SURFACE-ACIDITY OF V2O5 AL2O3 CATALYSTS - IR AND TPD STUDIES/

V2O5/Al2O3 catalysts of different vanadium contents were prepared by c ontacting powdered gamma-Al2O3 with aqueous solutions of different con centrations of NH4VO3. Calcined catalysts with vanadium loadings below the monolayer coverage of Al2O3 possessed adsorbed V-oxide layer whic h resisted dissolution in an ammoniacal solution. On the other hand, t hose catalysts of higher vanadium loadings had adsorbed V-oxide layer as well as precipitated crystalline V2O5. The latter preferentially di ssolved in NH4OH solution. Infrared studies of chemisorbed NH3 on V-Al -oxide catalysts as well as temperature programmed desorption (TPD) of NH3 from these materials are reported. These studies revealed the exi stence of two kinds of both Lewis and Bronsted acidic sites. Below the monolayer coverage, Lewis acidity originated from Al ions uncovered w ith vanadia, whereas at high loadings of vanadia, this acidity is attr ibuted to the unsaturated vanadyl groups. Regarding the Bronsted acidi ty, the V-OH species of the adsorbed V-oxide and the V-OH species of p recipitated V2O5 were shown to be responsible for this acidity. TPD re sults showed that the NH3 was desorbed from Lewis acid sites by heatin g at approximate to 440 K and from Bronsted acid sites of adsorbed V-o xide at approximate to 640 K. At higher loadings of vanadium, the Lewi s acidity was diminished due to coverage with precipitated V2O5, where as Bronsted acidity of these precipitates was predominant. Ammonia was desorbed from the Bronsted acidic sites of V2O5 particles by heating at approximate to 530 K. The various kinds of surface acidic sites wer e quantified by analysing the desorbed NH3 from each kind of these sit es. The present results assisted in confirming a proposed structure of the V-Al-oxide catalyst.