No abstract

In chemical kinetics research, kinetic models containing hundreds of species and tens of thousands of elementary reactions are commonly used to understand and predict the behavior of reactive chemical systems. Reaction Mechanism Generator (RMG) is a software suite developed to automatically generate such models by incorporating and extrapolating from a database of known thermochemical and kinetic parameters.Here, we present the recent version 3 release of RMG and highlight improvements since the previously published description of RMG v1.0. One important change is that RMG v3.0 is now Python 3 compatible, which supports the most up-to-date versions of cheminformatics and machine learning packages that RMG depends on. Additionally, RMG can now generate heterogeneous catalysis models, in addition to the previously available gas-and liquid-phase capabilities. For model analysis, new methods for local and global uncertainty analysis have been implemented to supplement first-order sensitivity analysis. The RMG database of thermochemical and kinetic parameters has been significantly expanded to cover more types of chemistry. The present release also includes parallelization for reaction generation and on-the-fly quantum calculations, and a new molecule isomorphism approach to improve computational performance. Overall, RMG v3.0 includes many changes which improve the accuracy of the generated chemical mechanisms and allow for exploration of a wider range of chemical systems.

A novel approach is presented for generating microkinetic mechanisms in heterogeneous catalysis. The open-source software RMG-CAT automatically develops a detailed list of elementary surface reactions, including thermodynamic properties for the adsorbates and parametrized rate coefficients for the reactions. The software proposes numerous possible surface intermediates and reactions, but it only retains those species that have a sufficiently high rate of formation. RMG-CAT was tested on the dry reforming of methane on nickel. The software correctly found the same set of elementary reactions as in a previously compiled microkinetic mechanism, as well as a few missing reactions. These results demonstrate the potential of this approach for predicting the dominant pathways in heterogeneous catalysis.

The automatic microkinetic mechanism generator for heterogeneous catalysis, RMG-Cat, has been extensively updated. Density functional theory calculations were performed for 69 adsorbates on Pt(111), and the resulting thermodynamic properties were added to RMG-Cat. The thermo database is significantly more accurate; it includes nitrogen-containing adsorbates for the first time as well as better capabilities for predicting the thermochemistry of novel adsorbates. Additionally, RMG-Cat can now simultaneously pursue a mechanism expansion both on the surface and in the gas phase. This heterogeneous/homogeneously coupled capability is tested on the catalytic combustion of methane on platinum. The results confirm that under some conditions the catalyst is capable of inducing thermal ignition in the gas phase.

Despite the industrial importance of the process, the 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, TiO2Cl3, Ti2O2Cl3, Ti2O2Cl4, Ti2O3Cl2, Ti2O3Cl3, Ti3O4Cl4, and Ti5O6Cl8 were calculated using density functional theory (DFT). The use of isodesmic and isogyric reactions was shown to be important for determining standard enthlapy of formation (Delta(f)H(degree)(298K)) values for these transition metal oxychloride species. TiOCl2, of particular importance in this mechanism, was also studied with CCSD(T) and found to have Delta(f)H(degree)(298K) = -598 +/- 20 kJ/mol. Finally, equilibrium calculations were performed 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.