Investigation of Sulfate Interaction With Zirconium Oxide By X-Ray Absorption Spectroscopy

A large literature exists on the characterization of sulfated ZrO₂ acid catalysts, but X-ray absorption spectroscopy (XAS) of the Zr, O and S elements, presumed to comprise the active site, have not been used in these characterization. In the usual case of crystalline ZrO₂ used as the support, bulk Zr would dominate and the minority surface Zr, associated with surface sulfate, would make this unfeasible. Stabilizing ~ 2 nm ZrO₂ crystallites on a multi-walled carbon nanotube (MWCNT) support renders both Zr K-edge and L-edge XANES spectra sensitive to sulfate interaction. Sulfur K-edge XANES and O and C K-edge NEXAFS can complement the Zr XANES to describe electron transfer between Zr and S, relative to references such as ZrO₂, Zr(SO₄)₂, and (NH₄)₂SO₄. This does not allow a structural model of the acid site, but does provide qualitative molecular orbital insight into the origin of acidity.