Structures, Energetics, and Reaction Barriers for CH_x Bound to the Nickel (111) Surface

To provide a basis for understanding and improving such reactive processes on nickel surfaces as the catalytic steam reforming of hydrocarbons, the decomposition of hydrocarbons at fuel cell anodes, and the growth of carbon nanotubes, we report quantum mechanics calculations (PBE flavor of density functional theory, DFT) of the structures, binding energies, and reaction barriers for all CH_x species on the Ni(111) surface using periodically infinite slabs. We find that all CH_x species prefer binding to μ3 (3-fold) sites leading to bond energies ranging from 42.7 kcal/mol for CH_3 to 148.9 kcal/mol for CH (the number of Ni-C bonds is not well-defined). We find reaction barriers of 18.3 kcal/mol for CH_(3,ad) → CH_(2,ad) + H_(ad) (with ΔE = +1.3 kcal/ mol), 8.2 kcal/mol for CH_(2,ad) → CH_(ad) + H_(ad) (with ΔE = -10.2 kcal/mol) and 32.3 kcal/mol for CH_(ad) → C_(ad) + H_(ad) (with ΔE = 11.6 kcal/mol). Thus, CH_(ad) is the stable form of CH_x on the surface. These results are in good agreement with the experimental data for the thermodynamic stability of small hydrocarbon species following dissociation of methane on Ni(111) and with the intermediates isolated during the reverse methanation process

Development and Validation of ReaxFF Reactive Force Field for Hydrocarbon Chemistry Catalyzed by Nickel

To enable the study of hydrocarbon reactions catalyzed by nickel surfaces and particles using reactive molecular dynamics on thousands of atoms as a function of temperature and pressure, we have developed the ReaxFF reactive force field to describe adsorption, decomposition, reformation and desorption of hydrocarbons as they interact with the nickel surface. The ReaxFF parameters were determined by fitting to the geometries and energy surfaces from quantum mechanics (QM) calculations for a large number of reaction pathways for hydrocarbon molecules chemisorbed onto nickel (111), (100) and (110) surfaces, supplemented with QM equations of state for nickel and nickel carbides. We demonstrate the validity and accuracy of ReaxFF by applying it to study the reaction dynamics of hydrocarbons as catalyzed by nickel particles and surfaces. For the dissociation of methyl on the (111), (100), and stepped (111) surfaces of nickel, we observe the formation of chemisorbed CH plus subsurface carbide. We observe that the (111) surface is the least reactive, the (100) surface has the fastest reaction rates, and the stepped (111) surface has an intermediate reaction rate. The importance of surface defects in accelerating reaction rates is highlighted by these results

Competing, Coverage-Dependent Decomposition Pathways for C_2H_y Species on Nickel (111)

Competing, coverage-dependent pathways for ethane (CH_3CH_3) decomposition on Ni(111) are proposed on the basis of quantum mechanics (QM) calculations, performed by using the PBE flavor of density functional theory (DFT), for all C_2H_y species adsorbed to a periodically infinite Ni(111) surface. For CH_2CH_3, CHCH_3, and CCH_3, we find that the surface C is tetrahedral in each case, with the surface C forming bonds to one, two, or three Ni atoms with bond energies scaling nearly linearly (E_(bond) = 32.5, 82.7, and 130.8 kcal/mol, respectively). In each of the remaining six C_2H_y species, both C atoms are able to form bonds to the surface. Three of these (CH_2CH_2, CHCH_2, and CCH_2) adsorb most favorably at a fcc-top site with the methylene C located at an on-top site and the other C at an adjacent fcc site. The bond energies for these species are E_(bond) = 19.7, 63.2, and 93.6 kcal/mol, respectively. The remaining species (CHCH, CCH, and C_2) all prefer binding at fcc-hcp sites, where the C atoms sit in a pair of adjacent fcc and hcp sites, with binding energies of E_(bond) = 57.7, 120.4, and 162.8 kcal/mol, respectively. We find that CHCH_(ad) is the most stable surface species (ΔH_(eth) = −18.6), and an important intermediate along the lowest-energy decomposition pathway for ethane on Ni(111). The second most stable species, CCH_3, is a close competitor (ΔH_(eth) = −18.2 kcal/mol), lying along an alternative decomposition pathway that is preferred for high-surface-coverage conditions. The existence of these competing, low- and high-coverage decomposition pathways is consistent with the experiments. The QM results reported here were used as training data in the development of the ReaxFF reactive force field describing hydrocarbon reactions on nickel surfaces [Mueller, J. E.; van Duin, A: C. T.; Goddard, W. A. J. Phys. Chem. C 2010, 114, 4939−4949]. This has enabled Reactive dynamics studying the chemisorption and decomposition of systems far too complex for quantum mechanics. Thus we reported recently, the chemisorption and decomposition of six different hydrocarbon species on a Ni_(468) nanoparticle catalysts using this ReaxFF description [Mueller, J. E.; van Duin, A: C. T.; Goddard, W. A. J. Phys. Chem. C 2010, 114, 5675−5685]

