Quantum dot functionalized Titania systems for photocatalytic oxidative NOx storage

Abstract

Increasing activities of industrial combustion systems, volcanic eruptions, agriculture activities and utilization of stationary and mobile fossil and biomass combustion systems are known to be the major causes of toxic nitrogen oxides (NOx) pollution. These pollutants are not only highly hazardous for the ecosystem but also can trigger the formation of secondary pollutants such as acid rain and tropospheric ozone. Abatement of toxic NOx gases can be achieved by thermal catalytic processes or physical/chemical adsorption systems. However, environmentally friendly, cost-efficient and sustainable alternative photocatalytic systems can also be designed which can exploit readily abundant solar radiation. One of the most well-known benchmarks for environmental photocatalysts is titanium dioxide with a wide band gap typically varying within 3.0-3.2 eV that can be activated via UV photons. This wide band gap prevents efficient absorption of visible light, which corresponds to around 5 times higher intensity compared to UV light. In order to increase the photocatalytic efficiency of the titanium dioxide, its visible-light exploitation capability should be enhanced. Although, this can be done by doping of TiO2 with nonmetal main group elements, recently the research focus has shifted towards utilization of semiconductor quantum dots (QDs) for this purpose. Visible response of the QDs can be modified by tuning their particle size. Furthermore, QDs provide additional advantages such as the generation of hot electrons or multiple charge carriers with a single high-energy photon. In the present work, CdTe QDs were employed as a direct band gap semiconductor (1.44 eV) compatible with the visible window of the solar spectrum to promote titania based photocatalysts. Due to its higher conduction band, CdTe can transfer its conduction band electrons to the conduction band of TiO2 and the hole that is created on the valence band of TiO2 can be transferred to the valence band of CdTe; leading to efficient electron-hole separation. Thus, visible light exploitation capacity of TiO2 can be enhanced along with its photocatalytic activity. Current photocatalytic activity results on QD functionalized titania systems exhibited much higher NOx storage in solid state and an enhancement of NO conversion values as compared to that of P25 titania benchmark photocatalyst. In addition, various reference materials were prepared and photocatalytically tested in order to shed light on the mechanism of this photocatalytic enhancement. These results provided insight regarding the functionality of the different structural components of the photocatalytic architecture in the photocatalytic NOx oxidative storage process. The influence of each structural component in this catalytic architecture was studied. Control experiments were conducted with the dispersant water and the capping agent thioglycolic acid. The results revealed that NO conversion and selectivity can be enhanced by the adsorbed water on titania surface. By adding thioglycolic acid on titania, the NO conversion is suppressed but the selectivity of the system increased. Finally, by replacing titania with a photocatalytically inactive material alumina, it was shown that only in the presence of CdTe quantum dots, there can be NO oxidation/conversion. In overall, utilizing CdTe quantum dots are advantageous for exploiting more of the solar irradiation; however, they suffer from low stability and short catalytic life time.