Exchange-coupled CrV and MoV ions on the surface of CrO x /ZrO2 and MoO x /ZrO2 : an electron paramagnetic resonance study

Abstract

Samples with different Cr or Mo loading have been prepared by contacting the ZrO2 support with CrO3 solutions at pH = 1 (Cr content 0.05–5 wt.%) or (NH4)6Mo7O24 solutions at pH = 8 (Mo content 0.05–7 wt.%). In CrOx/ZrO2 samples heated with O2 at 773 K, a weak line at 1642 G is observed in the EPR spectra together with a strong signal due to mononuclear CrV species (53Cr). Analogously, in MoOx/ZrO2 samples heated with O2 at 773 K and reduced with H2 at 673–773 K, a weak line at 1674 G is detected together with a strong signal of mononuclear MoV(95Mo). The two half-field signals (1642 and 1674 G, respectively) are substantially broader in the samples enriched with 53Cr and 95Mo because of hyperfine interactions, whose structure is not, however, resolved. These lines arise from surface species as shown by experiment with O2. Relying also on chemical evidence (reversibility of the species in redox experiments), the two weak lines are assigned to the half-field transition of the state S= 1 of exchange-coupled CrV and MoV ions on the surface of ZrO2. It is suggested that the lines arise from isolated pairs. From the position of the line: (i) the D-term of the pair, (ii) the distance between ions in the pair and (iii) the probability of the half-field transition, are calculated.For both CrV and MoV, the concentration of the isolated mononuclear species and that of the isolated pairs measured by EPR are found to be in agreement with the corresponding values calculated on the assumption of a random distribution of Cr (or Mo) on the ZrO2 surface.