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

MoO3 has been deposited to TiO2 (anatase) from which P and K impuritie s have been largely removed by washing. The MoO3 was introduced by imp regnation using aqueous oxalic acid solutions of (NH4)(6)Mo7O24.4H(2)O . The results are compared with those previously obtained using the sa me support without washing. Temperature-programmed reduction (TPR) pro files at low MoO3 contents suggest that the impurities inhibit the for mation of reducible tetrahedral Mo6+ species in the monolayer phase, p erhaps 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 an d 982 cm(-1) only when the impurities are present. Variation of the Mo /Ti signal intensity ratio in X-ray photoelectron spectroscopy (XPS) a s 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''.