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.

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Carbon dioxide (CO 2 ) capture and valorization have great potential for mitigating emissions of this greenhouse gas and accordingly for preserving the environment for future generations. In this regard, hydrogenation of CO 2 to methanol is highly attractive because this product is a valuable energy carrier and can also be used for production of various everyday commodities. Although many research papers on this topic have been published in the past decades, there is still a lack of fundamentals relevant to control catalyst performance. Herein, we demonstrate how statistically validated Big-Data analysis of available literature data identified hidden descriptors that can be applied for purposeful catalyst development and for identification of optimal reaction conditions. In view of catalyst development, the kinds of structural promoters or supports for bulk or supported Cu-, In-, or Pd-based catalysts are the most important descriptors for methanol selectivity, with Ce and Zr being the most efficient promoters. The type and the parameters of the preparation methods as well as the kind of active component precursors are also important in this regard. To validate the conclusion about the structural promoter, a series of supported CuZn-containing catalysts were prepared. The best-performing CuZn/CeO 2 catalyst outperformed the state-of-the-art CuZn-based catalysts tested at a total pressure of up to 30 bar using a feed with the ratio of H 2 /CO 2 of 3. In addition to the catalyst composition and the preparation method, our analysis suggests that the most often used Cu-based catalysts lose their methanol selectivity due to the decomposition of this product to CO. Our control experiments with the developed CuZn-based catalysts proved that this undesired reaction can be hindered when the catalyst support contains Ce or through increasing H 2 partial pressure. This knowledge is important for further catalyst development.

Ru supported on mayenite electride, [Ca24Al28O64]4+(e−)4 a calcium aluminum oxide denoted as C12A7e−, are described in the literature as highly active catalysts for ammonia synthesis, especially under conditions of low absolute pressure. In this study, we investigated the application of recently reported plasma arc melting synthesized C12A7e− (aluminum solid reductant) as supports of Ru/C12A7e− catalysts in ammonia synthesis up to pressures of 7.6 MPa. Together with the plasma-arc-melting-based catalyst support, we investigated a similar plasma-synthesized C12A7e− (graphite solid reductant) and a vacuum-sintering-based C12A7e−. Complementary to the catalytic tests, we applied 2H solid-state NMR spectroscopy, DRUVVis-spectroscopy, thermal analysis and PXRD to study and characterize the reactivity of different plasma-synthesized and vacuum-sintered C12A7e− towards H2/D2 and H2O. The catalysts showed an immediate deactivation at pressures > 1 MPa, which can be explained by irreversible hydride formation at higher pressures, as revealed by reactivity tests of C12A7e− towards H2/D2. The direct formation of C12A7:D from C12A7e− is proven. It can be concluded that the application of Ru/C12A7e− catalysts at the industrial scale has limited prospects due to irreversible hydride formation at relevant pressures > 1 MPa. Furthermore, we report an in-depth study relating to structural changes in the material in the presence of H2O.