Zirconia-modified
copper catalysts are being actively discussed
to be beneficial in CO2/H2 conversion to methanol.
The nature of the promotional effect of ZrO2 is still an
intense matter of debate. Here, we report a density functional theory
study investigating the CO2 hydrogenation reaction using
a tetragonal ZrO2(101)-supported copper nanowire model
of the copper–zirconia interface. We calculated adsorption
energies of reactants and intermediates relevant for CO2 hydrogenation at different copper and zirconia adsorption sites.
We suggest that in general adsorption at the Cu/ZrO2 interface
and at zirconia facets is far too strong and that these sites will
therefore be largely inactive for the hydrogenation of CO2 to methanol. We observe that adsorption at zirconia-supported copper
sites further from the support is increased compared to that on unsupported
Cu(111) and Cu(211) surfaces. We thus speculate that CO2 hydrogenation on these types of systems has the highest activity
in close proximity to but not directly at the Cu/ZrO2 interface.
The manufacturing of technical catalysts generally involves a sequence of different process steps, of which co-precipitation is one of the most important. In this study, we investigate how continuous co-precipitation influences the properties of Cu/ZnO/ZrO2 (CZZ) catalysts and their application in the direct synthesis of dimethyl ether (DME) from CO2/CO/H2 feeds. We compare material characteristics investigated by means of XRF, XRD, N2 physisorption, H2-TPR, N2O-RFC, TEM and EDXS as well as the catalytic properties to those of CZZ catalysts prepared by a semi-batch co-precipitation method. Ultra-fast mixing in continuous co-precipitation results in high BET and copper surface areas as well as in improved metal dispersion. DME synthesis performed in combination with a ferrierite-type co-catalyst shows correspondingly improved productivity for CZZ catalysts prepared by the continuous co-precipitation method, using CO2-rich as well as CO-rich syngas feeds. Our continuous co-precipitation approach allows for improved material homogeneity due to faster and more homogeneous solid formation. The so-called “chemical memory” stamped during initial co-precipitation is kept through all process steps and is reflected in the final catalytic properties. Furthermore, our continuous co-precipitation approach may be easily scaled-up to industrial production rates by numbering-up. Hence, we believe that our approach represents a promising contribution to improve catalysts for direct DME synthesis.
Catalyst systems for the conversion of synthesis gas, which are tolerant to fluctuating CO/CO2 gas compositions, have great potential for process-technical applications, related to the expected changes in the synthesis gas supply.
In order to enable science-and knowledge-based adaptation of catalyst materials to new demands, e.g., methanol synthesis from CO 2 , a modified method to prepare Cu/Zn-based catalysts based on the strict consecutive execution of co-precipitation and aging is investigated. By successfully stabilizing the initial co-precipitate, two mixing regimes are revealed: regarding slow mixing, the particle size of the co-precipitate decreases with increasing volume flow. By contrast, co-precipitation is no longer influenced by mixing for sufficiently high volume flows. While aging can be accelerated by forming smaller aggregates in the co-precipitation, the final state is found to be defined by thermodynamic equilibrium alone. Furthermore, the microstructure of the final catalyst was influenced and the performance in direct dimethyl ether synthesis was improved by adjusting the mixing in the co-precipitation. We believe that the approach could be scaled-up to industrial production rates and, hence, is promising to make methanol synthesis from CO 2 more effective and sustainable.
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