ReaxFF Reactive Force Field for Molecular Dynamics Simulations of Hydrocarbon Oxidation

Abstract

To investigate the initial chemical events associated with high-temperature gas-phase oxidation of hydrocarbons, we have expanded the ReaxFF reactive force field training set to include additional transition states and chemical reactivity of systems relevant to these reactions and optimized the force field parameters against a quantum mechanics (QM)-based training set. To validate the ReaxFF potential obtained after parameter optimization, we performed a range of NVT-MD simulations on various hydrocarbon/O 2 systems. From simulations on methane/O 2 , o-xylene/O 2 , propene/O 2 , and benzene/O 2 mixtures, we found that ReaxFF obtains the correct reactivity trend (propene > o-xylene > methane > benzene), following the trend in the C-H bond strength in these hydrocarbons. We also tracked in detail the reactions during a complete oxidation of isolated methane, propene, and o-xylene to a CO/CO 2 /H 2 O mixture and found that the pathways predicted by ReaxFF are in agreement with chemical intuition and our QM results. We observed that the predominant initiation reaction for oxidation of methane, propene, and o-xylene under fuel lean conditions involved hydrogen abstraction of the methyl hydrogen by molecular oxygen forming hydroperoxyl and hydrocarbon radical species. While under fuel rich conditions with a mixture of these hydrocarbons, we observed different chemistry compared with the oxidation of isolated hydrocarbons including a change in the type of initiation reactions, which involved both decomposition of the hydrocarbon or attack by other radicals in the system. Since ReaxFF is capable of simulating complicated reaction pathways without any preconditioning, we believe that atomistic modeling with ReaxFF provides a useful method for determining the initial events of oxidation of hydrocarbons under extreme conditions and can enhance existing combustion models.

Chemomechanics control of tearing paths in graphene

“…However, close-range non-bonded interactions are excluded by using a shielding term. As tested on a number of hydrocarbon-oxygen systems, the ReaxFF has been adequately shown to give energies, reaction pathways, transition states, and reactivity trends that are in great agreement with quantum mechanical calculations and experiments 18,19 , while capable of treating thousands of atoms. The ReaxFF force field has been widely used to study the graphene oxide 20 , graphene peeling from a substrate 21 , and graphene ripping 22 .…”

Reactive Molecular Dynamics Simulations of the Silanization of Silica Substrates by Methoxysilanes and Hydroxysilanes

“…The atomic forces are computed using ReaxFF 27, 28 . Specifically, we use our parameters of ReaxFF 29 which were optimized for the polycondensation of alkoxysilanes.…”

Formation of chlorobenzenes by oxidative thermal decomposition of 1,3-dichloropropene

“…HCl formation mechanisms, and their relation to the formation of chlorobenzenes, are discussed in greater detail below. The formation of CO 2 , CO and H 2 O from combustion of small saturated and unsaturated hydrocarbons has already been studied on a number of occasions [32,33,34]. These reactions are initiated by hydrogen abstraction by O 2 , forming HO 2 radicals.…”

“…It follows that the manner in which 1,3-D decomposition is initiated is temperature dependent. Subsequent reactions are then driven by HO 2 and OH radicals [32,33]. H 2 O is formed directly by OH-mediated hydrogen abstraction, whereas CO and CO 2 are formed primarily from reaction between aldehyde radicals and O 2 .…”

“…A number of previous investigations have shown that small aromatic molecules can form via cyclisation of unsaturated hydrocarbons (e.g., 1,3,5-hexatriene [28], hexachloropropene [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37]). …”