Date of Award

Winter 1-1-2010

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Alan W. Wiemer

Second Advisor

Mark D. Allendorf

Third Advisor

Will Medlin

Abstract

Production of renewable hydrogen is achievable via two-step redox cycles using metal oxide-based intermediates. Concentrated solar energy is capable of decomposing the metal oxide in the first high temperature step, and in the second step water is reacted with the reduced metal oxide to produce H2 and regenerate the starting material.

The thermodynamics of relevant ferrite-based water splitting cycles has been investigated using the thermodynamics software package FactSage. The effect of different metal substitutions in MxFe3-xO4, has been explored, and indicates that Co and Ni based ferrites are both superior to Fe3O4. Additionally, it is shown that increasing the inert gas concentrations has a direct effect on the reduction temperature. Increasing the amount of cobalt results in lowering the thermal reduction requirements, but does not necessarily translate to more H2 production. For values of x > 1, the amount of reducible iron decreases, and results in less H2 production at elevated reduction temperatures. Oxidation of reduced species is shown to be achievable at temperatures greater than when ΔGrxn > 0 if large excesses of water are introduced. More H2 is expected to be present at equilibrium for ferrite based reactions compared to ceria based water splitting cycles, because the degree of reduction is approximately three times greater.

Atomic layer deposition (ALD) has been used as a means to synthesize thin films of iron oxide, which can be used as reactive intermediates in solar redox cycles. Conformal films of amorphous iron (III) oxide and α-Fe2O3 have been coated on zirconia nanoparticles (26 nm) in a fluidized bed reactor by atomic layer deposition. Ferrocene and oxygen were alternately dosed into the reactor at temperatures between 367 oC and 534 oC. Self-limiting chemistry was observed via in situ mass spectrometry, and by means of induced coupled plasma – atomic emission spectroscopy analysis. Film conformality and uniformity were verified by high resolution transmission electron microscopy, and the growth rate was determined to be 0.15 Å per cycle.

Iron oxide (γ-Fe2O3) and cobalt ferrite (CoxFe3-xO4) thin films have also been synthesized via ALD on high surface area (50 m2/g) m-ZrO2 supports. The oxide films were grown by sequentially depositing iron oxide and cobalt oxide, and adjusting the number of iron oxide cycles relative to cobalt oxide to achieve desired stoichiometry. Samples were chemically reduced in a flow reactor equipped with in situ x-ray diffraction. They were also subjected to chemical reduction and oxidation in a stagnation flow reactor to test activity for use in chemical looping cycles to produce H2 via water splitting. γ-Fe2O3 films chemically reduced in mixtures of H2, CO, and CO2 at 600 °C formed Fe3O4 and FeO phases, and exhibited a trend-wise decrease in H2 production rates upon cycling. Co0.85Fe2.15O4 films were successfully cycled without deactivation and produced four times more H2 than γ-Fe2O3, principally due to the formation of a CoFe alloy upon reduction. For comparison, a mechanically milled mixture of α-Fe2O3 and ZrO2 powders with similar iron loading to the thin films did not maintain high activity to water splitting due to sintering and grain growth.

Cobalt ferrites are deposited on Al2O3 substrates via ALD, and the efficacy of using these in a ferrite water splitting redox cycle to produce H2 is studied. Experimental results are coupled with thermodynamic modeling, and results indicate that CoFe2O4 deposited on Al2O3 is capable of being reduced at lower temperatures than CoFe2O4 (200oC-300oC) due to a reaction between the ferrite and substrate to form FeAl2O4. Significant quantities of H2 are produced at reduction temperatures of only 1200 oC, whereas, CoFe2O4 produced little or no H2 until reduction temperatures of 1400 oC. CoFe2O4/Al2O3 was capable of being cycled at 1200 oC reduction/ 1000 oC oxidation with no obvious deactivation.

Cobalt ferrite (Co0.9Fe2.1O4) and iron oxide (Fe3O4) thin films deposited via ALD on m-ZrO2 supports are utilized in a high temperature water splitting redox cycle to produce H2. Both materials were thermally reduced at 1450 oC and oxidized with H2O (20-40%) at temperatures between 900 oC and 1400 oC in a stagnation flow reactor. Oxidation of iron oxide was more rapid than the cobalt ferrite, and the rates of both materials increased with temperature, even up to 1400 oC. At elevated oxidation temperatures (T > 1250 oC) we observed simultaneous production of H2 and O2, due to both thermal reduction and water oxidation operating in equilibrium. A kinetic model was developed for the oxidation of cobalt ferrite from 900 oC to 1100 oC, in which there was an initial reaction order limited regime, followed by a slower diffusion limited regime characterized well by the parabolic rate law. The activation energy and H2O reaction order during the reaction order regime were 119.76 ± 8.81 kJ/mole and 0.70 ± 0.32, respectively, and the activation energy during the diffusion limited regime was 191 ± 19.8 kJ/mol.

The feasibility of using commercially available, un-doped, ceria (CeO2) felts in a thermochemical redox cycle to produce H2 has been explored, and a detailed kinetic analysis of the oxidation reaction is discussed. Reduction is achieved at 1450 oC, and the subsequent H2 producing step is studied from 700 to 1200 oC and H2O mole fractions of 0.04 to 0.32. The O2 and H2 equilibrium compositions remain constant for up to 30 redox cycles, and sintering appears to be abated by microscopy analysis. The average amount of H2 produced is 280.9 ± 45.8 μmoles/g CeO2. The re-oxidation rates are faster on a per mass basis than similar ferrite based-cycles because the surface area is largely unaffected by thermal cycling. The oxidation reaction is governed by a first order reaction mechanism (1-α) at low temperatures and conversions, but at higher temperatures the mechanism transitions to a second order reaction (1-α)2. This is attributed to the onset of the thermodynamically favored reverse reaction at elevated temperatures. The activation energy is calculated between 700 and 900 oC from 0.2<α<0.5, and determined to be 35.5 ± 13.3 kJ/mol. An Arrhenius expression, coupled with a first order reaction mechanism is used to model the experimentally observed reaction rates where the forward reaction was predominant.

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