Theoretical Studies of Heterogeneous Catalysis of Molybdates

• Main Author: Allison, Janet Noel
• Format: Others
• Language: en
• Published: 1985
• Online Access: https://thesis.library.caltech.edu/5305/1/Allison_jn_1985.pdf; Allison, Janet Noel (1985) Theoretical Studies of Heterogeneous Catalysis of Molybdates. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/wpmx-ff05. https://resolver.caltech.edu/CaltechTHESIS:10192009-083321412 <https://resolver.caltech.edu/CaltechTHESIS:10192009-083321412>

<p>Chapter 1: We use thermochemical results from <i>ab initio</i> quantum chemical calculations (generalized valence bond) to examine the reaction mechanism for</p> <p>H₃COH + ½ O₂ → H₂CO + H₂O</p> <p>as catalyzed by MoO₃. We find that surface dioxo sites</p> <p>[Chemical structure; see abstract in scanned thesis for details.]</p> <p>are critical to activating the methanol, but we find that the Trifiro proposal of a single site-single step process</p> <p>[Chemical structure; see abstract in scanned thesis for details.]</p> <p>is not favorable (∆H ≃ +31.5 kcal). Our conclusion is that the catalytic site involves two adjacent surface <i>dioxo units</i> (the <i>dual dioxo site</i>), 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.</p> <p>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 <i>ab intitio</i> 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.</p> <p>Chapter 3: Extensive <i>ab initio</i> 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.</p> <p>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 4dδ_xy and Mo 4dδ_x²-y² orbitals. The first excited state (⁴Π) corresponds to the 5p_π ← 5s excitation. The calculated properties of R_e = 1.60 Å, ω_e = 1100 cm^-1, D_e = 4.07 eV, and ∆E (⁴Π-⁴∑^- = 2.128 eV are in good agreement with recent experimental results (R_e = 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 R_e = 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.</p>