The energy demand of the world has been exponentially increasing over recent years caused by economic development and increasing population. Fossil fuels have been the dominant sources of power. However, distribution of fossil fuels is largely geographically dependent. Combustion of fossil fuels generates a variety of air pollutants, which contribute to global warming and have caused major environmental concerns. Hydrogen has emerged as an alternative energy source to replace fossil fuels because it can be produced from a variety of sources including renewable sources and combustion of hydrogen produces only water, making it environmentally friendly. Catalytic hydrogen production technology is an integral part of the future energy portfolio. Research concentrates on improving process efficiencies and reducing emission levels for hydrogen production from fossil fuel and investigating effective ways to generate hydrogen from renewable sources.
This work examines catalytic hydrogen production from water-gas shift reaction via coal-derived synthesis gas and steam reforming of propane and bio-ethanol. Efforts have been made in catalyst development, performance evaluation and catalyst characterization to elucidate active sites and reaction network during catalytic reactions.
Fe-Cr catalysts are used in commercial high temperature water-gas shift reactors. This catalyst requires high temperatures to have sufficient activity. However, high temperatures lead to catalyst sintering and limit the conversion due to thermodynamic equilibrium. This brings about the need for a second shift reactor, which will operate at a lower temperature and push the thermodynamic limitation for the conversion. The two-stage operation makes this process costly and impractical. A small amount of Cr6+ is contained in the commonly used Fe-Cr formulation and causes environmental concerns.
In this research, Fe-Al-Cu was developed to replace Fe-Cr catalysts for water-gas shift reaction. Catalyst preparation methods were found to play a critical role in determining catalytic performance. Different from conventional impregnation and coprecipitation methods, sol-gel technique was introduced for Fe-Al-Cu catalyst preparation. Sol-gel catalysts demonstrated high activities over a wide range of reaction temperatures compared with commercial catalysts. Characterization results revealed that Cu promoter was uniformly distributed in the iron oxide structure for sol-gel prepared catalysts and iron oxides with different crystal structures were formed in sol-gel catalysts as well. These properties contribute to high catalyst activity and stability. Cu loading amount was optimized as an important synthesis parameter and catalyst performance was further improved. In addition to these efforts, catalysts were evaluated in the presence of H2S, which is a major impurity from coal derived synthesis gas. A series of Fe-based catalysts were compared for their sulfur resistance and a deactivation mechanism was proposed based on characterization studies for both fresh and poisoned catalysts. The Cu component in Fe-Al-Cu catalysts was found to be more susceptible to H2S and sulfidation of Cu leads to initial catalyst activity loss.
Another focus of this research is on hydrogen production from steam reforming. Propane steam reforming was studied over Ni-Al2O3 catalysts that were prepared by a conventional impregnation method and a one-step sol-gel technique. Impregnated Ni-Al2O3 catalysts showed high initial activity but deactivated severely with time-on-stream of propane steam reforming whereas sol-gel Ni-Al2O3 catalysts maintained relatively stable performance under the same testing conditions. It was found that sol-gel preparation yields strong interaction between the active metal and the support, thus forming highly dispersed small Ni crystallites on the surface. This was proved to be beneficial for coke suppression and catalyst stability. Similarly in Co-ZrO2 ethanol steam reforming studies, sol-gel prepared Co-ZrO2 catalysts exhibited higher coking resistance. The phase evolution with temperature for sol-gel Co-ZrO2 catalysts illustrates the crystallite transition from cubic to tetragonal and finally to monoclinic ZrO2. Co was found capable of stabilizing cubic ZrO2 phase, which may possibly explain the stability of sol-gel Co-ZrO2 catalysts. It was demonstrated that support basicity/acidity correlates with product distribution and catalyst stability. CeO2 was combined with ZrO2 to support Co metals. This mixed oxides catalyst showed less coking and higher activity and stability during ethanol steam reforming.