Doctor of Philosophy, University of Akron, 2022, Polymer Science

Photoelectrochemical (PEC) conversion of biomass (e.g., lignin) to hydrogen, a carbon-negative emission technology, is characterized by four key processes: (i) photo-generated electron-hole pairs, (ii) electron transport from the anode to the cathode, (iii) hydrogen generation at the cathode and (iv) biomass oxidation by photogenerated holes in the valence band. Overall performance of the photoelectrochemical cell is governed by step (i), electron-hole generation, followed by step (iv), charge transfer at the semiconductor/electrolyte interface. This dissertation will discuss the development of an in situ infrared spectroscopic (IR) approach to study charge dynamics during PEC reactions. Accumulated photogenerated electrons on the semiconductor surface in PEC reactions exhibit a structureless, and featureless spectrum centered around 2000 cm-1. The intensity and rate of the IR profile of photogenerated electrons at this wavenumber correlates to the charge transport in the PEC process, qualitatively characterizing the efficiency of the catalyst. Electron accumulation can also be observed under dark conditions with negative voltage bias. Adsorbed water on the semiconductor surface serves as a hole scavenger and shields the catalyst surface from oxygen, preventing electron-hole recombination, while simultaneously promoting the formation of a double layer of electrons and protons on the semiconductor surface. The effect of voltage on the performance of the PEC cell is investigated through the analysis of the IR profile (i.e., relative concentration) of photogenerated electrons. The results of charge dynamics shed a light on the PEC mechanism and provide a scientific basis for devising novel approaches to enhance the PEC efficiency. The observations of the dynamics of accumulated electrons and water coverage in PEC reactions revealed the applicability of the in situ IR approach to electro-swing CO2 capture in liquid monoethanolamine (MEA). CO2 reacts with amines (open full item for complete abstract)

Committee: Steven S.C. Chuang (Advisor); Toshikazu Miyoshi (Committee Member); Zhenmeng Peng (Committee Member); Mesfin Tsige (Committee Member); Yu Zhu (Committee Member) Subjects: Alternative Energy; Analytical Chemistry; Polymers

Master of Science in Engineering, University of Akron, 2010, Chemical Engineering

Heterogeneous photocatalytic oxidation allows many applications in industry use. Titanium dioxide (TiO2) is one of the most efficient photocatalysts due to its chemical stability, non-toxicity and low cost. Many studies on photocatalytic activity have been done on various types of surface-modified TiO2 catalysts but not on SiH4 modified TiO2. In this study, TiO2 (P25, Degussa) was treated with SiH4 and its photocatalytic activity has been observed and compared with that of non-treated TiO2. In order to understand some of the observations, various techniques such as introduction of varied ratio of EtOH/O2 pulses, variation of temperature, H2 and O2 exposure, phasing exposure of TiO2 to UV illumination and dark have been incorporated. From the results, the increase on the absorbance intensity at 2000 cm-1 (i.e. I2000, background shift), was observed on SiH4-treated TiO2. This could be due to inhibition of electron-hole recombination process by blockage/removal of electron hole recombination centers such as surface hydroxyl groups and adsorbed water. Excessive coverage of SiO2 on TiO2 surface lowered the photocatalytic oxidation rate due to blockage of oxidation sites. Quick drops followed by rises of the intensity of the background shift after EtOH/O2 pulses indicated faster consumption rate of photogenerated electrons by O2 comparing to the rate of accumulation of photogenerated electrons caused by the reaction of CH3CH2OHad and CH3CH2Oad with holes. This suggests that surface holes are more readily active than electrons and the majority of the photogenerated electrons are in the bulk. Increasing the ratio of O2 resulted in increased CO2 from different path of reactions. The background shift under abundant O2 was low due to electron scavenging. H2 can cause generation of trapped Ti atom as Ti3+. The trapped Ti3+ atoms can be exited resulting in separation of electron and hole and, in turn, causing higher backgrou (open full item for complete abstract)

Committee: Steven Chuang Dr. (Advisor) Subjects: Chemical Engineering