Thermodynamic complexity of sulfated zirconia catalysts

A series of sulfated zirconia (SZ) catalysts were synthesized by immersion of amorphous zirconium hydroxide in sulfuric acid of various concentrations (1–5 N). These samples were fully characterized by X-ray diffraction (XRD), thermogravimetric analysis and mass spectrometry (TGA-MS), and aqueous sulfuric acid immersion and high temperature oxide melt solution calorimetry. We investigated the enthalpies of the complex interactions between sulfur species and the zirconia surface (ΔH_SZ) for the sulfated zirconia precursor (SZP), ranging from −109.46 ± 7.33 (1 N) to −42.50 ± 0.89 (4 N) kJ/mol S. ΔH_SZ appears to be a roughly exponential function of sulfuric acid concentration. On the other hand, the enthalpy of SZ formation (ΔH_f), becomes more exothermic linearly as sulfur surface coverage increases, from −147.90 ± 4.16 (2.14 nm⁻²) to −317.03 ± 4.20 (2.29 nm⁻²) kJ/mol S, indicating formation of energetically more stable polysulfate ​species.