MODELING OF EXTENDED DEFECTS IN THE VANADIUM PHOSPHATE CATALYST FOR BUTANE OXIDATION, (VO)(2)P2O7

X-ray diffraction patterns of catalysts based on (VO)(2)P2O7 show peak broadening effects which cannot be explained by strain and crystallit e size effects. We show that the extensive broadening of peaks where t he h or I indices are odd is due to extended defects. These defects ca n be modeled as stacking faults perpendicular to the a and c axes. The proposed defects also explain the streaking effects observed by singl e crystal X-ray diffraction and by electron diffraction. One of the pr oposed defects can explain the presence of some V5+ in this phase. The amount of V5+ found both by titration and by P-31 NMR spin echo mappi ng correlates well with defect concentration. The NMR studies also con firm that the V5+ present is associated with (VO)(2)P2O7 rather than a second phase. The defects are most prominent in samples prepared to b e optimum catalysts for the oxidation of n-butane to maleic anhydride. Our work suggests a new explanation for the fact that good VPO cataly sts must be prepared by dehydration of a V2P2O9 . xH(2)O precursor. (C ) 1996 Academic Press, Inc.