Lecture 24 March 02, 2011 Metal Oxide Catalysis Bucky tube overni

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1 Lecture 24 March 02, 2011 Metal xide Catalysis Bucky tube overni Nature of the Chemical Bond with applications to catalysis, materials science, nanotechnology, surface science, bioinorganic chemistry, and energy William A. Goddard, III, 316 Beckman Institute, x3093 Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics, California Institute of Technology Teaching Assistants: Wei-Guang Liu Caitlin Scott Ch120a-Goddard-L24 copyright 2011 William A. Goddard III, all rights reserved 1

2 Last time Ch120a-Goddard-L24 copyright 2011 William A. Goddard III, all rights reserved 2

3 Now want to study Reactive Dynamics CH 3 H + 2 H 2 C= + H 2 over V 2 5 Problem: QM methods not practical to determine mechanism and describe catalysis for mixture of reactants at experimental T and pressure 3

4 Problem QM methods not practical to determine mechanism and describe catalysis for mixture of reactant at experimental T and pressure Solution: ReaxFF reactive force field Describes reaction mechanisms (transition states and barriers) at nearly the accuracy of QM at computation costs nearly as low as ordinary force fields Val Coul E = E + E + E VdW Valence energy Electrostatic energy short distance Pauli Repulsion + long range dispersion (pairwise Morse function) Allow bonds to break and form smoothly, describe barriers for reactions. Allows charges to change continuously as reactions proceed (QEq method) 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 Applied to catalysis for realistic temperatures and pressures 4

5 Critical element ReaxFF: charges flow as geometry changes 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) atomic interactions E J ij { χ iq ηij η kl i ( q ) = + i} J ij ( qi, q j, rij ) i< j E int Erf r k l k l ( r Q Q ) ηij + ηkl ij,, = Q Q ij i j r ij i k 1/r ij i 1 + J 2 i q Charge Equilibration for MD Simulations; Rappé, Goddard J. Phys. Chem. 95, 3358 (1991) 2 i Keeping: q i = Q Hardness (IP-EA) Electronegativity (IP+EA)/2 I i r i 0 +r j 0 Three universal parameters for each element: r ij 1991: use experimental IP, EA, R i ; ReaxFF get from fitting QM χ o i, J, o i R c i 5

6 Bond order nd Essential element of ReaxFF Bond distance bond order forces General analytic form. Get parameters from fitting QM bond breaking for systems with single, double, and triple bonded. All parameters from σ σ σ π π ππ QM Bond order Bond order (uncorrected) Sigma bond Pi bond Double pi bond E bond = D B f ( B ) D Distance (A) Interatomic distance (Å) Interatomic distance (Å) Valence Terms (E Val ) based on Bond rder: dissociates smoothly Forces depend only on geometry (no assigned bond types) Allows angle, torsion, and inversion terms (where needed) Describes resonance (benzene, allyl) Describes forbidden (2 s + 2 s ) and allowed (Diels-Alder) reactions Atomic Valence Term (sum of Bond rders gives valency) e ij Bond energy (kcal/mol) Bond energy ij e B ij D Sigma energy Pi energy e Double pi energy Total bond energy B ππ ij Distance (A) Adri van Duin 6

7 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 in 3, Si 2, H 2 C, Hb 2, BaTi 3 ) (we do not pre-designate the C bond in H 2 C as double and the C bond in H 3 CH as single or in C as triple, ReaxFF figures this out) ReaxFF determines equilibrium bond lengths, angles, and charges solely from the chemical environment. Require that ReaxFF reproduces all the QM data Most theorists (including me) thought that this would be impossible, hence it would never have been funded by NSF, DE, or NIH since it was far too risky. (DARPA came through, then NR, then AR). 7

8 Current applications of ReaxFF Catalysts: Pt-Co Fuel cell cathode, Pt-Ru FC anode Vx catalyzed oxidative dehydrogenation: propane to propene MoVNbTaTe x 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-Hf2, Si-Zr2, Si-Si x N y interfaces) Ferroelectric oxides (BaTi 3 ) domain structure, Pz/Ez Hysteresis Loop of BaTi 3 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 8

9 All ReaxFF parameters are published with applications ~60 papers, also used by ~100 groups around the world - H/C/ - van Duin, Dasgupta, Lorant and Goddard, JPC-A 2001, 105, van Duin and Sinninghe Damste, rg. Geochem.2003, 34, Chen, Lusk, van Duin and Goddard PRB 2005, 72, Han, Kang, Lee, van Duin and Goddard Appl. Phys. Lett. 2005, 86, ) - Chenoweth, van Duin and Goddard, accepted in JPC-A -Si/Si 2 /SiC - van Duin, Strachan, Stewman, Zhang, Xu and Goddard, JPC-A 2003, 107, Chenoweth, Cheung, van Duin, Goddard and Kober, JACS 2005, 127, Buehler, van Duin and Goddard, PRL 2006, 96, Buehler, Tang, van Duin and Goddard, PRL 2007, 90, High energy -Strachan, van Duin, Chakraborty, Dasgupta and Goddard, PRL 2003,91, Strachan, van Duin, Kober and Goddard, JCP 2005,122,054502; - van Duin, Dubnikova, Zeiri, Kosloff and Goddard, JACS 2005, 127, 11053, 58 - Nomora, Kalia, Nakano, Vashista, van Duin and Goddard PRL , Al/Al Zhang, Cagin, van Duin, Goddard, Qi and Hector, PRB 2004,69, Ni/Cu/Co/C - Nielson, van Duin, xgaard, Deng and Goddard, JPC-A 2005, 109, Su, Nielsen, van Duin and Goddard, PRB 75, Pt/PtH/PtC - Ludwig, Vlachos, van Duin and Goddard, JPC-B 2006 Nanotube growth - Sanz-Navarro, Astrand, Chen, Ronning, van Duin, Jacob and Goddard, accepted in JPC-A - Na/Al/Mg/H - Cheung, Deng, van Duin and Goddard, JPC-A 2005, 109, jwang, van Santen, Kramer, van Duin and Goddard, in preparation. - B/N - Han, Kang, Lee, van Duin and Goddard, JCP 2005, 123, Han, Kang, Lee, van Duin and Goddard, JCP 2005, 123, Li/LiC - Han, van Duin and Goddard, JPC-A 2005, 109, Mo/V/Bi//C/H - Goddard et al Topics in Catalysis 2006, 38 (1-3) Chenoweth, van Duin, xgaard, Cheng and Goddard, in preparation. - Cu/Zn//H - Raymand, van Duin, Baudin and Hermannsson, accepted in Surface Science - van Duin et al., in preparation. - Y/Zr/Ba//H - van Duin,, Merinov, Jang. and Goddard, W.A. accepted in JPC-A. - van Duin, Merinov, Han, Dorso, Goddard, W.A. submitted to JPC-A riginal ReaxFF Combustion; Appendix contains current equations Siloxane polymers Crack propagation Parallel ReaxFF Fuel cell anode H-storage HC-oxidation catalysis Fuel cell membrane 9

