2,391 research outputs found
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
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
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]
ReaxFF Reactive Force-Field Modeling of the Triple-Phase Boundary in a Solid Oxide Fuel Cell
In our study, the Ni/YSZ ReaxFF reactive force field was developed by combining the YSZ and Ni/C/H descriptions. ReaxFF reactive molecular dynamics (RMD) were applied to model chemical reactions, diffusion, and other physicochemical processes at the fuel/Ni/YSZ interface. The ReaxFF RMD simulations were performed on the H_2/Ni/YSZ and C_4H_(10)/Ni/YSZ triple-phase boundary (TPB) systems at 1250 and 2000 K, respectively. The simulations indicate amorphization of the Ni surface, partial decohesion (delamination) at the interface, and coking, which have indeed all been observed experimentally. They also allowed us to derive the mechanism of the butane conversion at the Ni/YSZ interface. Many steps of this mechanism are similar to the pyrolysis of butane. The products obtained in our simulations are the same as those in experiment, which indicates that the developed ReaxFF potential properly describes complex physicochemical processes, such as the oxide-ion diffusion, fuel conversion, water formation reaction, coking, and delamination, occurring at the TPB and can be recommended for further computational studies of the fuel/electrode/electrolyte interfaces in a SOFC
Being More Realistic About Reasons: On Rationality and Reasons Perspectivism
This paper looks at whether it is possible to unify the
requirements of rationality with the demands of normative
reasons. It might seem impossible to do because one depends
upon the agent’s perspective and the other upon features of
the situation. Enter Reasons Perspectivism. Reasons
perspectivists think they can show that rationality does consist
in responding correctly to reasons by placing epistemic
constraints on these reasons. They think that if normative
reasons are subject to the right epistemic constraints, rational
requirements will correspond to the demands generated by
normative reasons. While this proposal is prima facie plausible,
it cannot ultimately unify reasons and rationality. There is no
epistemic constraint that can do what reasons perspectivists
would need it to do. Some constraints are too strict. The rest
are too slack. This points to a general problem with the
reasons-first program. Once we recognize that the agent’s
epistemic position helps determine what she should do, we
have to reject the idea that the features of the agent’s situation
can help determine what we should do. Either rationality
crowds out reasons and their demands or the reasons will make
unreasonable demands
Regolith Advanced Surface Systems Operations Robot (RASSOR)
Regolith is abundant on extra-terrestrial surfaces and is the source of many resources such as oxygen, hydrogen, titanium, aluminum, iron, silica and other valuable materials, which can be used to make rocket propellant, consumables for life support, radiation protection barrier shields, landing pads, blast protection berms, roads, habitats and other structures and devices. Recent data from the Moon also indicates that there are substantial deposits of water ice in permanently shadowed crater regions and possibly under an over burden of regolith. The key to being able to use this regolith and acquire the resources, is being able to manipulate it with robotic excavation and hauling machinery that can survive and operate in these very extreme extra-terrestrial surface environments. In addition, the reduced gravity on the Moon, Mars, comets and asteroids poses a significant challenge in that the necessary reaction force for digging cannot be provided by the robot's weight as is typically done on Earth. Space transportation is expensive and limited in capacity, so small, lightweight payloads are desirable, which means large traditional excavation machines are not a viable option. A novel, compact and lightweight excavation robot prototype for manipulating, excavating, acquiring, hauling and dumping regolith on extra-terrestrial surfaces has been developed and tested. Lessons learned and test results will be presented including digging in a variety of lunar regolith simulant conditions including frozen regolith mixed with water ice
Quantum simulation of battery materials using ionic pseudopotentials
Ionic pseudopotentials are widely used in classical simulations of materials
to model the effective potential due to the nucleus and the core electrons.
Modeling fewer electrons explicitly results in a reduction in the number of
plane waves needed to accurately represent the states of a system. In this
work, we introduce a quantum algorithm that uses pseudopotentials to reduce the
cost of simulating periodic materials on a quantum computer. We use a
qubitization-based quantum phase estimation algorithm that employs a
first-quantization representation of the Hamiltonian in a plane-wave basis. We
address the challenge of incorporating the complexity of pseudopotentials into
quantum simulations by developing highly-optimized compilation strategies for
the qubitization of the Hamiltonian. This includes a linear combination of
unitaries decomposition that leverages the form of separable pseudopotentials.
Our strategies make use of quantum read-only memory subroutines as a more
efficient alternative to quantum arithmetic. We estimate the computational cost
of applying our algorithm to simulating lithium-excess cathode materials for
batteries, where more accurate simulations are needed to inform strategies for
gaining reversible access to the excess capacity they offer. We estimate the
number of qubits and Toffoli gates required to perform sufficiently accurate
simulations with our algorithm for three materials: lithium manganese oxide,
lithium nickel-manganese oxide, and lithium manganese oxyfluoride. Our
optimized compilation strategies result in a pseudopotential-based quantum
algorithm with a total runtime four orders of magnitude lower than the previous
state of the art for a fixed target accuracy
Evolution of Parton Fragmentation Functions at Finite Temperature
The first order correction to the parton fragmentation functions in a thermal
medium is derived in the leading logarithmic approximation in the framework of
thermal field theory. The medium-modified evolution equations of the parton
fragmentation functions are also derived. It is shown that all infrared
divergences, both linear and logarithmic, in the real processes are canceled
among themselves and by corresponding virtual corrections. The evolution of the
quark number and the energy loss (or gain) induced by the thermal medium are
investigated.Comment: 21 pages in RevTex, 10 figure
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