Surface characterization of acidic ceria-zirconia prepared by direct sulfation

Acidic ceria-zirconia (SCZ) solid acid catalysts with a nominal surface density of ca 2 SO₄²⁻/nm² were prepared by a simple route consisting in soaking high specific surface area Ce_xZr_1−xO₂ (with x = 0.21 and 0.69) mixed oxides solutions in 0.5 M sulphuric acid. Characterizations by TPD-MS, TP-DRIFTS and FT-Raman revealed that most of surface structures generated by sulfation are stable at least up to 700 °C under inert atmosphere and consist mainly as isolated sulfates located on defects or crystal planes and to a lesser extent as polysulfates. Investigations by pyridine adsorption/desorption have stated that: SCZ possess both strong Brønsted (B) and Lewis (L) acid sites, some of them being presumably superacidic; the B/L site ratio was found to be more dependent on the temperature and hydration degree than on the composition of the ceria-zirconia. By contrast, the reactivity of the parent Ce_xZr_1−xO₂ materials towards pyridine is mostly driven by redox properties resulting in the formation of Py-oxide with the participation of Lewis acid sites of moderate strength (cus Ce^x+ and Zr^x+ cations). Basicity studies by CO₂ adsorption/desorption reveal that SCZ surfaces are solely acidic whereas the number and strength of Lewis basic sites increases with the Ce content for the parent Ce_xZr_1−xO₂ materials.