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Ch121a Atomic Level Simulations of Materials and Molecules. BI 115 Hours: 2:30-3:30 Monday and Wednesday Lecture or Lab: Friday 2-3pm (+3-4pm). Lecture 11, April 25, 2011 Reactive Force Fields – 1: ReaxFF. William A. Goddard III, [email protected]

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Ch121a atomic level simulations of materials and molecules

Ch121a Atomic Level Simulations of Materials and Molecules

BI 115

Hours: 2:30-3:30 Monday and Wednesday

Lecture or Lab: Friday 2-3pm (+3-4pm)

Lecture 11, April 25, 2011

Reactive Force Fields – 1: ReaxFF

William A. Goddard III, [email protected]

Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics, California Institute of Technology

Teaching Assistants Wei-Guang Liu, Fan Lu, Jose Mendoza, Andrea Kirkpatrick


Reaxff first principles force field

Bridging the Gap between QM and MD to describe reactive processes ranging from combustion to catalysis, fuel cells, nanoelectronics, and shock induced chemistry

ReaxFF: first principles force field

471. ReaxFF: A Reactive Force Field for Hydrocarbons A.C.T. van Duin, S. Dasgupta, F. Lorant and W. A. Goddard III J. Phys. Chem. A, 105: (41) 9396-9409 (2001)

514. ReaxFF sio Reactive Force Field for Silicon and Silicon Oxide Systems Adri C. T. van Duin, Alehandro Strachan, Shannon Stewman, Qingsong Zhang, Xin Xu, and William A. Goddard, III J. Phys. Chem. A, 107, 3803 (2003)

  • 533. Shock waves in high-energy materials: The initial chemical events in nitramine RDX Strachan A, van Duin ACT, Chakraborty D, Dasgupta S, Goddard WA Physical Review Letters, 91 (9): art. no. 098301 (2003)


Ch121a atomic level simulations of materials and molecules

Motivation: Design Materials, Catalysts, Pharma from 1st Principles so can do design prior to experiment

time

ELECTRONS ATOMS GRAINS GRIDS

simulations real devices and full cell (systems biology)

hours

millisec

nanosec

picosec

femtosec

Continuum

(FEM)

Micromechanical modeling

Protein clusters

MESO

MD

Deformation and Failure

Protein Structure and Function

QM

distance

Å nm micron mm yards

To connect 1st Principles (QM) to Macro work use an overlapping hierarchy of methods (paradigms)

Need accurate force fields (potentials) with parameters derived fromfirst-principles QM

Big breakthrough making FC simulations practical:

reactive force fields based on QM

Describes: chemistry,charge transfer, etc. For metals, oxides, organics.

Accurate calculations for bulk phases and molecules(EOS, bond dissociation)

Chemical Reactions (P-450 oxidation)


Ch121a atomic level simulations of materials and molecules

r

E = kA(A-A0)

A

E = kr(r-r0)

Ordinary Force Fields

Bonds, angles, torsions described as elastic springs

Fixed charges, Empirical vdw nonbond terms,

Bonds cannot be broken, making the model unsuitable for modeling reactions.

Examples: MM3, Dreiding, Amber, Charmm, Gromos, UFF

ReaxFF:

Allow bonds to break and form and describe barriers for reactions.

All parameters from quantum mechanics, no empirical data


First principles reactive force fields strategy

First Principles Reactive force fields: strategy

  • Describe Chemistry (i.e., reactions)of molecules

  • Fit QM Bond dissociation curves for breaking every type of bond

    • (XnA-BYm), (XnA=BYm), (XnA≡BYm)

  • Fit angle bending and torsional potentials from QM

  • Fit QM Surfaces for Chemical reactions (uni- and bi-molecular)

  • Fit Ab initio charges and polarizabilities of molecules

  • Pauli Principle: Fit to QM for all coordinations (2,4,6,8,12)

