Origins of the Photocatalytic NO _x Oxidation and Storage Selectivity of Mixed Metal Oxide Photocatalysts: Prevalence of Electron-Mediated Routes, Surface Area, and Basicity

MgO, CaO, SrO, or BaO-promoted TiO2/Al2O3 was utilized in the photocatalytic NOx oxidation and storage reaction. Photocatalytic performance was investigated as a function of catalyst formulation, calcination temperature, and relative humidity. Onset of the photocatalytic activity in TiO2/Al2O3 coincides with the transition from the anatase to rutile phase and increasing number of paramagnetic active centers and oxygen vacancies. Disordered AlOx domains enable the formation of oxygen vacancies and paramagnetic centers on titania domains, hindering the nucleation and growth of titania particles, as well as increasing specific surface area (SSA) to store oxidized NOx species away from titania active sites. Both e(-)- and h(+)-mediated pathways contribute to photocatalytic NO conversion. Experiments performed using an e(-) scavenger (i.e., H2O2), suppressing the e(-)-mediated route, attenuated the photocatalytic selectivity by triggering NO2(g) release. Superior NOx storage selectivity of 7.0Ti/Al-700 as compared to other TiO2/Al2O3 systems in the literature was attributed to an interplay between the presence of electrons trapped at oxygen vacancies and superoxide species allowing a direct pathway for the complete NO oxidation to HNO3/NO3- species, and the relatively large SSA of the photocatalyst prevents the rapid saturation of the photocatalyst with oxidation products. Longevity of the 7.0Ti/Al-700 was improved by the incorporation of CaO, emphasizing the importance of the surface basicity of the NOx storage sites