framework is able to use affordable density functional theory (DFT) calculations to deliver highly accurate estimates of the standard enthalpy of formation, comparable to high-level quantum chemistry methods for both hydrocarbons and transition metal complexes

A kinetic mechanism for the thermal decomposition of titanium tetraisopropoxide

This work presents the first systematically derived and thermodynamically consistent mechanism to describe the thermal decomposition of titanium tetraisopropoxide (TTIP). The mechanism is based on an analogy between the decomposition of the isopropoxide branches and the decomposition of isopropanol. Flux and sensitivity analyses were used to identify the main reaction pathways in the proposed mechanism as the step-wise release of C3H6 via four-member ring transition states, the successive abstraction of CH3 radicals via C–C bond cleavage followed by hydrogen abstraction to form C = C double bonds, and hydrogen abstraction from the isopropoxide methyl groups followed by the release of C3H6. The final decomposition product was titanium hydroxide, Ti(OH)4. Rate constants were calculated using conventional and variational transition state theories for reactions in the first two pathways. The calculated rates are similar to the rates calculated for the corresponding isopropanol reactions, providing support for the analogy with isopropanol. The mechanism was used to simulate the ignition delay of isopropanol and TTIP. Excellent agreement was observed with experimental data for isopropanol. However, the mechanism over predicted the ignition delay for TTIP. The discrepancy was shown to be unlikely to be caused by the modest difference between the true reaction rates for the TTIP system and those assumed based on the analogy with isopropanol. It was found that the sensitivity of the TTIP decomposition to the presence of water must be caused by additional chemical pathways than the ones given by isopropanol analogy.This project is partly funded by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme. The authors thank Huntsman Pigments for financial support

A big data framework to validate thermodynamic data for chemical species

The advent of large sets of chemical and thermodynamic data has enabled the rapid investigation of increasingly complex systems. The challenge, however, is how to validate such large databases. We propose an automated framework to solve this problem by identifying which data are consistent and recommending what future experiments or calculations are required. The framework is applied to validate data for the standard enthalpy of formation for 920 gas-phase species containing carbon, oxygen and hydrogen retrieved from the NIST Chemistry WebBook. The concept of error-cancelling balanced reactions is used to calculate a distribution of possible values for the standard enthalpy of formation of each species. The method automates the identification and exclusion of inconsistent data. We find that this enables the rapid convergence of the calculations towards chemical accuracy. The method can exploit knowledge of the structural similarities between species and the consistency of the data to identify which species introduce the most error and recommend what future experiments and calculations should be considered.This project is partly funded by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme. The authors thank Huntsman Pigments and Additives for financial support

Skeletal chemical mechanism of high-temperature TEOS oxidation in hydrogen-oxygen environment

© 2016 The Combustion Institute. This paper improves the tetraethoxysilane (TEOS) oxidation mechanism proposed by Nurkowski et al. (2015) [17] by refining the rate parameters of the key reaction channels in the mechanism. A skeletal version of the mechanism is proposed for hydrogen-oxygen environment. The rates of ethylene-loss from (tetra-, tri-, di- and dimethyldi-) ethoxysilane are computed using transition state theory. The energetics of the main pathways are refined by performing detailed ab initio calculations using the CBS-Q technique. An analysis of ethanol formation via silicates is also performed resulting in the addition of 27 new silica species to the model. Thermodynamic properties for these species are calculated via the balanced reactions method. Reasonably good agreement between the improved model and available experimental data is observed. The subsequent elimination of unimportant species and reactions is achieved via a three-stage reduction procedure. The first and second stages involve the Direct Relation Graph with Error Propagation (DRGEP) method, whereas the third stage analyses rate of progress of each reaction. The investigated conditions are taken from the experimental studies of TEOS oxidation in oxygen-hydrogen flames. The final skeletal mechanism comprises 70 species and 457 reactions and retains good reproduction of the key model properties across the chosen operating conditions as compared to the full mechanism

Identification of recruitment and retention strategies for rehabilitation professionals in Ontario, Canada: results from expert panels

