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ReaxFF for Vanadium and Bismuth Oxides
Kim Chenoweth Force Field Sub-Group Meeting January 20, 2004
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Overview Significance of a Bi/V force field ReaxFF: general principles
Force field optimization for V Force field optimization for Bi Future work
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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 O 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
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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 metals 1Argyle et al, J. Catal. 2002, 208, 139
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ReaxFF Bridging the gap between QC and EFF
Time Distance Ångstrom Kilometers 10-15 years QC ab initio, DFT, HF Electrons Bond formation MD Empirical force fields Atoms Molecular 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
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ReaxFF: Energy of the System
2-body multi-body 3-body 4-body Similar to empirical non-reactive force fields Divides the system energy into various partial energy contributions
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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
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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
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Strategy for Parameterization of ReaxFF
Identify important interactions to be optimized for relevant systems Build QC-training set for bond dissociation and angle bending cases for small clusters Build QC-training set for condensed phases to obtain equation of state Force field optimization using Metal training set Metal oxide clusters and condensed phases Applications
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1st row transition metal (4s23d3)
Vanadium Training Set 1st row transition metal (4s23d3) Cluster Bonds -Normal, under-, and over-coordinated systems Angles O-V=O, V-O-V, O=V=O Condensed Phase Metal BCC, A15, FCC, SC, Diamond Metal Oxide VO (II) FCC V2O3 (III) Corundum VO2 (IV) Distorted rutile V2O5 (V) Layered octahedral Successive bond dissociation of oxygen in V4O10
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Bulk Metal - Vanadium QC ReaxFF
ReaxFF reproduces EOS and properly predicts instability of low-coordination phases (SC, Diamond)
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Bond Dissociation in VO2OH
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V=O Bond Dissociation in V4O10
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Angle Distortion in V2O5 V-O-V Angle O-V=O Angle
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Angle Distortion in VO2 O=V=O Angle
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Angle Distortion in V2O6 V-O-O Angle
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Charge Analysis for VxOy Clusters in Training Set
1 2 3 4 1 2 3 1 2 3 5 7 6 4
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Charge Analysis for VxOY Clusters in Literature (QC data taken from Calatayud et al, J. Phys. Chem. A 2001, 105, 9760.)
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Common oxidation states: 3, 5
Bismuth Training Set Common oxidation states: 3, 5 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 a-Bi2O3 (III) Monoclinic b-Bi2O3 (III) Distorted cubic Bi2O4 (BiIIIBiVO4) BiO2 (IV) Cubic
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Bulk Metal - Bismuth QC ReaxFF
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Relative Stabilities of V and Bi Bulk Phases
Bismuth Vanadium
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Application: Melting Point of Vanadium
55 molecules 900 K 1700 K 2500 K 1700 K 900 K 1900 K Melting point of Vanadium = 2163 K Melting point obtained from simulation ~ 1900 K
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Application: Melting Point of Vanadium
147 molecules 900 K 1700 K 2500 K 1700 K 900 K 2000 K Melting point of Vanadium = 2163 K Melting point obtained from simulation ~ 2000 K
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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
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