Enhanced photocatalytic NOx oxidation-storage over titania-metal oxide physical mixtures under UV and visible light

Abstract

Developing new technologies for the abatement of gaseous nitrogen oxides (NO, NO2, etc.) will still be one of the popular research fields; because fossil fuels (mainly coal and natural gas) will remain as the main energy sources for many decades to come. Although various technologies have been developed and implemented for DeNOx processes, alternative approaches are still open to discussion. Among these; Photocatalytic NOx Oxidation-Storage (PhoNOS) can offer promising opportunities to overcome this environmental challenge, as it can be utilized under ambient conditions with the help of UV and visible light irradiation. In this study; firstly, a new performance analysis method was developed other than the photonic efficiencies used in previous works. In this analysis method, a “DeNOx Index” was utilized. This index indicates the net change in total air pollution due to NOx species by comparing the relative contributions of NO and NO2 along with NO conversion and solid state NOx storage selectivity. This new method was first applied on previously studied TiO2-Al2O3 binary oxide samples (P2) synthesized by sol-gel co-precipitation method in comparison with commercially available Degussa P25 TiO2. Furthermore, TiO2-Al2O3 (P2) binary oxides were also physically/mechanically mixed with an alkaline earth oxide, CaO. Addition of CaO to P2 binary oxides decreased the NO conversion while enhancing the NOx storage. In order to alleviate the loss of NO conversion in CaO+P2 systems, physical mixtures of P25 TiO2 with two different commercial metal oxides (CaO and γ-Al2O3) were prepared and investigated. While CaO provides “higher alkalinity” (i.e. a desirable property for the solid state storage of acidic gaseous NOx species) than γ-Al2O3, mesoporous γ-Al2O3 can provide a higher porosity and specific surface area for the adsorption and storage of the oxidation products in the solid state. Considering these, binary or ternary mixtures with various compositions were prepared and catalytically tested under UV and Visible light irradiation. It was found out that the boosting effect of CaO on NOx storage is more significant than that of γ-Al2O3 for the binary oxides. On the other hand, it should be noted that ternary mixtures containing smaller amounts of titania with high performance can also be obtained by incorporating alumina into the mixture. In addition to these, performances of selected samples were studied under different humidity conditions and experimental durations. These experiments yielded interesting implications regarding NOx adsorption-oxidation phenomena on the investigated mixed oxide surfaces. Current findings indicate that further experiments are required to fully understand the fundamental mechanisms of photocatalytic NO oxidation and storage at the molecular level.