Browsing by Author "Klemm, Elias"

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Open Access
Assembling metal organic layer composites for high‐performance electrocatalytic CO2 reduction to formate
(2022) Liu, Hang; Wang, Hongguang; Song, Qian; Küster, Kathrin; Starke, Ulrich; Aken, Peter A. van; Klemm, Elias
2D metal-organic-framework (MOF) based composites have emerged as promising candidates for electrocatalysis due to their high structural flexibility and fully exposed active sites. Herein, a freestanding metal-organic layer (MOL) with a 2D kgd (kagome dual) lattice was constructed with abundant surface oxygenate groups serving as anchoring sites to immobilize diverse guests. Taking Bi as an example, tetragonal Bi2O3 nanowires can be uniformly grown on MOLs after solvothermal treatment, the structural evolution of which was followed by ex situ electron microscopy. The as-prepared Bi2O3/MOL exhibits excellent CO2 electroreduction activity towards formate reaching a specific current of 2.3 A mgBi−1 and Faradaic efficiencies of over 85 % with a wide potential range from -0.87 to -1.17 V, far surpassing Bi2O3/UiO (a 3D Zr6-oxo based MOF) and Bi2O3/AB (Acetylene Black). Such a post-synthetic modification strategy can be flexibly extended to develop versatile MOL composites, highlighting the superiority of optimizing MOL-based composites for electrocatalysis.
Open Access
CHEMampere : technologies for sustainable chemical production with renewable electricity and CO2, N2, O2, and H2O
(2022) Klemm, Elias; Lobo, Carlos M. S.; Löwe, Armin; Schallhart, Verena; Renninger, Stephan; Waltersmann, Lara; Costa, Rémi; Schulz, Andreas; Dietrich, Ralph‐Uwe; Möltner, Lukas; Meynen, Vera; Sauer, Alexander; Friedrich, K. Andreas
The chemical industry must become carbon neutral by 2050, meaning that process‐, energy‐, and product‐related CO2 emissions from fossil sources are completely suppressed. This goal can only be reached by using renewable energy, secondary raw materials, or CO2 as a carbon source. The latter can be done indirectly through the bioeconomy or directly by utilizing CO2 from air or biogenic sources (integrated biorefinery). Until 2030, CO2 waste from fossil‐based processes can be utilized to curb fossil CO2 emissions and reach the turning point of global fossil CO2 emissions. A technology mix consisting of recycling technologies, white biotechnology, and carbon capture and utilization (CCU) technologies is needed to achieve the goal of carbon neutrality. In this context, CHEMampere contributes to the goal of carbon neutrality with electricity‐based CCU technologies producing green chemicals from CO2, N2, O2, and H2O in a decentralized manner. This is an alternative to the e‐Refinery concept, which needs huge capacities of water electrolysis for a centralized CO2 conversion with green hydrogen, whose demand is expected to rise dramatically due to the decarbonization of the energy sector, which would cause a conflict of use between chemistry and energy. Here, CHEMampere's core reactor technologies, that is, electrolyzers, plasma reactors, and ohmic resistance heating of catalysts, are described, and their technical maturity is evaluated for the CHEMampere platform chemicals NH3, NOx, O3, H2O2, H2, CO, and CxHyOz products such as formic acid or methanol. Downstream processing of these chemicals is also addressed by CHEMampere, but it is not discussed here.
Open Access
Comparison of electrochemically deposited Bi and Sn catalysts onto gas diffusion electrodes for the electrochemical CO2 reduction reaction to formate
(2023) Manolova, Mila; Hildebrand, Joachim; Hertle, Sebastian; Sörgel, Şeniz; Kassner, Holger; Klemm, Elias
In this publication, we report about the selectivity and stability of bismuth (Bi)- and tin (Sn)-based electrocatalysts for the electrochemical CO2 reduction reaction (eCO2RR) for formate production. Bismuth and tin were successfully electrodeposited using the pulse plating technique on top of and inside of the gas diffusion layers (GDLs). The distribution of the catalyst throughout the thickness of the gas diffusion electrodes (GDEs) was investigated by using scanning electron microscopy and computer tomography; it was found that the catalyst morphology determines the performance of the electrode. Inhomogeneous deposits, with their enlarged catalyst surface area, provide more active centres for the eCO2RR, resulting in increased Faraday efficiency (FE) for formate. The initial electrochemical characterisation tests of the bismuth- and tin-loaded GDEs were carried out under laboratory operating conditions at an industrially relevant current density of 200 mA·cm-2; complete Sn dissolution with a subsequent deformation of the GDL was observed. In contrast to these results, no leaching of the electrodeposited Bi catalyst was observed. An FE of 94.2% towards formate was achieved on these electrodes. Electrodes based on an electrodeposited Bi catalyst on an in-house prepared GDL are stable after 23 h time-on-stream at 200 mA·cm-2 and have very good selectivity for formate.
