Surface species during the crystallization of VOHPO4 ·0.5H2O

Leonard O'Mahony, Teresa Curtin, Dmitry Zemlyanov, Miroslav Mihov, B. Kieran Hodnett

Research output: Contribution to journalArticlepeer-review

Abstract

The synthesis of VOHPO4·0.5H2O by reaction of a reduced suspension/solution of V2O5 in alcohol and o-H3PO4 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 V2O5 and formation of VOPO4·2H2O, VOPO4· H2O, and VOHPO4·0.5H2O. 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 2p3/2 peak at 515.5 eV and by the O 1s peak at 531.2 eV. V2O5 exhibited the V 2p3/2 peak at 517.4 eV and the O 1s peak at 530.0 eV. The dihydrate phase VOPO4·2H2O and hydrate phase VOPO4·H2O were monitored by the V 2p3/2 peak at 518.1 eV and the O 1s peak at 531.2 eV. The VPO catalyst precursor VOHPO4· 0.5H2O, the V4+ oxidation state, shows the V 2p3/2 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. VOPO4·2H2O and VOPO4·H2O were successfully identified by XPS as a metastable phase, which forms at short synthesis times. As the hydrate phase concentration decreases the concentration of VOHPO4·0.5H2O increases. All XPS data were consistent with the earlier proposed mechanism, which supposed that VOPO4 · 2H2O dehydrates to VOPO4 · H2O, delaminates and the delaminated edges of VOPO4 · H2O serve as the nucleation point for growth of VOHPO4 · 0.5H2O.

Original languageEnglish
Pages (from-to)270-281
Number of pages12
JournalJournal of Catalysis
Volume227
Issue number2
DOIs
Publication statusPublished - 25 Oct 2004

Keywords

  • Crystal morphology
  • Growth from solutions
  • In situ XRD
  • Nucleation
  • Phosphates
  • Surfaces
  • Vanadates
  • Vanadium phosporus oxide catalyst
  • XPS

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