10 Derive one FF for V to describe all coordinations in the metal and oxide and all oxidation states Metal QM ReaxFF FCC, BCC,HCP,A15, SC, Diamond QM: SeqQuest (SNL Gaussianbased periodic DFT) Metal oxides Heat of formation (kcal/unit) 4, 6, 8 xygen coordination V(bcc) V V 2 V 2 3 V 2 5 QM V 2 V 2 5 V V(bcc) V 2 3 ReaxFF Energy difference for oxidation changes is in good agreement with QM data Indicates that ReaxFF is able to capture energetics of redox reactions 10

11 ReaxFF Development: Bulk oxides Te 2 Heat of formation (kcal/mol) ReaxFF QM Density (kg/dm 3 ) Same ReaxFF describes: Te 0, Te II, Te IV, Te VI, Bi 0, Bi III, Bi V V 0, V III, V IV, V V, Mo 0, Mo II,Mo IV, Mo V, Mo VI, 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 11

12 ReaxFF Development: Propane propene on V 4 10 ReaxFF QM V propane Binding of 2 displaces propene product Cheng, Chenoweth, xgaard, van Duin, Goddard JPC-C 2007, 111, V H propene ReaxFF Kimberly reproduces Chenoweth the MSC, QM Caltech energies for the entire reaction pathway 12

13 ReaxFF Reactive Dynamics Simulation CH 3 H + ½ 2 H 2 C= + H 2 over V layer V 2 5 (001) periodic slab 30 methanol molecules Total number of atoms = 684 Slab Temp = 650K CH 3 H Temp = 2000K Time step = 0.25 fs Temperature damping = 100 fs Total simulation time = 250ps 13

14 ReaxFF RD Simulation Methanol xidative DeHydrogenation (DH)/V H 3 CH Methanol Formaldehyde H2CH Radical Water thers Surface species 5 0 H 2 C= H Time (ps) methanol converts to formaldehyde with production of water thers include the number of hydrocarbons bound to the surface Expt.: Major product is formaldehyde (also H 2, C x ) 14

15 ReaxFF Mechanistic Details Formation of H 2 C-H radical 3.45ps 8.80ps Formation of formaldehyde H-abstraction by surface vanadyl groups 15

16 Desorption of Water from catalyst Snapshots from simulation showing atoms within 5.5Å of V bound to H Water bound to V III, bond very strong, will not desorb 2 nd layer has V V = pointing at V III of top layer 2 nd layer bonds to top V get V IV --V IV H 2 bonds weakly to V IV now desorbs 16

17 Selective (Amm)xidation of Propene Yun Hee Jang June C CH3CHCH 2 (propene) + 2 CH 2CHCH (acrolein) + H2 3 o C CH3CHCH 2 (propene) + NH3 + 2 CH 2CHCN (acrylonitrile) + 3 H2 2 Several generations of catalysts developed refined at SHI in 1950 s to 1980 s (Grasselli, Callahan, Brazdil, Burrington, and coworkers) Mechanistic experimental studies reported by Grasselli, Burrington, and coworkers in 1980 s Basis for developing paradigms in catalysis Site isolation Phase cooperation Early quantum mechanics studies in 1985 (Allison, Goddard) Update using modern methods (Jang and Goddard 2000) First do oxidation then ammoxidation o 17

18 Selective xidation of Propene on Bismuth Molybdates SHI (1959) discovery of Bi-Mo- catalyst SHI (1970) Fe-Sb- SHI (1970) USb 10 SHI Bi 9 PMo = α-bi 2 Mo 3 12 and γ-bi 2 Mo 6 Multiphase, Multicomponent mixed metal oxides SHI (K,Cs) a (Ni,Mg,Mn) 7.5 (Fe,Cr) 2.3 Bi 0.5 Mo 12 x -Si 2 SHI K a Ni b Co c Fe d BiMo 12 x catalyst surface area (m 2 /g) relative activity specific activity selectivity to acrolein (%) Bi 2 (Mo 4 ) 3 (α) 2 ~ Bi 2 Mo 2 9 (β) 1 ~ CoMo 4 10 ~ Mo-Bi-Co- 4 ~ Mo-Bi-Co-Mg- 4 ~ Mo-Bi-Co-Fe- 6 ~ Moro-oka and Ueda, Adv. Catal. 40, 233 (1994) 18

19 Primary function of each component Grasselli, Burrington, and Brazdil, Faraday Diss. 72, 203 (1981) Bi: Propene activated to allyl through α-hydrogen abstraction Mo: chemisorption of Allyl (from Propene) into Mo=/Mo=N; NH 3 activation Fe: Redox couple (Fe 2+ /Fe 3+ ); dioxygen chemisorption; lattice oxygen trasfer (Ni,Co,Mg,Mn) 2+ : provide host structure for Fe 2+ -molybdate propene allyl alcohol catalyst % selectivity % conversion % conversion (M 2+ ) a (M 3+ ) b Bi x Mo y z Bi 2 Mo 2 9 (β) Bi 2 Mo 3 12 (α) Bi 2 Mo 6 (γ) Bi 2 3 No -insertion 100% to dimer Mo No C-H activation 19

20 SHI Simplified Mechanism Burrington, Katriseck, and Grasselli, J. Catal. 81, 489 (1983) ammoxidation oxidation 20

21 Calculation methods (First Principles QM) B3LYP: Density Functional Theory (DFT) including Generalized Gradient Approximation (GGA) LACVP**: Transition Metals described with Core-Valence Effective Core Potential (Mo has explicit electrons) cc-pvtz(-f): Triple zeta basis for C,N,,H ptimize geometry and calculate vibrational frequencies Get zero-point energy, enthalpy, entropy versus temperature ΔG 673K = E + ZPE + ΔΔG 0to673K Compare Bond dissociation energy: calculation vs. experiment in kcal/mol ΔE ΔZPE ΔΔH D 0 (calc) D 0 (exp) D 0 (CH 3 -H) ± 0.1 D 0 (CH 2 CHCH 2 -H) ± 2.1 D 0 (Bi-Bi) ± 2 D 0 (Bi-) ± 3 D 0 (Mo-Mo) ± 5 D 0 (Mo-) ± 5 (1984) QM Good to ~ 2 kcal/mol for (amm)oxidation processes (1998) 21

22 Bi x Mo y z / Mo 3 / Bi 2 3 : Crystal structures Active catalyst may have reconstructed site, perhaps between two phases Crystal suggests likely possibilities, use theory to test chemistry for each plausible site Bi 2 Mo 2 9 (β-phase) α-mo 3 Bi 2 Mo 3 12 (α-phase) Bi 2 Mo 6 (γ-phase) α-bi

