Lecture 16-17 February 8-10, 2011 Nature of the Chemical Bond with applications to catalysis, materials science, nanotechnology, surface science, bioinorganic chemistry, and energy Course number: Ch120a Hours: 2-3pm Monday, Wednesday, Friday William A. Goddard, III, wag@wag.caltech.edu 316 Beckman Institute, x3093 Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics, California Institute of Technology Teaching Assistants: Caitlin Scott <cescott@caltech.edu> Hai Xiao xiao@caltech.edu; Fan Liu <fliu@wag.caltech.edu> NiCHx Ch120a-1
CH x /Ni(111) Structures, Energetics, and Reaction Barriers for CHx Bound to the Nickel (111) Surface Mueller, JE; van Duin, ACT and Goddard, WA J. Phys. Chem. C, 113 (47): 20290-20306 (2009) wag 828 2
3 views of periodic N(111) surface A B C A A B C A FCC is ABCABC HCP IS ABABAB 3
H/Ni(111) fcc site 65.7 kcal hcp site 65.4 kcal bridge site 62.6 kcal On-top site 52.7 kcal 4
fcc site 42.7 kcal hcp site 42.3 kcal CH3/Ni(111) bridge site 39.3 kcal On-top site 37.2 kcal 5
fcc site 89.3 kcal hcp site 88.6 kcal CH 2 /Ni(111) bridge site 83.9 kcal On-top site 66.0 kcal 6
fcc site 148.0 kcal hcp site 148.9 kcal CH/Ni(111) bridge site 139.4 kcal On-top site 99.5 kcal 7
fcc site 153.2 kcal hcp site 154.8 kcal C/Ni(111) bridge site 143.1 kcal On-top site 103.6 kcal 8
CH3 0 kcal CH3ad Had + CH2ad H-CH2 TS 18.4 kcal Had-CH2ad 8.2 kcal adj Had-CH2ad 1.3 kcal next 9
CH 2ad 0 kcal CH 2ad H ad + CH ad H-CH TS 8.3 kcal H ad -CH ad -6.5 kcal adj Had-CHad -10.2 kcal next 10
Energy surface for CH 2ad H ad + CH ad 11
CH ad 0 kcal CH ad H ad + C ad H-C TS 32.8 kcal H ad -CH ad 19.3 kcal adj Had-CHad 11.6 kcal next 12
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Stop L16, Feb 8 14
C2Hy species on Ni(111) Competing, Coverage-Dependent Decomposition Pathways for C 2 H y Species on Nickel (111) Mueller, JE; van Duin, ACT; Goddard, WA J. Phys. Chem. C, 114 (47): 20028-20041 (2010) wag 894 15
H3C-CH2/Ni(111) 16
CH3C-CH/Ni(111) 17
H3C-C/Ni(111) 18
H2C=CH2/Ni(111) 19
C=CH 2 /Ni(111) 20
HC=CH2/Ni(111) 21
(HC=CH)/Ni(111) 22
C=CH/Ni(111) 23
C=C/Ni(111) 24
Decomposition pathwas Most stable 25
Bond Distances in (HC=CH)ad and (H2C=CH2)ad TABLE 5: a adsorbate distance experiment QM DFT(PBE) HC=CH C-C 1.44±0.15 Å 1.40 Å HC=CH Cfcc-Nisurface 1.36±0.04 Å 1.41 Å HC=CH Chcp-Nisurface 1.37±0.04 Å 1.41 Å H2C=CH2 C-C 1.60±0.18 Å 1.45 Å H2C=CH2 C-Nisurface 1.90±0.02 Å 2.07 Å 26
Validation of ReaxFF for CH x and CH x Me on Ni(111) Energy of Formation (kcal/mol) 0-5 -10-15 -20-25 -30-35 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) 40 35 30 25 20 15 10 5 0 CH4 --> CH3 + H Barrier heights ReaxFF QM CH3 --> CH2 + H CH3 --> CH2 + H CH --> C + H 27
Energy of Formation (kcal/mol) Energy of Formation (kcal/mol) 90 80 70 60 50 40 30 20 10 0-10 70 60 50 40 30 20 10 0 Validation of ReaxFF for Ni and NiC crystals Ni Crystals QM: Ni Crystal EOS fcc bcc diamond a15 sc 7 12 17 22 Volume per Nickel Atom (cubic angstroms) ReaxFF: Ni Crystal EOS fcc bcc diamond a15 sc 7 12 17 22 Volume per Ni (cubic Angstroms) Energy of Formation (kcal/mol) Energy of Formation (kcal/mol) 350 300 250 200 150 100 50 0 Ni x C Crystals QM: NiC Inverse Density vs Energy NiC: B1 NiC: B3 Ni2C 5 25 45 NiC: B2 NiC: B4 Ni3C Volume per Unit Cell (cubic Angstroms) ReaxFF: NiC Inverse Density vs Energy 350 NiC: B1 NiC: B2 300 250 200 150 100 50 0 NiC: B3 Ni2C 5 25 45 Volume per Unit Cell (cubic Angstroms) NiC: B4 Ni3C 28
Validation of ReaxFF for H, C, CHx binding to Ni(111) H, C & CH x Binding to 4 Layer Ni111 Slab Energy of Formation (kcal/mol) 100 80 60 40 20 0-20 -40 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 ReaxFF QM 29
Validation of ReaxFF for CC bonded species on Ni(111) Energy of Formation (kcal/mol) C 2 H y Binding to 4 Layer Ni111 Slab 50 40 30 20 10 0-10 -20-30 ReaxFF QM CC fcc-hcp CCH fcc-hcp CCH fcctop CCH2 fcc-top CHCH fcc-hcp CHCH2 2f-top CH2CH2 fcc-top CH2CH2 top-top Enegy of Formation Relative to Graphene on Ni111 (kcal/mol) 40 30 20 10 0-10 C f cc C-C Bond Formation CC fcchcp C chain CH chain ReaxFF QM CHCH fcchcp CH 30
Reactions of hydrocarbons on Ni 468 nanoparticle New paper on ReaxFF Jan. 20, 2010 6 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 31
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. 33
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 34
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 7 6 46 37
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. 38
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 + 1000 K, 1500 K, 2000K, 2500 K 0.5 fs time- step 100 fs T-damping 39
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 40
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 ) 41
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 140 120 100 80 60 40 Ni C H Cross section side view 20 0 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 42
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. Hofmann, S. et al. Nano Lett. 2007, 7, 602. 43