Metal-organic frameworks (MOFs) are commended as photocatalysts for H2 evolution and CO2 reduction as they combine light-harvesting and catalytic functions with excellent reactant adsorption capabilities. For dynamic processes in liquid phase, the accessibility of active sites becomes a critical parameter as reactant diffusion is limited by the inherently small micropores. Our strategy is to introduce additional mesopores by selectively removing one ligand in mixed-ligand MOFs via thermolysis. Here we report photoactive MOFs of the MIL-125-Ti family with two distinct mesopore architectures resembling either large cavities or branching fractures. The ligand removal is highly selective and follows a 2-step process tunable by temperature and time. The introduction of mesopores and the associated formation of new active sites have improved the HER rates of the MOFs by up to 500%. We envision that this strategy will allow the purposeful engineering of hierarchical MOFs and advance their applicability in environmental and energy technologies.
Achieving light-driven
splitting of water with high efficiency
remains a challenging task on the way to solar fuel exploration. In
this work, to combine the advantages of heterogeneous and homogeneous
photosystems, we covalently anchor noble-metal- and carbon-free thiomolybdate
[Mo
3
S
13
]
2–
clusters onto photoactive
metal oxide supports to act as molecular co-catalysts for photocatalytic
water splitting. We demonstrate that strong and surface-limited binding
of the [Mo
3
S
13
]
2–
to the oxide
surfaces takes place. The attachment involves the loss of the majority
of the terminal S
2
2–
groups, upon which
Mo–O–Ti bonds with the hydroxylated TiO
2
surface
are established. The heterogenized [Mo
3
S
13
]
2–
clusters are active and stable co-catalysts for the
light-driven hydrogen evolution reaction (HER) with performance close
to the level of the benchmark Pt. Optimal HER rates are achieved for
2 wt % cluster loadings, which we relate to the accessibility of the
TiO
2
surface required for efficient hole scavenging. We
further elucidate the active HER sites by applying thermal post-treatments
in air and N
2
. Our data demonstrate the importance of the
trinuclear core of the [Mo
3
S
13
]
2–
cluster and suggest bridging S
2
2–
and
vacant coordination sites at the Mo centers as likely HER active sites.
This work provides a prime example for the successful heterogenization
of an inorganic molecular cluster as a co-catalyst for light-driven
HER and gives the incentive to explore other thio(oxo)metalates.
We
present a novel approach for the separation and recovery of
Pt and Pd leached from a spent automotive catalyst relying on conventional
and polymerized supported ionic liquid phases (SILPs and polySILPs,
respectively). A variety of parameters with possible effects on the
separation behavior, namely, acidity and concentration of the platinum
group metal (PGM) containing solution, as well as different SILP and
polySILP loadings, were evaluated for the separation of PGMs in the
presence of high concentrations of Al, Fe, Zn, and Ce. The polySILP
material demonstrated the ability to separate the PGMs from major
accompanying interferences in a single separation step, while problems
arising from ionic liquid leaching in the case of SILPs could be avoided.
Moreover, the use of supported ionic liquid phases allowed the drastic
reduction of the amount of required ionic liquid compared to conventional
liquid–liquid separation, while avoiding problems arising from
emulsion formation. Subsequent stripping experiments lead to further
purification of the PGMs and finally desorption from the solid material
into a pure solution. Eventually, the concept of chemisorbed polySILPs
provides a new and convenient approach for the recycling of platinum
group metals.
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