Graduation Year

2016

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Chemical Engineering

Major Professor

Babu Joseph, Ph.D.

Co-Major Professor

Venkat Bhethanabolta, Ph.D.

Committee Member

John Kuhn, Ph.D.

Committee Member

Shengqian Ma, Ph.D.

Committee Member

Arash Takshi, Ph.D.

Keywords

CO2 conversion, photocatalyst, metal-organic-framework (MOF), perovskite, photosynthesis

Abstract

Solar energy is a sustainable resource which has substantial potential to meet the increasing demand for renewable energy. Though there has been some success in harvesting solar energy for electricity production, converting solar energy to chemical energy as fuels is still a challenge due to low efficiency.

Since the discovery of TiO2 photocatalysts for splitting water (4) and reducing CO2 (5) to form useful chemical feedstock such as H2, CO and CH4, much research has been done to increase the efficiency of photocatalysts. However, the current conversion efficiency of photocatalysts remains low (~5%) (6, 7). Issues being addressed include the wide bandgap and mismatched band edge for reactions (thermodynamic energy for reaction), poor quantum efficiency of the photon collector systems, high recombination of e-/h+ pairs and limitation in the rate of charge transfer from photocatalyst to reactants.

This work focuses on improving efficiency of photocatalysts for fuel production through several approaches: (1) engineering a metal-organic-framework (MOF) to have proper band gaps and band edges for targeted reactions and for enhancing photoadsorption in the visible light range, (2) tuning an ABO3-type perovskite for desired bandgaps and thermodynamically favored bandedges for CO2 reduction with water in visible light range.

A porphyrin-based Ti-MOF is studied for CO2 photoreduction to gaseous chemical fuels such as CH4 and CO. The porphyrin linkers allow porphyrin-based MOF-525 to achieve narrow bandgap (Eg = ~1.7eV) to absorb visible light, indicating its ability to harvest more solar energy than conventional TiO2. Ti/Zr-MOF-525 also exhibited the appropriate energy level alignment for CO2 and H2O redox reaction for CO and CH4 production. Its CO2 photoreactivity under visible light was demonstrated in a photoreaction, illuminated by 150W Xenon solar simulator. Interestingly, Ti/Zr-MOF-525 demonstrates a selectivity toward CH4, a more valuable fuels than CO. The gas phase reaction condition is an advance over liquid photoreaction. The catalyst stability was also studied and presented. After 3 cycles of reactions, Ti/Zr-MOF-525 is relatively stable for CO2 photoreduction and able to maintain its photoreactivity at about 60-65% of fresh catalyst. The reduction of reactivity is due to a less stable fresh catalyst.

When investigating LaCr1-xFexO3 perovskite oxides for photocatalyst, it was found that when replacing Cr ions at the B sites of LaCrO3 by Fe ions, the bandgap does not follow a linear trend in regards to metal ratio composition but rather reflects the smaller bandgap of LaFeO3. Bandedges were successfully measured for the new synthesized materials. At x = 0.25, the conduction band potential remains similar with x = 0. However, at x = 0.75, the conduction band potential was more negative than either perovskites at x = 0 or x = 1. Future simulation of density of state could address this interesting observation. CO2 reduction relativities of each perovskites were predicted well by their measured bandgaps and bandedges. Among five studied perovskites, synthesized LaCr0.25Fe0.75O3 (x = 0.75) is the most active for CO2 photoreduction under visible illumination at room temperature thanks to its small bandgap (2.0 eV) and its suitable bandedges for CO2 photoreduction.

Available for download on Saturday, November 18, 2017

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