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Theoretical Studies of Heterogeneous Catalysis of Molybdates

Citation

Allison, Janet Noel (1985) Theoretical Studies of Heterogeneous Catalysis of Molybdates. Dissertation (Ph.D.), California Institute of Technology. https://resolver.caltech.edu/CaltechTHESIS:10192009-083321412

Abstract

Chapter 1: We use thermochemical results from ab initio quantum chemical calculations (generalized valence bond) to examine the reaction mechanism for

H3COH + ½ O2 → H2CO + H2O

as catalyzed by MoO3. We find that surface dioxo sites

[Chemical structure; see abstract in scanned thesis for details.]

are critical to activating the methanol, but we find that the Trifiro proposal of a single site-single step process

[Chemical structure; see abstract in scanned thesis for details.]

is not favorable (∆H ≃ +31.5 kcal). Our conclusion is that the catalytic site involves two adjacent surface dioxo units (the dual dioxo site), with each dioxo site extracting one H in a sequence of steps. The required dual dioxo site exists on the (010) surface of MoO3 but does not exist on the other low index surfaces. This mechanism is supported by atmospheric pressure experimental studies which indicate that MoO3 (010) is selective for CH2O products. A detailed sequence of reaction steps and the associated thermochemistry is proposed.

Chapter 2: Molybdates involving various metal additives play a dominant role in such industrially important catalytic processes as selective oxidation (propene to acrolein) and ammoxidation (propene to acrylonitrile); however, the details of the reaction mechanism and of the surface sites responsible are yet quite uncertain. In order to establish the thermo-chemistry and detailed mechanistic steps involved with such reactions, we have performed ab intitio quantum chemical calculations [generalized valence bond (GVB) and configuration interaction (CI)]. These studies indicate a special importance of multiple surface dioxo Mo sites (possessing two Mo-O double bonds and hence spectator oxo groups) arranged together so as to provide the means for promoting the sequence of transformations.

Chapter 3: Extensive ab initio calculations have been carried out on molybdenum (VI, V and IV) complexes containing oxygen and nitrogen. A detailed description of the bonding of oxo, nitrido and imido terminally attached ligands to molybdenum is presented. These results are used to explain the preferred geometries of complexes as well as the periodic trends as ligands to molybdenum are varied from O, N, NH.

Chapter 4: We find that the ground state of MoN (4-) has a covalent triple bond where the σ bond is dz2-like on the Mo, leading to a quartet state with unpaired electrons in the Mo 5s, Mo 4dδxy and Mo 4dδx2-y2 orbitals. The first excited state (4Π) corresponds to the 5pπ ← 5s excitation. The calculated properties of Re = 1.60 Å, ωe = 1100 cm-1, De = 4.07 eV, and ∆E (4Π-4- = 2.128 eV are in good agreement with recent experimental results (Re = 1.63 Å and ∆E = 2.011 eV). Particularly interesting is a dramatic nonmonotonic change of dipole moment with distance (µ = -3.123 D at Re = 1.60 Å, -5.982 D at R = 2.60 Å and µ = -0.176 D at R = 5.0 Å. This effect is explained.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Chemistry
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Gray, Harry B.
Thesis Committee:
  • Goddard, William A., III (chair)
  • Beauchamp, Jesse L.
  • Baldeschwieler, John D.
  • Sparks, Randal K.
  • Gray, Harry B.
Defense Date:28 November 1984
Funders:
Funding AgencyGrant Number
Fannie and John Hertz FoundationUNSPECIFIED
Record Number:CaltechTHESIS:10192009-083321412
Persistent URL:https://resolver.caltech.edu/CaltechTHESIS:10192009-083321412
Related URLs:
URLURL TypeDescription
https://doi.org/10.1016/0021-9517(85)90242-8DOIArticle adapted for Chapter 1.
https://doi.org/10.1021/bk-1985-0279.ch002DOIArticle adapted for Chapter 2.
https://doi.org/10.1016/0301-0104(83)85320-8DOIArticle adapted for Chapter 4.
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:5305
Collection:CaltechTHESIS
Deposited By: Tony Diaz
Deposited On:03 Nov 2009 23:27
Last Modified:02 Dec 2020 01:32

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