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 CrO_x/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 Cr^V species (⁵³Cr). Analogously, in MoO_x/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 Mo^V(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 Cr^V and Mo^V 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 Cr^V and Mo^V, 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.