VPO catalyst transformations were investigated using in situ laser Raman sp
ectroscopy. During reduction-oxidation step changes, (VO)(2)P2O7 was readil
y converted to alpha(II)-, delta-VOPO4, and ultimately to beta-VOPO4 in O-2
/N-2; these V5+ phases were eliminated in n-butane/N-2. A wet N-2 feed (5-1
0% H2O in N-2) transformed (VO)(2)P2O7 and alpha(I)-, alpha(II)-, beta-, de
lta-, gamma-VOPO4 to V2O5 at temperatures above 400 degrees C. The presence
of water vapor facilitated the loss of oxygen atoms involved in V-O-P bond
ing, and separated vanadium oxide and phosphorus oxide species were formed.
The isolated vanadium oxide species could be transformed to V2O5; phosphor
us species likely diffused from the catalyst lattice in the form of acid ph
osphates.