Spray-dried nanoporous supraparticles as adjustable support structures for catalytically active materials

Heterogeneous catalysis is a fundamental aspect of the chemical industry, used to reduce energy requirements and improve the sustainability of fabrication processes. Typically, heterogeneous catalysts consist of an active catalyst species and a support material. Their structural characteristics, including the distribution of the catalyst and the pore framework properties of the support, are decisive for the final catalytic performance of the materials. They determine the accessibility of the catalytically active species and the mass transport through the material. The controlled adjustment of the heterogeneous catalyst structure is, therefore, a fundamental synthetic aspect that must be addressed in the design of the materials. In this thesis, spray-dried supraparticles are utilized as heterogeneous catalysts and their structural properties are deliberately adjusted to modulate their catalytic functionality. Supraparticles are micron-sized entities, which are hierarchically composed of various colloidal building blocks. This class of material offers the advantage of creating multicomponent systems with adjustable structures and multifunctional property profiles. An essential feature of supraparticles is their three-dimensional pore network, which is provided by the interstitial spaces between the constituent building blocks. The interconnected pore network and the potential for combining a multitude of materials during the fabrication of supraparticles make these systems well-suited for the development of heterogeneous catalysts. Using spray-drying as the synthetic approach for supraparticles allows the fabrication of versatile, adjustable materials in a scalable manner. In Chapter 3.1, the utilization of catalytically active spray-dried supraparticles in the propane dehydrogenation and the color change reduction reaction of a dye by H2 gas is presented. The objective is the examination of fundamental parameters of the nanoparticles and supraparticles that impact the catalytic performance of the materials. These parameters include the incorporation and stabilization of the catalyst nanoparticles within the supraparticle framework, the catalyst type and distribution, as well as the pore framework properties of the support. In Section 3.1.1, supraparticles composed of GaPt and SiO2 nanoparticles are presented as dehydrogenation catalysts. The successful entrapment of the GaPt nanoparticles inside the SiO2 framework is demonstrated by employing the modular approach of spray-drying to create supraparticles. The fixation of the GaPt nanoparticles, which become liquid metal solutions during the catalytic reaction, in the nanoporous SiO2 framework enhances the physical stability of the catalyst. The entrapment facilitates a superior activity in propane dehydrogenation compared to conventional support strategies. In Sections 3.1.2 and 3.1.3, gasochromic supraparticles are reported that combine SiO2 support nanoparticles with various catalyst nanoparticles (Pt, Pd, and Au-Pd) and resazurin dye molecules to form a material with the ability to indicate gaseous H2 via a color change reduction reaction. The influence of the catalyst material type and its agglomeration state within the SiO2 framework on the H2 gas indication performance are discussed (Section 3.1.2). The underlying catalytic reduction of the indicator dye is found to be strongly dependent on the intrinsic H2-activation ability of the catalyst type utilized and its distribution in the supraparticle framework. The latter is significantly reduced by an increased agglomeration state of the catalyst nanoparticles. This change in distribution compromises the catalytic functionality significantly. The modular synthesis approach of spray-drying to fabricate supraparticles facilitates the modification of the pore framework by varying the SiO2 nanoparticle size (Section 3.1.3). In the case of H2 indicator supraparticles, an increasing pore size results in a deceleration of the catalytic dye reduction reaction. A lower water adsorption capacity of larger pore sizes, which reduces the dye molecule mobility in the pores, is rationalized as the reason. Conducting a systematic variation of the SiO2 nanoparticle size demonstrates the occurrence of particle segregation between SiO2 and the catalyst nanoparticles within the supraparticles. This segregation is caused by the size difference between the different nanoparticle species and yields a strong agglomeration of the catalyst nanoparticles. As a result, the distribution of the catalyst nanoparticles within the supraparticle reduces significantly, thereby reducing the catalytic conversion of the dye molecules. Chapter 3.2 outlines the development of a synthetic approach with the aim to deliberately control the structure of multicomponent supraparticles in spray-drying. This approach is based on a salt-induced preliminary agglomeration of the precursor nanoparticles prior to spray drying. Firstly, a model system composed of SiO2 nanoparticles with different sizes (40 and 400 nm) is studied to reveal the impact of the preliminary agglomeration on the morphology and internal structure of the spray-dried supraparticles (Section 3.2.1). The formation of agglomerates allows for obtaining well-mixed supraparticles instead of a core/shell morphology when no deliberate agglomeration is induced. An increase in the concentration and valence of the salt cation promotes these morphological adaptations. Simultaneously, the morphological changes affect the internal structure of the supraparticles. By adjusting the extent of agglomeration, the position of nanoparticles and the resulting pore framework can be modified. Secondly, the feasibility of transferring the salt-induced preliminary agglomeration approach from a model SiO2 based supraparticle system to a catalytically active hybrid Pt/SiO2 supraparticle system is demonstrated. Therefore, the use of SiO2 support nanoparticles in combination with Pt nanoparticles, which serve as a hydrogenation catalyst, is presented. By inducing preliminary agglomeration via salt addition, the impact of the changing Pt distribution and the tunable SiO2 pore framework on the hydrogenation performance is elaborated and discussed. It is demonstrated that increasing the extent of agglomeration by higher quantities of added salt results in a continuously improving Pt catalyst distribution. This enhances the hydrogenation performance of Pt/SiO2 supraparticles by providing a higher accessibility of the Pt nanoparticle surface. Additionally, controlling the extent of agglomeration facilitates the modification of the pore framework properties of the SiO2 support, including the pore size, pore size distribution, as well as pore interconnectivity. The ability to adjust these properties allows for the deliberate adjustment of the hydrogenation performance by controlling the mass transport through the pore network of the Pt/SiO2 supraparticles (Section 3.2.2). The findings of this thesis demonstrate the potential of supraparticles functioning as heterogeneous catalysts. They facilitate a flexible and versatile adjustability of their structural properties, thereby allowing precise control over their catalytic performance. A variety of catalyst materials can be entrapped within a porous support matrix, and the structural properties can be modified in a modular manner, specific to the intended applications of the materials. Deliberate electrostatic manipulation of the nanoparticle precursor allows for the control of material distribution and pore framework properties, which are essential performance-determining parameters of heterogeneous catalysts. From a broader perspective, the intentional design and modification of the nanoparticle precursors represent crucial steps toward the controlled structural arrangement within multicomponent supraparticles. The findings of this thesis thus provide important insights into the essential criteria of the structural design of supraparticles to fundamentally and deliberately control their functionalities.