Reports: UNI550385-UNI5: The Role of Defects in Sulfur Removal Over Titanium Dioxide

Lauren Benz, PhD, University of San Diego

The second year of this grant has proven to be very successful. We completed the study of the four organosulfur molecules discussed in the last reporting period: thiophenol, thioanisole, tetrahydrothiophene, and thiophene. This study of the interaction of these representative organosulfur compounds with titanium dioxide and defects therein was recently published in Langmuir (please see citation). We summarize the main findings below, and discuss the positive impact of this work on the students involved.

Two sets of structurally analogous organosulfur molecules (thiophenol/thioanisole and thiophene/tetrahydrothiophene) were selected for study with the titanium dioxide surface as they are representative of some of the core sulfur-containing structures present in petroluem. We started with a nearly stoichiometric TiO2 substrate which contained 94% Ti4+ and only 6% Ti3+ defect sites. All adsorbed molecules (90K) desorbed upon heating to ~400K, as analyzed by temperature programmed reaction spectroscopy (TPRS) and X-ray photoelectron spectroscopy (XPS), with the exception of thiophenol which experienced ~25% decomposition into CxHy and S fragments. No evidence of additional reaction products was revealed with TPRS (capable of scanning up to 300 amu). We also examined the interaction of these species with a defect-rich TiO2 surface. The surface was prepared with argon bombardment to induce the formation of additional Ti3+ defects (~80% Ti4+ and 20% Ti3+ determined using XPS). On this surface, we observed the formation of benzene from thiophenol at 400K, in addition to a small amount of decomposition similar to that observed over the nearly stoichiometric surface. We identified a distinct shift in the S 2p XPS signal correlating to the desorption of benzene which led us to propose the following mechanism: A fraction of the adsorbed thiophenol binds to the bombardment-induced Ti3+ defects leading to the formation of phenylthiolate intermediates. These intermediates decompose upon heating to 400K to release benzene. Atomic sulfur remains on the surface (in addition to CxHy fragments as described previously). This study points to the relative reactivity of S-H bonds in comparison to S-C bonds, highlighting the difficulty in the removal of S from thioethers in petroleum. We are currently exploring larger organosulfur molecules and the addition of active metals to TiO2 to enhance the binding affinity and/or reactivity of thioethers. In order to do this we have modified our vacuum chamber to allow for the introduction of larger, low vapor pressure organics and metals to the substrate.

The above findings were reported at the in both the 243rd ACS National meeting in March of 2012 (San Diego) and the 244th ACS National meeting in August of 2012 (Philadelphia). Two students who worked on this project, Michelle Mezher and Aileen Park, presented a poster in San Diego which won an award from the ACS Division of Colloid and Surface Chemistry. The students attended an award luncheon alongside of several distinguished researchers in the field. One of these students, Michelle, will begin graduate studies in chemistry in the fall, while the other continues with the research (graduating in May 2014). In addition, I gave a talk on this work in Philadelphia in the ACS Division of Colloid and Surface Chemistry in a session on molecular processes at surfaces, which received some positive feedback.

The funding of this work by ACS has been instrumental in the development of my lab, in particular with regard to the purchase and installation of the XPS, and also to the success of my students.