(345h) Characterization of K-Promoted Ru Catalysts for Hydrogen Production Via Ammonia Decomposition

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 350ºC.

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).