In situ studies of atomic, nano- and macroscale order during VOHPO4•0.5H2O transformation to (VO)2P2O7

Transformation of VOHPO4.0.5H(2)O precursor to well-crystallized (VO)(2)P2O7, for n-butane oxidation to maleic anhydride was studied by in situ Raman and XRD techniques. Atomic scale changes observed in the precursor structure 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 the specificity of the in situ conditions. Under catalytic reaction conditions 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. Simultaneously, a disordered layer ca. 2 nm thick which was covering the surface (1 0 0) planes of (VO)(2)P2O7 disappeared yielding a solid with high steady-state catalytic performance. Only (VO)(2)P2O7 was observed both at room temperature and reaction temperature in the equilibrated VPO catalysts. Specific surface termination of the (1 0 0) planes of (VO)(2)P2O7 in the equilibrated VPO catalysts is believed to be responsible for high activity and selectivity of these catalysts for maleic anhydride formation.(C) 2001 Elsevier Science B.V. All rights reserved.