Intermetallic compounds are highly investigated, as they combine or enhance the properties of their constituting elements or even bring forth new properties. Especially at the nanoscale, these features can be exploited in heterogeneous catalysis.[1] Various methods have been developed so far to synthesize intermetallic particles, each with its own benefits and drawbacks. A facile procedure is the polyol process, which was first introduced by the group of FIÉVET and FIGLARZ in 1989. Here, the polyol serves a threefold purpose. First, it serves as the primary solvent. With respect to metal salts, it displays a solvation behavior similar to water due to the chelating properties. Second, the polyol can act as a surface-capping agent, which prevents agglomeration, resulting in finely dispersed particles. Third and last, the main attribute of the polyol is its reductive property, which increases with temperature, enabling the reduction of multiple metal cations. Compared to other synthetic routes, the polyol process can be performed with cheap starting materials, such as metal salts or oxides. The utilization of a laboratory microwave can further improve the process. A homogeneous heat distribution and contactless heating diminish side reactions. Additionally, the extreme heating rates foster homogeneous nucleation, resulting in a uniform product. Furthermore, the precise control over the temperature profile enables good reproducibility, making this setup ideal for efficient syntheses as well as investigations of reaction pathways.
In this PhD thesis, formation mechanisms of Bi-M particles (M = Ni, Ir, Rh; ) were elucidated, revealing different mechanisms depending on the metal combination as well as various intermediates. Additionally, the influence of reaction parameters, such as metal precursor, anion, and pH value was investigated.
In the case of BiNi particle formation, a successive reduction of bismuth and nickel cations was observed. Bismuth cations are reduced first producing bismuth particles, which act as nucleation sites for the subsequent nickel reduction. The particles grow on the surface of the bismuth core resulting in a core-shell structure. Diffusion of nickel results in Bi3Ni and eventually BiNi after full depletion of the nickel shell. The choice of nickel precursor substantially influences the required reaction time. Nickel acetate requires the shortest reaction time, whereas nickel nitrate necessitates drastically longer reaction times due to a decreased reductivity. Nickel chloride is not reducible in neat ethylene glycol due to the formation of a stable dinuclear nickel complex in solution.
The overall formation kinetics are substantially promoted by increasing the pH value or temperature, leading to a higher reduction strength or improved diffusion dynamics, respectively.
The study of Bi2Ir particle formation revealed a two-stage scenario. First, the starting materials are partially co-reduced to the new intermetallic suboxide Bi4Ir2O. In a second step, at higher temperatures, the suboxide is fully reduced to Bi2Ir. So far, only the combination of bismuth nitrate and iridium acetate results in the suboxide, whereas the introduction of chloride ions, i.e., iridium chloride or potassium hexachloroiridate, merely results in BiOCl and elemental iridium. A structural model of Bi4Ir2O was established based on diffraction data and quantum chemical calculations. Edge-sharing [IrBi6] octahedra form corrugated layers stacked along the c-axis, which are separated by oxide ions. The calculated band structure and DOS suggest metallic behavior within the layers, whereas a band gap was found along the stacking order, thus, making the compound a pseudo 2D metal.The formation of Bi2Rh particles follows a two-step mechanism. For rhodium acetate, the process starts with a direct co-reduction of rhodium and bismuth cations resulting in the formation of BiRh. Increasing the temperature further leads to a gradual transition into Bi2Rh via reduction of residual bismuth cations in solution followed by diffusion. In the case of rhodium nitrate, a bismuth-glycolato complex precipitates, which undergoes a reaction with rhodium at high temperatures. The addition of a base promotes the reactions by lowering the necessary reduction temperatures and preventing the precipitation of the bismuth glycolate. Rhodium chloride does not yield intermetallic phases in the desired purity and yield.
These results allowed for a comparison and assessment of reactions in the synthesis of γ-BiPd particles. Similar to the above discussed reactions, chlorides resulted in the formation of BiOCl. An increased pH value was beneficial by preventing precipitation of intermediates, i.e., BiOCl or bismuth glycolates, and improved reduction strength.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:84147 |
| Date | 20 March 2023 |
| Creators | Smuda, Matthias Adam |
| Contributors | Doert, Thomas, Kaskel, Stefan, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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