Title

Desulfurization of diesel for fuel cell applications: Utilizing catalysts and adsorbents synthesized by supercritical carbon dioxide route

Date of Completion

January 2005

Keywords

Engineering, Chemical

Degree

Ph.D.

Abstract

Hydrogen, for fuel cell applications, can be produced on-board or on-site, from a wide variety of sources such as methanol, gasoline and diesel. Diesel fuel is preferred over other fuels due to its high energy density and wide availability. However, diesel has a high concentration of refractory organosulfur compounds such as dibenzothiophenes (DBTs). The presence of such compounds at such concentrations has a detrimental effect on the performance of catalysts used in fuel processing and fuel cell stacks. Therefore, there is a need to develop efficient desulfurization technologies for diesel that would make it a suitable fuel for fuel cells. ^ Two approaches were investigated for desulfurization of diesel. The first approach is hydrodesulfurization (HDS), where the organosulfur compounds are converted to hydrogen sulfide and sulfur-free organic compound(s) by reaction with hydrogen in the presence of a catalyst. Pt/Al2O3 catalysts with Pt loadings ranging from 0.58 to 5.7 wt.% were synthesized by supercritical carbon dioxide (scCO2) deposition method and tested for HDS of commercial and model diesel at atmospheric pressure. Transmission Electron Microscopy images showed that the synthesized catalysts contained small Pt nanoparticles (1–3 nm in diameter) with a narrow size distribution, no observable agglomeration, and uniformly dispersed on the alumina support. The catalysts were found to be active towards HDS reactions, without sulfiding the metal phase. However, sulfided platinum was found to be more active. The HDS reaction proceeded only via the direct hydrogenolysis route in the temperature range 310–400°C and at atmospheric pressure. The presence of toluene inhibited the catalyst activity presumably due to competitive adsorption between DBT and toluene. Commercial diesel HDS under the studied conditions using the same catalysts was found to be feasible, however with some compromise. ^ The second approach investigated for sulfur removal was desulfurization by selective adsorption, where diesel fuel is brought in contact with a solid adsorbent which selectively adsorbs sulfur-containing compounds over other hydrocarbons. There is a need to develop adsorbents with high capacity and selectivity toward organosulfur compounds. Carbon aerogels (CAs), which are porous materials with a high surface area, were synthesized via scCO 2 route. CAs with two different average pore sizes (4 nm and 22 nm) were synthesized, characterized and tested for adsorption of DBT from n-hexadecane at ambient temperature and atmospheric pressure. The approach-to-equilibrium was monitored and adsorption isotherms were measured. The CA with the larger average pore size had a higher sulfur adsorption rate and a higher capacity for DBT. According to Langmuir isotherm, the adsorption capacities of the 4 nm CA and 22 nm CA were 11.2 and 15.1 mg S/g dry CA, respectively. CAs were found to selectively adsorb DBT over naphthalene when both present in solution. However, the presence of naphthalene reduced the amount of DBT adsorbed. ^