The main goal of the research presented in this thesis is the synthesis of suitable structural and functional models for the enzyme [NiFe] hydrogenase, which can reduce protons into dihydrogen. A... Show moreThe main goal of the research presented in this thesis is the synthesis of suitable structural and functional models for the enzyme [NiFe] hydrogenase, which can reduce protons into dihydrogen. A brief survey of the roles of all the known nickel containing enzymes in biological systems with a focus on the [NiFe] hydrogenases. Structure, function, physicochemical and catalytic properties of the [NiFe] hydrogenase itself and of the reported model complexes are presented. Many new Nickel, [NiFe], [NiRu] and [NiCu] complexes have been synthesized and studied in view of better catalysts for proton electroreduction into dihydrogen. Show less
With the energy transition toward a renewable energy supply and a CO2-neutral economy, electrification of the energy system is rising in importance, which leads to the challenge of long-term... Show moreWith the energy transition toward a renewable energy supply and a CO2-neutral economy, electrification of the energy system is rising in importance, which leads to the challenge of long-term storage of renewable electricity. A promising option is the electrochemical conversion of biomass or carbon dioxide in chemicals as energy carrier. In this research, catalysis of the electrochemical CO2 reduction was studied to obtain liquid fuels. In this fundamental study we discovered that so-called disproportionation reactions may occur simultaneously with the CO2 reduction reaction influencing the product spectrum. Moreover, we focused on metalloprotoporphyrins immobilized on a graphite surface. We found that the selectivity can be steered toward formic acid with rhodium, tin or indium metal centers. Apart from intrinsic catalyst parameters, we studied the influence of parameters related to the immobilization and the composition of the electrolyte. We showed that the substrate and its pretreatment as well as encapsulation of the catalyst in polymers can have a signifcant influence on the electrocatalysis of CO2 reduction. The results obtained in this thesis provide insight in the energy efficiency, reaction rate and selectivity of the CO2 reduction reaction, and play an important role for the development of an industrially viable process. Show less
Electrocatalysis allows for storing electricity or converting it into chemical bonds, producing chemical building blocks and fuels using renewable resources. Therefore, it plays an important role... Show moreElectrocatalysis allows for storing electricity or converting it into chemical bonds, producing chemical building blocks and fuels using renewable resources. Therefore, it plays an important role in the transition towards a more sustainable future for our society through electrification. Still, to bring electrochemical technologies to industrial scale and make them competitive, optimization of various aspects of electrocatalytic reactions are needed. Many fundamental studies focus on understanding the catalyst surface, however, different components of the electrolyte, as pH and cations, have also shown to significantly affect the reaction activity and selectivity. In view of that, in this thesis, various aspects of the electrode-electrolyte interface are studied at different scales, using Scanning Electrochemical Microscopy (SECM), stationary and rotating-disc electrode voltammetry techniques, and bulk electrolysis. Show less
In this dissertation iron-based homogeneous catalysts were synthesized, characterized and investigated for water oxidation activity. The catalysts were studied under electrochemical conditions in... Show moreIn this dissertation iron-based homogeneous catalysts were synthesized, characterized and investigated for water oxidation activity. The catalysts were studied under electrochemical conditions in order to compare the electrochemical approach to the catalysis based on the use of sacrificial oxidants. The mechanisms under which these catalysts operate have been explored with particular attention to the O−O bond formation step. The combination of electrochemical techniques and in situ characterization techniques allowed for the identification of the active intermediates responsible for catalysis. The influence of the presence of water oxidation catalysts in solution on the evolution of carbon dioxide from the surface of a pyrolytic graphite working electrode was also investigated. Overall, the results of this work demonstrate that the combination of in operando and in situ (spectro)electrochemical techniques allows for a complete investigation of the catalytic mechanism of the water oxidation reaction. Show less
This thesis discusses the parameters affecting the catalysis for the electrochemical conversion of water into oxygen. The slow kinetics for the oxygen evolution reaction (OER) is one of the major... Show moreThis thesis discusses the parameters affecting the catalysis for the electrochemical conversion of water into oxygen. The slow kinetics for the oxygen evolution reaction (OER) is one of the major bottlenecks in the solar energy-to-fuels conversion process, which reduces the efficiency for the photo-electrochemical fuels generation (artificial photosynthesis). The work shows that to enhance the kinetics for the oxygen evolution reaction, one should not only look at the catalysts but also consider the synergy between catalyst and electrolyte. A more general approach that considers the electrochemical interface as a whole (electrode + electrolyte) is therefore the most promising route towards optimal activity. Show less
