PhD Oral Exam - Alireza Haghighatmamaghani, Building Engineering
When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
Energy saving policies applied to modern buildings, tighter building envelopes, and decline in outdoor air quality promote the escalation in levels of indoor air pollutants. In recent years, concerns regarding the impact of indoor air quality (IAQ) on human health especially in well-insulated buildings in metropolitans have created a pressing need for reliable air remediation technologies. Heterogeneous ultraviolet photocatalytic oxidation (UV-PCO) has been put forward as an efficient method to ameliorate IAQ via catalytic degradation of harmful organic compounds to benign final products.
In this context, developing highly photoactive and durable photocatalysts that can perform satisfactorily under realistic conditions is sought after to expedite UV-PCO systems commercialization. Photocatalyst's performance is a function of several interconnected features: crystallinity, crystal phase and size, surface area, porosity, surface chemistry, light-harvesting ability, exposed facet, and charge-separation efficiency. In this regard, thanks to their unique morphology and multimodal porosity, hierarchical photocatalysts can incorporate high molecular diffusion/transport, superior light harvesting, and large surface area. Template-free hydrothermal/solvothermal route is a green, versatile, and less costly strategy for fabricating hierarchical photocatalysts with controlled characteristics.
Taking into account the above-mentioned merits of hierarchical photocatalysts and hydrothermal synthesis method, the core objectives of this research are:
- Synthesize hierarchically porous titanium dioxide materials with enhanced photoactivity with respect to commercially available photocatalysts (e.g. P25) via hydrothermal method.
- Explore the complex preparation-property-performance relationships for titanium dioxide photocatalysts utilized in photocatalytic air purification.
- Examine the role of photocatalyst's morphology in photocatalytic degradation by developing several potentially promising structures and assess their performance.
- Investigate the improvement in photocatalytic filter activity and stability with combining hierarchical titanium dioxide with carbonaceous support.
- Evaluate the activity of photocatalysts under realistic operating conditions to provide reliable information regarding the capability of UV-PCO air purifiers.
To accomplish these goals, a large number of TiO2 photocatalysts are synthesized by systematically varying the hydrothermal preparation parameters (e.g. time, temperature, pressure, pH, solvent, titanium precursor, calcination temperature, etc.). Accordingly, by means of comprehensive characterization, the influence of synthetic methodology on textural, morphological, and crystallographic properties of photocatalysts is investigated. In the last stage, the performance of photocatalyst is evaluated by degradation of two prevalent indoor air pollutants (toluene and methyl ethyl ketone (MEK)) at wide ranges of operating conditions (e.g. relative humidity, airflow rate, concentration, light intensity, etc.).
Based on the Fourier transform infrared (FTIR) analysis of photocatalysts after interaction with toluene/MEK, it was deduced that terminal isolated hydroxyl groups (Ti–OH) play critical role in adsorption of volatile organic compounds (VOCs) in air. Among operating conditions, airflow rate has the most adverse impact on removal efficiency and by-products generation; therefore, for implementation in air-handling units, the bottleneck of UV-PCO performance is the short residence times. Among investigated hydrothermal preparation variables (i.e. time, temperature, pressure, pH, type of acid/base, type of solvent, etc.), hydrothermal temperature and pH of reaction medium offered the best control over textural and crystalline properties. N2 adsorption-desorption and electron microscopy analyses confirmed that some of the hydrothermally-prepared photocatalysts have pores at micro, meso, and macro scales. This hierarchical porosity can facilitate VOCs diffusion/entrapment and light penetration, and provide a great number of accessible active sites. All hydrothermally prepared photocatalysts possessed sufficient crystallinity, large surface area (77.6-237.2 m2/g), mesoporous structure, and good activity compared to P25. Among developed morphologies, titanium dioxide hollow spheres and nanosheets exhibited the optimum MEK removal efficiencies, which were respectively 1.8 and 2.0 times higher than that of P25. Solvothermally-prepared pure anatase TiO2 showed excellent thermal stability with anatase-to-rutile phase transformation temperature greater than 650 °C, a huge improvement over sol-gel derived amorphous titania (transformation temperature=550 °C). Combination of hierarchical titanium dioxide with activated carbon filter (ACF) not only boosted pollutant removal efficiency (from 36.3 to 62.1%) and lowered gaseous by-products (from 217.5 to 180.0 ppb), but also enabled in-situ regeneration of ACF for extended operational life.