Spectroscopic characterisation of MoO3/TiO2 deNOx-SCR catalysts: Redox and coordination properties

MoO3/TiO2 catalysts prepared by impregnation of anatase with ammonium heptamolybdate have been investigated with the aim of understanding their redox chemistry and the coordination state of surface active species. Adsorption of ammonia has been followed by FTIR and by EPR on thermally treated, partially reduced samples. FTIR has shown that surface isolated molybdenylic species (with one short MoO bond) are present at the surface which can coordinate an NH3 molecule in their coordination sphere. The effect of thermovacuum treatments at increasing temperature is the formation of Mo(V) centres whose amounts depend on the total Mo loading. Several families of Mo(V) have been identified by EPR, differing in their coordinative environment. Reoxidation by O2 of the reduced samples follows a unique pathway involving formation of partially reduced Ti3+ ions in the matrix. This pathway is different from that followed by silica-supported MoO3 systems and the difference has been explained in terms of the semiconducting nature of titanium dioxide.