Surface species during the crystallization of VOHPO 4 ⋅ 0.5H 2O

Abstract

The synthesis of VOHPO 4 ⋅ 0.5H 2O by reaction of a reduced suspension/solution of V 2O 5 in alcohol and o-H 3PO 4 has been studied by in situ X-ray diffraction (XRD) and ex situ X-ray photoelectron spectroscopy (XPS) and focused ion beam (FIB) microscopy, including cross-sectioning. XPS, XRD, and microscopy evidence is presented for the temporal dissolution of V 2O 5 and formation of VOPO 4 ⋅ 2H 2O, VOPO 4 ⋅ H 2O, and VOHPO 4 ⋅ 0.5H 2O. The XPS technique allows us to trace the development of surface vanadium, oxygen, and carbon states as well as the surface P:V ratio. Four vanadium species were identified. The oxygen vacancies on the surface were characterised by the V 2p 3/2 peak at 515.5 eV and by the O 1s peak at 531.2 eV. V 2O 5 exhibited the V 2p 3/2 peak at 517.4 eV and the O 1s peak at 530.0 eV. The dihydrate phase VOPO 4 ⋅ 2H 2O and hydrate phase VOPO 4 ⋅ H 2O were monitored by the V 2p 3/2 peak at 518.1 eV and the O 1s peak at 531.2 eV. The VPO catalyst precursor VOHPO 4 ⋅ 0.5H 2O, the V 4+ oxidation state, shows the V 2p 3/2 peak at 516.6 eV and the O 1s peak at 531.2 eV. The O 1s peak at 532.9 eV is assigned to crystal water. In situ monitoring of the synthesis by XRD was in a good agreement with the ex situ XPS analysis. VOPO 4 ⋅ 2H 2O and VOPO 4 ⋅ H 2O were successfully identified by XPS as a metastable phase, which forms at short synthesis times. As the hydrate phase concentration decreases the concentration of VOHPO 4 ⋅ 0.5H 2O increases. All XPS data were consistent with the earlier proposed mechanism, which supposed that VOPO 4 ⋅ 2H 2O dehydrates to VOPO 4 ⋅ H 2O, delaminates and the delaminated edges of VOPO 4 ⋅ H 2O serve as the nucleation point for growth of VOHPO 4 ⋅ 0.5H 2O.