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 (Delta H-SZ) for the sulfated zirconia precursor (SZP), ranging from - 109.46 +/- 7.33 (1 N) to -42.50 +/- 0.89 (4 N) S. Delta H-SZ appears to be a roughly exponential function of sulfuric acid concentration. On the other hand, the enthalpy of SZ formation (Delta H-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. (C) 2016 Elsevier Inc. All rights reserved.