| 总结: | <p>Our current reliance on fossil fuels is highly unsustainable due to their limited supply
and environmental impact. There is an urgent global need to revolutionise the way in
which we source and store our energy. Hydrogen is undoubtedly going to play a role
in re-designing the transportation sector in particular, but there are significant barriers
preventing its widespread implementation. The reversible chemical storage of hydrogen as
ammonia provides all the benefits of hydrogen as a fuel, whilst also having characteristics
enabling some of the challenges faced by hydrogen to be overcome.</p>
<p>It is the catalytic cracking of NH<sub>3</sub> to release the intrinsic H2, enabling the provision of hydrogen on demand for use in fuel cells or combustion engines that is arguably the most
viable solution to developing a sustainable energy system. The main reason the potential
of ammonia as a carbon free hydrogen vector at the point of use has yet to be realised is
largely due to the unavailability of an efficient, cheap and effective means for accessing its hydrogen content at scale.</p>
<p>Earth abundant Group 1 and 2 metal amides are among the most highly active and effective NH<sub>3</sub> decomposition catalysts at modest temperatures and have matched or surpassed
the performance of optimised transition metal based catalysts. A progression of different
systems have been studied to investigate the ability of mixed-metal amides to tailor the
active form and activity of the catalyst towards decomposing ammonia.</p>
<p>A wide range of mixed-metal amides can be successfully synthesised by straightforward
ball-milling of the constituent single metal amides in the appropriate ratios. The structure and behaviour of these high purity samples was then investigated under inert gas at a
range of temperatures through both ex-situ laboratory based, and in-situ neutron diffraction experiments. Correlating changes in the structures of these mixed-metal amides with
variations in the mass of the sample as a function of temperature have enabled a complete
description of the behaviour and dynamics of the systems to be obtained.</p>
<p>Analysis of the low temperature behaviour of Li<sub>3</sub>Na(NH<sub>2</sub>
)<sub>4</sub> and Li<sub>3</sub>K(NH<sub>2</sub>
)4 under argon, showed initially NaNH<sub>2</sub> or KNH<sub>2</sub> melts and separates out of the structure via nonstoichiometric Li<sub>3</sub>R<sub>(1-x)</sub>(NH<sub>2</sub>
)<sub>(4-x)</sub> (R=Na or K). There is then a small temperature range
for which the amides are present as separate components, before the solid LiNH<sub>2</sub> dissolves
into the liquid NaNH<sub>2</sub> or KNH<sub>2</sub>
to form an inter-mixed molten phase with a composition
approaching that of the constituent amides. LiNa<sub>2</sub>
(NH<sub>2</sub>
)3 and LiK<sub>2</sub>
(NH<sub>2</sub>
)3
cannot support this non-stoichiometry as they are line phases so they simultaneously separate and
melt in one step, but ultimately form analogous inter-mixed molten phases. NaK<sub>2</sub>
(NH<sub>2</sub>
)3
does not separate on melting and instead forms molten NaK<sub>2</sub>
(NH<sub>2</sub>
)3 which is stable to at
least 300 °C, and only separates above this temperature into a molten phase approaching
NaNH<sub>2</sub>+ 2KNH<sub>2</sub>
.</p>
<p>At high temperatures it is the decomposition of the two amides within the inter-mixed
molten phases that is observed, and there is a degree of interaction between these two
concurrent processes. It is ultimately H<sub>2</sub> and N<sub>2</sub>
release that is observed, in ratios dependent
on those of the constituent single metal amides, rather than NH<sub>3</sub> which is seen for the
decomposition of LiNH<sub>2</sub> alone. The decomposition temperatures of the single metal amides,
when part of this combined melt phase are also significantly lower than those seen during
their individual decomposition.</p>
<p>A thorough analysis of the phase space between the two end members of the Li-K-N-H
