A ReaxFF Potential for Modeling Organic Matter Degradation with Oxybromine Oxidants.

Oxidation of organic matter with oxybromine oxidants is ushering in a new era of enhanced hydrocarbon recovery. While these potent reagents are being tested in laboratory and field experiments, there is a pressing demand to delineate the molecular processes governing oxidation reactions at geological depth. Here, we parameterize a ReaxFF potential to model the oxidative decompositions of aliphatic and aromatic hydrocarbons in the presence of water-NaBr solutions that contain oxybromine (BrOn)- oxidizers. Our parameterization results in a reliable empirical bond-order potential that accurately calculates bond energies, exhibiting an RMSE of ∼1.18 eV, corresponding to 1.36 % average error. Reproducing bond dissociation and binding energies from Density Functional Theory (DFT), our parameterization proves transferable to aqueous environments. This H/C/O/Na/Br ReaxFF potential accurately reproduces the oxidation pathways of small hydrocarbons with oxybromine oxidizers. This force field captures proton and oxygen transfer, C-C bond tautomerization, and cleavage, leading to ring-opening and chain fragmentation. Molecular dynamic simulations demonstrate the oxidative degradation of aromatic and aliphatic kerogen-like moieties in bulk solutions. We envision that such reactive force fields will be useful to understand better the oxidation reactions of organic matter formed in geological reservoirs for enhanced shale gas recovery and improved carbon dioxide treatments.

Amorphous Si1-yCy composite anode materials: ab initio molecular dynamics for behaviors of Li and Na in the framework.

Although amorphous Si/C composite anode materials with various types of nanostructures Si/C materials have been experimentally proposed for rechargeable ion batteries for their structural durability, the atomistic mechanism primarily suggesting Li and Na monovalent ion intercalation into an amorphous Si/C composite matrix has not theoretically been understood to explore the thermodynamic and kinetic features of the a-Si/C composite phase regarding the effects on the carbon addition to an amorphous Si matrix. In this work, systematic ab initio molecular dynamics calculations (AIMDs) were conducted to identify electrochemical intercalation reactions involved in nanostructure evolutions, which correspond to favorable ion-intercalated formations, volume expansions, pair correlations, charge transfers, and diffusion behaviors of metals in a-MxSi1-yCy (Mx: Lix and Nax) alloys with increasing x contents of atomic concentrations. AIMDs using the a-Si1-yCy composite phase might allow one to have an atomic-level understanding of the composite phase and further insightful comprehension of any implementations such as the controlled ratio of the Si1-yCy composite and multivalent ions inserted into the framework.

Modified potential for atomistic simulation of the growth of carbon materials from binary alloy catalysts.

A new hybrid bond order potential has been developed and implemented to describe carbon-bimetallic alloy interactions, which are involved in the catalytic growth of carbon materials such as graphene and carbon nanotubes on the surface of binary alloy catalysts. In carefully adjusting the parameters, the potential energy fitting correlated with the results calculated from the density functional theory (DFT) method leads to a high quality empirical force field with an average error of <4.5% only. With the PES accuracy, in total 16 (n,m) have been successfully obtained from the MD trajectories in this work, and the structural evolution including random chirality and diameter formation has been identified. The newly modified force field is expected to be useful for modelling the spontaneous growth of carbon materials, particularly tubes on binary alloy clusters, giving an idea of how these C-C, C-M, and M-M interactions affect the growth behavior of carbon nanotubes. In addition, the new FF is only valid for liquid alloy nanoparticles at this time, but the use of solid alloy nanocatalysts with the new FF can be further employed for 2-D material growth such as graphene layer growth.

Modified reactive empirical bond-order potential for heterogeneous bonding environments.

An improvement to the AIREBO potential for hydrocarbons is presented in which contributions to the bond order are determined by the local bonding environment around the bond, rather than the average of the environments around the two constituent atoms. This bond-centric approach decreases the errors by ~80% in the fullerene-type systems for which the original approach leads to the most severe errors. With the newly developed and parameterized method, energy errors are less than 0.7 eV for a collection of hydrocarbon molecules not used in the fitting. This modified AIREBO potential is expected to be more useful not only for the molecular hydrocarbons and fullerene isomers studied here, but also for the full range of carbon and hydrocarbon systems to which the AIREBO potential has been applied.

Reactive force fields for modeling oxidative degradation of organic matter in geological formations.

In an attempt to better explore organic matter reaction and properties, at depth, to oxidative fluid additives, we have developed a new ReaxFF potential to model and describe the oxidative decompositions of aliphatic and aromatic hydrocarbons in the presence of the oxychlorine ClO n - oxidizers. By carefully adjusting the new H/C/O/Cl parameters, we show that the potential energies in both training and validation sets correlate well with calculated density functional theory (DFT) energies. Our parametrization yields a reliable empirical reactive force field with an RMS error of ∼1.57 eV, corresponding to a 1.70% average error. At this accuracy level, the reactive force field provides a reliable atomic-level picture of thermodynamically favorable reaction pathways governing oxidative degradation of H/C/O/Cl compounds. We demonstrate this capability by studying the structural degradation of small aromatic and aliphatic hydrocarbons in the presence of oxychlorine oxidizers in aqueous environments. We envision that such reactive force fields will be critical in understanding the oxidation processes of organic matter in geological reservoirs and the design of the next generation of reactive fluids for enhanced shale gas recovery and improved carbon dioxide adsorption and sequestration.