Molecular complexes can be used as electrocatalysts for oxygen reduction, water oxidation, and/or hydrogen peroxide production. However, in situ degradation of these catalyst is a major issue. This... Show moreMolecular complexes can be used as electrocatalysts for oxygen reduction, water oxidation, and/or hydrogen peroxide production. However, in situ degradation of these catalyst is a major issue. This dissertations describes the analysis of degradation processes as well as the performance of various molecular electrocatalysts. In addition, complexes have been structurally modified to perform structure-activity studies that could to mechanistic insight. In addition, it is described how molecular catalysts can be beneficial to heterogeneous electrocatalysis as well. Show less
This project has dealt with the mechanistic study of the electrocatalytic nitrite reduction, the selectivity-determining step of nitrate reduction. Nitrate is a polluting ion targeted by wastewater... Show moreThis project has dealt with the mechanistic study of the electrocatalytic nitrite reduction, the selectivity-determining step of nitrate reduction. Nitrate is a polluting ion targeted by wastewater remediation; electrochemistry strives to achieve selectivity to harmless products (N2). A multi-pronged approach has been followed, aimed at establishing the influence of several variables (electrocatalyst material, surface structure, pH and electrode potential) on the catalytic activity and the product distribution, which has been determined with in situ analytical techniques (mass spectrometry and infrared spectroscopy). The molecular underpinnings of nitrite reduction have thereby been unravelled for transition metals, showing that an optimal catalytic performance is achieved when metals intermediate affinities to reaction intermediates (Sabatier Principle). The all-important concept of structure sensitivity also applies to nitrite reduction at Pt electrodes, although only in alkaline media: a Pt(100) single-crystal is the sole Pt surface able to achieve the desired direct conversion of nitrite into 100% N2. Such selectivity is unparalleled for a simple monometallic surface and is an outstanding finding. Additionally, the nitrite-reducing performance of bio-inspired catalysts, (electroactive metalloporphyrins) was investigated. A further side-project of this PhD thesis has also been the electrochemical characterization of preferentially-oriented cuboid Pt nanoparticles synthesized with the innovative __cathodic corrosion__. Show less
The growing demand of energy indicates that global energy resources in the form of fossil fuels will not be sufficient in the future. In order to solve potential future energy problems development... Show moreThe growing demand of energy indicates that global energy resources in the form of fossil fuels will not be sufficient in the future. In order to solve potential future energy problems development of a sustainable hydrogen economy is highly desirable. Researchers are looking for new and cleaner ways for the production of dihydrogen gas. The structure and function of hydrogenases have raised the attention of synthetic chemists in the past decades, since new catalysts for proton reduction may be developed by using biomimetic, functional models of hydrogenases. Three types of hydrogenases are known, being the [FeFe], [Fe] and [NiFe] hydrogenases.A significant amount of data has been gathered over the years concerning the enzyme redox states and the reaction mechanism for the reversible heterolytic splitting of dihydrogen at the [NiFe] hydrogenase active site. The [NiFeSe] hydrogenases form a subclass of the [NiFe] hydrogenases, in which one of the cysteines (Cys) in the active site of the enzyme is replaced by selenocysteine (Sec). This thesis deals with the synthesis and characterization of new structural and functional models of the nickel-containing enzymes [NiFe] and [NiFeSe] hydrogenases for electrocatalytic hydrogen evolution. Show less
Gezer, G.; Durán Jiménez, D.; Siegler, M.A.; Bouwman, E. 2017
This thesis has shed light on some of the ways in which the local electrolyte composition can differ from the bulk and how these changes in the local reaction environment can determine the activity... Show moreThis thesis has shed light on some of the ways in which the local electrolyte composition can differ from the bulk and how these changes in the local reaction environment can determine the activity and/or selectivity of two important electrocatalytic reactions, namely, electrochemical CO2 reduction reaction (CO2RR) and hydrogen evolution reaction (HER). Show less
