Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Alan W. Weimer

Second Advisor

Charles B. Musgrave

Third Advisor

J. William Medlin

Fourth Advisor

John Falconer

Fifth Advisor

Sean Shaheen

Abstract

Light driven metal oxide catalysts can be used to avert looming energy and environmental challenges by generating hydrogen from water and degrading aqueous organic pollutants. This work reports on investigations of metal oxide catalyzed solar thermal water splitting (STWS) and photocatalytic organic pollutant degradation.

Two-step STWS is a process where solar heat is used to drive the endothermic water splitting process; a metal oxide serves as an O-carrier, where in the first step of the STWS process O2 is released by the metal oxide at high temperatures and in the second step, the O atoms from H2O molecules are re-incorporated into the metal oxide releasing H2. The hercynite cycle is a promising STWS material because of its relatively low reduction temperature, high melting point and H2 production capacity (over 150 μmol of H2/g per cycle). Density functional theory (DFT) calculations in combination with high-temperature XRD and EDS analyses show that the hercynite cycle operates via an O-vacancy mechanism, were the O2 that is released comes from the formation of O vacancies in the doped hercynite according to the reaction: CoxFe1 Al2O4→ CoxFe1-xAl2O4-δ + δ/2 O2.

The hercynite cycle was investigated for use in isothermal solar thermal water splitting where the reduction and oxidation step occur at the same temperature. Isothermal operation was previously thought to not be thermodynamically allowed. Not only does hercynite split water under isothermal conditions, but the H2 production yields at 1350°C reduction are, respectively, >3 and >12 times that of hercynite and ceria on per mass of active material basis when reduced at 1350°C and re-oxidized at 1000°C. A new set of thermodynamic models were developed which more accurately predict STWS behavior, including isothermal modes of operation.

The kinetics of the oxidation step of isothermal hercynite solar thermal CO2 splitting were investigated. Due to complicating reactor and materials behavior, namely CO2 thermolysis on the reactor walls and hence the simultaneous oxidation of hercynite by CO2 and O2, an extended formulation of solid state kinetic theory was developed which enabled the modeling of multiple simultaneous gas solid reactions. A second-order surface reaction model in relation to extent of unreacted material, and a 2.4th order model in relation to CO2 concentration, were found to best describe the CO generation behavior of the doped hercynite.

TiO2 can be used as a photocatalyst for the degradation of organic pollutants. The reaction is relatively slow due to the inability of O2, the electron acceptor, to adsorb to the TiO2 surface. Pt catalysts and material dopants are added to increase the overall rate of reaction. As the content of Pt increase the reaction rates increase and then subsequently decrease. DFT calculations were used to probe the O2 reduction reaction on the TiO2 and Pt decorated TiO2 surface. Pt enables O2 adsorption and reduction by providing high energy electron density which can form an O2-Pt bond, increasing the photocatalytic rate of TiO2. However, Pt also bridges the TiO2 band gap which increases electron/hole recombination, decreasing the photocatalytic rate of TiO2. At low Pt loadings the increased O2 reduction rate is more significant than the electron-hole recombination but at high Pt loadings the increased electron hole recombination is more significant.

Additionally, DFT calculations predict that non-metal near surface TiO2 dopants can serve as a source of high energy electron density to enable O2 adsorption and reduction as long as the energy of the band gap states produced by doping are higher in energy that the empty O2 π* state of the adsorbing O2. B and interstitial C atoms facilitate O2 reduction and adsorption but substitutional C and both interstitial and substitutional N dopants do not. N dopants can even hinder O2 adsorption and the photocatalytic rate by creating electron/hole recombination sites.

Share

COinS