The 31 P{ 1 H} MAS NMR of SZO300 after triethylphosphineoxide (TEPO) adsorption at 100 o C contains signals at 71 ppm and broad signal centered at ~77 ppm that is typical of Brønsted sites interacting with TEPO. This spectrum also contains a sharp signal at 96 ppm assigned to the TEPO*SO3 adduct (Figure ). To place this value in context, similar 31 P chemical shifts are usually observed for TEPO adducts of silylium-like ions, which are exceptionally strong Lewis acids. This trend holds with silylium-like ions on surfaces, TEPO contacted [ i Pr3Si][SZO] has a 31 P NMR chemical shift of 93 ppm. To rationalize these experimental observations, and to build support for the formation of these SO3 adducts, periodic DFT calculations at the PBE-D3BJ level were carried out on tetragonal ZrO2 surface models modified by sulfate and pyrosulfate species. This model also contains Brønsted sites that have similar structures to those reported previously. An excerpt of this model is shown in Figure . This model contains a pyrosulfate flanked by three sulfate anions to approach the experimental values obtained from ICP-OES data. Adduct formation between TEPO or pyridine to the pyrosulfate site results in formation of the TEPO*SO3 (Eads = -50.2 kcal mol -1 ) or pyridine*SO3 (DEads = -39.7 kcal mol -1 ) adducts, respectively. These adduct formation energies are more exothermic than those calculated from adsorption of TEPO or pyridine onto bare Zr-Lewis sites on a model containing fewer sulfate ions to allow binding to accessible Zr (DEads(py) = -32.3 kcal mol -1 ; DEads(TEPO) = -45.4 kcal mol -1 ). The calculated 31 P NMR chemical shift of TEPO*SO3 is 96 ppm, in excellent agreement with experimental values (Table ).