Mesoporous WCx Films with NiO‐Protected Surface: Highly Active Electrocatalysts for the Alkaline Oxygen Evolution Reaction

Metal carbides are promising materials for electrocatalytic reactions such as water electrolysis. However, for application in catalysis for the oxygen evolution reaction (OER), protection against oxidative corrosion, a high surface area with facile electrolyte access, and control over the exposed active surface sites are highly desirable. This study concerns a new method for the synthesis of porous tungsten carbide films with template-controlled porosity that are surface-modified with thin layers of nickel oxide (NiO) to obtain active and stable OER catalysts. The method relies on the synthesis of soft-templated mesoporous tungsten oxide (mp. WOx) films, a pseudomorphic transformation into mesoporous tungsten carbide (mp. WCx), and a subsequent shape-conformal deposition of finely dispersed NiO species by atomic layer deposition (ALD). As theoretically predicted by density functional theory (DFT) calculations, the highly conductive carbide support promotes the conversion of Ni2+ into Ni3+, leading to remarkably improved utilization of OER-active sites in alkaline medium. The obtained Ni mass-specific activity is about 280 times that of mesoporous NiOx (mp. NiOx) films. The NiO-coated WCx catalyst achieves an outstanding mass-specific activity of 1989 A gNi−1 in a rotating-disc electrode (RDE) setup at 25 °C using 0.1 m KOH as the electrolyte.BMBFDFG SPP 2080 priority programPeer Reviewe

Mesoporous WCx Films with NiO‐Protected Surface: Highly Active Electrocatalysts for the Alkaline Oxygen Evolution Reaction

Metal carbides are promising materials for electrocatalytic reactions such as water electrolysis. However, for application in catalysis for the oxygen evolution reaction (OER), protection against oxidative corrosion, a high surface area with facile electrolyte access, and control over the exposed active surface sites are highly desirable. This study concerns a new method for the synthesis of porous tungsten carbide films with template‐controlled porosity that are surface‐modified with thin layers of nickel oxide (NiO) to obtain active and stable OER catalysts. The method relies on the synthesis of soft‐templated mesoporous tungsten oxide (mp. WOx) films, a pseudomorphic transformation into mesoporous tungsten carbide (mp. WCx), and a subsequent shape‐conformal deposition of finely dispersed NiO species by atomic layer deposition (ALD). As theoretically predicted by density functional theory (DFT) calculations, the highly conductive carbide support promotes the conversion of Ni2+ into Ni3+, leading to remarkably improved utilization of OER‐active sites in alkaline medium. The obtained Ni mass‐specific activity is about 280 times that of mesoporous NiOx (mp. NiOx) films. The NiO‐coated WCx catalyst achieves an outstanding mass‐specific activity of 1989 A gNi−1 in a rotating‐disc electrode (RDE) setup at 25 °C using 0.1 m KOH as the electrolyte.BMBF, 03EK3052A, Verbundvorhaben ATO-KAT: Atomar dünn beschichtete poröse Elektroden als neuartige Katalysatoren für die Wasser-Elektrolyse: - leitfähige Träger und Elektrochemie -BMBF, 03EK3052C, Verbundvorhaben ATO-KAT: Quantenchemische Berechnung beschichteter dotierter Metalloxide als Katalysatoren für die OER.DFG,358713534, SPP 2080: Katalysatoren und Reaktoren unter dynamischen Betriebsbedingungen für die Energiespeicherung und -wandlun

Influence of Phase Composition and Pretreatment on the Conversion of Iron Oxides into Iron Carbides in Syngas Atmospheres

CO2 Fischer–Tropsch synthesis (CO2–FTS) is a promising technology enabling conversion of CO2 into valuable chemical feedstocks via hydrogenation. Iron–based CO2–FTS catalysts are known for their high activities and selectivities towards the formation of higher hydrocarbons. Importantly, iron carbides are the presumed active phase strongly associated with the formation of higher hydrocarbons. Yet, many factors such as reaction temperature, atmosphere, and pressure can lead to complex transformations between different oxide and/or carbide phases, which, in turn, alter selectivity. Thus, understanding the mechanism and kinetics of carbide formation remains challenging. We propose model–type iron oxide films of controlled nanostructure and phase composition as model materials to study carbide formation in syngas atmospheres. In the present work, different iron oxide precursor films with controlled phase composition (hematite, ferrihydrite, maghemite, maghemite/magnetite) and ordered mesoporosity are synthesized using the evaporation–induced self–assembly (EISA) approach. The model materials are then exposed to a controlled atmosphere of CO/H2 at 300 °C. Physicochemical analysis of the treated materials indicates that all oxides convert into carbides with a core–shell structure. The structure appears to consist of crystalline carbide cores surrounded by a partially oxidized carbide shell of low crystallinity. Larger crystallites in the original iron oxide result in larger carbide cores. The presented simple route for the synthesis and analysis of soft–templated iron carbide films will enable the elucidation of the dynamics of the oxide to carbide transformation in future work.DFG, 406695057, Fe-basierte Katalysatoren für die Umwandlung von CO2 zu höheren Kohlenwasserstoffen unter dynamischen BedingungenBMBF, 03EK3052A, Verbundvorhaben ATO-KAT: Atomar dünn beschichtete poröse Elektroden als neuartige Katalysatoren für die Wasser-Elektrolyse: - leitfähige Träger und Elektrochemie -DFG, 414044773, Open Access Publizieren 2021 - 2022 / Technische Universität Berli