This dissertation focused on computational methods based on first principles calculations using the Density Functional Theory (DFT) framework. Emphasis was laid on affordable methods that can... Show moreThis dissertation focused on computational methods based on first principles calculations using the Density Functional Theory (DFT) framework. Emphasis was laid on affordable methods that can provide a tradeoff between computational expense and accuracy. Specifically, we investigated solvation effects near the surface of the electrode, used thermodynamic cycles to compute solution-phase energies and also proposed a workflow to detect gas-phase errors on the free energies of target molecules. We used these simple methods to study complex adsorption processes at the PdMLPt(111) electrode surface. DFT and experimental studies (performed by Dr. Chen from our group) were crucial to guide the investigation forward. Show less
Cathodic corrosion is a relatively unknown phenomenon that can severely etch metallic electrodes at cathodic (negative) potentials. In spite of these remarkable changes that are caused by cathodic... Show moreCathodic corrosion is a relatively unknown phenomenon that can severely etch metallic electrodes at cathodic (negative) potentials. In spite of these remarkable changes that are caused by cathodic corrosion, the phenomenon is stil not fully understood. Cathodic corrosion is therefore the focus of this PhD thesis. The first three experimental chapters of the thesis focus on characterizing platinum, rhodium and gold electrodes before and after cathodic corrosion in a variety of working solutions. In doing so, these chapters establish surprisingly mild corrosion onset potentials and reveal an etching anistropy that depends on the cation in the working solution. Additional density functional theory calculations suggest a similarly significant role for adsorbed hydrogen. These result suggest the existence of ternary metal hydrides during cathodic corrosion. The role of hydrides is further studied in the fourth experimental chapter through X-ray absorption spectroscopy. These four fundamental chapters are followed by two more applied chapters. The first of these tailors the activity of a platinum single crystal towards oxygen reduction, by using cathodic corrosion. The second applied chapter uses cathodic corrosion to create and thoroughly characterize alloyed nanoparticles. Combined, these fundamental and applied chapters provide valuable new information towards understanding and applying cathodic corrosion. Show less
Advanced sensing techniques require graphene with high quality and well-controlled surface chemistry. The intrinsic high mobility, low electrical noises and uniform graphitic crystallinity are the... Show moreAdvanced sensing techniques require graphene with high quality and well-controlled surface chemistry. The intrinsic high mobility, low electrical noises and uniform graphitic crystallinity are the prerequisites for high-performance graphene electronics. More importantly, chemical functionalization contributes to unlock the sensing potential of the graphene basal plane. This thesis focuses on manipulating the surface chemistry of a graphene monolayer and explores the impacts on the electrical and electrochemical properties for sensing applications. Heteroatoms like hydrogen, nitrogen and oxygen were systematically introduced into the graphene lattice as defect sites to modify the surface chemistry, and consequently the electronic properties and sensing performance. In summary, a correlation between the in-plane electron transport and the electrochemical activity of hydrogenated graphene was studied by modulating the density of H-sp3 defects. Moreover, cleaning effect on the graphene surface caused by hydrogenation process and the corresponding mechanism were discussed. The electrocatalysis of oxygen reduction reaction on nitrogen doped monolayer graphene was conducted to pinpoint the catalytic active sites. The mechanics of a centimeter-scale graphene floating on water was characterized by biaxial compression. Finally, the chemically modified graphene was tested for field-effect sensing of gas molecules. Show less
The electrochemical oxidation of ammonia to dinitrogen is a model reaction for the electrocatalysis of the nitrogen cycle, as it can contribute to the understanding of the making/breaking of NN, NO... Show moreThe electrochemical oxidation of ammonia to dinitrogen is a model reaction for the electrocatalysis of the nitrogen cycle, as it can contribute to the understanding of the making/breaking of NN, NO, or NH bonds. Moreover, it can be used as the anode reaction in ammonia electrolyzers for H2 production or in ammonia fuel cells. We study here the reaction on the N2-forming Pt(1 0 0) electrode using a combination of electrochemical methods, product characterization and computational methods, and suggest a mechanism that is compatible with the experimental and theoretical findings. We propose that N2 is formed via an ∗NH + ∗NH coupling step, in accordance with the Gerischer-Mauerer mechanism. Other NN bond-forming steps are considered less likely based on either their unfavourable energetics or the low coverage of the necessary monomers. The NN coupling is inhibited by strongly adsorbed ∗N and ∗NO species, which are formed by further oxidation of ∗NH. Show less
