On the metastability of 'monolayer coverage' in the MoO3/ZrO2 dispersions

Sintering of differently prepared MoO3/ZrO2 supported systems and solid-solid wetting (SSW) in the mixtures of these oxides upon their heating was studied. It has been shown that metastable 'monolayer' coverage of zirconia surface by polymeric molybdate species exists. It contains about 5 Mo atoms per nm(2) of surface and shows Mo/Zr XPS ratio of 0.13-0.15. The 'monolayer' coverage can be obtained either from sintering of high surface area dispersions, or from heating of mixtures of bulk oxides owing to SSW phenomenon. Formation of this metastable structure provides a linear proportionality between the surface area and Mo content for the samples calcined at 800 degrees C, below 12 wt.% of MoO3 loading, whatever the preparation method and initial surface area. Since the 'monolayer' dispersion can be formed by SSW mechanism, the thermal stability of zirconia surface area can be increased just by mixing with a small amount of bulk MoO3. Qualitative thermodynamic discussion of the observed effect is presented.