Reducibility, heats of re-oxidation, and structure of vanadia supported on TiO2 and TiO2-Al2O3 supports used as vanadium traps in FCC

被引:10
作者
Martínez, S
Morales, R
Cárdenas-Galindo, MG
Rodríguez, AG
Pedraza, F
Handy, BE
机构
[1] Univ Autonoma San Luis Potosi, Fac Ciencias Quim, CIEP, San Luis Potosi 78210, Mexico
[2] Univ Autonoma San Luis Potosi, Inst Invest Comunicac & Opt, San Luis Potosi 78216, Mexico
[3] Inst Mexicano Petr, Mexico City 07730, DF, Mexico
关键词
calorimetry; TPR; vanadia; titania; alumina; titania-alumina;
D O I
10.1016/j.tca.2005.01.055
中图分类号
O414.1 [热力学];
学科分类号
摘要
V/TiO2 and V/TiO2-Al2O3 (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 V2O5 and amorphous vanadia. In V/TiO2-Al2O3 samples, vanadium coverages were 6-8 mu mol 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. (C) 2005 Elsevier B.V. All rights reserved.
引用
收藏
页码:74 / 80
页数:7
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