The effect of varying the duration of the butane/air pretreatment on the morphology and reactivity of (VO)2P2O7 catalysts

Three vanadium pyrophosphate catalysts have been prepared by calcining vanadium hydrogen phosphate hemihydrate (VOHPO 4 .0.5H 2 O), prepared in an organic medium, for different lengths of time (40, 100 and 132 h) in a n-butane (0.75%)/air mixture at 673 K. The catalysts were designated VP040, VPO100 and VPO132. Increasing the duration of reaction with n-butane/air mixture led to an increase in the total surface area from 21.3 (VP040) to 24.9 m 2 g -1 (VPO100) and 27 m 2 g -1 (VPO132). It also led to the complete removal of the VOPO 4 phase from catalysts VPO100 and VPO132, this VOPO 4 phase having been seen as a minor component of catalyst VP040. Scanning electron microscopy showed that longer periods of pretreatment in the n-butane/air mixture produced catalysts with increasing amounts of a characteristic rosette-type of agglomerate. Increasing the duration of the butane/air pretreatment at 673 K resulted in a lowering in the amount of oxygen desorbed on temperature programming, indicating that the butane/air pretreatment was effectively reducing. However, and apparently paradoxically, increasing the duration of the butane/air pretreatment resulted in an increase in the amount of oxygen removed from the catalysts by temperature programmed reduction in H 2 , rising from 11 monolayers for VP040 to 14 monolayers for VPO100 and to 15 monolayers for VPO132. All three catalysts showed a reduction peak with a maximum temperature of ∼1000 K for the removal of 11 monolayers of oxygen, while catalysts VPO100 and VPO132 showed a second reduction peak at ∼1100 K for the removal of an additional three (VPO100) and four monolayers (VPO132) of oxygen.