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Origins of the Photocatalytic NO<sub><i>x</i></sub> 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 site