Date of Award

May 2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Ying Li

Committee Members

Jin Li, Junhong Chen, John Reisel, Ben Church

Abstract

Due to fossil fuel usage, booming industry and other human activities, greenhouse gases associated with global warming and drinking water shortage severely threaten sustainable development of human society. It is emergent and critical to address and solve both of them.

Greenhouse gases will trap heat and cause global warming, carbon dioxide (CO2) from fossil fuel combustion is the major contribution to greenhouse gas emission. In order to control CO2 emission, different technologies have been invented. Recently, photoreduce CO2 using solar energy with photocatalyst catches a lot of attention. Because on the one hand this technology can reduce CO2 in atmosphere, on the other hand alternative fuel can be produced with solar energy such as CO, methane, methanol, etc.

For the drinking water shortage problem, membrane filtration technology has been proved as one of the most efficient and reliable methods to provide clean drinking water. However, membrane fouling caused by deposition of contaminants on membrane surface has been recognized as one of the major obstacles inhibiting the application of membrane technologies. Membrane fouling may dramatically shorten the lifetime of membrane module, deteriorate the quality of water produced and increase the operation cost. With the help of the photocatalyst, contaminates in water and on membrane can be degraded under light irradiation. Membrane fouling caused by contaminates can be significantly mitigated.

Among all photocatalysts that have been investigated, TiO2 is a promising high efficient photocatalyst for both environmental and energy application, due to the low cost, high redox potential and nontoxicity. However, because of the large bandgap, fast hole/electron recombination process and limited visible light absorption, those characters significantly limit the application of TiO2. In this study, different TiO2 modification strategies were carried out to improve the efficiency of TiO2 photoactivity.

One objective of this study is to demonstrate visible light functional iodine doped titanium oxide (I-TiO2) for CO2 photoreduction. I-TiO2 nanoparticles have been synthesized by hydrothermal method. I-TiO2 shows photocatalitically responsive to visible light illumination. The structure and properties of I-TiO2 nanocrystals prepared with different iodine doping levels and/or calcination temperatures were characterized by X-ray diffraction, transmission electron microscopy and diffraction, X-ray photoelectron spectroscopy, and UV–vis diffuse reflectance spectra. The three nominal iodine dopant levels (5, 10, 15 wt.%) and the two lower calcination temperatures (375, 450◦C) produced mixture of anatase and brookite nanocrystals, with small fractions of rutile found at 550◦C. The anatase phase of TiO2 increased in volume fraction with increased calcination temperature and iodine levels. A high CO2 reduction activity was observed for I-TiO2 catalysts (highest CO yield equivalent to 2.4 μmol g−1 h−1 ) under visible light, and they also had much higher CO2 photoreduction efficiency than undoped TiO2 under UV–vis irradiation. I-TiO2 calcined at 375◦C has superior activity to those calcined at higher temperatures. Optimal doping levels of iodine were identified under visible and UV–vis irradiations, respectively.

Along with promising nonmetal-doped TiO2 results, our study also entails a new metal-nonmetal ion co-modified TiO2 nanoparticles fabricated through a combined hydrothermal and wet-impregnation process. Under UV–vis irradiation, the activity of the co-modified catalyst (Cu–I–TiO2) was higher than that of the single ion-modified catalysts (Cu–TiO2 or I–TiO2). Under visible light irradiation, the addition of Cu to I–TiO2 did not lead to significant improvements in CO2 reduction. Methyl chloride (CH3Cl) was detected as a reaction product when CuCl2 was used as the precursor in the synthesis, thus suggesting that methyl radicals are reaction intermediates. When CuCl2 was used as the Cu precursor, a three-fold increase in CO2 photoreduction activity was observed, as compared to when Cu(NO3)2 was used as the Cu precursor. These differences in activities were probably due to enhanced Cu dispersion and the hole-scavenging effects of the Cl ions.

The water treatment with membrane filtration technology will always face membrane fouling. It is one of the major obstacles inhibiting the wide application of membrane technologies for water treatment. Membranes with surface modification of titanium dioxide (TiO2) nanoparticles or TiO2 nanowire membranes (Ti–NWM) have demonstrated reduced membrane fouling due to the photocatalytic capability of TiO2 in degrading foulants on the membrane surface. However, the wide band gap of TiO2 makes it only absorb ultraviolet light, which limits its applications under solar irradiation. In this study, our work entailed a novel membrane made of interwoven iron oxide (Fe2O3) nanowires and TiO2 nanowires (FeTi–NWM) has demonstrated superior anti-fouling capability in removing humic acid (HA) from water. Results showed that under simulated solar irradiation the FeTi–NWM achieved nearly complete HA removal during a 2 h short-term test at an initial HA concentration of 200 mg/L, compared with 89% HA removal by Ti–NWM. During a 12 h long-term test, the FeTi–NWM maintained 98% HA removal, while the Ti–NWM showed only 55% removal at the end. Without solar irradiation, the FeTi–NWM was severely contaminated and by contrast, a clean surface was maintained under solar irradiation after the 12 h test and the transmembrane pressure change was minimal. The improved HA removal by FeTi–NWM compared with Ti–NWM and its excellent anti-fouling capability under solar irradiation can be attributed to (1) the enhanced HA absorption by Fe2O3 nanowires and (2) the formed Fe2O3/TiO2 heterojunctions that increase photo-induced charge transfer and improve visible light activity.

Future work includes further improvement of FeTi-NWM membrane with other materials such as graphene etc. Also design and test multi-stage FeTi-NWM membranes system for real industry application.

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