and Na-K-N-H systems led to the identification of two new phases, nominally “LiK(NH<sub>2</sub>
)<sub>2</sub>
”
and “Na3K(NH<sub>2</sub>
)<sub>4</sub>
”. The synthesis of a novel triple mixed-metal amide with a composition of
Li<sub>6</sub>NaK(NH<sub>2</sub>
)<sub>8</sub> has been proven and all these species are close to being crystallographically
characterised.</p>
Imides generally have higher melting points than their corresponding amides, so an imide
which forms at low temperatures and remains solid over a wide range of operating conditions
would be an ideal candidate for a practical NH<sub>3</sub> decomposition catalyst. K<sub>2</sub>Mg(NH<sub>2</sub>
)<sub>4</sub> and
Na<sub>2</sub>Mg(NH<sub>2</sub>
)<sub>4</sub> are metastable kinetic products and do not readily separate, instead displaying a single melt temperature. They are stable to much higher temperatures before they decompose to novel molten mixed amide-imide phases, KMg(NH<sub>2</sub>
)(NH) or NaMg(NH<sub>2</sub>
)(NH)
which crystallise out of solution at 367 °C and 413 °C respectively and do not themselves
decompose until over 500 °C.</p>
<p>It is the correlation of the behaviour of each of these mixed-metal amides under inert gas,
with their catalytic activity towards NH<sub>3</sub>
that is crucial to improving their performance and
understanding how to optimise NH<sub>3</sub> decomposition conditions. The initial crystal structure
of the mixed-metal amides does not affect the NH<sub>3</sub> decomposition ability of the intra Group
1 mixed-metal catalysts, as these have separated and melted into a molten phase of the
constituent metal amides at the temperatures required for NH<sub>3</sub> decomposition.
LiNH<sub>2</sub> decomposes to the imide under NH<sub>3</sub> whereas NaNH<sub>2</sub> and KNH<sub>2</sub> decompose to
the respective metals. It is therefore Li<sub>2</sub>NH, Na, K that are the active forms of the catalysts
and it is the stoichiometry to which these are present, that ultimately affects the overall NH<sub>3</sub>
decomposition ability of the system. However, K<sub>2</sub>Mg(NH<sub>2</sub>
)4 and Na<sub>2</sub>Mg(NH<sub>2</sub>
)4 decompose
to KMg(NH<sub>2</sub>
)(NH) and NaMg(NH<sub>2</sub>
)(NH) which are the active forms of the catalysts for
these systems.</p>
<p>Mixed-metal amide systems can out-perform single metal amides across the entire temperature range. LiNa2
(NH<sub>2</sub>
)<sub>3</sub> and LiK<sub>2</sub>
(NH<sub>2</sub>
)<sub>3</sub> are currently the best low temperature
catalysts and Li<sub>3</sub>Na(NH<sub>2</sub>
)<sub>4</sub> and Li<sub>3</sub>K(NH<sub>2</sub>
)<sub>4</sub>
show the best conversion at high temperatures
to date. It is the imide forming component which appears crucial to optimising the NH<sub>3</sub>
decomposition ability. This provides evidence that there is added value of the mixed-metal
amides as NH<sub>3</sub> decomposition catalysts compared to single metal amides.</p>
<p>The percentage recovery of the catalyst is crucially important in order to maintain
efficiency and improve lifetimes. Na and K are volatile so the Li-Na-N-H and Li-K-N-H
systems suffer from poor containment and reversibility of such materials is clearly limited
under these conditions. The NH<sub>3</sub> decomposition ability of the K-Mg-N-H and Na-Mg-N-H
systems is entirely different to that of the Group 1 mixed-metal amides, but significantly
worse. However, K2Mg(NH<sub>2</sub>
)4 and Na2Mg(NH<sub>2</sub>
)4
show the highest catalytic mass recovery
rates of close to 90 %, due to stabilisation provided by the amide-imide complexes.</p>
<p>Future research efforts will therefore identify elements which combine the important
properties of high activity towards NH<sub>3</sub> decomposition with high mass recovery. This
is likely to occur within a system which has an unstable amide and a stable imide or
amide/imide in order to optimise the best catalysts for commercial applications.</p>
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