Hydrogen fuel cells are expected to be pivotal in the energetic transition towards renewable energy sources such as solar and wind power. However, their industrial scalability is severely hindered... Show moreHydrogen fuel cells are expected to be pivotal in the energetic transition towards renewable energy sources such as solar and wind power. However, their industrial scalability is severely hindered by the high cost and degradation rate of platinum catalysts, one of their key components. Addressing this challenge necessitates developing better catalysts, which requires a better fundamental understanding of their reactivity and degradation mechanism. In this thesis, we investigate the (in)stability of model platinum surfaces submerged in liquid and under applied voltage, thus simulating the operational conditions of fuel cells. For this we use a home-build Electrochemical Scanning Tunneling Microscope (EC-STM), which allows us to observe, in real time, the surface structure at the atomic scale. Our findings elucidate the origin of the platinum surface roughening that takes place upon voltage cycling. Moreover, we demonstrate that closely-spaced atomic steps are prone to bunch together, resulting in steps with multi-atomic height. This structural change has a significant effect on the catalyst reactivity, as we explain in detail. Overall, this dissertation contributes to a deeper fundamental understanding of the surface processes that drive platinum surface restructuring as well as their implications for reactivity. Show less
Heterogeneous catalysis is one of the fundamental processes of modern life, being common in industrial refinery and hydrogen vehicles, all the way to the living cell. The dissociation of H2 on Cu... Show moreHeterogeneous catalysis is one of the fundamental processes of modern life, being common in industrial refinery and hydrogen vehicles, all the way to the living cell. The dissociation of H2 on Cu(111) is an important benchmark system for studying heterogeneous catalysis, with a large and varied amount of experimental and theoretical data available.In this thesis I present my recent advances in including the effects of surface temperature on the H2/Cu(111) reaction in not only classical dynamics, but also quantum dynamics. I show how we can include surface temperature effects by treating the surface as static, but distorted, and present how the neglect of energy exchange between the surface and the hydrogen molecule does not appear to affect the dissociation or (ro)vibrationally elastic scattering probabilities of the H2 molecule. Furthermore, I show how treating the hydrogen at a quantum dynamical level has some minor effects on the scattering probabilities when compared to classical dynamics, but in general agrees very well. Finally, I also discuss how including the surface temperature effects improves agreement with the experimentally obtained dissociation curves, but also how smaller features of the experimental results are not reproduced by our models. Show less
The external tissues of plants and animals are colonized by microbial communities termed microbiota. When organisms are exposed to environmental pollutants, these substances will therefore... Show moreThe external tissues of plants and animals are colonized by microbial communities termed microbiota. When organisms are exposed to environmental pollutants, these substances will therefore encounter microbiota at the exposure interface. Many antimicrobial substances have been found to disturb beneficial interactions between microbiota and the host, thereby impairing host health. Nanomaterials exhibit nanoscale properties that could affect host health in two additional, understudied, microbiota-dependent ways. Firstly, owing to their large surface area, adsorption interactions between nanomaterials, microbial metabolites and microbes could alter the identity and colloidal stability of nanomaterials, and may influence the dispersal of microbes. Secondly, the immuno-modulatory effects of microbiota could affect the sensitivity of hosts to immunotoxic nanomaterials. In this dissertation, we use a combination of computational techniques and zebrafish larvae experiments to unravel and quantify these interactions. We predict the affinity of microbial metabolites to carbon and metal nanomaterials, and show that titanium dioxide nanoparticles can affect the dispersal of microbes through aquatic ecosystems, and across different life stages of oviparous animals. Additionally, we provide insight into microbiota-dependent signaling pathways that affect the sensitivity of zebrafish larvae to particle-specific, immunotoxic effects of silver nanoparticles. Altogether, these results contribute to mechanistic pathways for microbiota-inclusive nanomaterial safety assessment. Show less
The research presented in this thesis makes use of small molecules (as H2 , D2 and O2 ) on well-defined single crystal surfaces (flat Pt(111), flat Cu(211) and curved Pt(111)) to elucidate the... Show moreThe research presented in this thesis makes use of small molecules (as H2 , D2 and O2 ) on well-defined single crystal surfaces (flat Pt(111), flat Cu(211) and curved Pt(111)) to elucidate the role of surface structure and degrees of freedom in the reactant in specific surface reactions. For D2 on Pt(111), we find at most a very weak signature of geometric corrugation at large polar angles. For D2 on Cu, we find an anomalous reduced dissociative sticking probability for the stepped Cu(211) surface compared to Cu(111). For hydrogen on curved Pt(111), the HD formation increases linearly with the step density at low incident energy. A surface reconstruction on curved Pt(111) surface is observed on both A- and B-step side when the crystal is annealed at 1200 K. For O2 on curved Pt(111), at low incident energy, steps dominate reactivity by providing an indirect dynamical trapping mechanism. At higher impact energy, a direct chemisorption mechanism dominates. The step facet favors molecules impacting with their internuclear axis parallel to its surface. Show less