23 Model cluster for Mo 3 Big and small models lead to similar energies Thus use small model for most calculations Following slides report ΔΔG at the catalyst temperature Molybdenum site Big model Mo 3 9 ΔE ΔZPE ΔΔ H ΔΔ G ΔH 298 ΔG 298 (1) Mo propene Mo 3 9 _Hon 2 + allyl (2) Mo allyl Mo 3 9 _allyl ~5.2 ~0.3 ~9.5 ~-18 ~-8 Mo allyl Mo 3 9 _allyl ~5.2 ~0.3 ~9.5 ~-16 ~-6 (3) Mo Mo Molybdenum site Small model Mo 2 2 big small (1) Mo propene Mo 2 2 _Hon + allyl (2) Mo allyl Mo 2 2 _allyl Mo allyl Mo 2 2 _allyl (3) Mo Mo

24 Model cluster for Bi 2 3 Choose finite cluster to model active sites of crystal phases Select clusters so that all geometric components are optimized (no Constraints) Bismuth site: Bi 4 6 Each Bi has 3 strong bonds plus lone pair Each has 2 strong bonds plus lone pairs 24

25 H-abstraction from propene: Experiment Experiments show that Propene activation (H-abstraction) occurs on bismuth site and cannot occur on Mo site propene allyl alcohol catalyst % selectivity % conversion % conversion (M 2+ ) a (M 3+ ) b Bi x Mo y z Bi 2 Mo 2 9 (β) Bi 2 Mo 3 12 (α) Bi 2 Mo 6 (γ) Bi % to dimer Mo

26 Molybdenum site? H-abstraction from propene Calculations find that Propene activation (H-abstraction) occurs on bismuth site and not occur on Mo site, agree with experiment Bismuth site? ~ Mo VI ~ propene ΔΔG 673 =29.8 H Mo ~ ~ + allyl high energy cost Bi III (6s 2 6p 0 ) ~ ~ Bi III III Bi ~ ~ propene ΔΔG 673 =41.6 ~ Bi ~ II H III Bi ~ ~ + allyl high energy cost 1/2 2 ΔG 673 =37.0 [ lower if there's other [] source such as Fe(II)/Fe(III) ] Bi V (6s 0 6p 0 ) ~ ~ Bi III V Bi ~ ~ propene ΔΔG 673 =2.5 ~ ~ Bi III H IV Bi ~ ~ + allyl Conclude H-abstraction best on Bi V site while Bi III and Mo VI bad expect Bi V in low concentration under oxidizing conditions (not proved yet by experiment) low energy cost 26

27 xidation of Bi III to Bi V: Energetics from Theory No spectroscopic evidence of Bi V in Bi-Mo- or Bi- catalysts Perhaps they are in very low concentration Maybe because Instantaneously consumed Bi III 1/2 2 or [] ~ ~ Bi ~ ~ ΔG 673 ~ 37.0 ~ ~ allyl, H propene V propene ~ H IV Bi ~ ~ allyl 27

28 xidation of Bi III to Bi V Experimental Evidence? R. K. Grasselli, J. Chem. Educ. 63, 216 (1986) 28

29 xidation of Bi III to Bi V : Experimental Evidence? H. Mizoguchi, et al. J. Mater. Chem. 7, 943 (1997) H. Mizoguchi, et al. Solid State Commun. 104, 705 (1997) Conclusion: It is plausible that Bi V may be present 29

30 xygen Insertion Calculations find Allyl adsorption and xygen insertion occur on molybdenum site; Consistent with Experiment propene catalyst % selectivity % conversion (M 2+ ) a (M 3+ ) b Bi x Mo y z Bi 2 Mo 2 9 (β) Bi 2 Mo 3 12 (α) Bi 2 Mo 6 (γ) Bi % to dimer 1.0 Mo

31 Allyl chemisorption: Calculations Bismuth site? N ~ Bi III ~ III Bi ΔΔG 673 ~ 32 (-41cm -1 ) allyl ~ Mo VI ~ ~ ~ ΔΔG 673 = 5.7 Molybdenum site? YES n pure Bi 2 3 Allyl does not chemisorb only Dimerize to hexadiene observed experimentally ~ ~ ~ Bi II Mo Bi ~ ~ CH 2 CHCH 2 CH 2 CHCH 2 III ~ n Bi-Mo- Allyl chemisorbs preferentially on molybdenum site: consistent with experiment 31

32 H abstraction: Calculations CHCH=CH 2 H - =CH-CH=CH Mo III 2 ΔG 673 =18.6 ΔG 673 = H Mo III Mo VI ΔG 673 = -4.5 H Mo V Mo VI CH 2 =CHCH 2 Mo V ΔG 673 =14.5 CHCH=CH 2 H Mo H - =CH-CH=CH 2 Mo ΔG 673 = Mo VI ΔG 673 = -1.4 H Mo V Mo IV Mo VI H CHCH=CH 2 H ΔG 673 =4.6 Mo V Mo IV - =CH-CH=CH 2 ΔG 673 = -6.4 Mo V Mo IV Conclusion: Need two adjacent molybdenum sites. 32

33 Bi III 1/2 2 or [] ~ ~ Bi ~ ~ ΔG 673 ~ 37.0 ~ ~ allyl, H propene propene H allyl IV Bi ~ ~ ~ 2 allyl V Summary: xidation Propene + 2 Acrolein + H 2 ΔG 673 = kcal/mol CH 2 CHCH 2 CHCHCH 2 H ~ Mo VI ~ ~ Mo VI ~ ~ Mo VI ~ ~ Mo V ~ ~ Mo V ~ ~ Mo IV ~ ΔG 673 = 11.4 CH 2 CHCH 2 ΔG 673 = 9.2 H CHCHCH 2 ~ Mo VI ~ ~ Mo VI ~ ~ Mo VI ~ ~ Mo V ~ ~ Mo V ~ ~ Mo IV ~ H 2 ΔG 673 ~ ΔG 673 = acrolein 1/2 2 or [] H 2 or 2[] H ~ Mo V ~ ~ Mo VI ~ ~ Mo V ~ ~ Mo IV ~ All in agreement with experiment, except for the role of Bi V H Mo V ~ ~ Mo VI ~ ~ ΔG 673 ~ H Mo V ~ ~ ~ Mo IV ~ 33

34 Ammoxidation: Experimental Study of Mechanism (BKG) 34

35 bservation: Kinetics (BKG) Intermediate (pc 3 H 6 = atm) Low feed pressure (pc 3 H 6 = atm) High (pc 3 H 6 = 0.14 atm) linear Quadratic Kinetics depends on the partial feed pressure (pc 3 H 6 ). 35

36 Model: di-oxo/oxo-imido/di-imido (BKG) Low feed pressure (pc 3 H 6 = atm) Intermediate (pc 3 H 6 = atm) High (pc 3 H 6 = 0.14 atm) Equation 8 Equation 9 Kinetics depends on the partial feed pressure (pc 3 H 6 ). Equation 10 36

