Photocatalytic oxidation, silica titania composite, volatile organic compound, trace contaminant control
This collection of studies focuses on a PCO system for the oxidation of a model compound, ethanol, using an adsorption-enhanced silica-Ti02 composite (STC) as the photocatalyst; studies are aimed at addressing the optimization of various parameters including light source, humidity, temperature, and possible poisoning events for use as part of a system for gaseous trace-contaminant control system in closed-environment habitats. The first goal focused on distinguishing the effect of photon flux (i.e., photons per unit time reaching a surface) from that ofphoton energy (i.e., wavelength) of a photon source on the PCO of ethanol. Experiments were conducted in a bench-scale annular reactor packed with STC pellets and irradiated with either a UV -A fluorescent black light blue lamp O·max=365 nm) at its maximum light intensity or a UV -C germicidal lamp O.·max=254 nm) at three levels of light intensity. The STC-catalyzed oxidation of ethanol was found to follow zero-order kinetics with respect to C02 production, regardless of the photon source. Increased photon flux led to increased EtOH removal, mineralization, and oxidation rate accompanied by lower intermediate concentration in the effluent. The oxidation rate was higher in the reactor irradiated by UV -C than by UV-A (38.4 vs. 31.9 nM s-1 ) at the same photon flux, with similar trends for mineralization (53.9 vs. 43.4%) and reaction quantum efficiency (i.e., photonic efficiency, 63.3 vs. 50.1 nmol C02 ~mol photons-1 ). UV-C irradiation also led to decreased intermediate concentration in the effluent compared to UV -A irradiation. These results demonstrated that STC-catalyzed oxidation is enhanced by both increased photon flux and photon energy. The effect of temperature and relative humidity on the STC-catalyzed degradation of ethanol was also determined using the UV-A light source at its maximum intensity. Increasing ii temperature from 25°C to 65°C caused a significant decrease in ethanol adsorption (47.1% loss in adsorption capacity); minimal changes in EtOH removal; and ·a dramatic increase in mineralization (37.3 vs. 74.8%), PCO rate (25.8 vs. 53.2 nM s-1 ), and reaction quantum efficiency (42.7 vs. 82.5 nmol C02 J..Lmol phontons-1 ); intermediate acetaldehyde (ACD) evolution in the effluent was also decreased. By elevating the reactor temperature to 45°C, a -32% increase in reaction quantum efficiency was obtained over the use ofUV-C irradiation at room temperature; this also allowed for increased energy usage efficiency by utilizing both the light and heat energy of the UV-A light source. Higher relative humidity (RH) also caused a significant decrease (16.8 vs. 6.0 mg EtOH g STCs-1 ) in ethanol adsorption and dark adsorption 95% breakthrough times (48.5 vs.16.8 hours). Trends developed for ethanol adsorption correlated well with studies using methanol as the target VOC on a molar basis. At higher RH, ethanol removal and ACD evolution were increased while mineralization, PCO rate, and reaction quantum efficiency were decreased. These studies allowed for the development of empirical formulas to approximate EtOH removal, PCO rate, mineralization, and ACD evolution based on the parameters (light intensity, temperature, and RH) assessed. Poisoning events included long-term exposure to low-VOC laboratory air and episodic spikes of either Freon 218 or hexamethylcyclotrisiloxane. To date, all poisoning studies have shown minimal (0-6%) decreases in PCO rates, mineralization, and minimal increases in ACD evolution, with little change in EtOH removal. These results, while studies are still ongoing, show great promise of this technology for use as part of a trace contaminant control system for niche applications such as air processing onboard the ISS or other new spacecrafts.
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Doctor of Philosophy (Ph.D.)
College of Sciences
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Dissertations, Academic -- Sciences,Sciences -- Dissertations, Academic
Coutts, Janelle, "Trace Contaminant Control: An In-depth Study Of A Silica-titania Composite For Photocatalytic Remediation Of Closed-environment Habitat Air" (2013). Electronic Theses and Dissertations. 2715.