Conventional fuels exist in limited reserves and have adverse environmental impacts. Researchers are striving hard to either introduce alternative sources to replace conventional fuels or use the existing sources efficiently. Towards attaining sustainable development, it is considered an obligation to use the existing energy reserves efficiently with reduced environmental issues. Fuel cells are energy conversion devices which convert chemical energy into electricity. They are promising in that they do not cause pollution at source and can operate at relatively high energy efficiencies. The only by-product coming out of a fuel cell is water. Use of the fuel cell in transportation sector has been an area of active research. While car manufacturers are rigorously looking at challenges in replacing internal combustion engine with fuel cell stack, it has attracted also attracted a great deal of attention from heavy duty trucks. Trucks generally idle about 20-40% of the time the engine is running, depending on season and operation. During idling of trucks, the internal combustion engine is known to operate at much lower efficiencies, typically less than 5%. It is here where fuel cells can be used effectively as auxiliary power units (APUs). While the efficiency in a typical internal combustion engine of a truck rarely exceeds 20%, a fuel cell normally operates at around 40% to 60%. The maximum theoretical efficiency of a hydrogen fuel cell is around 83% and the higher efficiencies can be attributed to the fact that they do not operate on a themal cycle unlike internal combustion engines. Hydrogen used to run a fuel cell can be produced from transportation fuels. Given the advantages such as their availability, relatively low cost, and existing infrastructure for delivery and transportation, they are likely to play a major role in hydrogen production in the near future. Hydrogen production via transportation fuels such as diesel, gasoline and jet fuel is hindered by catalyst deactivation during its production via reforming. Deactivation of reforming catalyst used for hydrogen production from diesel or jet fuel creates a significant barrier to commercialization of fuel cell technologies. In order to design better catalysts, it is vital to understand the mechanisms of catalyst deactivation and then use the understanding for better catalyst development.
In an attempt to address the fundamental problem of catalyst deactivation, two catalyst systems, NM4 and 3J1, were extensively studied for deactivation. NM4 is a binary catalyst consisting of Ni and Rh while 3J1 consists of Ni, Pd and Rh. In the first part of the study, deactivation on NM4 was studied using n-hexadecane doped with thiophene as the surrogate fuel for diesel. 3J1 was the focus of study in the second part where a mixture of hydrocarbons (n-hexadecane, toluene, tetralin and 1-methylnapthalene) was used to represent jet fuel. The tests were also performed on real jet fuel (Jet A) obtained from the NASA Glenn Research Center. The basic difference between these fuels is the high content of aromatics and naphthenes in case of jet fuel in addition to high organic sulfur. The studies were aimed at understanding the effect of this difference on deactivation characteristics. Analysis of fresh catalysts showed the presence of two groups of particles, primarily distinguished by their size and composition; particles in the range of few nanometers to 6 nm were predominantly made of rhodium while nickel was generally seen as large crystallites. Palladium was seen in close proximity to nickel and generally present as large crystallites. Steam reforming caused sintering of crystallites containing nickel while no identifiable growth in rhodium crystallites was observed. The steam reforming activity was measured in terms of H2 yields. The catalysts showed superior activity and stability during steam reforming of paraffins represented by n-hexadecane. In the presence of naphthenic species such as 1-methylnaphthalene, the catalysts deactivated rapidly leading to quick drops in hydrogen yields. The catalysts were stable in the absence of sulfur but deactivated over a period of 10 h when sulfur was present at high loading. Stability and activity were higher with higher amounts of rhodium content when nickel was kept constant. Palladium was seen to cause excessive cracking and thereby leading to significant deactivation.
Sulfur compounds were converted to primarily hydrogen sulfide via hydrodesulfurization. It was found when complete conversion of sulfur compounds to hydrogen sulfide was observed, the catalyst lost only a small portion of its activity. Only when thiophene started to exit unconverted, the catalyst started to deactivate steadily. In addition, presence of sulfur led to significantly increased amounts of cracking products which indicated that sulfur deactivated primarily active sites responsible for steam reforming. STEM-EDS of used catalysts revealed preferential adsorption of sulfur on the surface of nickel crystallites; EDS and XRD analysis showed no bulk sulfide formation. No detectable sulfur was seen on rhodium crystallites. Excessive carbon deposition was observed during steam reforming of sulfur-containing fuel. Graphitic carbon was present at all stages of catalyst life irrespective of the catalyst composition and it was significantly higher for sulfur-containing fuels. Blocking of reactant species on the surface of the catalyst due to the formation of aromatic/polymeric carbon on the support was also seen, although higher rhodium content inhibited this phenomenon.