37 Calculations: Ammonia Activation (easier in more reducing condition) n Mo(VI) site Mo VI NH 3 ΔΔG 673 =8.9 NH 3 Mo NH 2H NH H 2 -H 2 Mo Mo ΔΔG 673 =26.3 ΔΔG 673 = -4.7 ΔΔG 673 = NH Mo VI ΔG 673 = 0.0 ΔG 673 = 8.9 ΔG 673 = 35.2 ΔG 673 = 30.5 ΔG 673 = 12.4 Mo IV NH 3 Mo NH 3 ΔΔG 673 = -2.4 Mo VI ΔΔG 673 =6.9 Mo NH 2 H Mo ΔΔG 673 =12.3 Mo NH H 2 Mo ΔG 673 = 0.0 ΔG 673 = -2.4 ΔG 673 = 4.5 ΔG 673 = 16.8 ΔΔG 673 = 11.7 ΔΔG 673 = H 2 H Mo NH 2 n Mo(IV) site Mo NH Mo IV ΔG 673 = 9.3 ΔG 673 = 12.3 Explains Grasselli experiments at low pc 3 H 6 and high pc 3 H 6 37

38 BKG: Assumptions in derivating of kinetic equations k NI >> k I (N insertion easier than insertion) k I >> k I ( insertion from di-oxo easier than from oxo-imido) 38

39 Calculation: Allyl adsorption Spectator Mo= Mo VI CH 2 =CHCH 2 Mo V ΔΔG 673 = 5.7 NH Mo VI ΔΔG 673 = -4.0 CH 2 CH=CH 2 NH Mo V Spectator Mo=NH NH CH 2 =CHCH 2 NH HN NH HN CH 2 CH=CH 2 NH Mo VI ΔΔG 673 =12.8 Mo V Mo VI ΔΔG 673 = 2.9 Mo V insertion N insertion Spectator effect: Mo= > Mo=NH by 7 kcal/mol Consistent with k I >>k I Chemisorption on Mo=NH is easier than on Mo= by 10 kcal/mol Consistent with the assumption k NI >> k I 39

40 ne-center or Multi-center? /NH Insertion ΔΔG 673 = -4.8 CHCH=CH 2 HN NH 2 Mo di-imido di-oxo CH 2 =CHCH 2 Mo ΔΔG 673 =18.6 ΔΔG 673 =14.5 Mo ΔΔG 673 = 4.6 Mo Mo H CHCH=CH 2 H CHCH=CH 2 Mo H CHCH=CH 2 Multi-center for di-oxo Multi-center for oxo-imido May be one or two center for di-imido All in agreement with experiment Mo CH 2 =CHCH 2 NH NH CH 2 =CHCH 2 NH Mo Mo ΔΔG 673 = 4.3 HN Mo NH ΔΔG 673 = -6.5 ΔΔG 673 ~ 4.4 ΔΔG 673 ~ 14.5 Mo NH ΔΔG 673 = -9.2 HN CHCH=CH 2 HN H Mo NH CHCH=CH 2 HN NH 2 HN NH Mo Mo CHCH=CH 2 Mo Mo H CHCH=CH 2 HN H Mo NH 2 oxo-imido CHCH=CH 2 HN Mo 40

41 Summary from QM calculations From QM calculations, the following points were suggested or confirmed. xidation Bi, probably Bi V, does propene activation. Mo dioxo site does allyl adsorption and -insertion. Further H abstraction needs two Mo dioxo sites. (two-center) Ammoxidation NH 3 activation is easier on Mo IV rather than on Mo VI. N-insertion is easier than -insertion (by 10 kcal/mol). Spectator oxo effect larger than spectator imido effect (by 7kcal/mol) N-insertion via oxo-imido species is the best reaction pathway. 41

42 Comparison with ther Catalysts α-h abstraction insertion Redox couple Example Bi 3+ (5d 10 6s 2 6p 0 ) Mo 6+ (4d 0 5s 0 ) Fe 2+ /Fe 3+ M II am III bbi x Mo y z Te 4+ (4d 10 5s 2 5p 0 ) Mo 6+ (4d 0 5s 0 ) Ce 3+ /Ce 4+ Te a Ce b Mo y z Sb 3+ (4d 10 5s 2 5p 0 ) Sb 5+ (5s 0 5p 0 ) Fe 2+ /Fe 3+ Fe x Sb y z U 5+ (5f 1 6d 0 7s 0 ) Sb 5+ (5s 0 5p 0 ) U 5+ /U 6+ USb 3 10 Se 4+ (3d 10 4s 2 4p 0 ) Te 6+ (5s 0 5p 0 ) Fe 2+ /Fe 3+ Fe a Se b Te c x R. K. Grasselli, App. Catal. 15, 127 (1985); J. Chem. Educ. 63, 216 (1986) Preliminary calculations underway on 2 activation on Fe,Bi,Mo mixed oxides 42

43 Ammoxidation on Multi-Metal xide Catalysts MM catalysts are used industrially for ammoxidation of propene (10 billion pounds per year, 1993) % yield Prefer to start from propane rather than propene ($, abundance) Mo-V-Nb-Te- x current yield from propane, 60%, need 70% M1 and M2 phases of Mo-V-Nb-Te- x believed to both be important for the catalyst. M1 phase is essential, but a symbiosis between the M1 and M2 phases currently produces the best catalytic results. V in M1 phase believed essential to activation of propane. M1 phase has at least four (maybe up to six) crystallographically unique sites with partial Mo/V occupation in the. Probably specific sites important for selective activation while others play role in nonselective reactions. Need to understand atomic details of mechanism to design synthetic procedures to optimize good sites. 43

44 Breakthrough 1995 (Mitsubishi, BP-America). Mixed Metal xide Catalysts (MoVNbTaTex +xxx) for propane ammoxidation CH 3 -CH 2 -CH 3 + NH CH 2 =CH-CN + 4 H 2 Progress: now know that 2 phases (M1 and M2) are involved in MM Catalysis. Have powder xray structures.. But 13 years of R&D still not commercial. No real idea about how atomic level structure affects the chemistry model for critical sites in M1 and M2 phases M To make rapid progress NEED THERY T UNDERSTAND Mechanism M2 M4 M5 Te1 M4 Need to model critical sites in M1 and M2 phases. But must use huge unit cell (>3000 atoms) because of partial occupation M3 Te2 M4 M4 M3 Not practical for QM M5 Te1 44

45 Crystal Structure of M1 Phase of MoVNbTe x Alternating layers of metal oxide and oxygen. Metal oxide layer has 10 types of metal sites forming 5, 6, and 7 membered rings. 7 Also 3 additional metal sites (one inside each type of ring). 45

46 Experimental ccupations in M1 Phase of MoVNbTe x Tot Mo : V 7.3: : :1.5 1 Mo : V 0.46: : :0.2 2 Mo : V 0.63: : :0.2 3 Mo : V 0.46: : :0.5 4 Mo : V 0.88: : :0.0 5 Mo : V 0.80: : :0.0 6 Mo Mo : V 0.62: : :0.5 8 Mo Nb Mo Mo Te Te H. Murayamai et al. Applied Catalysis A 318 (2007) 137. P. Desanto et al. Z. Kristallogr. A 219 (2004) 152. HP. DeSanto et al. Topics in Catalysis 23 (2003)