Open Access
Identifying monomeric Fe species for efficient direct methane oxidation to C1 oxygenates with H2O2 over Fe/MOR catalysts
(2022) Xu, Caiyun; Song, Qian; Merdanoglu, Nagme; Liu, Hang; Klemm, Elias
Exploring advanced catalysts and reaction systems operated at mild reaction conditions is crucial for conducting the direct methane oxidation reaction toward oxygenate products. Many efforts have been put into research on pentasil-type (MFI) zeolites based on mononuclear and/or binuclear iron sites, using H2O2 as the oxidant. In this work, we present a modified liquid ion-exchange method to better control Fe loading in a mordenite-type (MOR) zeolite with a Si/Al molar ratio of 9. The optimized Fe/MOR catalyst showed excellent performance in the direct methane oxidation reaction with turnover frequencies (TOFs) of 555 h-1 to C1 oxygenates, significantly better than the reported activity. Multiple comparative experiments were conducted to reveal the mechanism behind the performance. Strikingly, the active sites in the Fe/MOR catalyst were found to be mononuclear iron sites, confirmed by transmission electron microscopy (TEM), ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), and X-ray absorption spectroscopy (XAS). Increasing the iron loading led to the aggregation of the iron sites, which tend to trigger undesirable side reactions (i.e., H2O2 decomposition and over-oxidation), resulting in a significant decrease in TOFs to C1 oxygenates.
Open Access
Investigating the long-term kinetics of Pd nanoparticles prepared from microemulsions and the Lindlar catalyst for selective hydrogenation of 3-hexyn-1-ol
(2024) Tari, Faeze; Hertle, Sebastian; Wang, Hongguang; Fischer, Julian; Aken, Peter A. van; Sottmann, Thomas; Klemm, Elias; Traa, Yvonne
The effect of non-saturated corner and edge sites of Pd particles on the long-term selectivity of cis-3-hexen-1-ol in the hydrogenation of 3-hexyn-1-ol was studied in this work. Non-supported Pd agglomerates were synthesized through the microemulsion synthesis route and used at nalkynol/APdratios between 0.08 and 21 mol/m2for the catalytic conversion of 3-hexyn-1-ol for 20 h. The selectivity of the cis-hexenol product increased by reducing the quantity of Pd catalytic sites (increasing the nalkynol/APdratio) without introducing any modifier or doping agent to poison the nonselective sites. Then, Pd aggregates with fused primary particles and, thus, fewer corner and edge sites were produced through thermal sintering of the agglomerates at 473-723 K. By comparing the catalytic performance of the agglomerates and aggregates, it was observed that at a rather similar kinetic behavior (99.99% conversion and 85-89% selectivity to cis-hexenol), the sintered aggregates could stay selective despite a catalytic surface area about seven times larger. This emphasizes the role of low-coordinated edge and corner sites on the final selectivity of the cis product and demonstrates that thermal sintering allows the number of non-selective sites to be reduced without any need for toxic or organic doping agents or modifiers.
Open Access
PEMFC anode durability : innovative characterization methods and further insights on OER based reversal tolerance
(2021) Bentele, Dominik; Aylar, K.; Olsen, K.; Klemm, Elias; Eberhardt, S. H.
