The disproportionation of H₂O₂ to H₂O and molecular O₂ catalyzed by platinum nanocatalysts is technologically very important in several energy conversion technologies, such as steam propellant thrust applications and hydrogen fuel cells. However, the mechanism of H₂O₂ decomposition on platinum has been unresolved for more than 100 years and the kinetics of this reaction were poorly understood. Our goal was to quantify the effect of reaction conditions and catalyst properties on the decomposition of H₂O₂ by platinum nanocatalysts and determine the mechanism and rate-limiting step of the reaction. To this end, we have characterized two commercial platinum nanocatalysts, known as platinum black and platinum nanopowder, and studied the effect of different reaction conditions on their rates of H₂O₂ decomposition. These samples have different particle size and surface chemisorbed oxygen abundance, which were varied further by pretreating both samples at variable conditions. The rate of H₂O₂ decomposition was studied systematically as a function of H₂O₂ concentration, pH, temperature, particle size and surface chemisorbed oxygen abundance. The mechanism of H₂O₂ decomposition on platinum proceeds via two cyclic oxidation-reduction steps. Step 1 is the rate limiting step of the reaction. Step 1: Pt + H₂O₂ → H₂O + Pt(O). Step 2: Pt(O) + H₂O₂ → Pt + O₂ + H₂O. Overall: 2 H₂O₂ → O₂ + 2 H₂O. The decomposition of H₂O₂ on platinum follows 1st order kinetics in terms of H₂O₂ concentration. The effect of pH is small, yet statistically significant. The rate constant of step 2 is 13 times higher than that of step 1. Incorporation of chemisorbed oxygen at the nanocatalyst surface resulted in higher initial rate of H₂O₂ decomposition because more sites initiate their cyclic process in the faster step of the reaction. Particle size does not affect the kinetics of the reaction. This new molecular-scale understanding of the decomposition of H₂O₂ by platinum is expected to help advance many energy technologies that depend on the rate of H₂O₂ decomposition on nanocatalysts of platinum and other metals. / Ph. D. / Platinum nanomaterials are indispensable to catalyze a variety of industrial and technological processes ranging from catalytic conversion of carbon monoxide (CO), hydrocarbons, and nitrogen oxides (NO_x) in modern automobiles to energy production by hydrogen fuel cells and thrust generation in steam propellers. These technological innovations have a tremendous impact in modern society, including the areas of transportation, energy supply, soil and water quality, environmental remediation and global climate change. The decomposition of hydrogen peroxide (H₂O₂) to water (H₂O) and oxygen (O₂) on platinum nanomaterials is of particular importance because it affects the efficacy of many technological applications, such as hydrogen peroxide steam propellers and hydrogen fuel cells. However, the reaction pathway and kinetics of H₂O₂ decomposition on platinum were only partly understood. My goal was to understand how the reaction conditions and the nanocatalyst properties control the mechanism and kinetics of platinum-catalyzed hydrogen peroxide decomposition. To do that we characterized the atomic scale structural and chemical properties of two different platinum nanocatalysts, known as platinum black and platinum nanopowder and evaluated the effect of their properties in their catalytic activity. Our characterization studies were used to understand the reactivity of these two platinum nanocatalysts in the decomposition of H₂O₂, which we evaluated separately in laboratory studies. Establishing relationships between the catalyst properties and their activity, as we have done in this work for platinum nanocatalysts in the decomposition of hydrogen peroxide, has the potential to improve nanocatalyst design and performance for those applications.