47 Grasselli Mechanism for Ammoxidation of Propane #1 Paraffin activation β-h Abstraction β-h Abstraction #2 α-h Abstraction propene NH Insertion R.K. Grasselli et al. Topics in Catalysis 23 (2003) 5. 47

48 Grasselli Proposed Active Centers on Surface Assume active centers contains all components required for complete reaction: =V 5+ for paraffin activation =Te 4+ for α-h abstraction Mo 6+ --Mo 6+ -NH for NH insertion R.K. Grasselli et al. Topics in Catalysis 23 (2003) 5. R.K. Grasselli et al. Catalysis Today (2004)

49 Experimental Limitation Computational Solution Critical to resolve the partial occupations to obtain whole atoms at each site in order to elucidate the atomistic chemical mechanism of the MoVNbTe x catalyst This would allow one to identify which sites are important for selectivity and activity This would suggest modifications of procedures for synthesis of catalysts CURRENT EXPERIMENTAL PRCEDURES ARE NT ABLE T RESLVE THE STRUCTURAL UNCERTAINTIES Given the experimental distributions, there are 12,825,612,800 unique configurations M1! To solve this problem we combine the ReaxFF with a Monte Carlo approach in which atoms in equivalent partially occupied lattice sites are swapped until we converge at a low energy structure. 49

50 ReaxFF Development for Bi, Te, V, Nb, Mo oxides Double Bond Dissociation ReaxFF DFT - Singlet DFT - Triplet Te(H) n Te(H) n-1 + H Hydrogen shift in V 4 10 H Bi Bi Bi Bi Bi Bi Bi Bi V H V H V V H H V H H V H V V H V H V V V H H H -Nb- Angle Bending Mo--V Angle Bending Charge Distributions (V 2 H) MSC, Caltech 50

51 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 (periodic DFT Gaussian basis) Metal oxides Heat of formation (kcal/unit) 4, 6, 8 xygen coordination V(bcc) V V 2 V 2 3 V 2 5 QM V(bcc) V V 2 V 2 3 V 2 5 ReaxFF Energy difference for oxidation changes is in good agreement with QM data Indicates that ReaxFF is able to capture energetics of redox reactions 51

52 ReaxFF Development: Bulk oxides Te 2 Heat of formation (kcal/mol) ReaxFF QM Density (kg/dm 3 ) Same ReaxFF describes: Te 0, Te II, Te IV, Te VI, Bi 0, Bi III, Bi V V 0, V III, V IV, V V, Mo 0, Mo II,Mo IV, Mo V, Mo VI, 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 52

53 ReaxFF Validation: Reaction of Propene on Bi 2 3 and Mo 3 Propene + Bi 2 3 Slab Propene + Mo 3 Slab 1100K Get abstraction of allylic hydrogen by bridging oxygen on amorphous Bi 2 3 surface No formation of oxide products Agree with experiment No abstraction of allylic hydrogen by Mo 3. No formation of oxide products 53 Agree with experiment

54 ReaxFF Validation: xidation of Propene on Bi 2 Mo 3 12 (010) H abstracted by Mo= bond of =Mo--Bi unit Had expected Bi= bond to be involved Allyl subsequently is trapped on a different Mo= bond Much Longer times required to observe oxidation of allyl radical to form acrolein Grasselli et al Goddard, van Duin, Chenoweth, Cheng, Pudar, xgaard, Merinov, Jang, Persson Topics in Catal. 38, 2006,

55 Propene conversion on the equilibrated (010) surface of α- Bi 2 Mo 3 12 from ReaxFF Simulations - details Propene reacts with Mo dioxo-bi cluster to form allyl which is trapped by Mo-oxo groups to form chemisorbed propenol. Mo Bi ReaxFF simulations at T=625K nly the atoms within 10 Å of the propene CH3 carbon are displayed 55

56 General problem with Crystal structures of Mixed Metal xide catalysts: partial occupancy M1 phase of the MoVTeNb x catalyst To get mechanism, need whole atoms at each site. Use ReaxFF to find stable whole atom structures for large supercell Trigonal Mo 3 V x catalyst 56

57 Illustrate ReaxFF resolution of partial occupation using Mo 3 V x Propane ammoxidation with 25% (2006) Propane oxidation in 5% selectivity to AA and 5-20% to (2006) Acrolein to acrylic acid in 90% 463K (2007) Alcohol 353K (2008) Primary alcohol yield aldehydes in 90% selectivity (10% selective for olefin) Secondary alcohol yield olefins (4-22% selective for aldehyde) Cyclic alcohols yield ketones in 94% selectivity Published by Ueda et al. 57

58 Mo 3 V x Trigonal Crystal Structure L = heptagonal and S = hexagonal channels M. Sadakane, N. Watanabe, T. Katou, Y. Nodasaka, W. Ueda Angew. Chem., Int. Ed. 2007, 46,

59 Convergence of ReaxFF Monte Carlo partial occupancy resolution Use large enough unit cell to resolve all partial occupations in terms of whole atoms move atoms at same crystallographic site, randomly to alternative equivalent sites, evaluate the energy, use Monte Carlo criteria to determine whether keep new choice or go back, continue until energy converges 59

60 Compare Calculated x-ray powder diffraction intensities from final ReaxFF structure with experimental peaks Crystal-x-ray From Predicted Structure Annealed at 300K Conclusion: predicted structure with resolved partial occupations is consistent with experimental x-ray diffraction intensities Peak at 22 is indicative of the layered structure. 60

61 Final Configuration trigonal Mo 3 V x catalyst from ReaxFF-MC-RD C7 1 metals in highest oxidation state C7 2 and C7 3 containing some reduced metals C6 1 and C6 2 contain some reduced metal sites. C7 2 C6 2 C7 1 C7 3 C6 1 vanadyl oxygens are shown in blue. C6 2 C7 3 C7 1 C6 1 C7 2 61

62 Metal Coordination: location of V V Double bond, can activate propane C6 2 C7 3 C7 2 C7 1 C6 1 62

63 Metal Coordination: location of V IV C6 2 C7 3 C7 2 C7 1 C6 1 63

64 Metal Coordination: location of Mo VI Two M=, get spectator oxo activity C6 2 C7 3 C7 2 C7 1 C6 1 64

65 Metal Coordination: location of Mo V Mo IV... =Mo VI Mo V ----Mo V disproportionation C6 2 C7 3 C7 2 C7 1 C6 1 65

66 Vanadium Coordination M3 site, V= vanadyl groups align along the c-axis, similar to bulk V 2 5 M1 2 site has V= Vanadyl groups pointing into the C7 2 channel 66

67 Donor-acceptor Network a) Network of donoracceptor interactions (Mo VI =---Mo VI ) in the Mo 3 V x catalyst where b) Mo VI (purple) facilitates the continuation of the network while other metal sites disrupt the network. c) Network is more complete in the c- direction compared to the a-b plane due the chains of Mo VI. 67