    • Metals: fcc, hcp, bcc, a15, sc, diamond

    • Defects (vacancies, dislocations, surfaces)

    • cover high pressure(to 50% compression or 500GPa)

  • Generic: use same parameters for all systems(same O in O3, SiO2, H2CO, HbO2, BaTiO3)

Require that One FF reproduces all the ab-initio data (ReaxFF)

Most theorists (including me) thought that this would not be possible, but we claim to have achieved and validated it for many systems


Many chemical processes bond breaking for 5000 millions atoms this is far too large for dft

Many Chemical processes: bond breaking for 5000millions atoms. This is far too large for DFT

Solution: ReaxFF first principles reactive force field

short distance Pauli Repulsion + long range dispersion

(pairwise Morse function)

Valence energy

Electrostatic energy

  • Based completely on First Principles QM (no empirical parameters)

  • Valence Terms (EVal) based on Bond Order: dissociates smoothly

    • Bond distance  Bond order  Bond energy

    • Forces depend only on geometry (no assigned bond types)

    • Allows angle, torsion, and inversion terms (where needed)

    • Describes resonance(benzene, allyl)

    • Describes forbidden(2s + 2s)and allowed(Diels-Alder)reactions

    • Atomic Valence Term(sum of Bond Orders gives valency)

  • Pair-wiseNonbond Terms between all atoms (no “bond” exclusions)

    • Short range Pauli Repulsion plus Dispersion(EvdW)

    • QEq Electrostatics allows charges to flow depending on environment and external fields


Reaxff reactive force field for reactive dynamics rd

ReaxFF reactive force field for Reactive Dynamics (RD)

Adri van Duin

Allow bonds to break and form, describe barriers for reactions.

All parameters from quantum mechanics (no empirical data)

ReaxFF describes reactive processes (from oxidation to combustion to catalysis to shock induced chemistry)for 1000s to millions atoms

We use ReaxFF to prepare the structures of complex heterogeneous systems by processes similar to experimental synthesis (DLC, SiO2/Si)


Bond order dependent potentials

Sigma Bond 1.5Å - 2.3Å

First Pi Bond 1.2Å - 1.75Å

Second Pi Bond 1.0Å - 1.4Å

Bond Order Dependent Potentials


Bond order dependence on r cc

Bond Order Dependence on r (CC)



s


Ch121a atomic level simulations of materials and molecules

Bond distance bond order

C-C bond



Bond order

s

Bond distance (Å)

Bond order bond energy

Bond distance bond order bond energy

Use general functional form. Determine parameters from fitting QM bond breaking for many single, double, and triple bonded systems.

Bond distance (Å)


Van der waals

Van der Waals

Include vdW for 1-2 and 1-3 interactions since bonded atoms may dissociate and nonbonded atoms may bond during the dynamics

We use a Morse function rather than LJ12-6 (which is too stiff in the inner wall region

Or exponential-6 which has problems for small R

Here f7 was introduced to modify interactions for small R

This was a mistake and should be eliminated


Vdw energy

vdW Energy


Coulomb interacions

Coulomb interacions

Electronegativity

Hardness

Include Coulomb for 1-2 and 1-3 interactions since bonded atoms may dissociate and nonbonded atoms may bond during the dynamics

This f7 function provides shielding so that the Coulomb potential goes to a constant, Cqiqj gw at small R, where gw is related to the size of the atoms.

ReaxFF uses the Electron Equilibration Method (EEM) of Mortier to determine charges rather than rather than QEq

Mortier, W. J.; Ghosh, S. K.; Shankar, S. J. Am. Chem. Soc. 1986,108, 4315; Janssens, G. O. A.; Baekelandt, B. G.; Toufar, H.; Mortier, W. J.;Schoonheydt, R. A. J. Phys. Chem. 1995, 99, 3251.


Discussion about charges in reaxff

Discussion about charges in ReaxFF

I believe that the use of a shielding denominator in the Coulomb energy and the use of EEM in ReaxFF are inconsistent and a fundamental mistake.

