Low-temperature NOx trapping on alkali or alkaline earth metal modified TiO2 photocatalyst

摘要

Low-temperature NOx trapping was demonstrated on alkali (Na, K, Rb, and Cs) or alkaline earth metal (Mg, Ca, Sr, and Ba)-modified anatase TiO2 photocatalysts (M-TiO2). The modification enhanced the NOx-trapping efficiency of bare TiO2 as well as the NOx storage capacity (NSC). The NSC depended on the modified metal species. Cs-TiO2 and Ba-TiO2 showed a comparable capacity (179 and 187 mu mol g(-1), respectively) with a conventional NOx-trapping catalyst, Pt/Ba/Al2O3 (176 mu mol g(-1)). XRD and XPS revealed the modification effect on the structures and the surface properties of M-TiO2. Alkali TiO2 promoted the sintering of TiO2 and significantly increased the electron density of the TiO2 surface, while alkaline earth TiO2 suppressed the sintering and slightly changed the surface properties. The adsorption strength of NOx was evaluated from the NOx-TPD of the as-synthesized catalyst. For alkaline earth TiO2, stronger NOx adsorption increased the NOx-trapping efficiency. In addition, in-situ DRIFT measurements clearly showed the difference between adsorbed NOx species in M-TiO2. Alkaline earth TiO2 traps NOx on Ti sites, whereas alkali TiO2 traps NOx mainly on alkali sites. NOx trapping on Ti sites for alkaline earth TiO2 was effective for photocatalytic NOx trapping with high efficiency at low temperature.