No / Here we report a hierarchical composite structure composed of few-layers molybdenum disulfide nanosheets supported by vertical graphene on conductive carbon cloth (MDNS/VG/CC) for high-performance electrochemical hydrogen evolution reaction (HER). In the fabrication, 3D vertical graphene is first prepared on carbon cloth by a micro-wave plasma enhanced chemical vapor deposition (MPCVD) and then few-layers MoS2 nanosheets are in-situ synthesized on the surface of the vertical graphene through a simple hydrothermal reaction. This integrated catalyst exhibits an excellent HER electrocatalytic activity including an onset potential of 50 mV, an overpotential at 10 mA cm(-2) (eta(10)) of 78 mV, a Tafel slop of 53 mV dec(-1), and an excellent cycling stability in acid solution. The excellent catalytic performance can be ascribed to the abundant active edges provided by the vertical MoS2 nanosheets, as well as the effective electron transport route provided by the graphene arrays on the conductive substrate. Moreover, the vertical graphene offers robust anchor sites for MoS2 nanosheets and appropriate intervals for electrolyte infiltration. This not only benefits hydrogen convection and release but also avoids the damaging or restacking of catalyst in electrochemical processes. / This work was financially supported by the National Natural Science Foundation of China (Grant nos. 61176007, 51372213, and 51402343).

M. Tech. (Department of Chemistry, Faculty of Applied and Computer Sciences), Vaal University of Technology. / Organic water pollutants such as dyes are difficult to biodegrade. In this study Fenton, photo-Fenton and photocatalysis were used to degrade methylene blue dye in the presence of copper oxide catalysts. The copper oxide catalysts were prepared with a precipitation reduction method. The effect of different preparation parameters on the catalyst properties and catalytic activity were investigated. The reducing agents, ascorbic acid (ASC, C6H8O6), hydrazine (N2H4), sodium boron hydride (NaBH4) and glucose (C₆H₁₂O₆) could be used to obtain the desired Cu2O phase. ASC, N2H4 and NaBH4 were able to reduce copper (II) to copper (I) at room temperature whereas glucose required a higher reduction temperature. Stoichiometric amounts of the reducing agents ASC, N2H4 and glucose and double the stoichiometric amount of NaBH4 were required to obtain Cu2O. A further increase in the amounts of NaBH4 and N2H4 resulted in the formation of copper metal (Cu (0)). High amounts of ASC did not over-reduce the copper. ASC also functioned as capping molecule and anti-oxidant preventing the oxidation of the Cu2O to CuO in air after preparation. Hydrazine was thus not able to protect the Cu2O against oxidation. The SEM results showed that an increase in the amount of the precipitating agent, NaOH, resulted in an increase in the particle sizes. The particle shapes changed from spherical to cubic when a high amount of NaOH was used with hydrazine as reducing agent. Smaller particle sizes were obtained when CuCl2 was used instead of CuSO4 and Cu(NO3)2. Larger crystallites formed when the preparation temperature was increased from room temperature to 100°C with glucose as reducing agent. TEM and XRD analyses showed that the micro-particles seen in SEM analyses are made up of nano-particles. The catalysts were not active for photocatalysis which may be explained by the oxidation of these nano-particles to form the photocatalytic inactive CuO. The catalysts were shown to be active for Fenton and photo-Fenton degradation. The addition of graphene and activated carbon to the Cu2O catalysts were detrimental to the catalytic activity. The percentage degradation of methylene blue by the Fenton reaction increased with an increase in the BET surface area from 1.5 m2/g to 10 m2/g and a further increase in the surface area resulted in a decrease in the percentage degradation. A direct correlation between the Fenton catalytic activity and the pore size were found which indicate that the reaction was mass transfer limited.