We should use QEq. In QEq the shielding of charges on different centers is built in both in calculating the Coulomb energy and in calculating the charges.

Separating them leads to potential problems for small R, where the charges may be affected by the 1/R form in the potential.

In QEq this shielding is based on the covalent radius of the atom and hence need not be optimized.


Charge equilibration qeq

Charge Equilibration (QEq)

Charge Equilibration for Molecular Dynamics Simulations

A. K. Rappé and W. A. Goddard III

J. Phys. Chem. 95, 3358 (1991)

I

atomic

interactions

Keeping:

Jij

Hardness (IP-EA)

Electronegativity (IP+EA)/2

1/rij

rij

ri0 +rj0

  • Self-consistent Charge Equilibration (QEq)

  • Describe charges as distributed (Gaussian)

  • Thus charges on adjacent atoms shielded

  • (interactions  constant as R0) and include interactions over ALL atoms, even if bonded (no exclusions)

  • Allow charge transfer (QEq method)

2

Three universal parameters for each element:

1991: use experimental IP, EA, Ri; ReaxFF get from fitting QM


Cc bond dissociation energy

Experiment

Mol. Ebre

C2H6 90.4 - 1.533 C3H8 85.8 - 1.526 C4H10 86.5 - 1.532

CC Bond Dissociation Energy

In ReaxFF the BO-BE term is monotonic and attractive, becoming a constant at small R

This is balance by the vdW term which is large and repulsive at small R

To finally give the normal bonding function.


Coordination

Deviation of bond orders from saturation

Over-coordination

Coordination


Under coordination

Under-coordination


Angle

Angle


Calculation of 0

Calculation of 0

SBO2 = 2; SBO>2

SBO2 = 2-(2-SBO)5; 1<SBO<2

SBO2 = SBO 5; 0<SBO<1

SBO2 = 0; SBO  0

2j = j; j < 0

2j = 0; j  0


E angle

Eangle


Torsion

Torsion

  • V2 diminishes rapidly when central bond deviates from BO=2

  • Sin terms ensure this terms goes to 0 when the angles approach 180

  • f4 accounts for over-coordinated central C-C atoms

  • f3 allows for proper dissociation behavior


Ch121a atomic level simulations of materials and molecules

f3


Ch121a atomic level simulations of materials and molecules

f4


Ch121a atomic level simulations of materials and molecules

V2


E tors

Etors


Conjugation

Conjugation


Heats of formation for 40 compounds radicals

Heats of Formation for 40 compounds/radicals


Bond dissociation energies

Bond dissociation energies


Bond lengths and r 0

Bond lengths and r0


Rdx concerted ring breaking

RDX concerted ring breaking


Rdx n n dissociation

RDX N-N dissociation


Hono elimination

HONO elimination


Dft calculation

DFT Calculation


Ch121a atomic level simulations of materials and molecules

EoS


Ch121a atomic level simulations of materials and molecules

EoS


Ch121a atomic level simulations of materials and molecules

Over-coordination energy and van der Waals

Over-coordination penalty

Over-coordination energy

Energy (kcal/mol)

Total bond order

Shielded van der Waals

Unshielded van der Waals

Energy (kcal/mol)

Shielded vdW (w= 0.8)

C-C distance (Å)


Ch121a atomic level simulations of materials and molecules

Key features of ReaxFF

  • To get a smooth transition from nonbonded to single, double and triple bonded systems ReaxFF employs a bond length/bond order relationship1,2. Bond orders are updated every iteration.

  • Nonbonded interactions (van der Waals, Coulomb) are calculated between every atom pair, irrespective of connectivity. Excessive close-range nonbonded interactions are avoided by shielding.

  • All connectivity-dependent interactions (i.e. valence and torsion angles) are made bond-order dependent, ensuring that their energy contributions disappear upon bond dissociation.

