HRTEM characterization of phase changes and the occurrence of maghemite during catalysis by an iron oxide

• Published in: Applied catalysis. A, General Vol. 258; no. 1; pp. 7 - 16
• Main Authors: Miser, Donald E., Shin, Eun-Jae, Hajaligol, Mohammad R., Rasouli, Firooz
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 10.02.2004; Elsevier
• Subjects: Catalysis; Catalysis mechanism; Catalyst intermediates; Catalytic reactions; Chemistry; Exact sciences and technology; General and physical chemistry; HRTEM; Iron oxide; NANOCAT; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Iron oxide; Catalyst intermediates; NANOCAT; Catalysis mechanism; HRTEM; Catalytic reaction; Catalyst regeneration; Deactivation; Nanoparticle; Transition metal Compounds; Carbon dioxide; Pyrocatechol; Heterogeneous catalysis; Phase transformation; Transmission electron microscopy; Phenols; Oxidation; Carbon monoxide
• ISSN: 0926-860X; 1873-3875
• DOI: 10.1016/j.apcata.2003.07.011

Nanoparticle iron oxide catalyst was studied to determine the phase changes that occur during catalysis experiments and to determine if these changes could explain the oxidative catalysis and deactivation mechanisms. The starting material was characterized as a mixture of glassy material, poorly crystalline iron hydroxides, and a small amount of highly crystalline γ-Fe 2O 3. Under oxidative (with respect to phase changes in iron oxide) heating conditions (3% oxygen and higher) this material transformed to a much coarser α-Fe 2O 3, γ-Fe 2O 3, or Fe 3O 4 phase depending on the conditions of the experiment. When the material was pre-heated to intentionally transform it to α-Fe 2O 3, and then later exposed to catechol vapors below 450 °C, the α-Fe 2O 3 was further transformed to either an oxygen deficient γ-phase or to magnetite even though input oxygen tension remained well above the magnetite (Fe 3O 4), hematite (α-Fe 2O 3) phase boundary. These two phases (γ-Fe 2O 3 and magnetite), both of which are spinels, do not seem to be catalytically active for either the oxidation of CO or the oxidative degradation of catechol. It is proposed that the appearance of these phases are evidence of an intermediate reduction step in the oxidative catalysis mechanism performed by iron oxide. Specifically, this provides ex situ analytical evidence that a mechanism similar to the Mars–van-Krevelan mechanism operated for the catalysis by bulk crystalline iron oxide. Subsequent oxidation of the reduced iron oxide would normally regenerate the α-phase completing the Mars–van-Krevelan cycle. However, in cases where the rate of reduction exceeds the rate of oxidation, the iron oxide permanently transforms to a spinel and catalysis halts even though one of these spinels, maghemite (γ-Fe 2O 3), has no stability field in P– T space with respect to magnetite and hematite. The appearance of magnetite is similar to the results expected for reducing conditions and further attests to the role of α-Fe 2O 3 as an oxygen source during catalysis rather than as an electron acceptor.