Durability is a major lever for commercial success of proton exchange membrane fuel cells (PEMFCs). The introduction of OER catalyst to the PEMFC anode has been established as a material based mitigation strategy for reversal events caused by gross fuel (i.e. H2) starvation. We investigated the degradation of two different OER based reversal tolerant anodes during short-term recurring reversal operation to mimic field occurrence of reversal events realistically. PEMFC failure during normal operation can be observed whereas OER activity during reversal operation is unaffected. This result is in contrast to findings for commonly applied prolonged reversal accelerated stress tests (ASTs) and indicates an OER catalyst recovery effect for short and recurring reversal events. Combining the developed AST with cyclic voltammetry, electrochemical impedance spectroscopy and hydrogen pump, tests failures during normal operation is mainly assigned to hydrogen oxidation mass transfer increase indicating carbon corrosion and structural change within the anode catalyst layer. Consequently, the developed combination of AST and further characterization methods enables in situ distinction between catalyst and structural degradation, highlighting to be a good basis to investigate future aspects regarding anode degradation caused by cell reversal.
Open Access
Rapid aging as a key to understand deactivation of Ni/Al2O3 catalysts applied for the CO2 methanation
(2021) Beierlein, Dennis; Häussermann, Dorothea; Traa, Yvonne; Klemm, Elias
We developed a rapid aging method for Ni/Al2O3 methanation catalysts mimicking the real aging in the actual application. The method is based on hydrothermal deactivation of the catalyst at 600 or 700 °C, which leads to a catalyst with nearly constant conversion after a much shorter time period compared to normal aging. The hydrothermally aged catalysts are characterized by N2 adsorption, X-ray powder diffraction, temperature-programmed reduction and H2 chemisorption. The catalytic performance of the aged catalysts is comparable to the one of a catalyst deactivated in a long-term measurement with up to 720 h on stream. The time needed for reaching a stable conversion can be diminished by rapid aging by a factor of 10. The investigations also showed that the long-term deactivation is caused by Ni particle sintering and that the support pores limit the Ni particle size.
Open Access
Remarkable enhancement of catalytic activity of Cu‐complexes in the electrochemical hydrogen evolution reaction by using triply fused porphyrin
(2022) Chandra, Shubhadeep; Singha Hazari, Arijit; Song, Qian; Hunger, David; Neuman, Nicolás. I.; Slageren, Joris van; Klemm, Elias; Sarkar, Biprajit
A bimetallic triply fused copper(II) porphyrin complex (1) was prepared, comprising two monomeric porphyrin units linked through β-β, meso-meso, β′-β′ triple covalent linkages and exhibiting remarkable catalytic activity for the electrochemical hydrogen evolution reaction in comparison to the analogous monomeric copper(II) porphyrin complex (2). Electrochemical investigations in the presence of a proton source (trifluoroacetic acid) confirmed that the catalytic activity of the fused metalloporphyrin occurred at a significantly lower overpotential (≈320 mV) compared to the non‐fused monomer. Controlled potential electrolysis combined with kinetic analysis of catalysts 1 and 2 confirmed production of hydrogen, with 96 and 71 % faradaic efficiencies and turnover numbers of 102 and 18, respectively, with an observed rate constant of around 107 s-1 for the dicopper complex. The results thus firmly establish triply fused porphyrin ligands as outstanding candidates for generating highly stable and efficient molecular electrocatalysts in combination with earth‐abundant 3d transition metals.
Open Access
Ruthenium complexes of polyfluorocarbon substituted terpyridine and mesoionic carbene ligands : an interplay in CO2 reduction
(2023) Stein, Felix; Nößler, Maite; Singha Hazari, Arijit; Böser, Lisa; Walter, Robert; Liu, Hang; Klemm, Elias; Sarkar, Biprajit
In recent years terpyridines (tpy) and mesoionic carbenes (MIC) have been widely used in metal complexes. With the right combination with a metal center, both of these ligands are individually known to generate excellent catalysts for CO2 reduction. In this study, we combine the potentials of PFC (PFC=polyfluorocarbon) substituted tpy and MIC ligands within the same platform to obtain a new class of complexes, which we investigated with respect to their structural, electrochemical and UV/Vis/NIR spectroelectrochemical properties. We further show that the resulting metal complexes are potent electrocatalysts for CO2 reduction in which CO is exclusively formed with a faradaic efficiency of 92 %. A preliminary mechanistic study, including the isolation and characterization of a key intermediate is also reported.