Les matériaux hydroxydes doubles lamellaires (HDL) sont une classe d'argiles anioniques synthétiques dont la structure est basée sur celle du brucite Mg(OH)2 dans lesquelles une partie des cations métalliques divalents sont été remplacés par des ions trivalents donnant ainsi des feuillets chargés positive. Cette charge est équilibrée par l'intercalation d'anions dans la région interlamellaire hydratée. Les identités et les rapports des cations di- et trivalents et l'anion interlamellaire peuvent être varié sur une large gamme, donnant lieu à une large classe de matériaux isostructurales. Le matériau d’origine de cette classe est l’hydrotalcite (HT) et les HDL sont par conséquent également connus comme des matériaux de type hydrotalcite. Bien que les caractéristiques de base de la structure soient bien comprises, des aspects structurels détaillés ont fait l'objet de certaine controverse dans la littérature afin de maîtriser leurs propriétés et leurs applications potentielles. Dans ce travail de thèse nous avons retenu deux types de HDL MgAl et ZnAl qui ont été largement introduits dans diverses applications, tels que la sorption des molécules d'intérêt biologique (enzyme et médicament) et l'élaboration d'électrodes. La spécificité de ce travail repose sur l’immobilisation d’une enzyme modèle, la lactate déshydrogénase dans ces deux matrices ainsi qu’un médicament anti-bactérien, la berbérine, afin d’étudier les interactions entre ces deux biomolécules et la phase HDL introduite et de répondre à leurs exigences d'applications dans le domaine médical. Dans un second temps nous avons tenté d’étudier les deux phases mentionnées de plus en plus fine en termes de structure, morphologie et profil électrochimique en vue de les employer en tant que matériaux d’électrode pour le développement de biopile / DHs are a class of synthetic anionic clays whose structure is based on brucite-like layers Mg(OH)2 inwhich some of the divalent cations have been replaced by trivalent ions giving positively-charged sheets.This charge is balanced by intercalation of anions in the hydrated interlayer regions. The identities andratios of the di- and trivalent cations and the interlayer anion may be varied over a wide range, giving rise toa large class of isostructural materials. The parent material of this class is the naturally occurring mineralhydrotalcite and LDHs are consequently also known as hydrotalcite-like materials. Although the basicfeatures of the structure are well understood, detailed structural aspects have been the subject of somecontroversy in the literature. In this thesis, we have selected two types of LDH, MgAl and ZnAl, which havebeen widely introduced in various applications, such as sorption of molecules of biological interest (enzymeand drug) and the development of electrodes. The specificity of this work lies on the immobilization of amodel enzyme, lactate dehydrogenase in both matrices as well as an anti-bacterial drug, berberine, inorder to study the interactions between these two biomolecules and the introduced LDH phase and tobetter address their challenges of applications in the medical field. Second, we have tried to study the twophases mentioned above more and more accurately in terms of structure, morphology and electrochemicalprofile in order to use them as electrode materials for microbial fuel cell device

Phosphatases of the haloacid dehalogenase superfamily are one of the cell's tools for dephosphorylation of many diverse endogenous and exogenous compounds. This work is aimed at enzymes Tt82 and cytosolic purine 5'-nucleotidase II (cN-II), two members of this large enzyme superfamily. The Tt82 originates in the hyperthermophilic archaeon Thermococcus thioreducens. Up to date, there is only a small amount of knowledge about properties and biological function of this enzyme. Based on its sequence and structure, it was predicted that the Tt82 should possess a phosphatase catalytic activity. Consequently, potential substrates of the Tt82 were proposed by the molecular docking. In this work, the phosphatase activity of the Tt82 was confirmed together with several of its substrates: AMP, D-glucose 1-phosphate, D-glucose 6-phosphate and p-nitrophenyl phosphate (pNPP). Activity towards AMP and pNPP was then characterized by steady-state kinetics at 37 řC and 60 řC. In consistence with its thermophilic origin, the Tt82 showed markedly higher activity towards both substrates at 60 řC. Nonetheless, the effectivity of the Tt82 catalytic activity towards these substrates was actually very low. This leads to assumption, that the identified substrates are probably not biologically relevant. On the other hand, it is quite...

