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

Electron paramagnetic resonance (EPR) and electron spin echo envelope modul ation (ESEEM) spectroscopies have been used to characterize paramagnetic ce nters in crystalline and gel forms of vanadium pentoxide as well as vanadia supported on silica, titania, and magnesia. A number of different paramagn etic 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 V-51 Larmor fre quency. This modulation originates from interlayer VO2+ species in gel samp les and from V4+ centers in crystalline oxides. The intensity of the V-51 m odulation varies considerably from sample to sample depending on the specif ic nature of the paramagnetic center. For the supported vanadia samples, co mplex EPR spectra are obtained that show two classes of signals. Signals wi th narrow linewidths and resolved hyperfine structure are observed suggesti ng magnetically isolated V4+ in addition to dipolar exchange broadened sign als. ESEEM spectroscopy reveals that the spectra observed for vanadia suppo rted on silica are from surface bound vanadyl species that interact with pr edominantly diamagnetic surface vanadia clusters. In contrast, for vanadia supported on titania, the V4+ signals observed by pulsed EPR show no modula tion. The absence of modulation is attributed to well dispersed surface van adyl species and subsurface V-6c(4+) centers. Vanadium supported on magnesi a interacts to form new magnesium vanadate phases containing V-6c(4+). It i s shown that ESEEM is a valuable tool for characterizing vanadia supported on various oxides.