(134e) CO,Sub>x-Free Hydrogen Via NH3 Decomposition On K-Promoted Ru Catalysts Derived From KRu4O8

Discovery of an ammonia
decomposition catalyst that is active at low temperatures is essential to ensuring
the viability of ammonia for use as a chemical carrier for on-board hydrogen
production.  Toward this end, an extensive search of materials and synthesis
conditions was undertaken to identify promising catalysts. The most dramatic
improvement in NH₃ conversion was achieved by promoting Ru supported
on g-Al₂O₃ with K; at 350°C and
GHSV_NH3=4,000 g·mL/h, NH₃ conversion was increased from
10% with unpromoted Ru to 65% with K promotion.

             Extensive
characterization of this catalyst throughout the synthesis process revealed the
formation of an mixed-metal Ru oxide phase after calcination of the catalyst. XRD,
EDS, bright field TEM and HAADF imaging were used to identify this crystalline
phase as hollandite (KRu₄O₈), which manifested itself as
a network of ?whiskers? dispersed on the surface of the g
-Al₂O₃ support. Upon exposure to NH₃, the hollandite
whiskers reduced to Ru^o particles. NH₃ decomposition was
observed, however it was unknown if or how the hollandite contributed to the active
sites.  KRu₄O₈ whiskers were isolated from the supported
catalyst and tested for activity; the whiskers were reduced to Ru^o
particles that showed significant NH₃ conversion.

            Based
on these results, we proposed that the reduction of the oxide hollandite to a
well-dispersed Ru phase was the mechanism by which the K-promoted Ru catalyst
was able to achieve high NH₃ conversion. Recent high resolution
microscopy provides strong supporting evidence; atomic scale images of the
hollandite after initiation of the degradation process show the onset of
formation of Ru^o particles with high dispersion. Furthermore, by
exploring preparation treatments to the K-promoted Ru system, a correlation was
found between hollandite formation after calcination and high NH₃
conversion in reaction conditions. The behavior of hollandite as a sacrificial
structure to a well-dispersed active phase may indicate a novel method of
achieving nanoparticle catalysts derived from a unique precursor material.