Publications

Predicting solvation energies for kinetic modeling

Tue, 15 Jun 2010 17:29:46 -0600

A. Jalan, R. W. Ashcraft, R. H. West, and W. H. Green. Predicting solvation energies for kinetic modeling. Annu. Rep. Prog. Chem., Sect. C, 106: 211–258, 2010. doi:10.1039/b811056p

Ab initio and empirical methods for predicting solvation energies are reviewed, focusing on the challenge of predicting the solvation energies of reactive low-concentration species (and transition states) needed for kinetic models. Several rather different approaches are being pursued with success, but none of the purely a priori methods have yet achieved the accuracy needed to quantitatively predict solution-phase kinetics. Empirical methods are quite accurate at predicting the variation of a molecules solvation energy with changes in solvent. Some a priori approaches based on these empirical methods are discussed. Several effects which are poorly-predicted by existing a priori methods and need further work are highlighted.

A Detailed Model for the Sintering of Polydispersed Nanoparticle Agglomerates

Wed, 22 Jul 2009 10:00:08 -0600

M. Sander, R. H. West, M. S. Celnik, and M. Kraft. A detailed model for the sintering of polydispersed nanoparticle agglomerates. Aerosol Sci. Technol., 43(10):978–989, 2009. doi:10.1080/02786820903092416

In this study the coagulation, condensation, and sintering of nanoparticles is investigated using a stochastic particle model. Each stochastic particle consists of interacting polydisperse primary particles that are connected to each other. In the model sintering occurs between each individual pair of neighboring primary particles. This is important for particles in which the range of the size of the primary particles varies significantly. The sintering time is obtained from the viscous flow model. The model is solved using a stochastic particle algorithm. The particles are represented in a binary tree that contains the connectivity as well as the degree of sintering information. Particles are forme, coagulate, sinter, and experience condensation according to known rate laws. The particle binary tree, along with it the degree of sintering, is updated after each time step according to the rates of the different processes. The stochastic particle method uses the technique of fictitious jumps and linear process deferment. The theoretical results are fitted against experimental values for the formation of SiO2 nanoparticles and computer generated TEM pictures are presented and compared to experiments.

First-Principles Thermochemistry for Silicon Species in the Decomposition of Tetraethoxysilane

Wed, 15 Jul 2009 10:00:04 -0600

W. Phadungsukanan, S. Shekar, R.A. Shirley, M. Sander, R.H. West, and M. Kraft. First-principles thermochemistry for silicon species in the decomposition of tetraethoxysilane. J. Phys. Chem. A, 113(31):9041—9049, 2009. doi:10.1021/jp905494s

Tetraethoxysilane (TEOS) is used as a precursor in the industrial production of silica nanoparticles using thermal decomposition methods such as ﬂame spray pyrolysis (FSP). Despite the industrial importance of this process, the current kinetic model of high-temperature decomposition of TEOS to produce intermediate silicon species and eventually form amorphous silica (R-SiO2) nanoparticles remains inadequate. This is partly due to the fact only a small proportion of the possible species is considered. This work presents the thermochemistry of practically all of the species that can exist in the early stages of the reaction mechanism. In order to ensure that all possible species are considered, the process is automated by considering all species that can be formed from the reactions that are deemed reasonable in the standard ethanol combustion model in the literature. Thermochemical data for 180 species (over 160 of which have not appeared in the literature before) are calculated using density functional theory with two different hybrid functionals, B3LYP and B97-1. The standard enthalpy of formation (∆fH298.15K) values for these species are calculated using isodesmic reactions. It is observed that internal rotation may be important because the barriers to rotation are reasonably low. Comparisons are then made between the rigid rotor harmonic oscillator approximation (RRHO) and the RRHO with some of the vibrational modes treated as hindered rotors. It is found that full treatment of the hindered rotors makes a significant difference to the thermochemistry and thus has an impact on equilibrium concentrations and kinetics in this system. For this reason, all of the species are treated using the hindered rotor approximation where appropriate. Finally, equilibrium calculations are performed to identify the intermediates that are likely to be most prevalent in the high-temperature industrial process. Particularly, Si(OH)4, SiH(OH)3, SiH2(OH)2, SiH3(OH), Si(OH)3(OCH3), Si(OH)2(OCH3)2, the silicon dimers (CH3)3-SiOSi(CH3)3 and SiH3OSiH3, and the smaller hydrocarbon species CH4, CO2, C2H4, and C2H6 are highlighted as the important species.