  • ReaxFF uses a geometry-dependent charge calculation scheme (EEM) that accounts for polarization effects.

1:Tersoff, PRB 1988;2: Brenner PRB 1990


Ch121a atomic level simulations of materials and molecules

General rules ReaxFF

MD-force field; no discontinuities in energy or forces even during reactions.

User does not pre-define reactive sites or reaction

pathways; ReaxFF automatically handles

coordination changes associated with reactions.

Each element is represented by only 1 atom type in the force field;

force field determine equilibrium bond lengths,

valence angles etc. from chemical environment.


Reaxff uses generic rules for all parameters and functional forms

ReaxFF uses generic rules for all parameters and functional forms

ReaxFF automatically handles coordination changes and oxidation states associated with reactions, thus no discontinuities in energy or forces.

User does not pre-define reactive sites or reaction pathways (ReaxFF figures it out as the reaction proceeds)

Each element leads to only 1 atom type in the force field. (same O in O3, SiO2, H2CO, HbO2, BaTiO3)(we do not pre-designate the CO bond in H2CO as double and the CO bond in H3COH as single or in CO as triple, ReaxFF figures this out)

ReaxFF determines equilibrium bond lengths, angles, and charges solely from the chemical environment.

Require that One FF reproduces all the ab-initio data (ReaxFF)

Most theorists (including me) thought that this would be impossible, hence it would never have been funded by NSF, DOE, or NIH since it was far too risky. (DARPA came through, then ONR, then ARO).


Current applications of reaxff

Catalysts: Pt-Co Fuel cell cathode, Pt-Ru FC anode

VOx catalyzed oxidative dehydrogenation: propane to propene

MoVNbTaTeOx ammoxidation catalysts (propaneacrylonitrile)

Ni,Co,Mo catalyzed growth of bucky tubes

Metal alloy phase transformations (crystal-amorphous)

Si-Al-Mg oxides: Zeolites, clays, mica, intercalated polymers

Gate oxides (Si-HfO2, Si-ZrO2, Si-SiOxNy interfaces)

Ferroelectric oxides (BaTiO3) domain structure,

Pz/Ez Hysteresis Loop of BaTiO3 at 300K, 25GHz by MD

Decomposition of High Energy (HE) Density Materials (HEDM)

MD simulations of shock decomposition and of cook-off

MD elucidation of the origins of sensitivity in HE materials

Reaction Kinetics from MD simulations (oxidations)

ADP-ATP hydrolysis

Enzyme Proteolysis

Current applications of ReaxFF


Applications to energetic materials

Applications to energetic materials

Sergey Zybin, Peng Xu, Yi Liu, Qing Zhang,

Luzheng Zhang, Adri van Duin and William A. Goddard III

  • Force field development

    • Training sets for HE-materials

    • Treating organic crystals

  • Overview of past and ongoing applications

    • Predicting chemistry : cookoff of RDX, HMX and TATB and carbon cluster analysis

    • Prediction of sensitivity for HE materials

    • Energy release: ISP prediction for RDX/Al and hydrazine materials

    • New HE-materials: all-Nitrogen


  • Training set for nitramine potential

    Training set for nitramine potential

    Bond and angle distortion

    Reaction barriers

    C-N bond dissociation in H2C=NH

    Nitromethane C-N-O angle bending

    First-row elements

    Charge distributions

    Condensed phase


    Ch121a atomic level simulations of materials and molecules

    ReaxFF gives accurate description of complex chemical reactions: Decomposition Mechanism RDX (gas-phase)

    NO2 dissociation

    concerted

    New mechanism

    QM

    ReaxFF

    8 membered ring

    HONO elimination

    Concerted, NO2 and HONO-

    dissociation pathways

    (part of the original training set)

    ReaxFF MD simulation found New unimolecular reaction, confirmed by QM, More important than concerted pathway

