In situ studies of atomic, nano- and macroscale order during VOHPO4 centerdot 0.5H(2)O transformation to (VO)(2)P2O7

Transformation of VOHPO4.0.5H(2)O precursor to well-crystallized (VO)(2)P2O 7, for n-butane oxidation to maleic anhydride was studied by in situ Raman and XRD techniques. Atomic scale changes observed in the precursor structur e at 583 K provided new insights into its transformation to (VO)(2)P2O7 In addition to (VO)(2)P2O7, nanocrystalline oxidized delta -VOPO4 invisible to XRD was detected during transformation in n-butane/air, possibly due to th e specificity of the in situ conditions. Under catalytic reaction condition s the disordered nanocrystalline (VO)(2)P2O7 in the fresh catalysts (ca. 10 -20 nm domains) gradually transformed into well-crystallized (VO)(2)P2O7 in the equilibrated VPO catalysts (> 30 nm domains) with time on stream. Simu ltaneously, a disordered layer ca. 2 nm thick which was covering the surfac e (1 0 0) planes of (VO)(2)P2O7 disappeared yielding a solid with high stea dy-state catalytic performance. Only (VO)(2)P2O7 was observed both at room temperature and reaction temperature in the equilibrated VPO catalysts. Spe cific surface termination of the (1 0 0) planes of (VO)(2)P2O7 in the equil ibrated VPO catalysts is believed to be responsible for high activity and s electivity of these catalysts for maleic anhydride formation.(C) 2001 Elsev ier Science B.V. All rights reserved.