Development of a Photo-Fenton Catalyst Supported on Modified Polymer Films

The work presented in this thesis is a part of the European project INNOWATECH. The global objective of this project was to provide effective technological solutions for the treatment of industrial wastewater, to propose new concepts in wastewater treatment with potential benefits for the protection of the environment. In particular photo-assisted Fenton oxidation was investigated. It is a promising technology to decontaminate industrial wastewater as it makes use of the natural energy that provides the sun, abundant chemicals (iron ions and hydrogen peroxide) and does not produce toxic waste. Apart from short detour about homogenous photo-Fenton reaction (chapter 2) where the influence of pollutant physico-chemical properties on reactivity is studied, this thesis focuses on photo-Fenton treatment using a new solid catalysis. The immobilization of iron oxide on a suitable support is a strategy proposed to overcome the practical limitation related to homogeneous photo-Fenton treatment (i.e. the limited operational pH range and the problems caused by the separation of catalyst from the effluent). The preparation and the surface characterization of new photo-Fenton catalysts based on iron oxide supported on modified polymer films is described in chapters 3 and 4. The photocatalytic activities of prepared materials were evaluated mainly toward organic pollutant degradation both at laboratory (chapter 3-5) and at pilot (chapter 6) scales. In detail, chapter 2 focuses on the effect of contaminant physico-chemical properties on the reactivity via photo-assisted Fenton catalysis. Several para-substituted phenols were used in order to cover a wide range of electronics effects. Many physico-chemical descriptors were correlated with the initial Fenton and photo-Fenton degradation rates (r0). Electronic descriptors such as calculated zero point energy (Ezero) and energy of the highest occupied orbital were found to be the most adequate to predict Fenton and photo-assisted Fenton reactivity. The preparation of iron oxide-coated polymer films is described in chapter 3. Polyvinyl fluoride (PVF) film surface was functionalized by different methods, either by Vacuum-UV radiation, radio-frequency plasma, photo-Fenton oxidation or TiO2 photocatalysis. These pre-treatments were performed to increase iron oxide adhesion to polymer surface. Afterward the functionalized polymers films were immersed in an aqueous solution for the deposition of iron oxide layer by hydrolysis of FeCl3. The catalytic activities of resulting materials were compared during hydroquinone degradation in presence of H2O2 and under simulated solar light illumination. The most efficient and stable catalyst obtained was prepared by means of TiO2 photocatalytic functionalization of polymers followed by iron oxide coating (leading to so called Pf-TiO2-Fe oxide), therefore this preparation procedure was selected in chapters 4-6. Chapter 4 focuses on the study of the mechanisms involved during the preparation and use of Pf-TiO2-Fe oxide. In particular, the modifications induced by TiO2 photocatalysis on polymer surface such as oxygen group formation and deposition of TiO2 particles were characterized by x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and UV-visible spectrophotometry. The photocatalytic activity of Pf-TiO2-Fe oxide in presence of H2O2 and under simulated solar light radiation was evaluated toward HQ degradation. The occurrence of important synergistic effects between TiO2 and iron oxide was discussed. Finally, the effect of preparation parameters on photocatalytic activity of Pf-TiO2-Fe oxide/H2O2/light system was determined allowing the optimization of preparation procedure. Hence highly efficient photocatalysts for hydroquinone degradation and E. Coli inactivation were obtained. The degradation of hydroquinone and nalidixic acid (NA) mediated the system Pf-TiO2-Fe oxide/H2O2/light was examined (chapter 5). The contribution of homogeneous photo-Fenton oxidation in the degradation process was determined and the mechanisms involved in iron leaching are discussed. Besides, the effect of operational parameters on degradation rates was assessed. The rates are independent on initial pH and NaCl presence but were enhanced by increasing temperature. Long-term stability of Pf-TiO2-Fe oxide was evaluated by repetitive nalidixic acid degradation runs. The adaptation of Pf-TiO2-Fe oxide to pilot scale in a compound parabolic collector (CPC) solar photoreactor is described in chapter 6: solar photocatalytic degradation of phenol, nalidixic acid, mixture of pesticides, and another of emerging contaminants, in water was investigated. The influences of operational pH and pollutant structure on the degradation rates were evaluated. It was found that compounds with chelating moieties (or carboxylic acids) were the most quickly removed by Pf-TiO2-Fe oxide/H2O2/light and allowed the photo-Fenton catalyst to be efficient at higher pH values.