    Strachan, Kober, van Duin, Oxgaard and Goddard, J.Chem.Phys 2005


    Rdx shock simulations md with reaxff

    RDX shock simulations: MD with ReaxFF

    • Simulate High impact shock chemistry using MD simulations

    • 1st step: thermalize two semi-infinite slabs of RDX (1344 atoms/cell)

    • Add the shock velocity on top of the thermal velocities

    • Constant NVE molecular dynamics (adiabatic)

    ∞ Slab: 32 RDX molecules/cell on ∞ Slab:32 RDX molecules/cell

    Full-physics, full-chemistry simulations of shock induced chemistry


    Hydrocarbon combustion and metal oxide catalyzed hydrocarbon oxidation

    Hydrocarbon combustion and metal-oxide catalyzed hydrocarbon oxidation

    Adri, Kimberly Chenoweth, Sanja Pudar, Mu-Jeng Cheng, and wag

    Selective oxidation of propene using multi-metal oxide (MMO) catalysts

    Develop ReaxFF based on QM-data , use ReaxFF to perform high-temperature simulations on catalyst/hydrocarbon reactions

    First, establish that ReaxFF can describe non-catalytic hydrocarbon combustion

    Mixed metal oxide catalyst (BixMoyVzTeaOb)


    Force field development hydrocarbon oxidation

    Force field development: hydrocarbon oxidation

    QM

    ReaxFF

    Oxidation reactions

    QM: Jaguar/DFT/B3LYP/6-311G**

    Radical rearrangements

    Rotational barriers

    Angle strain

    H3C• + CO

    H3C-C=O

    - total training set contains about 1700 compounds


    Test reaxff cho description oxidation of o xylene

    Test ReaxFF CHO-description: oxidation of o-xylene

    • Oxidation initiates with OOH-formation

    • Final products dominated by CO, CO2 and H2O

    Consumed O2

    CO2

    H2O

    CO

    o-Xylene

    OOH

    OH

    • Exothermic reaction

    • Exothermic events are related to H2O and CO2 formation

    2 o-Xylene; 70 O2 in 20x20x20 Angstrom box

    ReaxFF NVT/MD at T=2500K


    O xylene oxidation detailed reaction mechanism

    o-Xylene oxidation: Detailed reaction mechanism

    O

    O

    O

    O

    O

    2

    2

    2

    2

    2

    H

    C

    O

    O

    H

    C

    H

    2

    C

    H

    2

    3

    H

    O

    2

    frame 174

    C

    H

    frame 128

    C

    H

    C

    H

    3

    3

    3

    frame 175

    O

    O

    H

    C

    =

    O

    H

    C

    =

    O

    H

    C

    O

    2

    2

    2

    O

    O

    H

    H

    O

    H

    O

    H

    C

    H

    3

    C

    H

    C

    H

    3

    3

    frame 176

    frame 176

    frame 177

    H

    O

    O

    O

    O

    O

    O

    O

    H

    O

    H

    C

    H

    C

    H

    frame 180

    2

    H

    C

    =

    O

    frame 179

    2

    O

    H

    2

    C

    H

    H

    C

    =

    O

    2

    O

    H

    frame 182

    H

    H

    O

    H

    H

    O

    H

    H

    H

    O

    H

    H

    H

    H

    H

    H

    frame 205

    C

    H

    C

    O

    frame 193

    2

    C

    O

    2

    C

    O

    2

    C

    O

    H

    O

    O

    H

    2

    2

    O

    H

    H

    O

    Kimberly Chenoweth

    2

    frame 209

    H

    H

    C

    O

    O

    H

    H

    O

    H

    H

    H

    O

    2

    O

    H

    O

    H

    C

    O

    H

    H

    H

    2

    O

    H

    H

    H

    H

    frame 232

    frame 232

    C

    O

    H

    H

    H

    O

    H

    2

    2

    C

    O

    frame 234

    C

