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Understanding the Nature of Chemical Bonding in Catalysis and Materials Science

Explore the fundamental principles of chemical bonding with applications in catalysis, materials science, nanotechnology, and energy. This lecture dives into the roles of surface science, bioinorganic chemistry, and more.

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Understanding the Nature of Chemical Bonding in Catalysis and Materials Science

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  1. Lecture 25, December 2, 2009 Nature of the Chemical Bond with applications to catalysis, materials science, nanotechnology, surface science, bioinorganic chemistry, and energy Course number: KAIST EEWS 80.502 Room E11-101 Hours: 0900-1030 Tuesday and Thursday William A. Goddard, III, wag@kaist.ac.kr WCU Professor at EEWS-KAIST and Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics, California Institute of Technology Senior Assistant: Dr. Hyungjun Kim: linus16@kaist.ac.kr Manager of Center for Materials Simulation and Design (CMSD) Teaching Assistant: Ms. Ga In Lee: leeandgain@kaist.ac.kr Special assistant: Tod Pascal:tpascal@wag.caltech.edu EEWS-90.502-Goddard-L15

  2. Schedule changes Dec. 2, Wednesday, 3pm, L25, additional lecture, room 101 Dec. 3, Thursday, 9am, L26, as scheduled Dec. 7-10 wag meeting Pasadena; no lectures, Dec. 14, Monday, 2pm, L27, additional lecture, room 101 Dec. 15, Final exam 9am-noon, room 101 EEWS-90.502-Goddard-L15

  3. Last time EEWS-90.502-Goddard-L15

  4. Summary: Oxidation Propene + O2 Acrolein + H2O DG673 = -74.0 kcal/mol All in agreement with experiment, except for the role of BiV

  5. Calculation: Allyl adsorption Spectator Mo=O Spectator Mo=NH O insertion N insertion • Spectator effect: Mo=O > Mo=NH • by 7 kcal/mol • Consistent with “kOI>>k´OI” • Chemisorption on Mo=NH is easier than on Mo=O • by 10 kcal/mol • Consistent with the assumption “kNI >> kOI ”

  6. One-center or Multi-center?O/NH Insertion di-oxo di-imido oxo-imido Multi-center for di-oxo Multi-center for oxo-imido May be one or two center for di-imido All in agreement with experiment

  7. Vanadium Coordination M12 site has V=O Vanadyl groups pointing into the C72 channel M3 site, V=O vanadyl groups align along the c-axis, similar to bulk V2O5

  8. Final Configuration from ReaxFF-RD of Mo3VOx with Propane final Configuration Top view Initial Configuration final Configuration Mo = purple V = green O = red 3 propane molecules all go into the C72 channel propane molecules Yellow: in channel blue-gray: exterior

  9. Cross-section final configuration from the propane/Mo3VOx ReaxFF-RD 3 Propane moved into C72 Heptagonal Channel Average channel radius = 4.6 Å length ~ 18Å Channel C71 is smaller with average radius = 4.1Å and remains empty

  10. 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=O chain, just like V2O5 and VOPO that can break CH bond (Eact ~ 28 kcal/mol) After the activation, a 2nd 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=O chain has already been de-activated so that it can undergo selective activation of allylic CH bond followed by trapping on a M=O bond to form M-O-CH2-CH-CH2 and then it continues the same as for propene selective oxidation in BiMoOx etc We think that the unselective oxidation to CO2 occurs at the surfaces and grain boundaries, where there may be multiple V=O sites, leading to rapid oxidation Thus to obtain increased selectivity want to poison the surface V=O sites but not the channel V=O chains. This might be done with bulky groups

  11. Hemoglobin Blood has 5 billion erythrocytes/ml Each erythrocyte contains 280 million hemoglobin (Hb) molecules Each Hb has MW=64500 Dalton (diameter ~ 60A) Four subunits (a1, a2, b1, b2) each with one heme subunit Each subunit resembles myoglobin (Mb) which has one heme Hb Mb EEWS-90.502-Goddard-L15

  12. The action is at the heme or Fe-Porphyrin molecule Essentially all action occurs at the heme, which is basically an Fe-Porphyrin molecule The rest of the Mb serves mainly to provide a hydrophobic envirornment at the Fe and to protect the heme EEWS-90.502-Goddard-L15

  13. The heme group The net charge of the Fe-heme is zero. The VB structure shown is one of several, all of which lead to two neutral N and two negative N. Thus we consider that the Fe is Fe2+ with a d6 configuration Each N has a doubly occupied sp2s orbital pointing at it. EEWS-90.502-Goddard-L15

