Electron paramagnetic resonance and electron spin echo study of supported and unsupported vanadium oxides

Abstract

Electron paramagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM) spectroscopies have been used to characterize paramagnetic centers in crystalline and gel forms of vanadium pentoxide as well as vanadia supported on silica, titania, and magnesia. A number of different paramagnetic centers are observed in the bulk and supported vanadia phases that are characterized by their spin Hamiltonian parameters. ESEEM data for all the bulk vanadium oxide samples show intense modulation at the ⁵¹V Larmor frequency. This modulation originates from interlayer VO²⁺ species in gel samples and from V⁴⁺ centers in crystalline oxides. The intensity of the ⁵¹V modulation varies considerably from sample to sample depending on the specific nature of the paramagnetic center. For the supported vanadia samples, complex EPR spectra are obtained that show two classes of signals. Signals with narrow linewidths and resolved hyperfine structure are observed suggesting magnetically isolated V⁴⁺ in addition to dipolar exchange broadened signals. ESEEM spectroscopy reveals that the spectra observed for vanadia supported on silica are from surface bound vanadyl species that interact with predominantly diamagnetic surface vanadia clusters. In contrast, for vanadia supported on titania, the V⁴⁺ signals observed by pulsed EPR show no modulation. The absence of modulation is attributed to well dispersed surface vanadyl species and subsurface V⁴⁺_6c centers. Vanadium supported on magnesia interacts to form new magnesium vanadate phases containing V⁴⁺_6c. It is shown that ESEEM is a valuable tool for characterizing vanadia supported on various oxides.