68 Mo 3 V x (001) Surface Anneal 300K (1x1x4) Final Bulk (1x1x2) Final Slab (1x1x4) Bulk Layers of Slab Top Surface Bottom Surface Mo VI Mo V Mo IV V V V IV Bulk Unit cell: Mo 2.86 V or Mo Mo Mo V V Slab Bulk: Mo 2.86 V or Mo Mo Mo V V x 2- Slab Surface: Mo 2.86 V or Mo Mo Mo V V No significant different between surface and bulk, but have not calcined the system 68

69 ReaxFF Reactive Dynamics 6 layer Slab of Mo 3 V x Catalyst (300K) 12 Propane molecules (2000K) 1 st Reaction@ 42.6ps H abs by V= (V between 6 & 7 channel) Expanded to 2x2x8 69

70 Final Configuration from ReaxFF- RD of Mo 3 Vx with Propane Initial Configuration final Configuration final Configuration Top view Mo = purple V = green = red propane molecules Yellow: in channel blue-gray: exterior 3 propane molecules all go into the C7 2 channel 70

71 Cross-section final configuration from the propane/mo3vx ReaxFF-RD 3 Propane moved into C7 2 Heptagonal Channel Average channel radius = 4.6 Å length ~ 18Å Channel C7 1 is smaller with average radius = 4.1Å and remains empty 71

72 New material 72

73 Speculations about M1 selective oxidation from ReaxFF RD Simulations We believe that the migration of propane into the heptagonal channels found in the RD plays an important role in the selectivity. It has V= chain, just like V 2 5 and VP that can break CH bond (E act ~ 28 kcal/mol) After the activation, a 2 nd H can be transferred to any oxo group or ether group to form propene But this propene is in a protected site inside the channel where the V= chain has already been de-activated so that it can undergo selective activation of allylic CH bond followed by trapping on a M= bond to form M--CH2-CH-CH2 and then it continues the same as for propene selective oxidation in BiMox etc We think that the unselective oxidation to C 2 occurs at the surfaces and grain boundaries, where there may be multiple V= sites, leading to rapid oxidation Thus to obtain increased selectivity want to poison the surface V= sites but not the channel V= chains. This might be done with bulky groups 73

74 Experimental data M2 phase partial occupation at M3, M4, M5, and Te1, Te2 sites Formula: Mo 4.31 V 1.36 Te 1.81 Nb Unit cell ccupation M4 M3 M5 M4 M3 M4 M4 M5 Te2 Te1 All V are V IV, there is no V V DeSanto,.; Buttrey.; Grasselli,.; Lugmair.; Volpe.; Toby.; Vogt, Z. Kristallographie 2004, 219,

75 Initial Structure M2 phase To resolve partial occupations 2x3x4 super cell 25.26Å x 21.88Å x 16.08Å 28*24 = 672 atoms 10 different initial structures [010] [100] Te : 48 M3: Mo=13/V=11 M4: Mo=75/V=21 M5: Mo=24 Positions Num/unitcel ccupation Element l Ratio Mo V(Nb) Total error M1/M2 ~2 Te1/Te / % M3 1 Mo/V 0.54/ % M4 4 Mo/V 0.78/ % M5 1 Mo/Nb % 1.95x10 27 configurations, Use Monte Carlo

76 Initial Structure for M2 phase use super cell 24 times that from experiment: four layers each 2x3 Formula: Mo 4.31 V 1.36 Te 1.81 Nb Top view (2x3) Experimental structure has partial occupation of M3, M4, M5 sites 3.86 x configurations Carry out Monte Carlo optimization to find optimum distribution of V and Mo and Nb over the M3, M4, and M5 sites Use QM based ReaxFF reactive force field to assess the energies Build Initial Model: 2x3x4 supercell Assign V and Mo according to 76

77 Monte Carlo Swap Simulation 400,000 MC-steps Displacement temperature: 500K Displacement Details: step-size: 0.1 Å MC-swap temperature: 25000K Swap energy bias: 10.0 kcal/mol Swap frequency: 5 Analysis Method (001) face [001] [010] [100] Each Vertical Column

78 Distribution of Nb in M5 sites have 6 M5 sites in 2x3 supercell 8 Nb (open) and 16 Mo (filled) z 1 M5 M4 M4 M3 M3 M4 M4 M5 Nb column Mo Nb Nb prefer to stay one 78

79 b in M5 test Formula: Mo4.31 V1.36 Te1.81 Nb

80 Distribution of V,Mo in M3 and M4 sites when M5 site is Mo (not Nb) Types of Vertical Distribution y A Red VVVV B range VVVMo C Green VVMoMo E Blue VMoMoMo F Purple MoMoMoMo x unfavorable 1. V prefers to completely fill a single column in Z direction at both M3 and M4 sites 2. V prefers to congregate at the joint of M3 and M4 position 3. VMoVMo distribution along Z direction is never favorable 80

81 Distribution of V,Mo in M3 and M4 sites when M5 site is Nb (not Mo) Types of Vertical Distribution A Red VVVV B range VVVMo C Green VVMoMo E Blue VMoMoMo F Purple MoMoMoMo y Mo Nb Mo Mo x Mo Mo Nb decreases the fraction of VVVV May decrease activity for propane But may increase selectivity Mo prefers nucleate VVVV May be best for 81

82 Conformation of Te- chain Te2 BVS:4.119 M4 M 3 M4 Te1 BVS:3.372 No Vanadium M4 M5 M4 Te1 Te 2 M 5 M4 M4 M 5 M3 M4 M4 M3 M 3 M5 Tellurium has zigzag conformation in the center of channel 82

83 ost probablete2 position in Channel if 0 earby V top view A1 5 Most probable for no V in M

84 ost probablete2 position if 1 nearby 5 B B A B2 B one V in M5 one V in M4 Most probable A

85 ost probablete2 position if 2 nearby B A C B one V in M5 and in M4 two V in M A Most probable C2

86 M calculation on Te- chain, replacing in-plane e--m bonds with Te-H H 2 TeF 4 is hypervalent with in-plane bonds at ~95 and out-of-plane bonds at ~180. For M2 Phase we find Te--Te--Te- hypervalent chains H H H H Te Te Te Te H H H H H H H H Te Te H H Te Te H H H H

87 xpect that Te= can extract allylic ydrogen H H H H Te Te Te H HN Te C H H H H H H M All Te are Te IV before and after the H abstraction H H H H Te Te H H Te Te H H H H (-35. [-9.4 H C H HN Mo Mo Mo Te= extracts Allylic H to from TeH while allyl trapped on nearby Mo=NH H 2 Continue with previous mechanism for propene on BiMox

88 The final surface contains 71 oxygen atoms(74.0% coverage), 32 on the top and 39 on the bottom (totally 96 oxygen positions) Surface of M2 Build Process Top and Bottom surface oxygen

89 Active site for propene activation Use H atom to test activity of each site of the surface H added to the surface H-allyl bond energy:-88.0kcal/mol Te is the most active site of M2 phase for abstracting Te H

90 + + + Active center Mo Mo Te V Te Mo V V Mo Mo Te Mo M4 M4 M3 M4 M4 M4 M4 M5 M5 vertical oxygen are hidden Active site can contain one V, two V or three V one V IV one Te IV exists in the channel 2V 2Te, 3V 3Te M5 definitely would contain some Nb V

