The necessity for alternative energy solutions is motivated from increasing fuel prices, stringent emission regulations, and depleting fuel resources.  The most attractive option is H₂, as it has high-energy efficiency and H₂O is the only byproduct of its combustion. Traditionally, hydrogen generation involved the use of carbonaceous substances such as methanol and methane, but the combustion of these fuels give off undesired CO_x (x = 1,2) emissions. However, one alternative is ammonia because of its high hydrogen storage capacity (17.7 %), energy density (3000 Wh/kg), and the only byproduct of the decomposition is environmentally benign N₂. The decomposition reaction is endothermic (ΔH = +46 kJ/mol) and the reaction temperature is dependent on the catalyst used [1]. Therefore, the most effective catalysts are those that maximize the decomposition efficiency at the lowest possible temperatures.  To date, the best monometallic catalyst for ammonia decomposition is Ru, and this catalyst is further enhanced by the addition of alkali/alkaline earth metals [2,3].  Unfortunately, the mechanism of alkali/alkaline earth promotion is poorly understood.

Using high throughput experimentation, we have screened nearly twenty single metals and several bimetallic catalysts as a function of reaction temperature, calcination temperature, catalyst loading, inlet composition, promoter, support, and preparation solvent (~ 400 experiments).  Through these experiments, we have confirmed that Ru is the best catalyst amongst all the single metal and bimetallic combinations tested.  Furthermore, we have optimized the catalyst further by adding 12 wt% K to a 4 wt% Ru catalyst.  This addition of K provides an additional increase of ~35% in NH₃ decomposition efficiency at 350șC.

To better understand the enhanced decomposition efficiency, advanced characterization via FESEM and FETEM has been completed on the K-promoted Ru catalyst.  Using high spatial resolution, we hope to obtain a clear microscopic picture of the catalyst that will help provide insight towards the mechanism of K promotion.  Initial FESEM experiments on the pure Ru catalysts have revealed 20-100 nm particles.  However, on the K-promoted catalyst, we discovered nanowhiskers that have 20-50 nm diameters and lengths ranging from a few nm to several microns.  Selected-area electron diffraction shows that these whiskers have a KRu₄O₈ hollandite structure.  We believe that this structure is responsible for the enhanced decomposition behavior and a necessary component of our catalyst.  FESEM analysis completed before and after reaction shows that the K-promoted catalyst undergoes significant morphological changes during the reaction, but that the activity is maintained.  This observation suggests that the hollandite may be a structural intermediate that aides in dispersion of K and Ru throughout the catalyst.  Although the exact nature of the KRu₄O₈ hollandite is not yet known, these results appear to be the first identification of this structure in an ammonia decomposition catalyst and a first step towards understanding the role of K in Ru catalysts supported on Al₂O₃.  

[1] Yin, S.F., Xu, B.Q., Zhou, X.P., and Au, C.T., Appl. Catal. A: General 277, 1-9 (2004).

[2] Rarog-Pilecka, W., Szmigiel, D., Kowalczyk, Z., Jodzis, S., and Zielinski, J., J. Catal 218, 465-469 (2003).

[3] Kowalczyk, Z., Jodzis, S., Rarog W., Zielinski, J., Pielaszek, J., Appl. Catal. A: General 173, 153-160 (1998).