<p>Abstract</p> <p>Background</p> <p>Demand for rehabilitation services is expected to increase due to factors such as an aging population, workforce pressures, rise in chronic and complex multi-system disorders, advances in technology, and changes in interprofessional health service delivery models. However, health human resource (HHR) strategies for Canadian rehabilitation professionals are lagging behind other professional groups such as physicians and nurses. The objectives of this study were: 1) to identify recruitment and retention strategies of rehabilitation professionals including occupational therapists, physical therapists and speech language pathologists from the literature; and 2) to investigate both the importance and feasibility of the identified strategies using expert panels amongst HHR and education experts.</p> <p>Methods</p> <p>A review of the literature was conducted to identify recruitment and retention strategies for rehabilitation professionals. Two expert panels, one on <it>Recruitment and Retention </it>and the other on <it>Education </it>were convened to determine the importance and feasibility of the identified strategies. A modified-delphi process was used to gain consensus and to rate the identified strategies along these two dimensions.</p> <p>Results</p> <p>A total of 34 strategies were identified by the <it>Recruitment and Retention </it>and <it>Education </it>expert panels as being important and feasible for the development of a HHR plan for recruitment and retention of rehabilitation professionals. Seven were categorized under the <it>Quality of Worklife and Work Environment </it>theme, another seven in <it>Financial Incentives and Marketing</it>, two in <it>Workload and Skill Mix</it>, thirteen in <it>Professional Development </it>and five in <it>Education and Training</it>.</p> <p>Conclusion</p> <p>Based on the results from the expert panels, the three major areas of focus for HHR planning in the rehabilitation sector should include strategies addressing <it>Quality of Worklife and Work Environment</it>, <it>Financial Incentives and Marketing </it>and <it>Professional Development</it>.</p

Mu Insertions Are Repaired by the Double-Strand Break Repair Pathway of Escherichia coli

Mu is both a transposable element and a temperate bacteriophage. During lytic growth, it amplifies its genome by replicative transposition. During infection, it integrates into the Escherichia coli chromosome through a mechanism not requiring extensive DNA replication. In the latter pathway, the transposition intermediate is repaired by transposase-mediated resecting of the 5′ flaps attached to the ends of the incoming Mu genome, followed by filling the remaining 5 bp gaps at each end of the Mu insertion. It is widely assumed that the gaps are repaired by a gap-filling host polymerase. Using the E. coli Keio Collection to screen for mutants defective in recovery of stable Mu insertions, we show in this study that the gaps are repaired by the machinery responsible for the repair of double-strand breaks in E. coli—the replication restart proteins PriA-DnaT and homologous recombination proteins RecABC. We discuss alternate models for recombinational repair of the Mu gaps

Application of the bacteriophage Mu-driven system for the integration/amplification of target genes in the chromosomes of engineered Gram-negative bacteria—mini review

The advantages of phage Mu transposition-based systems for the chromosomal editing of plasmid-less strains are reviewed. The cis and trans requirements for Mu phage-mediated transposition, which include the L/R ends of the Mu DNA, the transposition factors MuA and MuB, and the cis/trans functioning of the E element as an enhancer, are presented. Mini-Mu(LR)/(LER) units are Mu derivatives that lack most of the Mu genes but contain the L/R ends or a properly arranged E element in cis to the L/R ends. The dual-component system, which consists of an integrative plasmid with a mini-Mu and an easily eliminated helper plasmid encoding inducible transposition factors, is described in detail as a tool for the integration/amplification of recombinant DNAs. This chromosomal editing method is based on replicative transposition through the formation of a cointegrate that can be resolved in a recombination-dependent manner. (E-plus)- or (E-minus)-helpers that differ in the presence of the trans-acting E element are used to achieve the proper mini-Mu transposition intensity. The systems that have been developed for the construction of stably maintained mini-Mu multi-integrant strains of Escherichia coli and Methylophilus methylotrophus are described. A novel integration/amplification/fixation strategy is proposed for consecutive independent replicative transpositions of different mini-Mu(LER) units with “excisable” E elements in methylotrophic cells