Reducibility, heats of re-oxidation, and structure of vanadia supported on TiO 2 and TiO 2–Al 2O 3 supports used as vanadium traps in FCC

Abstract

V/TiO 2 and V/TiO 2–Al 2O 3 (1:1 w/w basis) supports were characterized by TPR, Raman spectroscopy, and heats of re-oxidation of samples pre-reduced in CO at 770 K with a heat-flow calorimeter. Supports were pure anatase or rutile dispersed with hydrated aluminas (boehmite, gibbsite, bayerite) subsequently calcined at 870 K. Raman spectroscopy of fully oxidized, air-exposed samples show the presence of polymeric polyvanadate species, but not of isolated monomeric species. Sample loadings were 4 wt.% and show different reduction and structural features. During TPR, vanadia reduced to V(III) and V(IV) in V/rutile and V/anatase, respectively, and multiple reduction peaks were observed due to crystalline V 2O 5 and amorphous vanadia. In V/TiO 2–Al 2O 3 samples, vanadium coverages were 6–8 μmol V m −2 yielding well-dispersed, amorphous vanadia. Trends observed during TPR were: addition of bayerite phase to anatase or rutile increases H 2 consumption by 100%, implying formation of V(III) and V(II), respectively. However, with addition of boehmite or gibbsite to either titania phase, vanadia reduces only to V(IV). Oxygen doses at 473 K of pre-reduced samples titrated about one-third of total vanadia content. Re-oxidation heat values range from 400 to 500 kJ mol −1 O 2 and represent oxygen–vanadium ion bond strengths within the dispersed vanadia. The heat values are higher than expected for re-oxidation of a bulk phase, and are indicative of the degree of stabilization provided by the support.