    O

    O

    H

    2

    2

    C

    O

    H

    C

    O

    C

    O

    2

    H

    H

    H

    H

    H

    H

    H

    H

    H

    H

    H

    H

    H

    H

    H

    H

    H

    O

    O

    O

    O

    O

    O

    O

    O

    O

    O

    O

    O

    O

    O

    O

    H

    H

    2

    2

    2

    2

    2

    2

    2

    2

    2

    2

    2

    2

    2

    2

    2

    O

    H

    2

    O

    O

    H

    C

    =

    O

    • Reaction initiation with HO2-formation

    • Dehydrogenation occurs at methyl-groups, not at benzyl-hydrogens

    • Only after H2C=O is formed and dissociated the benzene ring gets oxidized

    • Ring opens shortly after destruction of aromatic system

    • Ring-opened structure reacts quickly with oxygen, forming CO2, H2O and CO

    • ReaxFF gives sensible predictions that can be directly tested against QM


    Determining the parameters for reaxff moox

    Determining the parameters for ReaxFF: MoOx

    ReaxFF

    diamond

    Simple cubic

    fcc

    A15

    bcc

    Step 1: get ReaxFF for Mo metal

    Need to describe the complicated bonding in MoxOy polymorphs

    QM

    diamond

    Energy (kcal/mol)

    Mo17O47-crystal

    Simple cubic

    fcc

    A15

    bcc

    Energy (kcal/mol)


    Oxidation of moo 2 slab by o 3

    Oxidation of MoO2 slab by O3

    Epot (kcal/mol)

    Time (ps)

    Initial reaction is fast

    Reaction slows down as MoO2 surface gets oxidized


    Analysis moo 2 o 3 simulation

    Analysis MoO2 + O3 simulation

    g(r)

    r (Å)

    Mo=O

    g(r)

    r (Å)

    Start: Mo64O128 [MoO2]

    End: Mo64O175 [MoO2.7]

    ReaxFF predicts correctly the formation of Mo=O surface double bonds


    Reaxff for reactions on pt catalysts

    ReaxFF for reactions on Pt catalysts

    Pt crystals

    DFT

    Energy (eV)

    diamond

    Simple cubic

    A15

    hcp

    ReaxFF gives a good description to the EOS of the stable phases (FCC, BCC, A15)

    ReaxFF properly predicts the instability of the low-coordination phases (SC, Diamond)

    bcc

    fcc

    ReaxFF

    diamond

    Energy (eV)

    Simple cubic

    A15

    hcp

    bcc

    fcc

    Volume/atom (Å3)


    Test of reaxff for pt metal clusters

    Test of ReaxFF for Pt metal clusters

    ReaxFF gives a good description for undercoordinated Pt-systems


    Hydrocarbon interaction with 35 atom pt cluster

    Hydrocarbon interaction with 35-atom Pt-cluster

    • ReaxFF can describe different C-Pt bonding modes


    Equation of state for pt bulk metal oxides

    Equation of State for Pt bulk metal oxides

    Energy (kcal/mol)

    equations of state for Pt-bulk oxides from ReaxFF are in good agreement with QM-data

    Energy (kcal/mol)


    Also do equation of state for bulk metal oxide phases

    Also do Equation of State for Bulk metal oxide phases

    ReaxFF MD-NVT simulation at T=300K

    MoO3

    Energy (kcal/mol)

    ReaxFF

    DFT

    Do similar calculations on:

    MoO2 equation of state

    Equilibrium structures of MoO2, Mo8O24, Mo5O8


    Bond dissociation valence distortions

    Bond dissociation, valence distortions

    DFT S=0

    ReaxFF

    ReaxFF

    DFT S=1

    DFT

    Mo=O bond dissociation in MoO3-cluster

    HO-Mo=O angle bending in cluster (HO)2MoO2-

    Energy (kcal/mol)

    Energy (kcal/mol)

    Also break Mo-OH bonds,

    Mo-CH3 bonds

    Mo-H bonds

    Other angle cases:

    Mo-O-Mo

    O=Mo=O

    HO-Mo-OH

    Mo-O-O

    Mo-O-H

    Mo-O-C


    Reactions in training set for reaxff

    Reactions in training set for ReaxFF

    ReaxFF

    DFT

    Energy (kcal/mol)

    Mo3O9

    3 MoO3

    Example test: Oxidation of MoO2 rutile using ozone

    Other reactions considered:

    MoO4 MoO3 +O  MoO2 + 2O  MoO+3O

    MoO3 + 0.5 O2 (O-O)MoO2

    Periodic (a=b=20.69 Å, c=55 Å)

    192-atom MoO2-slab + 50 O3

    62.5 ps. NVT MD at T = 1000K.

    Analyze final Mo oxidation state


    Reactions of h 2 and o 2 over pt 111 surfaces

    Reactions of H2 and O2 over Pt(111) surfaces

    Energy release

    8 H2 + 4 O2

    Pt(111) perfect

    96 atoms

    8 H2 + 4 O2

    Pt (111) stepped

    84 atoms

    perfect

    MD simulation at 1000K

    stepped

    Perfect surface generates H2O

    stepped surface gets oxidized

    Energy profile for perfect surface shows H2O generation events

    Have not yet done QM with stepped surface to compare with ReaxFF


    Vanadium oxide based catalysts

    Vanadium oxide based catalysts

    • Selective oxidation catalyst:

      • selective oxidation of o-xylene to phthalic anhydride

      • ammoxidation of alkyl aromatics (i.e. toluene, picolines)

      • oxidation of benzene, olefins, n-butane (poor selectivity)

      • oxidation of butane or hexane to maleic anhydride

    • Selective catalytic reduction:

      • SCR of NOx with NH3

      • controlling oxidation of SO2 to SO3 during SCR

    • Oxidative dehydrogenation(ODH):

      • convert alkane to olefin (i.e. propane to propene)

      • selective oxidation of methanol to formaldehyde


    Reaxff vanadium force field development

    ReaxFF – Vanadium Force Field Development

    Bond Dissociations

    Angle

    Dihedral

    Charge Distributions

    • Include bond dissociations, angle and dihedral distortion energies, charge distributions in training set for small clusters


    Reaxff development for bi te v nb mo oxides

    ReaxFF Development for Bi, Te, V, Nb, Mo oxides

    Double Bond Dissociation

    Te(OH)n Te(OH)n-1 + OH

    Hydrogen shift in V4O10H

    O-Nb-O Angle Bending

    Mo-O-V Angle Bending

    Charge Distributions (VO2OH)

    MSC, Caltech


    Derive one ff for v to describe all coordinations in the metal and oxide and all oxidation states

    Derive one FF for V to describe all coordinations in the metal and oxide and all oxidation states

    Metal

    FCC, BCC,HCP,A15, SC, Diamond

    QM: SeqQuest (SNL Gaussian-based periodic DFT)

    V(bcc)

    V(bcc)

    Metal oxides

    VO

    VO

    VO2

    VO2

    Heat of formation (kcal/unit)

    4, 6, 8 Oxygen coordination

    V2O3

    V2O3

    V2O5

    V2O5

    QM

    ReaxFF

    • Energy difference for oxidation changes is in good agreement with QM data

    • Indicates that ReaxFF is able to capture energetics of redox reactions


    Reaxff development bulk oxides

    ReaxFF Development: Bulk oxides

    TeO2

    ReaxFF

    QM

    Heat of formation (kcal/mol)

    Density (kg/dm3)

    Same ReaxFF describes: Te0, TeII, TeIV, TeVI, Bi0, BiIII, BiV

    V0, VIII, VIV, VV, Mo0, MoII,MoIV, MoV, MoVI,

    • Energy difference for oxidation changes is in good agreement with QM-data

    • ReaxFF able to capture the energetics of redox-reactions at metal oxide surfaces