M. Tech. (Department of Chemistry, Faculty of Applied and Computer Sciences), Vaal University of Technology. / Water polluted by recalcitrant organics, such as methylene blue (MB), can be treated with the Fenton reaction. The Fenton reaction degrades the pollutants through catalytic oxidation. Unsupported iron oxide nanoparticles (IONPs) were used as catalysts in this study. Iron oxide nanoparticles were synthesised using a precipitation-oxidation method and effects of various preparation parameters on the shape, size and catalytic activity of the iron oxide nanoparticles were studied. Parameters investigated include preparation temperature, type and amount of precipitating agent. The precipitating agents used are sodium hydroxide, tetramethyl ammonium hydroxide (TMAOH), tetraethyl ammonium hydroxide (TEAOH) and ethylamine. The iron oxide nanoparticles were found to be spherical for most of the preparation conditions as determined by TEM. However, irregular flower-like shapes (hexagonal with rod extensions) were obtained when the amounts of the TMAOH and TEAOH bases were more than the stoichiometric amounts. The nature and amount of precipitating agent also influenced the degree of particle agglomeration and growth, with an increase in alkyl chains in the base giving lesser agglomeration. The preparation temperature did not influence the nanoparticles’ size when NaOH was used as a precipitating agent. In contrast, when an amine was used as a precipitation agent, caused a slight increase in the size of the nanoparticles. Different crystal phases like hematite, magnetite, maghemite and goethite-hematite mixture were identified in the X-ray diffractograms. UV-Vis spectroscopy showed that all the catalysts were red-shifted except for B3 sample, which was blue-shifted from the bulk materials. The highest catalytic activities were obtained when NaOH was used as a precipitation agent instead of amine since catalyst has shown to contain the traces amounts of the base used on the surface. The lower catalytic activities for the catalysts prepared using amines may be due to amines adsorbed on the surface and blocking the catalytic active sites. FTIR spectra showed the presence of trace amounts of ammine functional groups on the nanoparticles No correlation was found between the crystallite size and the Fenton catalytic activity of the catalyst. In the same vein, operational parameters such as the amount of H2O2 and temperatures did not show a direct effect on the Fenton catalytic efficiency. Kinetic studies show that the degradation of methylene blue followed the first-order models for all the catalysts prepared with NaOH. Overall, the study shows that different preparation parameters had an effect on the size, shape, phase and the catalytic performance of the synthesised IONPs.

M.Tech. (Department of Chemistry, Faculty of Applied and Computer Sciences), Vaal University of Technology. / Several industries utilize species of synthetic dyes that are found in their wastewater, which is passed out in the environment. Methylene blue is one of the organic dyes that causes water pollution. It causes damage to the aquatic eco-system and health problems to human beings. It is non-biodegradable due to its chemical nature. Advanced oxidation processes (AOP’s) have been developed for the degradation of these dyes, however, some of these methods are limited due to their high cost and low efficiency. Among these methods, Fenton catalysis has been proven to be an effective method due to its low cost, high efficiency, and re-usability. Iron oxide nanoparticles have been mainly used in Fenton process however they are also limitated due to the forming of secondary pollutants, due to catalysts recovery difficulties, hence they require supporting materials. In this work, iron oxide-based catalyst supported on silica nanofibers were fabricated via electrospinning of silica sol incorporated with iron oxide, using three different routes, (a) Method 1 - wetness incipient impregnation, (b) Method 2 - direct addition of iron precursor to the silica sol and (c) Method 3 - incorporation of iron oxide nanoparticles into silica sol. The effect of iron oxide concentration loadings (1 wt%, 2 wt% and 5 wt %) was studied. Increase in iron content resulted in agglomeration of nanoparticles as embedded in the fibers as evident from their SEM images in method 3.1. The SEM results showed diameters from method 1, 2 and 3 ranging from the distribution ranges of 276 – 288 nm, 243 – 265 nm and 188 nm, respectively. EDS showed the presences of Si, P, Fe, O and P. XRD showed a crystalline phase of magnetite (9 nm) and goethite (32 nm) method 1 and 3, with vibrational modes at 3300 cm-1, 1100 cm-1, 950 cm-1 and 580 cm-1 ascribed to O-H, Si-O-Si, Si-O and Fe-O on the FTIR spectra, it showed both the presence of silica and iron oxide. The degradation of methylene blue was monitored by UV-Vis spectroscopy, the Fenton catalytic activity of the iron-oxide supported on silica nanofibers showed higher catalytic activity compared to the unsupported iron-oxide nanoparticles. The catalyst prepared by wetness incipient impregnation (method 1) had a degradation efficiency of 69.1%, the direct addition of iron precursor to the silica sol (method 2) had 75.2% and incorporation of iron oxide nanoparticles magnetite and goethite with the silica sol had 53.7% and 34.7%, respectively. The catalyst prepared by the direct addition of iron precursor in the sol (method 2) showed a high catalytic activity compared to the other catalyst prepared by other methods. Unsupported Iron oxide nanoparticles had a higher degree of leaching of 1.28 ppm magnetite, and 1.68 ppm goethite, compared to the supported iron oxide in method 1 and method 3. The catalyst incorporated with goethite showed a high degree of leaching, 3.95 ppm and 1.33 ppm. The catalyst with high catalytic activity showed a lower degree of leaching with 0.05 ppm.

