ReaxFF for Vanadium and Bismuth Oxides

Kim Chenoweth

Force Field Sub-Group Meeting

January 20, 2004

Overview

• Significance of a Bi/V force field

• ReaxFF: general principles

• Force field optimization for V

• Force field optimization for Bi

• Future work

Designing a Better Catalyst - I

• 85% of industrial organic chemicals are currently produced by catalytic processes

• 25% are produced by heterogeneous oxidation catalysis such as ammoxidation

CH2=CHCH3 + NH3 + 3/2 O2 CH2=CHCN + 3 H2O

• Bi-molybdates are currently used as the catalyst

• Use of alkanes as a cheaper feedstock requires design of a selective

catalyst

• Promising catalysts are complex oxides containing Mo, V, Te, X, and O

where X is at least one other element

Bismuth is one of the 19 elements listed in the Mitsubishi patent

Cat

Designing a Better Catalyst - II

• Low-MW alkenes (i.e. ethene and propene) can be formed via non-oxidative

dehydrogenation (ODH) of the corresponding alkane

• Supported vanadia is the most active and selective simple metal oxide for

alkane ODH1

Due to its reducible nature, it leads to rapid redox cycles necessary for catalytic

turnover

Local structure strongly influences ODH reaction rates and selectivity

• Force field would allow for the study of large and complex systems with

many atoms

Generate interesting structures for further study using QC methods

Optimize ratio of the various metals in the catalyst

Elucidate the purpose of the different metals1Argyle et al, J. Catal. 2002, 208, 139

ReaxFFBridging the gap between QC and EFF

Tim

e

DistanceÅngstrom Kilometers

10-1

5ye

ars

QC

ab initio,DFT,HF

ElectronsBond formation

MD

Empiricalforce fields

AtomsMolecular

conformations

MESO

FEA

Design

Grains

Grids

ReaxFF

Empirical methods:• Study large system• Rigid connectivity

QC Methods:• Allow reactions• Expensive

ReaxFF:

• Simulate bond formation in larger molecular systems

ReaxFF: Energy of the System

underover

torsvalCoulombvdWaalsbondsystem

EE

EEEEEE

2-body

multi-body

3-body 4-body

• Similar to empirical non-reactive force fields

• Divides the system energy into various partial energy contributions

Important Features in ReaxFF

• A bond length/bond order relationship is used to obtain smooth transition from non-bonded to single, double, and triple bonded systems. Bond orders are updated every iteration

• Non-bonded interactions (van der Waals, coulomb) Calculated between every atom pair Excessive close-range non-bonded interactions are avoided by shielding

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

• ReaxFF uses a geometry-dependent charge calculation scheme that accounts for polarization effects

ReaxFF as a Transferable Potential

General Rules: No discontinuities in energy or forces even during

reactions No pre-defined reactive sites or reaction pathways

Should be able to automatically handle coordination changes associated with reactions

One force field atom type per element Should be able to determine equilibrium bond lengths,

valence angles, etc from chemical environment

Strategy for Parameterization of ReaxFF

1. Identify important interactions to be optimized for relevant systems

2. Build QC-training set for bond dissociation and angle bending cases for small clusters

3. Build QC-training set for condensed phases to obtain equation of state

4. Force field optimization using

1. Metal training set

2. Metal oxide clusters and condensed phases

5. Applications

• Cluster Bonds

-Normal, under-, and over-coordinated systems

Angles O-V=O, V-O-V, O=V=O

Vanadium Training Set

• Condensed Phase Metal

BCC, A15, FCC, SC, Diamond

Metal Oxide VO (II)

• FCC

V2O3 (III) • Corundum

VO2 (IV) • Distorted rutile

V2O5 (V) • Layered octahedral

1st row transition metal (4s23d3)

• Successive bond dissociation of

oxygen in V4O10

Bulk Metal - Vanadium

ReaxFFQC

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tom

(kc

al/m

ol)

DiamondSCFCCA15BCC

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Vol./atom (Å^3)

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•ReaxFF reproduces EOS and properly predicts instability of low-coordination phases (SC, Diamond)

Bond Dissociation

in VO2OH

V=O Bond Dissociation

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190

0.5 1.5 2.5 3.5 4.5

Bond Distance (Å)

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ativ

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nerg

y (k

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)

QM (singlet)QM (triplet)ReaxFF

V-O Bond Dissociation

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0.5 1.5 2.5 3.5 4.5

Bond Distance (Å)

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ativ

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nerg

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QM (singlet)QM (triplet)ReaxFF

V=O Bond Dissociation in V4O10

V=O Bond Dissociation

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Bond Distance (Å)

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ativ

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nerg

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QM (singlet)QM (quintet)ReaxFF

