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.
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