The relationship between surface defects and surface reaction pathways on defective molybdenum sulfide(0001) single crystal surfaces.

摘要

Hydrodesulfurization (HDS), the removal of sulfur from organic compounds, is important in the processing of crude oil. The industrial catalyst used is sulfided molybdenum promoted with Co or Ni supported on alumina. The present investigation examines MoS₂ single crystals as model HDS catalysts. MoS₂(0001) was chosen because it has the closest structure to the real catalyst, and still can be easily characterized with standard surface science techniques.; This study is divided into two main areas of interest. The first area examined methods to create vacancies on the (0001) surface of MoS₂, since the MoS₂(0001) surface has been found to be relatively un-reactive for HDS. One way to create vacancies is to use hydrogen to remove sulfur from the surface. This would mimic the way defects are created in the real catalyst environment and develop a better understanding of hydrogen and hydrogen sulfide in HDS reactivity. The other method to create vacancies is to bombard the surface with argon ions to physically remove sulfur atoms from the surface, creating defect sites. Due to the limitation of the amount of hydrogen exposure needed to create sufficient vacancies to effect the surface reactivity, ion bombardment was favored to study the effects of sulfur defects on the HDS reactivity.; The second part of this study is to determine the effect of site vacancies on the changes in surface reactivity using two probe molecules. Methanethiol was chosen as the first test molecule because it is the simplest organosulfur molecule and would be the easiest to determine the reaction pathways. Methanethiol HDS on both MoS₂(0001) and defective MoS₂(0001) surfaces show products of hydrogen, methane, ethane, ethylene and methanethiol, desorbing at 125 K and 300 K. Thiophene was chosen because it is the simplest of the aromatic organosulfur compounds. Thiophene only showed HDS reactivity on defective MoS₂(0001) surfaces. Thiophene products were hydrogen, methane, ethylene, propadiene, and thiophene, desorbing at 170 K and 400 K. With short sputtering times the activity toward the probe molecules increase. At longer sputtering times the activity appeared to be transitioning to a more metal single crystal surface.; This is the first real study of the reactivity on a modified MoS ₂(0001) single crystal. It appears that it will be plausible that modified MoS₂(0001) surfaces can be used as an ideal catalyst to gain a better understanding of the surface reaction pathways of real HDS catalysts.