Angle Distortion in V2O5

O-V=O AngleV-O-V Angle

V-O-V Angle

O=V-O Angle

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Angle (Degrees)

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ativ

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nerg

y (k

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) ReaxFFQC

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Angle (Degrees)

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ativ

e E

nerg

y (k

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) ReaxFFQC

Angle Distortion in VO2

O=V=O Angle

O=V=O Angle

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Angle (Degrees)

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ativ

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nerg

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QC

Angle Distortion in V2O6

V-O-O Angle

V-O-O Angle

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nerg

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QC

Charge Analysis for VxOy Clusters in Training Set

-1.3

-0.8

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Atom Number

Mul

likan

Cha

rges 12

3

4

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Atom Number

Mul

likan

Cha

rges

1 2

3

-1-0.8-0.6-0.4-0.2

00.20.40.60.8

11.21.4

1 2 3 4 5 6 7Atom Number

Mul

lika

n C

harg

es

ReaxFFQC

12

3

5

7

6

4

Charge Analysis for VxOY Clusters in Literature(QC data taken from Calatayud et al, J. Phys. Chem. A 2001, 105, 9760.)

-0.8

-0.4

0

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1 2 3 4Atom Number

Mul

likan

Cha

rges

-0.8

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0

0.4

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1.2

1 2 3 4 5Atom Number

Mul

likan

Cha

rges

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1 2 3 4 5Atom Number

Mul

likan

Cha

rges

-0.8

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0

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1.2

1 2 3 4 5 6Atom Number

Mul

lika

n C

harg

es

-0.8

-0.4

0

0.4

0.8

1.2

1 2 3 4 5 6 7 8 9Atom Number

Mu

llik

an C

har

ges

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0

0.4

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1.2

1 3 5 7 9 11 13Atom Number

Mu

llik

an C

har

ges

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0

0.4

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1.2

1 2 3 4 5 6Atom Number

Mul

lika

n C

harg

es

-0.8

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1.2

1 2 3 4 5 6 7 8

ReaxFFQC

Bismuth Training Set

• Cluster Bonds

-Normal, under-, and over-coordinated systems

Angles Bi-Bi=O, O=Bi-O

• Condensed Phase Metal

HCP, SC, BCC, A15, FCC, Diamond

Metal Oxide BiO (II)

• Trigonal

-Bi2O3 (III)

• Monoclinic

-Bi2O3 (III)

• Distorted cubic

Bi2O4 (BiIIIBiVO4)

• Monoclinic

BiO2 (IV)

• Cubic

Common oxidation states: 3, 5

-10

0

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10 20 30 40 50 60 70 80Vol./atom (Å^3)

E/a

tom

(kc

al/m

ol)

Bulk Metal - Bismuth

ReaxFFQC

-5

0

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Vol./atom (Å^3)

E/a

tom

(kc

al/m

ol)

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Diamond SCFCC A15BCC HCP

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10 20 30 40 50 60 70 80Vol./atom (Å^3)

E/a

tom

(kc

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DiamondSC

FCC

A15

BCC-5

0

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Vol./atom (Å^3)

E/a

tom

(kc

al/m

ol)

Relative Stabilities of V and Bi Bulk Phases

Relative Energies (kcal/mol)ReaxFF QM

BCC 0.00 0.00A15 -2.51 -2.00FCC -6.34 -6.56SC -27.43 -24.18

Diamond -71.05 -63.19

Relative Energies (kcal/mol)ReaxFF QM

HCP 0.00 0.00SC -0.36 -0.42

BCC -0.60 -0.61A15 -1.53 -2.94

Diamond -6.12 -4.52

BismuthVanadium

Cohesive Energies (kcal/mol)ReaxFF Lit.

Vanadium 123.3 122.5Bismuth 50.4 50.3

QuickTime™ and aDV/DVCPRO - NTSC decompressor

are needed to see this picture.

Application: Melting Point of Vanadium

• Melting point of Vanadium = 2163 K• Melting point obtained from simulation ~ 1900 K

2500 K 1700 K900 K 900 K1700 K

1900 K

55 molecules

Application: Melting Point of Vanadium

• Melting point of Vanadium = 2163 K• Melting point obtained from simulation ~ 2000 K

QuickTime™ and aDV/DVCPRO - NTSC decompressor

are needed to see this picture.

2500 K 1700 K

2000 K

900 K 900 K1700 K

147 molecules

Future Work

• Bismuth oxide force field training set: Optimization of Bi oxide force field

Add bond dissociation and bond angles for clusters

Add bismuth oxide condensed phases

• Vanadium oxide force field training set: Further optimization of vanadium oxide force field

Add successive V=O bond dissociation for V4O10

Add vanadium oxide condensed phases

Add to training set and continue optimizing force field

Add to training set and continue optimizing force field