Browsing by Author "Liu, Hang"

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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
Exploring metal organic layer based composites for selective electrocatalytic CO2 reduction to formate
(2022) Liu, Hang; Klemm, Elias (Prof. Dr.-Ing.)
The massive consumption of fossil fuels has led to a significant accumulation of carbon dioxide in the atmosphere, and a CO2 concentration unprecedented over the past 3 million years. Capturing CO2 and further converting it into valuable chemicals or fuels could contribute to solving this dilemma. In recent years, electrochemical CO2 reduction technology has attracted increasing attention in research and development due to the tremendous advances in green and renewable electricity. This doctoral thesis provides a comprehensive summary of the progress in electrochemical CO2 reduction, especially for the formation of formic acid/formate, due to its advantages in meeting techno-economic requirements. Furthermore, in terms of the catalyst selection, we opt for metal-organic framework (MOF) based materials as our target catalysts, which have received much attention in recent years due to their unique structures. Firstly, considering the poor conductivity of most MOFs, electrocatalysts based on 2D-MOFs (also known as metal-organic layers, MOLs) would be a promising option. Based on the synthesis strategy and stability requirement, we decided to synthesize MOLs with a 2D-kgd lattice, consisting of Zr6-oxo clusters as metal nodes and tridentate linkers. Zr6-oxo-based MOFs, such as UiO-66, are usually found to exhibit good chemical stability. Furthermore, the addition of acetic acid as a capping agent can inhibit the stacking of the MOL layers and enable the direct synthesis of free-standing MOLs. As a result, Zr-TATB MOL was successfully constructed by a facile solvothermal process, using for the first time TATB (4,4’,4’’-s-triazine-2,4,6-triyl-tribenzoate) as a linker. Furthermore, multiple characterizations verified and validated its exact structure, which was in agreement with the expected MOL structure. Secondly, Zr-TATB MOL with multiple anchoring sites on its surface shows a great potential of immobilizing different catalytic sites by post-modification strategies to form MOL composites. As a representative, uniformly dispersed Bi2O3 nanowires were grown on the Zr-TATB MOL, denoted as Bi2O3/MOL. The structure of Bi2O3/MOL was exactly determined by advanced electron microscopy techniques. The post-modification process of Zr-TATB MOL was followed by ex-situ electron microscopy in order to determine the growth process from atomic level dispersion to Bi nanowires. The monolayer morphology of MOL facilitates the characterization of the structures, which provides a good basis for the rational design of MOF composites. Thirdly, electrocatalytic CO2 reduction (ECR) tests were conducted in a conventional H-type cell using carbon paper as a backing electrode. Bi2O3/MOL exhibited excellent ECR performance with Faradaic efficiencies of over 85% to formate at a wide potential window (~0.4 V), and the best partial current density reached an impressive value of 2.3 A·mgBi-1. This performance far exceeded that of Bi2O3/UiO (a typical 3D-MOF-based composite) and Bi2O3/AB (a conductive carbon based composite), highlighting the superiority of 2D-MOF composites. The Bi2O3/MOL also exhibited good structural stability, verified by multiple characterizations. In addition, further ECR tests were conducted using gas diffusion electrodes (GDEs), achieving industrially relevant current densities towards formate of over 300 mA·cm-2. Among all MOFs reported for ECR reactions, Bi2O3/MOL exhibited excellent formate generation activity, whether using carbon paper in H-cells or GDEs. Finally, we verified the universality of the post-modification strategy for the design of MOL-based composites. We were able to obtain a series of MOL composites bearing different metal species by tuning the reaction conditions, all of which exhibited good ECR activity. This research demonstrates the great potential of MOL-based composites as electrocatalysts and encourages further exploration of highly tunable MOLs for catalytic studies.
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
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.