Model Platinum Catalysts

Platinum is used by many industries because of its exceptional catalytic activity, electrochemical properties and corrosion resistance. It is widely used as a catalyst in the chemical production, in fuel cell and electrolyser electrodes and in sensors for analytical and medical applications. Catalysts are key to face grand technological challenges in the fields of energy and environmental protection and to ensure a sustainable supply of chemicals and materials. Catalysts are shaped in many ways. Due to the high cost of platinum, most efforts are directed towards maximizing the exposed surface area in order to increase the utilization of the platinum itself and thus to reduce the Pt loadings. This is conventionally achieved through reducing the size of the Pt particles to the nanoscale by dispersing them on porous high surface area supports. For some specific applications, and especially electrocatalysis and molecular sensing, unsupported nanostructured platinum architectures, combining high exposed surface area with electric conductivity, are needed. Understanding kinetic data by relating it with structural and compositional properties is essential for rational design of new catalysts. Gaining scientific insight when using industrial catalysts is very difficult because of the complexity and heterogeneous nature of such materials. They often contain a combination of phases that can vary in bulk and surface structure, particle size, shape, chemical composition, defects, impurities, promotors and interactions among the different phases. Usually supports themselves are also irregular with a heterogeneous surface. Characterization is inherently challenging for these catalysts and the active site is ill-defined. This complexity has been the driving force for many researchers to use model catalysts. Many model studies have been previously reported using single crystals or Pt particles on planar supports. Yet, examples of well-characterized 3D Pt model catalysts were only scarcely reported. The primary focus of this thesis was on the synthesis and characterization of generic 3D model platinum catalysts that can be used in kinetic investigations of many types of reactions under practically relevant reaction conditions. Compared to planar model catalysts, the structural complexity of such 3D model catalyst systems is more closely related to the industrial catalysts. A first model system was prepared by using a monodisperse, spherical alumina support to reduce the complexity. Spherical alumina particles with a particle size of about 1 µm were synthesized by a chemical precipitation method. Using 2D and 3D electron microscopy and 27Al solid-state NMR, the core-shell structure of the particles was fully characterized. The spherical alumina particles were then decorated with uniform Pt nanoparticles (~1 nm) using the strong electrostatic adsorption method. To establish its relevance as a model catalyst, the Pt alumina catalyst was combined with erionite zeolite having comparable morphology to generate a bifunctional catalyst. The suitability of such a physical mixture with a well-characterized Pt model catalyst was demonstrated in the hydroisomerization-hydrocracking of n-decane. The potential of atomic layer deposition (ALD) of platinum for creation of model catalysts was investigated. ALD could even be used to introduce acid sites next to platinum performing bifunctional catalysis with zeolites. Plasma-enhanced ALD of Ga2O3 (Ga-ALD) was performed on COK-14, an all-silica zeolite that lacks acid sites. After ALD, metal sites were introduced via Pt incipient wetness impregnation. Using electron microscopy, Pt nanoparticles were found to be preferentially located on the edges of the COK-14 plates. The optimum proximity between the Ga acid and Pt metal sites resulted in high activity and selectivity surpassing other large pore zeolites. To understand the potential of ALD for introducing metal sites in a zeolite, Pt-ALD was performed on ZSM-5 zeolite, which resulted in finely dispersed Pt nanoparticles of 1-2 nm. The hydroisomerization experiments confirmed Pt-ALD to be an adequate method for introducing metal sites on aluminosilicate zeolites. This work demonstrates how ALD-tailored materials can help in understanding the relationship between structure and performance. The final model system was a continuous unsupported platinum catalyst with an open and porous structure. The structure was prepared using the hard templating concept, consisting of filling the pores of a mesoporous template followed by removal of the template. Pt-ALD was chosen for depositing Pt in the pores and Zeotile-4 was selected as the template. By using Pt-ALD, the replica structure could be created over micrometer distances. Such larger structures are valuable in electrode and sensor applications. The structure was studied in detail with electron microscopy and tomography. The applicability was demonstrated in the electrocatalytic hydrogen evolution reaction (HER). Furthermore, the suitability of the structure for use in biomedical sensors was demonstrated.status: publishe