  14. Axial ligands to heme group EEWS-90.502-Goddard-L15

  15. Energies of the 5 Fe2+ d orbitals x2-y2 z2=2z2-x2-y2 yz xz xy EEWS-90.502-Goddard-L15

  16. Exchange stabilizations EEWS-90.502-Goddard-L15

  17. Summary 4 coord and 5 coord states EEWS-90.502-Goddard-L15

  18. Free atom to 4 coord to 5 coord EEWS-90.502-Goddard-L15

  19. Bond O2 to Mb EEWS-90.502-Goddard-L15

  20. Bonding O2 to Mb EEWS-90.502-Goddard-L15

  21. Role of exchange energy EEWS-90.502-Goddard-L15

  22. compare bonding of CO and O2 to Mb EEWS-90.502-Goddard-L15

  23. New EEWS-90.502-Goddard-L15

  24. Bonding in metallic solids Mosty of the systems discussed so far in this course have been covalent, with the number of bonds related to the number of valence electrons. Thus we have discussed the bonding of molecules such as CH4, benzene, O2, and Ozone.. The solids such as diamond, silicon, GaAs, are generally insulators or semiconductors We have also considered covalent bonds to metals such as FeH+, (PH3)2Pt(CH3)2, (bpym)Pt(Cl)(CH3), The Grubbs Ru catalysts We have also discussed the bonding in ionic materials such as (NaCl)n, NaCl crystal, and BaTiO3, where the atoms are best modeled as ions with the bonding dominated by electrostatics Next we consider the bonding in bulk metals, such as iron, Pt, Li, etc. where there is little connection between the number of bonds and the number of valence electrons. EEWS-90.502-Goddard-L15

  25. Elementary ideas about metals and insulators The first attempts to develop quantum theory started with the Bohr model H atom with electrons in orbits around the nucleus. With Schrodinger QM came the idea that the electrons were in distinct orbitals (s, p, d..), leading to a universal Aufbau diagram which is filled with 2 electrons in each of the lowest orbitals For example: O (1s)2(2s)2(2p)4 EEWS-90.502-Goddard-L15

  26. Bringing atoms together to form the solid As we bring atoms together to form the solid, the levels broaden into energy bands, which may overlap . Thus for Cu we obtain Energy Fermi energy (HOMO and LUMO Thus we can obtain systems with no band gap. Density states EEWS-90.502-Goddard-L15

  27. Metals vs inulators EEWS-90.502-Goddard-L15

  28. conductivity EEWS-90.502-Goddard-L15

  29. The elements leading to metallic binding There is not yet a conceptual description for metals of a quality comparable to that for non-metals. However there are some trends, as will be described EEWS-90.502-Goddard-L15

  30. Body centered cubic (bcc), A2 A2 EEWS-90.502-Goddard-L15

  31. Face-centered cubic (fcc), A1 EEWS-90.502-Goddard-L15

  32. Alternative view of fcc EEWS-90.502-Goddard-L15

  33. Closest packing layer EEWS-90.502-Goddard-L15

  34. Stacking of 2 closest packed layers EEWS-90.502-Goddard-L15

  35. Hexagaonal closest packed (hcp) structure, A3 EEWS-90.502-Goddard-L15

  36. Cubic closest packing EEWS-90.502-Goddard-L15

  37. Double hcp The hexagonal lanthanides mostly exhibit a packing of closest packed layers in the sequence ABAC ABAC ABAC This is called the double hcp structure EEWS-90.502-Goddard-L15

  38. Structures of elemental metals EEWS-90.502-Goddard-L15

  39. Binding in metals Li has the bcc structure with 8 nearest neighbor atoms, but there is only one valence electron per atom. Similarly fcc and hcp have 12 nearest neighbor atoms, but Al has only three valence electrons per atom. Clearly the bonding is very different than covalent One model (Pauling) resonating valence bonds Problem is energetics: Li2 bond energy = 24 kcal/mol 12 kcal/mol per valence electron Cohesive energy of Li (energy to atomize the crystal is 37.7 kcal/mol per valence electron. Too much to explain with resonance New paradigm: Interstitial electron model (IEM). Each valence electron localizes in a tetrahedron between four Li nuclei. Bonding like in Li2+, which is 33.7 kcal/mol per valence electron EEWS-90.502-Goddard-L15

  40. GVB orbitals of ring M10 molecules Get 10 valence electrons each localized in a bond midpoint Calculations treated all 11 valence electrons of Cu, Ag, Au using effective core potential. All electrons for H and Li R=2 a0 EEWS-90.502-Goddard-L15

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  43. Geometries of Li clusters EEWS-90.502-Goddard-L15

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