The recent advances in the development
of heterogeneous catalysts
and processes for the direct hydrogenation of CO2 to formate/formic
acid, methanol, and dimethyl ether are thoroughly reviewed, with special
emphasis on thermodynamics and catalyst design considerations. After
introducing the main motivation for the development of such processes,
we first summarize the most important aspects of CO2 capture
and green routes to produce H2. Once the scene in terms
of feedstocks is introduced, we carefully summarize the state of the
art in the development of heterogeneous catalysts for these important
hydrogenation reactions. Finally, in an attempt to give an order of
magnitude regarding CO2 valorization, we critically assess
economical aspects of the production of methanol and DME and outline
future research and development directions.
The development of active, cost-effective and stable oxygen-evolving catalysts is one of the major challenges for solar-to-fuel conversion towards sustainable energy generation. Iridium oxide exhibits the best available compromise between catalytic activity and stability in acid media, but it is prohibitively expensive for large-scale applications. Therefore, preparing oxygen-evolving catalysts with lower amounts of the scarce but active and stable iridium is an attractive avenue to overcome this economical constraint. Here we report on a class of oxygen-evolving catalysts based on iridium double perovskites which contain 32 wt% less iridium than IrO2 and yet exhibit a more than threefold higher activity in acid media. According to recently suggested benchmarking criteria, the iridium double perovskites are the most active catalysts for oxygen evolution in acid media reported until now, to the best of our knowledge, and exhibit similar stability to IrO2.
Depletion of crude oil resources and environmental concerns have driven a worldwide research on alternative processes for the production of commodity chemicals. Fischer-Tropsch synthesis is a process for flexible production of key chemicals from synthesis gas originating from non-petroleum-based sources. Although the use of iron-based catalysts would be preferred over the widely used cobalt, manufacturing methods that prevent their fast deactivation because of sintering, carbon deposition and phase changes have proven challenging. Here we present a strategy to produce highly dispersed iron carbides embedded in a matrix of porous carbon. Very high iron loadings (440 wt %) are achieved while maintaining an optimal dispersion of the active iron carbide phase when a metal organic framework is used as catalyst precursor. The unique iron spatial confinement and the absence of large iron particles in the obtained solids minimize catalyst deactivation, resulting in high active and stable operation.
The oxygen evolution reaction (OER)
and chlorine evolution reaction
(CER) are electrochemical processes with high relevance to water splitting
for (solar) energy conversion and industrial production of commodity
chemicals, respectively. Carrying out the two reactions separately
is challenging, since the catalytic intermediates are linked by scaling
relations. Optimizing the efficiency of OER over CER in acidic media
has proven especially difficult. In this regard, we have investigated
the OER versus CER selectivity of manganese oxide (MnOx), a known OER catalyst. Thin films (∼5–20 nm) of MnOx were electrodeposited on glassy carbon-supported hydrous
iridium oxide (IrOx/GC) in aqueous chloride solutions of
pH ∼0.9. Using rotating ring–disk electrode voltammetry
and online electrochemical mass spectrometry, it was found that deposition
of MnOx onto IrOx decreases
the CER selectivity of the system in the presence of 30 mM Cl– from 86% to less than 7%, making it a highly OER-selective
catalyst. Detailed studies of the CER mechanism and ex-situ structure studies using SEM, TEM, and XPS suggest that the MnOx film is in fact not a catalytically active phase, but functions
as a permeable overlayer that disfavors the transport of chloride
ions.
In
this combined in situ XAFS, DRIFTS, and Mössbauer
study, we elucidate the changes in structural, electronic, and local
environments of Fe during pyrolysis of the metal organic framework
Fe-BTC toward highly active and stable Fischer–Tropsch synthesis
(FTS) catalysts (Fe@C). Fe-BTC framework decomposition is characterized
by decarboxylation of its trimesic acid linker, generating a carbon
matrix around Fe nanoparticles. Pyrolysis of Fe-BTC at 400 °C
(Fe@C-400) favors the formation of highly dispersed epsilon carbides
(ε′-Fe2.2C, d
p = 2.5 nm), while at temperatures of 600 °C (Fe@C-600), mainly
Hägg carbides are formed (χ-Fe5C2, d
p = 6.0 nm). Extensive carburization
and sintering occur above these temperatures, as at 900 °C the
predominant phase is cementite (θ-Fe3C, d
p = 28.4 nm). Thus, the loading, average particle size,
and degree of carburization of Fe@C catalysts can be tuned by varying
the pyrolysis temperature. Performance testing in high-temperature
FTS (HT-FTS) showed that the initial turnover frequency (TOF) of Fe@C catalysts does not change significantly for
pyrolysis temperatures up to 600 °C. However, methane formation
is minimized when higher pyrolysis temperatures are applied. The material
pyrolyzed at 900 °C showed longer induction periods and did not
reach steady state conversion under the conditions studied. None of
the catalysts showed deactivation during 80 h time on stream, while
maintaining high Fe time yield (FTY) in the range of 0.19–0.38
mmolCO gFe
–1 s–1, confirming the outstanding activity and stability of this family
of Fe-based FTS catalysts.
Self-assembly of three molecular components results in a delivery platform, the release rate of which can be tuned after its production. A fluorophore-conjugated gelator can be hydrolyzed by an enzyme, resulting in the release of a fluorescent small molecule. To allow the release to be tunable, the enzyme is entrapped in liposomes and can be liberated by heating the system for a short period. Crucially, the heating time determines the amount of enzyme liberated; with that, the release rate can be tuned by the time of heating.
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