The Locus of Sulfate Sites on Sulfated Zirconia

Abstract

The surface of sulfated zirconia was probed using X-ray photoelectron spectroscopy. It was observed that the entire inventory of sulfur could be completely removed by sputtering the surface using an argon beam. Calibration using a TiO₂(110) standard resulted in a surface concentration of 2.85 sulfur atoms/nm². This is in reasonable agreement with a value of 4.15 sulfur atoms/nm² based on sulfur analysis on the assumption that all of the sulfur was located at the surface. These results suggest that most, if not all, of the sulfur is near or at the surface. When charging was taken into account, we observed that the oxidation state of sulfur did not change following catalyst deactivation during the isomerization of n-butane. We also determined that the entire inventory of sulfur was present as SO₄ ^2-. These results reinforce previous studies suggesting that catalyst deactivation occurs as the result of carbon deposition and not a change in the oxidation state of sulfur.