Mechanical Properties and Fracture Dynamics of Silicene Membranes

As graphene became one of the most important materials today, there is a renewed interest on others similar structures. One example is silicene, the silicon analogue of graphene. It share some the remarkable graphene properties, such as the Dirac cone, but presents some distinct ones, such as a pronounced structural buckling. We have investigated, through density functional based tight-binding (DFTB), as well as reactive molecular dynamics (using ReaxFF), the mechanical properties of suspended single-layer silicene. We calculated the elastic constants, analyzed the fracture patterns and edge reconstructions. We also addressed the stress distributions, unbuckling mechanisms and the fracture dependence on the temperature. We analysed the differences due to distinct edge morphologies, namely zigzag and armchair

Pilot, Rollout and Monte Carlo Tree Search Methods for Job Shop Scheduling

Greedy heuristics may be attuned by looking ahead for each possible choice, in an approach called the rollout or Pilot method. These methods may be seen as meta-heuristics that can enhance (any) heuristic solution, by repetitively modifying a master solution: similarly to what is done in game tree search, better choices are identified using lookahead, based on solutions obtained by repeatedly using a greedy heuristic. This paper first illustrates how the Pilot method improves upon some simple well known dispatch heuristics for the job-shop scheduling problem. The Pilot method is then shown to be a special case of the more recent Monte Carlo Tree Search (MCTS) methods: Unlike the Pilot method, MCTS methods use random completion of partial solutions to identify promising branches of the tree. The Pilot method and a simple version of MCTS, using the $\varepsilon$-greedy exploration paradigms, are then compared within the same framework, consisting of 300 scheduling problems of varying sizes with fixed-budget of rollouts. Results demonstrate that MCTS reaches better or same results as the Pilot methods in this context.Comment: Learning and Intelligent OptimizatioN (LION'6) 7219 (2012

Modeling of Hydrogen Storage Materials: A Reactive Force Field for NaH

Parameterization of a reactive force field for NaH is done using ab initio derived data. The parameterized force field(ReaxFFNaH) is used to study the dynamics governing hydrogen desorption in NaH. During the abstraction process of surface molecular hydrogen charge transfer is found to be well described by the parameterized force field. To gain more insight into the mechanism governing structural transformation of NaH during thermal decomposition a heating run in a molecular dynamics simulation is done. The result shows that a clear signature of hydrogen desorption is the fall in potential energy surface during heating

Cytomegalovirus viremia, pneumonitis, and tocilizumab therapy

To the Editor: Tocilizumab is a monoclonal antibody that competitively inhibits binding of interleukin-6 (IL-6) to its receptor. It is approved for treatment of rheumatoid arthritis (RA) as monotherapy or with methotrexate. We report a case of cytomegalovirus (CMV) disease complicating treatment with an IL-6 receptor antagonist