Metal oxide promoted TiO2 catalysts for photo-assisted selective catalytic reduction of NO with NH3

The photo-assisted selective catalytic reduction (SCR) of NO with NH3 (Photo-SCR) was performed over TiO2 modified by supporting 1 wt% of various transition metal (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ta or W) oxides aiming at the improvement of the photo-SCR activity. The addition of Nb, Mo or W oxide to TiO2 was found to enhance the photo-SCR activity. We have reported that the amount of acid sites on TiO2 is one of the key factors to the photo-SCR activity. The increase in the activity depends on the enhancement of acidity of catalyst by the addition of Nb, Mo or W oxide. In contrast, the addition of V, Cr, Mn, Fe, Co, Ni or Cu oxide to TiO2 lowered the photo-SCR activity, although addition of metal cations also changed the acidity of TiO2. We guess that the reduction of the activity was caused by two reasons; the first is that the sites newly formed on these transition metal oxides is not photoactive and the second is that TiO2 supporting V Cr, Mn, Fe, Co, Ni or Cu oxides had low stabilities under the reaction conditions, i.e., the chemical state of the cations changed during the reaction. Therefore, we concluded that the increase in the acid sites that are active sites for photo-SCR and the stability of the catalysts are important for the photo-SCR.