Bridging experiment and theory: enhancing the electrical conductivities of soft-templated niobium-doped mesoporous titania films

Theoretical calculations suggest a strong dependence of electrical conductivity and doping concentration in transition-metal doped titania. Herein, we present a combined theoretical and experimental approach for the prediction of relative phase stability and electrical conductivity in niobium-doped titania as model system. Our method paves the way towards the development of materials with improved electrical properties.TU Berlin, Open-Access-Mittel – 2021BMBF, 03EK3052A, Verbundvorhaben ATO-KAT: Atomar dünn beschichtete poröse Elektroden als neuartige Katalysatoren für die Wasser-Elektrolyse: - leitfähige Träger und Elektrochemie

A Systematic Approach to Study Complex Ternary Co-Promoter Interactions: Addition of Ir, Li, and Ti to RhMn/SiO₂ for Syngas Conversion to Ethanol

The direct conversion of synthesis gas could open up economically viable routes for the efficient production of ethanol. RhMn/SiO2 represents one of the most active systems reported thus far. Potential improvements were reported by added dopants, i.e., Ir, Ti, and Li. Yet, combining these elements leads to contradicting results, owing to the complexity of the interactions in a multi-promoted system. This complexity is often encountered in heterogeneous catalysis. We report a systematic data-driven approach for the assessment of complex multi-promoter interactions based on a combination of design-of-experiment, high-throughput experimentation, statistical analysis, and mechanistic assessment. We illustrate this approach for the system RhMn/SiO2 promoted with Ir, Li, and Ti. Using this approach, we investigate the impact of promoters’ interactions on a mechanistic level. Our analysis depicts the means to learn hidden correlations in the performance data and, additionally, high performance for ethanol yield for the RhMnIr/SiO2 catalyst. The method presented outlines an efficient way to also elucidate co-promoter interactions in other complex environments

Seawater Electrolysis Using All-PGM-Free Catalysts and Cell Components in an Asymmetric Feed

In arid coastal zones, direct seawater electrolysis appears particularly intriguing for green hydrogen production. State-of-the-art direct seawater electrolyzers, however, show unsatisfactory performance and rely on large amounts of platinum-group metals (PGMs) in the electrodes or hidden as transport layer coatings. Herein, we report an asymmetric-feed electrolyzer design, in which all cell components consist of PGM-free materials. Cobalt- and nickel-based phosphides/chalcogenides not only serve as active and robust electrocatalysts but also are put forth as porous transport layer (PTL) surface coatings enhancing selective seawater splitting performance. In a systematic design study at the single-cell level, we report the integration of our catalysts and PTLs into a membrane–electrode assembly (MEA) using a customized, terphenyl-based anion-exchange membrane (AEM). The presented entirely PGM-free electrolyzer achieves industrially relevant current densities of up to 1.0 A cm–2 below 2.0 Vcell in standardized alkaline seawater and dry cathode operation

Identifying Performance Descriptors in CO2CO_2 Hydrogenation over Iron‐based Catalysts Promoted with Alkali Metals

Alkali metal promoters have been widely employed for preparation of heterogeneous catalysts used in many industrially important reactions. However, the fundamentals of their effects are usually difficult to access. Herein, we unravel mechanistic and kinetic aspects of the role of alkali metals in CO$_2$ hydrogenation over Fe-based catalysts through the state-of-the-art characterization techniques, spatially resolved steady-state and transient kinetic analyses. The promoters affect electronic properties of iron in iron carbides. These carbide characteristics determine catalyst ability to activate H$_2$ , CO and CO$_2$. The Allen scale electronegativity of alkali metal promoter was successfully correlated with the rates of CO$_2$ hydrogenation to higher hydrocarbons and CH$_4$ as well as with the rate constants of individual steps of CO or CO$_2$ activation . The derived knowledge can be valuable for designing and preparation of catalysts applied in other reactions where such promoters are also used

Identifying Performance Descriptors in CO2 Hydrogenation over Iron‐based Catalysts Promoted with Alkali Metals

Alkali metal promoters have been widely employed for preparation of heterogeneous catalysts used in many industrially important reactions. However, the fundamentals of their effects are usually difficult to access. Herein, we unravel mechanistic and kinetic aspects of the role of alkali metals in CO(2) hydrogenation over Fe‐based catalysts through state‐of‐the‐art characterization techniques, spatially resolved steady‐state and transient kinetic analyses. The promoters affect electronic properties of iron in iron carbides. These carbide characteristics determine catalyst ability to activate H(2), CO and CO(2). The Allen scale electronegativity of alkali metal promoter was successfully correlated with the rates of CO(2) hydrogenation to higher hydrocarbons and CH(4) as well as with the rate constants of individual steps of CO or CO(2) activation. The derived knowledge can be valuable for designing and preparing catalysts applied in other reactions where such promoters are also used