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Bakterielle Zellhüllenproteine (S-Layer) können als formgebende Muster für die bottom-up Materialsynthese Verwendung finden. Auf S-Layern von Bacillus sphaericus und Sporosarcina ureae lassen sich nasschemisch Platin- bzw. Palladiumcluster abscheiden, die sich durch ihren gleichmäßig geringen Durchmesser und ihre hohe laterale Dichte auszeichnen. Am Beginn der vorliegenden Arbeit steht die Charakterisierung des Proteintemplates, welches grundlegenden Einfluss auf die sich bildenden Edelmetallcluster hat. Die Topographie der S-Layeroberfläche wird atomkraftmikroskopisch untersucht. Durch Photoemissions- und NEXAFS-Spektroskopie werden Aussagen zur elektronischen Struktur des Proteins gewonnen, die nach entsprechender Interpretation Erklärungen für das Verhalten des Proteintemplates liefern. Daneben sind Syntheseparameter ausschlaggebend für das Erscheinungsbild des dispersen Metalls. Insbesondere der Einfluss des Reduktionsmittels auf die Clustergröße wird elektronenmikroskopisch und durch Kleinwinkelstreuung untersucht. Die katalytische Aktivität von auf gamma-Al2O3 und SiC immobilisierten metallisierten S-Layern für die Oxidation ausgewählter Kohlenwasserstoffe und Kohlenmonoxid wird bestimmt. Außerdem werden Verfahren zur Erzeugung von Gold- und Silberclustern auf S-Layern vorgestellt.

Bakterielle Zellhüllenproteine (S-Layer) können als formgebende Muster für die bottom-up Materialsynthese Verwendung finden. Auf S-Layern von Bacillus sphaericus und Sporosarcina ureae lassen sich nasschemisch Platin- bzw. Palladiumcluster abscheiden, die sich durch ihren gleichmäßig geringen Durchmesser und ihre hohe laterale Dichte auszeichnen. Am Beginn der vorliegenden Arbeit steht die Charakterisierung des Proteintemplates, welches grundlegenden Einfluss auf die sich bildenden Edelmetallcluster hat. Die Topographie der S-Layeroberfläche wird atomkraftmikroskopisch untersucht. Durch Photoemissions- und NEXAFS-Spektroskopie werden Aussagen zur elektronischen Struktur des Proteins gewonnen, die nach entsprechender Interpretation Erklärungen für das Verhalten des Proteintemplates liefern. Daneben sind Syntheseparameter ausschlaggebend für das Erscheinungsbild des dispersen Metalls. Insbesondere der Einfluss des Reduktionsmittels auf die Clustergröße wird elektronenmikroskopisch und durch Kleinwinkelstreuung untersucht. Die katalytische Aktivität von auf gamma-Al2O3 und SiC immobilisierten metallisierten S-Layern für die Oxidation ausgewählter Kohlenwasserstoffe und Kohlenmonoxid wird bestimmt. Außerdem werden Verfahren zur Erzeugung von Gold- und Silberclustern auf S-Layern vorgestellt.