Westdijk, J.; Metz, B.; Spruit, N.; Tilstra, W.; Gun, J. van der; Hendriksen, C.; Kersten, G.F.A. 2017
Today, the energy sector is highly dependent on heterogeneous catalysis because a future solution to end our dependency on natural sources lies in generating hydrogen by splitting water. Several... Show moreToday, the energy sector is highly dependent on heterogeneous catalysis because a future solution to end our dependency on natural sources lies in generating hydrogen by splitting water. Several transition metals, such as Pt, are known to be good catalyst materials for water splitting reactions. They play a key role in understanding the fundamental aspects of the elementary interactions occurring on the surfaces of catalysts. These surfaces, however, are generally very complex and contain a wide distribution of structurally and chemically different sites with different activities. One of the key issues in optimizing the activity of the catalysts is to distinguish and specify the active sites on the surface. In this thesis we use highly corrugated Pt surfaces and UHV techniques (TPD, LEED, and STM) to explore the effects of surface defects on adsorption and desorption of water and related adsorbates. We elucidate to what extent the substrate type influences the structure of interfacial water both in the monolayer and thin film regime. Our studies also show that step geometry is the determining factor in low temperature oxygen dissociation. Show less
In this study we focused on the simulation of the interaction of water and its dissociation products hydrogen, oxygen and OH with platinum surfaces that contain a periodic arrangement of single... Show moreIn this study we focused on the simulation of the interaction of water and its dissociation products hydrogen, oxygen and OH with platinum surfaces that contain a periodic arrangement of single-atom-high steps. We simulate these interactions using the framework of density functional theory and employ high-performance computing resources such as the Cartesius supercomputer of SURFsara. We find that the two possible types of step edges exhibit increased binding strengths compared to the flat parts of the interface. This binding strength is a key factor in the chemical reactivity of these surfaces. Furthermore, we investigate the structure of higher coverages of water molecules around the step edges, where we observe surprising changes, namely the appearance of four, five and seven-membered rings, compared to the six-membered rings that are usually observed on the flat surfaces. Our predictions for these structures are confirmed using high-resolution scanning tunneling microscopy data. Our results are among the first simulations of high-coverage water adsorption on regularly stepped platinum surfaces, which will help advance our understanding of solvation effects at the step edges, which are relevant for the fields of solvation science, electrochemistry and surface science. Show less
Bukas, V.J.; Mitra, S.; Meyer, J.; Reuter, K. 2015
The growing field of bio-electronics holds many promises with regard to the integration of various organic molecules onto printed circuit-boards. These include a decrease in the cost of production,... Show moreThe growing field of bio-electronics holds many promises with regard to the integration of various organic molecules onto printed circuit-boards. These include a decrease in the cost of production, an increased sensitivity and specificity to molecular detection from various solutions (i.e. blood) and ultra-miniaturization. However, numerous challenges still face such prospects, chief among which is the retention of biological activity of the adsorbed molecules. To circumvent the possible harmful effects of the bare surfaces, we have made use of self-assembled molecular films that not only shield the proteins (i.e. azurin) off surfaces, but also help establish a spatially-defined conductive path to electrodes. At the same time, the protein itself was engineered such that the active cavity is directly connected via such molecular __wires__. Our results may help in the adsorption of more complex enzymes into future molecular devices, that will retain their activity on the surfaces and are able to integrate into biosensors. Show less
As nickel and platinum are in the same group of the periodic table, the Ni(111) and Pt(111) surfaces may be expected to show similar interaction with water and hydrogen. However in this thesis, we... Show moreAs nickel and platinum are in the same group of the periodic table, the Ni(111) and Pt(111) surfaces may be expected to show similar interaction with water and hydrogen. However in this thesis, we show these interactions for Ni(111) are quite different from those of Pt(111). Moreover, our results show that the Ni(111) surface is a unique surface with regards to its chemistry of water and hydrogen. Show less
