Surface acidity of VPO catalysts

In the study of vanadium-phosphorus catalysts and related substances with t he TPD/QMS of ammonia the higher resolution spectra have been obtained. The y show the presence at the catalysts surface of two well resolved bands of weakly bonded ammonia (120-140 degrees C) tentatively assigned to two kinds of surface phosphate groups of different acidity and the band of much stro nger bonded ammonia (200 degrees C) assigned to vanadate groups. Introducti on of bismuth atoms increases the number of active phosphate groups and the strength of their acid function. The effect of lanthanum additives is oppo site.