Geometric and Electronic Structure Sensitivity of Methyl and Methylene Reactions on α-Cr₂O₃ and α-Fe₂O₃ surfaces

Structural and electronic effects in hydrocarbon reactions over metal oxides have been examined by comparing the reactions of methyl (-CH₃) and methylene (=CH₂) fragments on three different oxide single crystal surfaces: α-Cr₂O₃(101̅2), α-Cr₂O₃(0001), and α-Fe₂O₃(101̅2). The intermediates have been generated through the decomposition of halogenated hydrocarbons. The primary reactions of methyl and methylene over α-Cr₂O₃ are methyl dehydrogenation to methylene, and methylene coupling (C-C bond formation) to ethylene (CH₂=CH₂). The different surface geometric structures of α-Cr₂O₃(101̅2) and (0001) lead to an increase in the activation barrier for methylene surface migration, a critical step in the coupling reaction, of 5.9 kcal/mol over the (0001) surface. For methyl dehydrogenation, differences in the local site pair (cation/anion) geometry and the proximity of surface lattice oxygen to the methyl group do not result in a significant difference in the barrier for dehydrogenation, suggesting that the surface anions play a minor role in the dehydrogenation of methyl on these surfaces. Electronic differences in the Fe³⁺ (𝑑⁵) and Cr³⁺ (𝑑³) cations on structurally-similar α-Cr₂O₃(101̅2) and α-Fe₂O₃(101̅2) surfaces lead to major differences in reaction selectivity. α-Cr₂O₃(101̅2) is nonreducible under the reaction conditions of this study, but α-Fe₂O₃(101̅2) is highly reducible due to the difference in the d electron configuration. Hydrocarbons are formed over α-Cr₂O₃(101̅2), but nonselective oxidation products (CO₂, CO, H₂O) are formed over the stoichiometric α-Fe₂O₃(101̅2) surface along with surface reduction. Reduction of the α-Fe₂O₃(101̅2) leads to a shift in the product selectivity towards formaldehyde (CH₂O) and ethylene. For the limited number of systems examined in this study, examples of geometric structure sensitive (methylene coupling) and structure insensitive (methyl dehydrogenation) reactions have been found on α-Cr₂O₃, and electronic effects are observed for the reactions on α-Cr₂O₃(101̅2) and α-Fe₂O₃. For the structure sensitive reaction, the differences in surface geometry impact the reactions kinetics over Cr₂O₃ but not the types of products formed, while the electronic differences give rise to dramatic changes in the selectivity associated with the very different products formed over α-Cr₂O₃(101̅2) and α-Fe₂O₃(101̅2).

Name thesis:degree_name

Ph. D.

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Chemical Engineering

Department dc:contributor.department

Chemical Engineering

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

2012

Author dc:creator

• Dong, Yujung

Chair dc:contributor.committeechair

• Cox, David F.

Committee members dc:contributor.committeemember

• Oyama, Shigeo Ted
• Davis, Richey M.
• Morris, John R.

• iron oxide
• metal oxide surface
• methyl
• methylene
• chromium oxide

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Dc Identifier Other

etd-09242012-120905

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/39469