Photocatalytic NOx oxidation and storage under ambient conditions for air purification

Abstract

Air pollution is one of the most serious environmental problems in both urban and rural settings with a direct impact on human health. A variety of chemical compounds can be associated with air pollution and gaseous nitrogen oxides (NOx), such as NO and NO2, are especially among the most hazardous environmental pollutants. NOx abatement can be efficiently performed at elevated temperatures (i.e. T > 300oC), however, an important challenge in air purification is the abatement of gaseous NOx species under ambient conditions (i.e. at room temperature and under regular atmospheric conditions). Photocatalytic systems offer promising opportunities in order to tackle this important environmental challenge, as these systems can be tailored to efficiently clean/purify air under ambient conditions with the help of ultraviolet (UV) and/or visible (VIS) light. In the current work, a hybrid technology for the photocatalytic oxidation and storage of gas phase NOx species is proposed where titania based powders are investigated as candidate photocatalytic materials. With this aim, various components of a thermally activated conventional NOx Storage/Reduction (NSR) catalyst is combined with a photocatalytically activated NOx oxidation catalyst to obtain a photocatalytically activated NOx oxidation and storage material. In this regard, three different sets of samples were prepared and investigated. The first set of photocatalysts was prepared by employing Al2O3, a high surface area support material, in order to disperse the photocatalytically active titania in an effective manner. Using a ―sol-gel co-precipitation method‖, TiO2/Al2O3 binary oxides were synthesized (where TiO2:Al2O3 mole ratio was chosen to be 0.25, 0.5, 1.0) and characterized by X-ray diffraction, Raman Spectroscopy and BET. For these samples, the effects of specific surface area, calcination temperature and the crystallinity of TiO2 were investigated in relevance to the photocatalytic NOx oxidation/storage reaction. Next, an alkali/alkaline earth oxide storage component is added to the TiO2- Al2O3 mixture and the incorporation of the storage component is achieved via two different routes; (i) either through ―incipient wetness impregnation‖ of 5 or 10% (w/w) metal nitrate [M(NO3)x] salts on TiO2-Al2O3 and a subsequent calcination to obtain alkali/alkaline earth oxides [MyO] or (ii) by physically grinding 5 or 10% (w/w) BaO powder with TiO2-Al2O3 binary oxide to obtain a ternary mixture. For these samples, the route of metal oxide incorporation (impregnation vs. physical mixture), the type of metal oxide storage component (alkali vs. alkaline earth metal) and the percentage of metal oxide loading (5% vs. 10%, w/w) were examined in photocatlytic NOx oxidation/storage reaction. The photonic efficiencies of these samples were tested using a continuous flow system, composed of mass flow controllers, a custom-made UVA-illuminated reaction cell and an ambient chemiluminescence NOx analyzer. Photocatalytic performance of all samples were compared with that of a commercially available Degussa P25 TiO2 benchmark catalyst. Photocatalytic preformance tests revealed that the TiO2-Al2O3 binary oxides had much higher NOx storage capacities compared to Degussa P25 and the further addition of an alkaline earth oxide (BaO) storage component on TiO2-Al2O3 by physical mixing significantly enhanced the NOx capture in solid state and decreased unwanted gaseous NO2 emission to an almost negligible level. On the other hand, the ―incipient wetness impregantion‖ of metal nitrates resulted in metal titanate (MxTiyOz) formation on TiO2-Al2O3 binary oxide and diminished the photooxidation ability of the catalyst.