    • ReaxFF slight systematic tendency to overestimate stability of metal oxide phases

    PBE GGA exchange-correlation functional with Gaussian basis sets as implemented in SeqQuest


    Reaxff development propane propene on v 4 o 10

    ReaxFF Development: Propanepropene on V4O10

    ReaxFF

    QM

    V4O10 + O2 + 2 propane

    Binding of O2 displaces propene product

    V4O10 + 2 H2O + 2 propene

    QM (B3LYP/LACVP**): Cheng, Chenoweth, Oxgaard, van Duin, Goddard JPC-C 2007, 111, 5115.

    ReaxFF reproduces QM energies for the entire reaction pathway

    MSC, Caltech

    Kimberly Chenoweth


    Reaxff md simulation conditions

    ReaxFF MD Simulation Conditions

    Started from a minimized structure

    30 methanol molecules

    3-layer V2O5 (001) periodic slab

    Total number of atoms = 684

    Slab Temp = 650K

    CH3OH Temp = 2000K

    Time step = 0.25 fs

    Temperature damping = 100 fs

    Total simulation time = 250ps


    Reaxff nvt md simulation methanol oxidative dehydrogenation odh

    ReaxFF NVT-MD Simulation: Methanol Oxidative DeHydrogenation (ODH)

    • methanol converts to formaldehyde with production of water

    • Others include the number of hydrocarbons bound to the surface

    • Expt.: Major product is formaldehyde (also H2O, COx)


    Reaxff mechanistic details

    ReaxFF Mechanistic Details

    Formation of H2C-OH radical

    3.45ps

    Formation of formaldehyde

    8.80ps

    H-abstraction by surface vanadyl groups


    Reaxff validation h 3 coh h 2 c o on v 2 o 5 001

    ReaxFF Validation: H3COH H2C=O on V2O5 (001)

    H2O desorption induced by interlayer binding to convert

    VIII… O=VV pair to VIV-O-VIV

    NVT-MD simulation at 650K with 30 CH3OH (at 2000K)

    Observe conversion of CH3OH to CH2O in dynamics

    • Observe CH3OH  CH2O + H2O

    • Longer simulation also leads to COx

    • Agrees with Experiment

    3.425ps

    3.450ps

    8.800ps

    Chenoweth, van Duin, Cheng, Persson, Oxgaard, Goddard, in preparation.


    Desorption of water from catalyst

    Desorption of Water from catalyst

    Snapshots from simulation showing atoms within 5.5Å of V bound to H2O

    1

    2

    3

    4

    5

    Water bound to VIII, bond very strong, will not desorb

    2nd layer has VV=O pointing at VIII of top layer

    2nd layer O bonds to top V get VIV-O-VIV

    H2O bonds weakly to VIV now desorbs


    Reaxff validation reaction of propene on bi 2 o 3 and moo 3

    ReaxFF Validation: Reaction of Propene on Bi2O3 and MoO3

    1100K

    Propene + Bi2O3 Slab

    Propene + MoO3 Slab

    Get abstraction of allylic hydrogen by bridging oxygen on amorphous Bi2O3 surface

    No formation of oxide products

    Agree with experiment

    No abstraction of allylic hydrogen by MoO3.

    No formation of oxide products

    Agree with experiment


    Reaxff validation oxidation of propene on bi 2 mo 3 o 12 010

    ReaxFF Validation: Oxidation of Propene on Bi2Mo3O12 (010)

    H abstracted by Mo=O bond next to Mo-O-Bi

    Had expected Bi=O bond to be involved

    Allyl subsequently is trapped on a different Mo=O bond

    Much Longer times required to observe oxidation of allyl radical to form acrolein

    Goddard, van Duin, Chenoweth, Cheng, Pudar, Oxgaard, Merinov, Jang, Persson Topics in Catal. 38, 2006, 93.

    Grasselli et al. 1984


    Stop lecture 11

    Stop lecture 11


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