Title page for ETD etd-01252001-125957


Type of Document Dissertation
Author Radhakrishnan, Rakesh
Author's Email Address rradhakr@vt.edu
URN etd-01252001-125957
Title Structure and Ozone Decomposition Reactivity of Supported Manganese Oxide Catalysts
Degree PhD
Department Chemical Engineering
Advisory Committee
Advisor Name Title
Oyama, Shigeo Ted Committee Chair
Cox, David F. Committee Member
Davis, Richey M. Committee Member
Dillard, John G. Committee Member
Gibbs, Gerald V. Committee Member
Keywords
  • Decomposition
  • Oxygen Chemisorption
  • Kinetics
  • Temperature Programmed Desorption
  • Ozone
  • Raman Spectroscopy
  • Manganese oxides
  • Mechanism
  • Ab initio
  • X-ray Absorption Fine Structure Spectroscopy
Date of Defense 2001-01-18
Availability unrestricted
Abstract
Manganese oxide catalysts supported on Al2O3, ZrO2, TiO2 and SiO2 supports were used to study the effect of support on ozone decomposition kinetics. X-ray diffraction (XRD), in-situ laser Raman spectroscopy, temperature programmed oxygen desorption, surface area measurements, extended and near edge x-ray absorption fine structure (EXAFS and NEXAFS) showed that the manganese oxide was highly dispersed on the surface of the supports. EXAFS spectra suggest that the manganese active centers on all of the surfaces were surrounded by five oxygen atoms. These metal centers were of a mononuclear type for the Al2O3 supported catalyst and multinuclear for the other supports. NEXAFS spectra for the catalysts showed a chemical shift to lower energy and an intensity change in the L-edge features which followed the trend Al2O3 > ZrO2 > TiO2 > SiO2. The trends provided insights into the positive role of available empty electronic states required in the reduction step of a redox reaction.

The catalysts were tested for their ozone decomposition reactivity and reaction rates had a fractional order dependency (n < 1) with ozone partial pressure. The apparent activation energies for the reaction was low (3-15 kJ/mol). The support influenced the desorption step (a reduction step) and this effect manifested itself in the pre-exponential factor of the rate constant for desorption. Trends for this pre-exponential factor correlated with trends in NEXAFS features and reflected the ease of electron donation from the adsorbed species to the active center.

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