Development and validation of a ReaxFF potential for hydrocarbon cracking reactions on Co and Fe-doped aluminosilicate catalysts

ABSTRACT: The objective of this Phase-II research effort is to further develop, validate and apply the ReaxFF force field parameters for hydrocarbon cracking catalysis on Fe- and Co-doped aluminosilicate systems. During Phase I we demonstrated the feasibility of the ReaxFF method for this catalyst/substrate system; we demonstrated that ReaxFF can reproduce DFT results for a wide range of systems, including equations of state, reaction energies and reaction barriers. We also performed initial application studies, showing, for the first time, a fully dynamical description of this complex catalyst/fuel interface. In Phase II we will focus on extension, validation and, eventually, application of this ReaxFF description. We will first bring the Co-ReaxFF description to the same level as the Phase-I Fe-parameters and will develop new parameters for alkali metals, which are important doping elements in zeolite-based cracking catalysts. The validation effort in Phase II will involve comparison of newly identified routes form high-temperature ReaxFF MD-simulations to DFT data, as well as an extensive interaction with experimental data. In order to enable experimental validation we will synthesize Fe- and Co-doped aluminosilicate materials and study their catalytic conversion, as a function of feedstock composition, temperature and pressure. Furthermore, we will perform calorimetry studies to evaluate the heat duty associated with the hydrocarbon cracking and EXAFS and thermogravimetric analysis studies to evaluate the composition and structure of the metal clusters in the zeolite. We will design a series of computational studies that can extrapolate towards the experimental conditions, thus enabling validation of our computational approach by both DFT and experiment. These validated parameters will then be used for an extensive MD-based study to map out the mechanisms, rates and long-term ageing effects of the cracking reaction for (a) various pressures and temperatures, (b) different composition of the hydrocarbon feedstock, and (c) varying dopant concentration and location. This research effort will extend the application of currently existing catalysts for the cracking of hydrocarbon fuels to the pressures found in high-heat fuel systems by providing detailed, atomistic-scale, insight in the key reactive events on the hydrocarbon/catalyst interface. BENEFIT: The development of a force field (FF) based reactive molecular dynamics (MD) simulation program to understand hydrocarbon cracking catalysis for high-heat sink fuels will play a key role in the research and development of scramjet engines. The proposed software will be capable of performing nanosecond-scale MD-simulations on large (>>1000 atoms), and the development of parallel ReaxFF will enable application to systems>1,000,000 atoms. Hence, the development of this software will enable researchers in educational, industry, and DoD research facilities to study the full complexity of a dynamic catalyst/fuel interface for a range of applications from propulsion/energy to materials. Groups that have an interest in ReaxFF include Exxon (fuel chemistry and catalysis), Lockheed Martin (carbon nanotube enforced polymers), CFDRC (catalyzed canbon nanotube growth), Intel (catalyzed carbon nanotube growth), Seiko-Epson (SiO2/Si interfaces), and Nissan (diamond-like carbon materials). Spectral Energies will work with our partners at Penn State and the Air Force Research Laboratory to ensure that the results of this work enable the development of advanced engine technology, as described in the work for Phase II, as well as follow-on Phase III activities.