91 Conclusion about M2 phase Vanadium prefers to stay as one column in M3 and M4 sites Nb has effect of segregating the Vanadium columns Te- in the center of channel in zigzag conformation, Te IV Surface coverage of oxygen on M2 surface is 74.0% Te hypervalent chain is the most active site for abstracting allyic H from propene

92 Determine whole atom structures for M1 phase M1 Phase of MoVNbTex Tot Mo : V 7.3: : :1.5 1 Mo : V 0.46: : :0.2 2 Mo : V 0.63: : :0.2 3 Mo : V 0.46: : :0.5 4 Mo : V 0.88: : :0.0 5 Mo : V 0.80: : :0.0 6 Mo Mo : V 0.62: : :0.5 8 Mo Nb Mo Mo Te Te H. Murayamai et al. Applied Catalysis A 318 (2007) 137. P. Desanto et al. Z. Kristallogr. A 219 (2004) 152. HP. DeSanto et al. Topics in Catalysis 23 (2003)

93 Procedure to obtain Structure of M1 Phase Expand to 2x1x4 super-cell Started with 13 random configurations and carried out ReaxFF- Monte Carlo-Reactive Dynamics Choose lowest energy structure and minimize geometry. Compute neighbor distances for each metal atom. Determine typical distances for single and double metal oxygen bonds. Enumerate metal oxygen bonds each metal atom is involved in. Calculate the oxidation state of the metal in usual way (counting each metal oxygen double bond as 2+ and each metal oxygen single bond and 1+). Jonathan Mueller

94 V= ( Å); V- ( Å); Mo= ( Å); Mo- ( Å) adial 1.2 distribution function of ReaxFF structure 1 Mo- 0.8 V oxygen density etermine the effective oxidation state from the Angstroms Use RDF to guide our choice of the distance range for double and single metal oxygen bonds.

95 Final Bulk Configuration of M1 Sites (Mo 12 V 4 ) Mo +6 Mo +6 V +5 Mo +7 Mo +6 Mo +6 V +4 Mo +5 Mo +6 Mo +7 V +5 Mo +6 Mo +6 Mo +6 V +4 Mo +6 V atoms cluster in a single VVVV column, with half +4 and half +5, All Mo atoms are +5 or +6. Suggests that V 5+ and V 4+ have similar energy allowing it to switch oxidation states easily to compensate for oxidation or reduction occurring elsewhere in the catalyst.

96 Final Bulk Configuration of M2 Sites (Mo 6 V 10 ) Mo +5 Mo +4 V +5 Mo +4 Mo +5 Mo +4 V +5 V +5 Mo +4 V +6 V +5 V +5 V +5 V +5 V +4 V +5 each of the four columns has a net oxidation of +19.

97 Final Bulk Configuration of M3 Sites (Mo 18 V 14 ) Mo +6 Mo +6 V +5 V +5 Mo +6 Mo +6 V +5 V +5 Mo +6 V +5 V +5 Mo +6 V +5 Mo +6 V +5 Mo +6 Mo +6 V +5 Mo +6 V +5 Mo +6 V +5 Mo +6 V +5 The highly oxidized states of both Mo and V at M3 sites suggests that M3 sites are likely particularly reactive. Mo +6 Mo +6 V +5 Mo +6 V +5 V +5 Mo +6 Mo +6

98 Final Bulk Configuration of M7 Sites (Mo 18 V 14 ) V +5 Mo +6 V +5 Mo +6 V +4 V +5 Mo +6 V +6 Mo +6 Mo +6 Mo +5 Mo +6 Mo +6 Mo +6 Mo +6 Mo +6 Mo +6 V +5 Mo +6 Mo +5 Each nearby pair of M7 sites has a net oxidation of 47+ or 48+, except for the pair in the center of the unit cell, which surround the only M2 column with only V, and has a net oxidation in the M7 sites of only 40+. V +6 V +4 Mo +6 V +5 V +5 V +5 V +6 V +5 V +6 Mo +6 Mo +6 Mo +6

99 Resolution of Partial ccupations for M1 phase ReaxFF MC (2 x 1 x 4 unit cell) 13 separate calculations to get statistical significance Best energy Final structure The experimental structure has four sites with partial populations in Mo and V (M1, M2, M3 and M7). Expanding to a super-cell with 4 unit cells per super-cell allows us to resolve these partial occupations within 2%

100 Super-cell Resolution of Partial ccupations The experimental structure has four sites with partial populations in Mo and V (M1, M2, M3 and M7). Expanding to a super-cell with 4 unit cells per super-cell allows us to resolve these partial occupations within 2% as follows: M1: V 0.25 /Mo 0.74 V 2 Mo 6 = V 0.25 Mo 0.75 M2: V 0.62 /Mo 0.38 V 5 Mo 3 = V /Mo M3: V 0.42 /Mo 0.58 V 7 Mo 9 = V Mo M7: V 0.42 /Mo 0.58 V 7 Mo 9 = V Mo

101 Current Monte Carlo Swap Procedure in ReaxFF MC Accept/Reject Criteria: If E new < E old Accept therwise choose random number (0-1): N; If N < exp[(e old -E new )/kt] Accept ReaxFF MC swap/min works as follows. 1. Minimize geometry and calculate energy of initial structure 2. Randomly select a swaplist 3. Randomly swap a pair of dissimilar atoms from the swaplist. 4. Minimize new geometry and calculate final energy. (start minimization from final/relaxed coordinates of previous minimization) 5. Accept or Reject new structure based on MC criteria. (always comparing energies for relaxed structures and start) 6. Return to

102 Strategy of Current Calculations In order to get an understanding the configurations of the partial occupations in the M1 crystal structure, we decouple degrees of freedom corresponding to important structural motifs. Assume coupling between partial occupation sites increases with decreasing distance (# of M- bonds) between the sites. This results in isolated clusters of strongly related sites. Consider structural motifs for these clusters of strongly interacting sites consistent with experimental partial occupations. Use the MC swap/min procedure to optimize all other partial occupation sites around each of these motifs, to determine their relative energies This procedure helps resolve the structure and to provide insights into structural patterns of the partial occupation sites

103 M1 Sites two M1 sites per unit cell. In the same plane the nearest site with partial occupation is an M3 site 4 bonds away. The nearest M2 and M7 sites are 6 bonds away, while the next M1 site in the same plane is 10 bonds away. However, in the unit cells directly above and below are M1 sites which are only 2 bonds away. Thus, we expect partial resolution of partially occupied M1 sites to show strongest correlation with M1 sites in the z direction directly above and below it. The simplest structural motifs for columns of M1 sites are columns of only V and columns of only Mo, or columns of alternating Mo and V atoms. In a 2x2x2 unit cell with 4 V atoms and 12 Mo atoms at M1 sites, this gives either: V Mo 2 & 6 R V Mo 4 V Mo & 4 Mo Mo 103

