Controlling Product Selectivity in Oxidative Desulfurization Using an Electrodeposited Iron Film

Sulfur-containing compounds must be removed from raw fuel oils before use and recently, there has been an effort to identify and optimize a more energy efficient method of oil processing. One promising route is electrochemical oxidative desulfurization (ODS), and in this work, we investigate an electrodeposited iron oxide film as a working electrode to catalyze the oxidation of dibenzothiophene (DBT). The electrodeposited Fe film displays unexpected selectivity for the DBT sulfoxide (DBTO) — departing from the catalytic behavior of gold, which favors the dimerization of DBT. In addition, we observe a morphological change within our Fe film from ϒ-FeOOH (lepidocrocite) to ϒ-Fe2O3 (maghemite). This change provides insight to the activity of each structure for ODS as the rate of oxidation increases after the incorporation of ϒ-Fe2O3. Our experimental observations are corroborated with DFT calculations, which suggest that the adsorption energy of DBT on Au is significantly greater than on the Fe film, favoring the formation of dimeric and oligomeric product. Calculations also demonstrate that DBT binds closer on ϒ-Fe2O3 (2.04 Å) when compared to ϒ-FeOOH (2.68 Å) due to the presence of a four-coordinate iron site in ϒ-Fe2O3. We propose that this closer surface interaction facilitates electron transfer and leads to difference in catalytic behavior between Au and the electrodeposited Fe film.