The_Catalyst_Review_June_2023 - 6

Process News
Researchers Use Lithium to Split Nitrogen, Make Ammonia
Using air, water and renewable electricity, researchers in Australia have pioneered a high-selectivity, lithium-mediated
electrochemical pathway to ammonia. The effort strengthens the path toward ammonia as a carbon-free fuel, and reportedly
achieves results at a rate that is commercially competitive with Haber-Bosch. Lithium can take the triple-bonded nitrogen
molecule (N2
in a catalyst-mediated process, then
) and, under ambient conditions, break it in two; the team aims to harness this chemistry to make ammonia (NH3
combine the atoms with protons (H+) sourced from water to form ammonia. That is the theory, at least; but it has only been
), instead of ammonia. Hydrogen is
).
The idea of pulling apart nitrogen molecules to make ammonia using an electrical current, rather than high temperatures and
pressures, goes back a century. The electrodes in an electrochemical cell can split N2
created in miniscule amounts, not enough to be commercially viable. The challenge lies in suppressing a side reaction in which
the cell takes the simpler path of combining pairs of protons to produce hydrogen gas (H2
usually the predominant product in the electrochemical process. The problem, known as the selectivity challenge, is described
by a metric called faradaic efficiency (FE): the amount of ammonia produced relative to the ammonia that could be generated
based on electrical input. Until a few years ago, ammonia selectivity of only 5%-20% FE had been reported. After trying a
number of possibilities, the team decided to try to break open the nitrogen molecule using lithium. The key chemistry of the
lithium-mediated process happens at the cathode of the electrochemical cell. Here, lithium and nitrogen react to form lithium
nitride (Li3
N). This intermediate reacts with protons (generated at the anode) to release ammonia and regenerate the lithium.
The team initially found that by adding a phosphorus-based proton shuttle to mediate proton delivery to the cathode, they
had reached 69% FE. A year later, they reported that by switching to an electrolyte that better supported the lithium-mediated
nitrogen-splitting step, they had hit almost 100% FE. Source: Nature, 5/24/2023.
Olefins-from-Coal Catalysis Procedure
Makes Strides
Researchers have reported a strategy to disentangle the
activity-selectivity tradeoff for direct conversion of syngas
into light olefins ethylene, propylene, and butylene. For
almost a century, the Fischer-Tropsch synthesis (FTS) was
used for direct syngas conversion with iron or cobaltbased
catalysts for synthesis of chemicals. However,
selectivity for light olefins remained a challenge. An
alternative process, OXZEO, developed six years ago
by the same research team using metal oxide-zeolite
catalyst improved light-olefin selectivity far beyond the
theoretical limit of FTS. Despite the significant progress
over the years, the activity is still limited by the activityselectivity
tradeoff. For example, when FTS is used to
convert syngas to light olefins, the yield amounts to
around 26%. Using traditional silicon containing zeotypes
within the OXZEO catalyst concept, the light-olefins yield
has so far maxed out at 27%. These limits originate from
activity-selectivity tradeoff, a long-standing challenge in
catalysis. This can be traced to the catalytic sites for both
the target and side reactions, which are usually entangled
on technical catalysts. Now, researchers have shown that
incorporating germanium-substituted aluminophosphates
within the OXZEO catalyst concept can disentangle the
desired target reaction from the undesired secondary
reactions. It enhances the conversion of the intermediates
to produce olefins by creating more active sites and in
turn generation of intermediates but without degrading
the selectivity of light olefins. With this new strategy,
researchers simultaneously achieved high CO conversion
and light-olefins selectivity and the yield reached an
unprecedented 48% under optimized conditions. Source:
Science Magazine, 5/18/2023.
Chemists Unravel Reaction Mechanism
for Clean Energy Catalyst
Using pulse radiolysis and time-resolved spectroscopy,
a team of researchers has unraveled the entire reaction
mechanism for the Cp*Rh water-splitting catalyst. The
breakthrough afforded a complete understanding of a full
catalytic cycle, thereby improving the prospects for the
production of pure hydrogen. Rapid intermediate steps
make it difficult for scientists to decipher exactly where,
when, and how the most important parts of a catalytic
reaction occur-and therefore, if the catalyst is suitable for
large-scale applications. Researchers were intrigued when a
catalyst, a pentamethylcyclopentadienyl rhodium complex,
commonly known as Cp*Rh complex, was demonstrating
reactivity in an area where molecules are usually stable. To
find out more, the team turned to pulse radiolysis, which
harnesses the power of particle accelerators to isolate rapid,
hard-to-observe steps within a catalytic cycle. Time-resolved
spectroscopy tools were used to monitor the chemical
reactivity after this rapid change occurs; the instrumentation
can resolve events at one millionth to one billionth of a
second. By combining pulse radiolysis and time-resolved
spectroscopy with more common electrochemistry and
stopped-flow techniques, the team was able to decipher
every step of the complex catalytic cycle, including the
details of the unusual reactivity occurring at the ligand
scaffold. The group determined that a hydride group, an
intermediate product of the reaction, jumped onto the Cp*
ligand, proving that the Cp* ligand was an active part of
the reaction mechanism. Capturing these precise chemical
details will make it significantly easier for scientists to
design more efficient, stable, and cost-effective catalysts for
producing pure hydrogen. Source: phys.org, 5/15/2023.
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