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Engineering of Supported Molten Salt Catalysts for Dehydrogenation Reactions and Hydrogen Production Technologies

Final Report Summary - H2-SMS-CAT (Engineering of Supported Molten Salt Catalysts for Dehydrogenation Reactions and Hydrogen Production Technologies)

Catalysis is the key concept for realizing chemical transformations in a sustainable manner. An ideal catalyst combines highest selectivity, catalytic activity and robustness with synthetic availability and processability. This requires the preparation of materials with structurally optimized, molecularly uniform sites, efficient ways of manufacturing these materials in technical quantities, and optimized reactor systems for applying them. The H2-SMS-CAT project aimed for engineering Supported Molten Salt (SMS) materials as novel catalysts for dehydrogenation reactions and H2 production technologies in the temperature range of 200 °C to 500 °C. Catalytic dehydrogenation reactions are currently gaining enormous interest as key steps in future chemical energy storage concepts aiming for the most effective use of excess renewable electricity for energy, mobility and industrial applications (Power-to-X technologies). Within the project, SMS catalyst materials were found to result in greatly enhanced performance compared to state-of-the-art catalysts in all four test reactions under investigation:

a. ammonia decomposition to nitrogen and hydrogen;
b. methanol steam reforming to CO2 and hydrogen;
c. hydrogen release from the Liquid Organic Hydrogen Carrier (LOHC) perhydro-N-ethylcarbazole;
d. butane dehydrogenation to butenes and hydrogen.

Even more relevant, the detailed investigation of SMS catalysts created new fundamental understanding of supported catalysts under high temperature conditions, e.g. with respect to the molecular nature of the active site, metal interaction with the supported salt film, the salt’s reactivity towards the support and doping effects. The newly gained insight went significantly beyond the state of current knowledge and is expected to open new avenues for future catalyst optimization in this technically highly relevant area.

As one important result it was found that stability of precious metal salt solutions in the selected molten salt media is limited with nanoparticle formation being observed at temperatures above 200 °C. In contrast, even highly concentrated salt solutions (up to 20 mol %) of transition metal salts, like Ni[NTf2]2 or Co[NTf2]2 in salts like Cs[NTf2] or [PPh4][NTf2], were stable for prolonged times at temperatures up to 350 °C. As a result of the thermal “decomposition” of supported precious metal salt solutions, catalytically highly active, “salt-born” supported precious metal nanoparticles were formed and this observation drew our attention to study in more general the effect of liquid salt coatings on classical heterogeneous catalysts. For both, the continuous ammonia decomposition and the continuous methanol steam reforming reaction, very strong beneficial effects on catalyst activity and selectivity were obtained by coating such catalysts with basic, hydrophilic alkali salts (Angewandte Chemie, International Edition 2013, 52(19), 5028–5032; ChemSusChem 7(9), 2014, 2516–2526; Applied Catalysis, A: General 2016, 510, 189–195). Insight into the chemical reasons for this step-change in catalytic performance has been gained by extensive analytic and spectroscopic studies, including solid-state-NMR, TPD, IR, XPS and XRD methods.
The dehydrogenation of perhydro-N-ethylcarbazole is a liquid phase reaction and showed a considerable higher sensitivity to limiting pore diffusion effects. In order to address the role of individual steps in the macrokinetics, detailed kinetic and mechanistic studies at the molecular level of the supported Pd- and Pt catalysts were carried out using XP and IR-spectroscopies (ACS Catalysis 2014, 4(2), 657–665; J. Phys. Chem. Lett. 2014, 5(8), 1498–1504; J. Chem. Phys. 2014, 140(20), 204711/1–204711/9). To overcome pore diffusion problems while still using pelletized catalyst materials, special egg-shell catalysts have been developed and explored (Energy & Environ. Sci., 2015, 8, 3013–3021). Also in the dehydrogenation of heteroatom-free LOHC compounds, salt coatings showed significantly improved activity and greatly reduced LOHC cracking.
For the dehydrogenation of alkanes, much higher temperatures (> 400 °C) are needed for thermodynamic reasons and catalyst deactivation through coke formation is the biggest technical challenge. For the first time we could show that supported liquid catalyst materials can extend catalyst lifetime by more than a factor of five at comparable or better activity and selectivity than their state-of-the-art counterparts.

To date, the H2-SMS-CAT project has produced 19 scientific papers in refereed journals (+ two more submitted) which have been cited 126 times (SCOPUS 24.2.2016). The project results have been presented in 36 oral presentations and 15 posters at national and international conferences.