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

Practical application of thermoreversibly Cross-linked rubber products

Currently, rubber products cannot simply be reprocessed after their product life, due to the irreversible cross-linking methods traditionally applied. The purpose of this work is to investigate how thermoreversible cross-linking of rubbers via Diels Alder chemistry can be used for the development of recyclable rubber products. Unfortunately, the applicability of the thermoreversible EPM-g-furan/BM system appears to be limited to room temperature applications, because of the rapid deterioration of the compression set at elevated temperatures compared to irreversibly cross-linked EPM. However, the use of EPM rubber modified with thiophene or cyclopentadiene moieties may extend the temperature application range and results in rubber products with acceptable properties. Finally, rubber products generally comprise fillers such as silica, carbon black or fibers. In this context, the reinforcing effect of short cut aramid fibers on the material properties of the newly developed thermoreversibly cross-linked EPM rubbers was also studied. The material properties of the resulting products were found to be comparable to those of a fiber reinforced, peroxide cured reference sample

Theory for incongruent crystallization: application to a ZBLAN glass

Equations which describe incongruent nucleation and subsequent crystal growth are derived. A ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) glass was used to test the validity of these equations. Nucleation rate measurements were fitted to theory and some growth rate measurements were in reasonable agreement with theor. predictions. Both nucleation theory and crystal growth theory were used for computer simulations of the crystn. behavior during heat treatments. Some heat treatments were performed in a DSC app. to verify the theories. The exptl. results were in good agreement with the numerical data. Using these theor. results it is possible to est. fiber scattering losses due to crystn. Depending on drawing temp., estd. losses can vary from 0.014 (310 Deg) to >=25 decibel/km (320 Deg). [on SciFinder (R)