STRUCTURE AND REACTIVITY OF TITANIA-SUPPORTED OXIDES .5. INFLUENCE OF PHOSPHORUS AND POTASSIUM IMPURITIES ON THE PROPERTIES OF MOLYBDENUM OXIDE-SUPPORTED ON TITANIA

MoO3 has been deposited to TiO2 (anatase) from which P and K impurities have been largely removed by washing. The MoO3 was introduced by impregnation using aqueous oxalic acid solutions of (NH4)(6)Mo7O24.4H(2)O. The results are compared with those previously obtained using the same support without washing. Temperature-programmed reduction (TPR) profiles at low MoO3 contents suggest that the impurities inhibit the formation of reducible tetrahedral Mo6+ species in the monolayer phase, perhaps by creating species such as K2MoO4 and a mixed POx-MoOx phase. These may be responsible for peaks seen in the Raman spectre at 972 and 982 cm(-1) only when the impurities are present. Variation of the Mo/Ti signal intensity ratio in X-ray photoelectron spectroscopy (XPS) as a function of MoO3 content is consistent with a structure in which, above the monolayer point, 10-20% of the surface is covered by MoO3 ''towers''.