In situ studies of atomic, nano- and macroscale order during VOHPO 4·0.5H 2O transformation to (VO) 2P 2O 7

Abstract

Transformation of VOHPO 4·0.5H 2O precursor to well-crystallized (VO) 2P 2O 7, 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) 2P 2O 7. In addition to (VO) 2P 2O 7, nanocrystalline oxidized δ-VOPO 4 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) 2P 2O 7 in the fresh catalysts (ca. 10–20 nm domains) gradually transformed into well-crystallized (VO) 2P 2O 7 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) 2P 2O 7 disappeared yielding a solid with high steady-state catalytic performance. Only (VO) 2P 2O 7 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) 2P 2O 7 in the equilibrated VPO catalysts is believed to be responsible for high activity and selectivity of these catalysts for maleic anhydride formation.