104 Status of MC swap/minimi zation of site types 2, 3 & 7, with type 1 sites fixed in either homogenous or alternating columns of V/Mo. After 15,000 MC iterations: homogeneous (separate -V--V-- and Mo-- Mo--) columns have a lower energy than alternating (-V--Mo--V--Mo- -) columns

105 MC swap/minimization of site types 2, 3 & 7, with type 1 sites fixed in either homogenous or alternating columns of V/Mo after 15,000 MC iterations

106 M3 Sites The M3 sites (stars) can be treated just like the M1 sites, since the nearest partial occupation sites in the same plane are 4 bonds away: M7 (triangles) and M1 (circles) sites. We do the same calculations as we did for the M1 sites, comparing homogenous and alternating column configurations

107 M2 Sites There are two M2 sites per unit cell (square). These are not only two bonds away from M2 sites in the cells above and below, but also to M7 sites on either side in the same layer. The M2 site is believed to be at the center of the active sites on M1 catalyst. While we don t expect the resolution of the M2 sites to be decoupled from the resolution of the M7 sites, we can nevertheless still consider homogeneous and alternating columns of M2 sites, using the MC swap/min procedure to resolve the M7 (triangles), M1 (circles) & M3 (stars) sites

108 Validation of ReaxFF for CH x and CH x Me on Ni(111) Energy of Formation (kcal/mol) Binding energies CH x Me y Binding to 4 Layer Ni111 Slab CH2Me f cc CH2Me top CMe2 2f CMe f cc ReaxFF QM Reaction Barrier (kcal/mol) Dehydrogenation Barriers on Ni(111) CH4 --> CH3 + H Barrier heights ReaxFF QM CH3 --> CH2 + H CH3 --> CH2 + H CH --> C + H 108

109 Energy of Formation (kcal/mol) Energy of Formation (kcal/mol) Validation of ReaxFF for Ni and NiC crystals Ni Crystals QM: Ni Crystal ES fcc bcc diamond a15 sc Volume per Nickel Atom (cubic angstroms) ReaxFF: Ni Crystal ES fcc bcc diamond a15 sc Volume per Ni (cubic Angstroms) Energy of Formation (kcal/mol) Energy of Formation (kcal/mol) Ni x C Crystals QM: NiC Inverse Density vs Energy NiC: B1 NiC: B3 Ni2C NiC: B2 NiC: B4 Ni3C Volume per Unit Cell (cubic Angstroms) ReaxFF: NiC Inverse Density vs Energy 350 NiC: B1 NiC: B NiC: B3 Ni2C Volume per Unit Cell (cubic Angstroms) NiC: B4 Ni3C 109

110 Validation of ReaxFF for H, C, CHx binding to Ni(111) H, C & CHx Binding to 4 Layer Ni111 Slab ReaxFF QM H fcc H hcp H 2f H top C hcp C fcc C 2f C top CH hcp CH fcc CH 2f CH top CH2 fcc CH2 hcp CH2 2f CH2 top CH3 fcc CH3 hcp CH3 2f CH3 top Energy of Formation (kcal/mol) 110

111 Validation of ReaxFF for CC bonded species on Ni(111) C 2 H y Binding to 4 Layer Ni111 Slab ReaxFF QM C-C Bond Formation C f cc CC fcchcp C chain CH chain ReaxFF QM CHCH fcchcp CH 111 CC fcc-hcp CCH fcc-hcp CCH fcctop CCH2 fcc-top CHCH fcc-hcp CHCH2 2f-top CH2CH2 fcc-top CH2CH2 top-top Energy of Formation (kcal/mol) Enegy of Formation Relative to Graphene on Ni111 (kcal/mol)

112 Reactions of hydrocarbons on Ni 468 nanoparticle New paper on ReaxFF Jan. 20, cases: 120 methane, 60 ethene, 60 ethyne, 40 propene, 20 benzene, 20 Cylclohexane Initial and Final structures for ReaxFF RD simulation of 40 propene molecules adsorbing and decomposing on a Ni 468 cluster Ni 468 particle, 21A diameter 112

113 ReaxFF: Acetylene Adsorption & Decomposition on Ni 468 nanoparticle Start: 60 C 2 H 2 end: 52 C ad + 2 C2H3 gas + 2 C 2 H 2 ad + C 2 Had+C 2 ad Conclusions 1. Both C-H bonds break before the C-C bond breaks 2. Formation of subsurface C helps break C-C bonds. 113

114 Ethyne detail Reaction of C with 2 nd layer Ni very important Build up surface Ni x C x in first few rows Dynamics of surface Ni plays important role in dissociating C2 Get some carbon into interior 114

115 ReaxFF: Benzene Adsorption & Decomposition on Ni Particle H 2 C 6 H x C 2 C 6 H 6ad Simplified sequence C 6 H 6 C 6 H 5 C 6 H 4 C 6 H 3 C 5 H 3 C 5 H 2 C 4 H 2 C 4 H C 3 H C 3 C 2 C At the end

116 C 6 H 6 chemisorbed C 6 H 3 -allyl tail in surface Benzene detail C 6 H 3 -allyl chemisorbed C 3 H with bare C in subsurface Benzene chemisorbs horizontally on the Ni particle surface through pi electrons. As H removed, get strong C-Ni sigma bonds, reorienting benzene vertically. C atoms denuded of H are swallowed by the particle by Pac- Man mechanism, for cleaving C-C bonds. C-H bonds far from the surface are protected until the C atoms separating them from the surface are eaten away. 116

117 Early Stages of CNT Growth from Acetylene Feedstock at 1500K on Ni 468 nanoparticle (21A) 2 nanosec NVT-RD Start with 100 gas phase C 2 H 2 molecules, add an additional 50 molecules every 200 ps. At end 350 C 2 H 2 NVT-RD 1 nanosecond K, 1500 K, 2000K, 2500 K 0.5 fs time- step 100 fs T-damping 117

118 CNT Nucleation Study (1ns at 2500K) 1 ns of ReaxFF reactive dynamics at 2500 K on a Ni 468 nanoparticle saturated with C from exposure to 350 acetylene molecules during previous 1 ns of RD. At the end of simulation have large carbon ring structure with 367 carbon atoms, and 78 hydrogen atoms 118

119 CNT Nucleation Study (after 2ns ReaxFF RD) At the end of the simulation we are left with a large carbon ring structure (C 367 H 78 ) 119

120 Subsurface Analysis of Acetylene Feedstock Decomposition on Ni nanoparticle 300 acetylene molecules after 2 ns RD at 2500K radial atom distribution after 2ns RD at 2500K # of atoms Ni C H Cross section side view radial distance (Angstroms) C atoms penetrate 10.5A to the core of the catalyst particle forming nickel carbide H penetrates only part way in, preferring the surface. Cross section head on 120

121 Experimental Confirmation of a Yarmulke Mechanism Atomic-scale, video-rate environmental transmission microscopy was used to monitor the nucleation and growth of single walled nanotubes. 121 Hofmann, S. et al. Nano Lett. 2007, 7, 602.