A detailed kinetic model for combustion synthesis of titania from TiCl4

Sat, 6 Jun 2009 10:00:12 -0600

R.H. West, R.A. Shirley, M. Kraft, C.F. Goldsmith, and W.H. Green. A detailed kinetic model for combustion synthesis of titania from TiCl4. Combust. Flame, 156(9):1764–1770, 2009. doi:10.1016/j.combustflame.2009.04.011

The combustion of TiCl4 to synthesize TiO2 nanoparticles is a multimillion tonne per year industrial process, the fundamental details of which are still not known. The gas-phase kinetic model presented by West et al. [R.H. West, M.S. Celnik, O.R. Inderwildi, M. Kraft, G.J.O. Beran, W.H. Green, Ind. Eng. Chem. Res. 46 (19) (2007) 6147–6156] is improved upon using density functional theory (DFT) and variational transition state theory (VTST) calculations. The pressure-dependent rate expression for the reaction TiCl3 + O2 <=> TiO2Cl3 is found using VTST, a stable Ti2O2Cl6 species is located on the minimum energy pathway for TiCl3 + TiO2Cl3 <=> 2 TiOCl3, and a number of new elementary reactions are added. Thermochemical data are provided for Ti2O2Cl6, Ti2O2Cl5 and TiCl2OCl. The new kinetic model is used to simulate a rapid compression machine (RCM) and a plug flow reactor (PFR) described in the literature. Agreement with the RCM measurements is good, but simulations of the PFR are less satisfying, suggesting that surface deposition on the reactor walls may have dominated these measurements, which have been the basis of many theoretical models. Finally, the gas-phase kinetic model is coupled to a particle population balance model (PBM) incorporating inception, coagulation, growth, and sintering.

A statistical approach to develop a detailed soot growth model using PAH characteristics

Wed, 11 Feb 2009 10:00:55 -0700

A. Raj, M.S. Celnik, R.A. Shirley, M. Sander, R.I.A. Patterson, R.H. West, and M. Kraft. A statistical approach to develop a detailed soot growth model using PAH characteristics. Combust. Flame, 156(4):896–913, 2009. doi:10.1016/j.combustflame.2009.01.005

A detailed PAH growth model is developed, which is solved using a kinetic Monte Carlo algorithm. The model describes the structure and growth of planar PAH molecules, and is referred to as the kinetic Monte Carlo–aromatic site (KMC-ARS) model. A detailed PAH growth mechanism based on reactions at radical sites available in the literature, and additional reactions obtained from quantum chemistry calculations are used to model the PAH growth processes. New rates for the reactions involved in the cyclodehydrogenation process for the formation of 6-member rings on PAHs are calculated in this work based on density functional theory simulations. The KMC-ARS model is validated by comparing experimentally observed ensembles on PAHs with the computed ensembles for a C2H2 and a C6H6 flame at different heights above the burner. The motivation for this model is the development of a detailed soot particle population balance model which describes the evolution of an ensemble of soot particles based on their PAH structure. However, at present incorporating such a detailed model into a population balance is computationally unfeasible. Therefore, a simpler model referred to as the site-counting model has been developed, which replaces the structural information of the PAH molecules by their functional groups augmented with statistical closure expressions. This closure is obtained from the KMC-ARS model, which is used to develop correlations and statistics in different flame environments which describe such PAH structural information. These correlations and statistics are implemented in the site-counting model, and results from the site-counting model and the KMC-ARS model are in good agreement. Additionally the effect of steric hindrance in large PAH structures is investigated and correlations for sites unavailable for reaction are presented.

Modelling soot formation in a premixed flame using an aromatic-site soot model and an improved oxidation rate

Wed, 24 Sep 2008 10:00:53 -0600

M.S. Celnik, M. Sander, A. Raj, R.H. West, and M. Kraft. Modelling soot formation in a premixed flame using an aromatic-site soot model and an improved oxidation rate. Proc. Combust. Inst., 32(1):639–646, 2009. doi:10.1016/j.proci.2008.06.062

An updated rate of O2 oxidation of one to four ring polyaromatic hydrocarbons in premixed flames is presented based on density function theory simulations of oxygen attack at different radical sites on various PAHs. The rate is in agreement with other rates found in the literature; however, it is several orders of magnitude lower than the currently accepted oxidation rate of multi-ring aromatic species, including soot. Simulations are presented of a premixed flame using this improved rate and a new advanced soot particle model, which is developed in this paper. This model includes unprecedented detail of the particles in the ensemble, including the aromatic content, C/H composition and primary-particle aggregate structure. The O2 oxidation rate calculated in this paper is shown to give a better prediction of particle number density and soot volume fraction for a premixed flame. The predicted particle size distributions are shown also to describe better the experimental data. Predicted C/H ratio and PAH size distributions are shown for the flame. Computed TEM-style images are compared to experimental TEM images, which show that the aggregate structure of the particles is well predicted.

Aromatic site description of soot particles

Thu, 29 May 2008 12:39:19 -0600

M.S. Celnik, A. Raj, R.H. West, R.I. Patterson, and M. Kraft. Aromatic site description of soot particles. Combust. Flame, 155(1-2):161–180, 2008. doi:10.1016/j.combustflame.2008.04.011

A new, advanced soot particle model is developed that describes soot particles by their aromatic structure, including functional site descriptions and a detailed surface chemistry mechanism. A methodology is presented for the description of polyaromatic hydrocarbon (PAH) structures by their functional sites. The model is based on statistics that describe aromatic structural information in the form of easily computed correlations, which were generated using a kinetic Monte Carlo algorithm to study the growth of single PAH molecules. A comprehensive surface reaction mechanism is presented to describe the growth and desorption of aromatic rings on PAHs. The model is capable of simulating whole particle ensembles which allows bulk properties such as soot volume fraction and number density to be found, as well as joint particle size and surface area distributions. The model is compared to the literature-standard soot model [J. Appel, H. Bockhorn, M. Frenklach, Combust. Flame 121 (2000) 122–136] in a plug-flow reactor and is shown to predict well the experimental results of soot mass, average particle size, and particle size distributions at different flow times. Finally, the carbon/hydrogen ratio and the distribution of average PAH sizes in the ensemble, as predicted by the model, are discussed.

