Hydrogen from Ammonia by Catalytic Spillover Membrane

Abstract

<p>One of the major challenges to be overcome before hydrogen fuelled vehicles can become commonplace is to store hydrogen with sufficient storage density to be practical. One approach to overcoming this challenge involves converting the hydrogen into a secondary fuel that can be stored more easily, such as ammonia. This introduces the challenge of efficiently retrieving the hydrogen from the secondary fuel with sufficient purity to be used in a polymer electrolyte membrane fuel cell. Putting the hydrogen producing reaction inside a membrane which is capable of filtering out hydrogen creates a membrane reactor which can increase hydrogen purity and can accelerate the reaction both kinetically and thermodynamically. The most effective materials currently known for hydrogen membranes are high palladium alloys of copper and silver. These are able to absorb hydrogen on the side with high hydrogen partial pressure and desorb that hydrogen on the side with low hydrogen pressure. Palladium metal is also able to interact with some catalysts by hydrogen spillover. Hydrogen is transported from the surface of the catalyst to the palladium surface more quickly than the hydrogen can desorb from the catalyst, this potentially accelerates both the catalysis and the hydrogen filtration. This research aimed to create a catalytic spillover membrane to extend the possibility of ammonia as a secondary fuel for hydrogen transport. In this research, several methods to produce a nickel catalyst on the surface of the palladium were explored: electrodeposition with and without a lithographic template; spray coating with nanoparticles; and preshaped nickel mesh and nickel foam. These potential catalysts were tested for ammonia decomposition. Templated electrodeposition created the most effective catalyst, but the nickel foam was most easily applied to the next stage of the research. The nickel foam catalyst was subsequently retested for ammonia decomposition in three scenarios: in contact with palladium foil; in a reactor with a palladium membrane; and in contact with a palladium membrane. The presence of a palladium membrane improved decomposition more than spillover contact between nickel foam catalyst and palladium, however, the combination of spillover contact with a palladium membrane increased the ammonia decomposition further. The rate of hydrogen flux through the palladium membranes was calculated for the experimental results. These were compared to flux values predicted by a model equation. The results showed that spillover contact between nickel catalyst and palladium membrane increased the hydrogen flux through the membrane.. The research outcomes have generated new knowledge and improved understanding of the morphology and role of nickel catalysts in accelerating ammonia decomposition. The research highlights the complex relationship between reactor design, gas flow paths, catalyst presentation and catalysis chemistry, suggesting promising areas for future research.</p>