The Effect of Thermal Aging and Addition of CeO x , NbO x , and SbO x on the Structure of Dispersed VO x in Low-Loaded Anatase-Supported Vanadia Catalysts

In situ vibrational spectroscopy (Raman, FTIR) is used at temperatures of 120–430 °C under oxidative dehydration conditions to study the effect of CeOx, NbOx, and SbOx modifiers, as well as the effect of thermal aging, on the structural and configurational properties of titania(anatase)-supported vanadia catalysts. The study pertains to catalysts with low vanadium coverage (i.e., ∼0.5 V nm–2) and an M/V molar ratio (M = Ce, Nb, Sb) equal to one. The dispersed (VOx)n phase consists of three species with distinct configurations: a majority Species-I with an O═V(−O–Ti)3 configuration (V═O terminal mode at 1024–1027 cm–1) followed by Species-II with an O═V(−O–Ti)4 configuration (V═O terminal mode at 1014–1016 cm–1) and a minority Species-III with an (O═V)2(−O–Ti)x configuration (symmetric stretching mode at ∼1000 cm–1). Lowering of the temperature in the 430–120 °C range results in reversible temperature-dependent structural transformations. First, in the 430–250 °C range, a Species-II to Species-I transformation takes place, followed by a Species-I to Species-III transformation at lower temperatures. Modifying the VOx/TiO2(anatase) catalysts with CeOx, NbOx, or SbOx results in the increased presence of Species-I at the expense of Species-II. Thermal aging of the studied catalysts at 580 °C for 100 h gives rise to structural transformation of the anatase carrier, resulting in an increased presence of (101) facets and severe lowering of the specific surface area, thereby causing an increase in Species-I at the expense of Species-II and in the formation of V–O–V and/or V–O–M linkages. The results are deemed important for gaining insight into the structure and the behavior of anatase-supported vanadia catalysts.