Xenophon Verykios

Abstract:

The concept of the "hydrogen economy" seems to be moving into the world of political and strategic planning as well as into business and enterprise. This is primarily due to the fact that it has been widely recognized and accepted that major improvements in energy efficiency of electric power generation must be achieved in order to reduce emissions of pollutants and, particularly, greenhouse gases such as CO₂ and CH₄. A major underlying principle in the utilization of hydrogen for power production is that of distributed power generation, which arises primarily due to severe difficulties in hydrogen storage and transportation. To overcome these difficulties, the idea of hydrogen production on the spot and on demand has been gaining ground. This implies that an appropriate hydrogen carrier, which could be liquid (methanol, ethanol, LPG, gasoline, etc.) or a gas (natural gas, biogas, etc.) can be used to extract hydrogen from. The process of small scale hydrogen production and power production by fuel cells is the heart of distributed power generation systems.
Results and Discussion
The fuel processor produces a hydrogen - rich stream which is suitable to be fed into the fuel cell. For PEM fuel cells, the most critical requirement is that the CO content in the feed stream is less than 30 ppm. Thus, the process comprises of the reformer, the water-gas shift reactors (low and high temperature) and the CO selective oxidation or selective methanation reactor(s) which minimize CO.
The reformer requires significant amounts of heat to be transported to the reformation zone. For this reason it is designed as a heat exchanger. In one concept, both, the reformation and the combustion catalysts are deposited on opposite sides of plates or tubes, in the form of thin films. This results in a very compact reformer with a high capacity for hydrogen production and high thermal efficiency. The reformation catalyst is normally Ni-based with a carrier designed to enhance its capacity to resist carbon deposition. To this effect, various promoters can also be added. The combustion catalyst is usually Pd-based with provisions to enhance its maintenance in the oxidized form, which is the most active in the fuel combustion process.
Concerning the water-gas shift reaction, it has been found that the catalytic performance of supported noble metal catalysts depends strongly on the nature of the metallic phase and the nature of the metal oxide support employed. Platinum catalysts are generally more active than Ru, Rh and Pd, and exhibit significantly improved activities (by 2 orders of magnitude) when supported on "reducible"(CeO₂, TiO₂, YSZ, MnO, Fe₂O₃, La₂O₃) rather than on "irreducible" (Al₂O₃, MgO, SiO₂) metal oxides. Specific reaction rate (TOF) does not depend on loading or crystallite size of the metallic phase. The activity of Pt/TiO2 catalysts can be further improved with decreasing the primary crystallite size of the support. Based on results of temperature programmed reduction (TPR), and in situ Raman and FTIR spectroscopies, this behaviour has been attributed to the higher reducibility of smaller titania crystallites. The catalytic performance of titania-supported platinum catalysts can be further improved by addition of small amounts of alkali (Na, K, Li, Cs) or alkaline earth (Ca, Ba, Sr, Mg) promoters. For optimized catalysts, specific activity (TOF) is about four times higher, compared to that of unpromoted Pt/TiO₂. Results of H₂^- and CO-TPD experiments demonstrate that promoters affect the population and strength of adsorption sites located at the metal support interface, which are suggested to be catalytically active.
The catalytic performance for the reaction of the selective methanation of CO depends strongly on the nature of the dispersed metallic phase and oxide support employed. Activity for CO/CO₂ hydrogenation is much higher for Ru and Rh catalysts, compared to Pd or Pt, and is significantly improved when supported on TiO₂, compared to Al₂O₃, CeO₂, YSZ or SiO₂. Both hydrogenation reactions are structure sensitive, and specific activity (TOF) increases substantially with increasing Ru crystallite size. Addition of up to 30% water vapour in the feed does not practically affect conversion of CO but retards CO₂ methanation, thereby expanding the temperature window of operation for the title reaction.

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