Modelling gas-phase synthesis of single-walled carbon nanotubes on iron catalyst particles

Sat, 15 Dec 2007 15:38:49 -0700

M.S. Celnik, R.H. West, N.M. Morgan, M. Kraft, A. Moisala, J. Wen, W.H. Green, and H. Richter. Modelling gas-phase synthesis of single-walled carbon nanotubes on iron catalyst particles. Carbon, 46(3):422–433, 2008. doi:10.1016/j.carbon.2007.12.004

A simple model for the gas-phase synthesis of carbon nanotubes on iron catalyst particles has been developed. It includes a growth model for the catalyst particles and describes nanotube growth processes through carbon monoxide disproportionation and hydrogenation. Models for particle–particle interactions and sintering are also included. When carbon arrives at a catalyst particle it can either dissolve in the particle until a saturation limit is reached, or form a graphene layer on the particle, or go on to form a nanotube. Two models for incipient nanotube growth are considered. The first allows nanotubes to form once a catalyst particle reaches the saturation condition. The second only allows nanotubes to form on the collision of two saturated particles. The particle system is solved using a multivariate stochastic solver coupled to the gas-phase iron chemistry using an operator splitting algorithm. Comparison with experimental data gives a good prediction of the nanotube length, and reasonable values of catalyst particle diameter and nanotube diameter. A parametric study is presented in which the carbon monoxide reaction rate constants are varied, as is the fraction of carbon allowed to form nanotubes relative to surface layers. The assumptions of the coagulation and sintering models are also discussed.

Toward a Comprehensive Model of the Synthesis of TiO2 Particles from TiCl4

Tue, 21 Aug 2007 10:04:42 -0600

R.H. West, M.S. Celnik, O.R. Inderwildi, M. Kraft, G.J.O. Beran, and W.H. Green. Toward a comprehensive model of the synthesis of TiO2 particles from TiCl4. Ind. Eng. Chem. Res., 46(19):6147–6156, 2007. doi:10.1021/ie0706414

The combustion of TiCl4 to synthesize TiO2 nanoparticles is a multimillion tonne per year industrial process. The objective of this paper is to further the understanding of this process. Work toward three aspects of this multiscale problem is presented herein: gas-phase chemistry, surface chemistry, and the solution of a multidimensional population balance problem coupled to detailed chemical mechanisms. Presented here is the first thermodynamically consistent mechanism with physically realistic elementary-step rate constants by which TiCl4 is oxidized to form a stable Ti2OxCly species that lies on the path to formation of TiO2 nanoparticles. Second, progress toward a surface chemistry mechanism based on density functional theory (DFT) calculations is described. Third, the extension of a stochastic two-dimensional (surface-volume) population balance solver is presented. For the first time, the number and size of primary particles within each agglomerate particle in the population is tracked. The particle model, which incorporates inception, coagulation, growth, and sintering, is coupled to the new gas-phase kinetic model using operator splitting, and is used to simulate a heated furnace laboratory reactor and an industrial reactor. Using the primary particle information, transmission electron microscopy (TEM)-style images of the particles are generated, demonstrating the potential utility of first-principles modeling for the prediction of particle morphology in complex industrial systems.

First-principles thermochemistry for the production of TiO2 from TiCl4

Thu, 19 Apr 2007 10:00:01 -0600

R.H. West, G.J.O. Beran, W.H. Green, and M. Kraft. First-principles thermochemistry for the production of TiO2 from TiCl4. J. Phys. Chem. A, 111(18):3560–3565, 2007. doi:10.1021/jp0661950

Despite the industrial importance of the process, detailed chemistry of the high temperature oxidation of titanium tetrachloride (TiCl4) to produce titania (TiO2) nanoparticles remains unknown, partly due to a lack of thermochemical data. This work presents the thermochemistry of many of the intermediates in the early stages of the mechanism, computed using quantum chemistry. The enthalpies of formation and thermochemical data for TiOCl, TiOCl2 , TiOCl3 , TiO2Cl2 , TiO2Cl2 , Ti2O2Cl3 , Ti2O2Cl4 , Ti2O3Cl2 , Ti2O3Cl3 , Ti3O4Cl4 , and Ti5O6Cl8 were calculated using density functional theory (DFT). Use of isodesmic and isogyric reactions was shown to be important for determining standard enthlapy of formation (∆fH298K) values for these transition metal oxychloride species. TiOCl2, of particluar importance in this mechanism, was also studied with CCSD(T) and found to have ∆fH298K = -598 +/-20 kJ/mol. Finally, equilibrium calculations are performed to try to identify which intermediates are likely to be most prevalent in the high temperature industrial process, and as a first attempt to identify the size of the critical nucleus.