reaxff for vanadium and bismuth oxides
DESCRIPTION
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. Cat. - PowerPoint PPT PresentationTRANSCRIPT
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
0
10
20
30
40
50
60
70
80
90
5 10 15 20 25 30
Vol/atom (Å^3)
E/a
tom
(kc
al/m
ol)
DiamondSCFCCA15BCC
0
10
20
30
40
50
60
70
80
90
5 10 15 20 25 30
Vol./atom (Å^3)
E/a
tom
(kc
al/m
ol)
•ReaxFF reproduces EOS and properly predicts instability of low-coordination phases (SC, Diamond)
Bond Dissociation
in VO2OH
V=O Bond Dissociation
-10
10
30
50
70
90
110
130
150
170
190
0.5 1.5 2.5 3.5 4.5
Bond Distance (Å)
Rel
ativ
e E
nerg
y (k
cal/
mol
)
QM (singlet)QM (triplet)ReaxFF
V-O Bond Dissociation
-10
10
30
50
70
90
110
130
150
170
190
0.5 1.5 2.5 3.5 4.5
Bond Distance (Å)
Rel
ativ
e E
nerg
y (k
cal/
mol
)
QM (singlet)QM (triplet)ReaxFF
V=O Bond Dissociation in V4O10
V=O Bond Dissociation
-10
10
30
50
70
90
110
130
150
170
190
0.5 1.5 2.5 3.5 4.5
Bond Distance (Å)
Rel
ativ
e E
nerg
y (k
cal/m
ol)
QM (singlet)QM (quintet)ReaxFF
Angle Distortion in V2O5
O-V=O AngleV-O-V Angle
V-O-V Angle
O=V-O Angle
-20
0
20
40
60
80
100
120
50 75 100 125 150 175
Angle (Degrees)
Rel
ativ
e E
nerg
y (k
cal/
mol
) ReaxFFQC
-5
0
5
10
15
20
75 100 125 150 175 200
Angle (Degrees)
Rel
ativ
e E
nerg
y (k
cal/
mol
) ReaxFFQC
Angle Distortion in VO2
O=V=O Angle
O=V=O Angle
-20
0
20
40
60
80
100
120
50 75 100 125 150 175
Angle (Degrees)
Rel
ativ
e E
nerg
y (k
cal/m
ol) ReaxFF
QC
Angle Distortion in V2O6
V-O-O Angle
V-O-O Angle
-5
0
5
10
15
20
25
30
50 75 100 125 150 175
Angle (Degrees)
Rel
ativ
e E
nerg
y (k
cal/m
ol) ReaxFF
QC
Charge Analysis for VxOy Clusters in Training Set
-1.3
-0.8
-0.3
0.2
0.7
1.2
1 2 3 4
Atom Number
Mul
likan
Cha
rges 12
3
4
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1 2 3
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
0.4
0.8
1.2
1 2 3 4Atom Number
Mul
likan
Cha
rges
-0.8
-0.4
0
0.4
0.8
1.2
1 2 3 4 5Atom Number
Mul
likan
Cha
rges
-0.8
-0.4
0
0.4
0.8
1.2
1 2 3 4 5Atom Number
Mul
likan
Cha
rges
-0.8
-0.4
0
0.4
0.8
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
-0.8
-0.4
0
0.4
0.8
1.2
1 3 5 7 9 11 13Atom Number
Mu
llik
an C
har
ges
-0.8
-0.4
0
0.4
0.8
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
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
10
20
30
40
50
60
70
80
10 20 30 40 50 60 70 80Vol./atom (Å^3)
E/a
tom
(kc
al/m
ol)
Bulk Metal - Bismuth
ReaxFFQC
-5
0
5
10
20 30 40 50
Vol./atom (Å^3)
E/a
tom
(kc
al/m
ol)
-10
0
10
20
30
40
50
60
70
80
10 20 30 40 50 60 70 80
Diamond SCFCC A15BCC HCP
-10
0
10
20
30
40
50
60
70
80
10 20 30 40 50 60 70 80Vol./atom (Å^3)
E/a
tom
(kc
al/m
ol)
DiamondSC
FCC
A15
BCC-5
0
5
10
20 30 40 50
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