CO2 hydrogenation to methanol
(CH3OH) is
widely accepted to proceed through two parallel reactions: (i) CH3OH formation and (ii) the reverse water gas shift (RWGS) reaction
to CO. The latter reaction causes the loss of CH3OH selectivity.
Our spatially resolved analysis of rates of product formation over
a classical CuZnAlO
x
catalyst in a broad
range of CO2 conversion degrees (from 0 to 90% of equilibrium
conversion) suggests revisiting this concept. In comparison with the
RWGS reaction, CH3OH decomposition to CO mainly contributes
to the loss of CH3OH selectivity with a rising degree of
CO2 conversion. Separate CH3OH decomposition
tests in a broad range of experimental conditions proved that this
side reaction is accelerated by H2O but negatively affected
by H2 and rising total pressure. Moreover, the decomposition
should occur on other sites rather than those participating in CH3OH synthesis from CO2 and can be practically suppressed
above certain partial pressures of CH3OH and H2O due to site saturation. The sites responsible for the hydrogenation
of CO2 to CH3OH, however, are not saturated.
Thus, this product can be further produced in downstream catalyst
layers. As this concept is valid for several CuZn-based catalysts,
we provide fundamentals for the design of selective CH3OH synthesis catalysts and for optimizing reaction conditions. Operating
under conditions, where the undesired CH3OH decomposition
reaction is hindered, enabled us to achieve 93% CH3OH selectivity
at 19% CO2 conversion (55% equilibrium conversion) at 50
bar and 200 °C using a feed with the ratio of H2/CO2 of 3.
Advanced analysis of 51V NMR chemical shift and quadrupolar tensor parameters revealed novel insights into the structure of vanadium species in MCM-41-based catalysts.
Industrially-applied mixed metal oxide catalysts often possess an ensemble of structural components with complementary functions. Characterisation of these hierarchical systems is challenging, particularly moving from binary to quaternary systems. Here a quaternary BiÀ MoÀ CoÀ Fe oxide catalyst showing significantly greater activity than binary BiÀ Mo oxides for selective propylene oxidation to acrolein was studied with chemical imaging techniques from the microscale to nanoscale. Conventional techniques like XRD and Raman spectroscopy could only distinguish a small number of components. Spatially-resolved characterisation provided a clearer picture of metal oxide phase composition, starting from elemental distribution by SEM-EDX and spatially-resolved mapping of metal oxide components by 2D Raman spectroscopy. This was extended to 3D using multiscale hard X-ray tomography with fluorescence, phase, and diffraction contrast. The identification and co-localisation of phases in 2D and 3D can assist in rationalising catalytic performance during propylene oxidation, based on studies of model, binary, or ternary catalyst systems in literature. This approach is generally applicable and attractive for characterisation of complex mixed metal oxide systems.
Hematite is a suitable precursor to obtain catalytically active iron (oxide) phases for CO2 Fischer‐Tropsch synthesis after a reductive pretreatment. As concluded from in situ Raman spectroscopy, in hydrogen atmosphere the transformation from α‐Fe2O3 to Fe3O4 is faster than the further reduction from Fe3O4 to Fe (metallic iron). The rate of these steps highly depends on the temperature. Starting from pure hematite, surface formate species are formed at a CO2/H2 gas mixture at elevated temperatures, observable by DRIFT‐spectroscopy. The exposure to CO2 and CO led to surface carbonate‐carboxylate surface species. Reduced samples with varying contents of Fe3O4 and Fe did not show any observable adsorbates at reaction conditions. The same behavior was found during the dosage of the single gases CO2 and CO to these reduced catalysts. The formation of carbonaceous species, detected by Raman spectroscopy, could indirectly hint to the occurrence of carbidation and was especially observed for a good performing catalyst with a medium reduction degree.
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