Influence of Sulfur on Liquid Fuel Reforming.

Abstract

The production of hydrogen and CO (synthesis gas) through reforming of petroleum distillates, such as commercial gasoline, is a growing technology with widespread potential impact on America’s energy efficiency. This dissertation describes the application of Ni-based catalysts to the Autothermal Reforming (ATR) of isooctane, a surrogate for gasoline. In this system, isooctane, air and water react to form an equilibrium-limited effluent, comprised chiefly of synthesis gas. Unfortunately, the widespread adoption of Ni-catalyzed ATR is limited by the tendency of the catalysts to lose activity when exposed to even low concentrations of sulfur. Experiments explored the effects of thiophene on isooctane reforming over Ni under varying reaction stoichiometries. As expected, the presence of thiophene led to lower production of synthesis gas for all conditions. One finding of this work was that the steam reforming performance of the catalyst was more adversely affected by the presence of sulfur than was partial oxidation activity. However, stable performance of the catalyst for at least 48 hours-on-stream was achieved at inlet conditions which favored high production of hydrogen (hydrogen molar fractions greater than 30%). Interestingly, these conditions also corresponded to those when thiophene was largely or completely converted to hydrogen sulfide. In an effort to examine the role of under-coordinated sites, an innovative approach was used to investigate the influence of Ni particle size during isooctane ATR. Under sulfur free conditions, catalysts comprised of roughly 5 nm Ni particles produced 32% less synthesis gas than catalysts with a mean Ni diameter of 50 nm. Although the bigger particles were more affected by sulfur exposure, they still had a 25% higher yield of synthesis gas than the smallest particles when thiophene was present. Finally, several bimetallic catalysts were designed and tested for their durability under high exposure of thiophene. Increased stability appeared possible by alloying Ni with either platinum or tungsten. This study has revealed new understanding about the performance of this reaction as determined by reaction conditions, sulfur content, Ni particle size, and bimetallic metal formulations. This new wisdom provides the foundation for the production sulfur-tolerant ATR reactors by employing inexpensive Ni-based catalysts.