Amorphous Molybdenum Sulfide Films as Hydrogen Evolution Catalysts in Water

Daniel Andreas Merki

Molecular hydrogen is a promising candidate to replace fossil fuels as the energy carrier. Hydrogen does not exist in its molecular form on earth and must therefore be generated, starting from hydrogen-rich compounds. Water would be a renewable resource for hydrogen. It can be split into oxygen and hydrogen. Water splitting, however, needs electrical energy. If this energy is produced from renewable resources, such as solar or wind power, water splitting would be a sustainable and green process. One key step in water splitting is the reduction of protons to form hydrogen. To achieve a high efficiency in this reaction, catalysts are required. This dissertation is devoted to the development of hydrogen evolution catalysts that are made of earth-abundant and inexpensive materials. First, transition metal complexes of tetrathiotungstate and tetrathiomolybdate were studied as potential homogeneous catalysts for hydrogen evolution in organic or aqueous solutions (chapter 1). The redox activity of these complexes was investigated by cyclic voltammetry in the absence and presence of an acid in organic solutions. The first reduction peak of (Pr4N)2[Ni(MoS4)2] became more intense and was shifted to less negative potentials upon addition of an acid, e.g. p-cyanoanilinium tetrafluoroborate. This was considered as a sign for catalytic proton reduction. Electrolysis of a solution containing (Pr4N)2[Ni(MoS4)2] and an acid produced hydrogen. However, the complex turned out to be a precursor of the real catalytically active species, which was deposited on the working electrode’s surface during the electrochemical experiment. Consecutive cyclic voltammetric scans were found to be a good method to deposit the catalytically active film in a controllable manner from an aqueous solution of (Pr4N)2[Ni(MoS4)2]. Furthermore, a catalytically active film could be deposited from a solution of MoS42-, i.e. in the absence of Ni2+. Chapter 2 describes the investigation and characterization of films made using MoS42- as the precursor. The films consist of amorphous molybdenum sulfide. They are efficient hydrogen evolution catalysts in water. The films were characterized by XPS and electron microscopy. Whereas the pre-catalysts could be MoS3 or MoS2, the active form of the catalysts was identified as amorphous MoSx, with x close to 2. Significant geometric current densities were achieved at low overpotentials (e.g., ca. 15 mA/cm2 at η = 200 mV) using these catalysts. The catalysis was compatible with a wide range of pH values. The current efficiency for hydrogen production was quantitative. A Tafel slope of 39 mV/dec was observed, suggesting a rate-determining Heyrovsky step. The turnover frequency per active site was estimated.

EPFL2012

Molecular Engineering of Electrode Surfaces for Electrocatalysis

Patrick Eduard Alexa

Electrocatalysts play an important part on the route towards environmental friendly energy sources. They support modern technologies like fuel cells to move further away from the utilization of fossil fuels and the resulting CO2 emission into our atmosphere. The main principle of such energy conversion technologies is to store chemical energy within chemical bonds and to release it as electric energy to fulfill the energy demand of today's society. However energy conversion chemistry in such technologies requires catalysts in order to be effective by providing an alternative reaction pathway that lowers activation energies and maximizes the energy output. Providing suitable catalyst surfaces requires a fundamental knowledge of the interaction of reactants and reaction intermediates with the catalyst surfaces. This concern is addressed in this thesis by fabricating specifically tailored molecular components on metal surfaces, which act as organic/inorganic hybrid electrodes for various energy conversion reactions in electrocatalysis. Characterizing the electrode surface is crucial in order to identify active sites of the catalyst. The molecular structure is investigated by surface sensitive techniques, such as STM and XPS in combination with a home-build transfer system, which enables electrode fabrication in UHV and electrocatalytic experiments in a liquid environment. A detailed characterization of organic polymer components on electrode surfaces is obtained by combining experimental data with MD simulations. The morphology of polymer networks can be successfully mimicked and depending on the chemical composition and the structure, spatial correlations on short length scales are reported. Hybrid electrodes with organic polymer co-catalysts reveal an increased catalytic activity for the HER by providing suitable docking sites that act as catalytic active sites. Reactants and reaction intermediates are stabilized via hydrogen bonding, which results in a higher catalytic turnover. The specific designed structure of the organic co-catalyst helps to identify a fraction of the reaction mechanism and highlights the importance of molecular components in electrocatalysis. The utilization of organic units on electrocatalysts provides also an excellent opportunity in order to use single metal atoms as catalysts. Engineering large pi-systems has no significant impact on the ORR activity, while the distance between active sites and the underlying electrode are crucial for enhancing the catalytic activity. A significant factor in using molecular components in electrocatalysis is the stability of the organics. Using electrografting allows to design organic/inorganic hybrid electrodes with covalent bonds between the molecular component and the metal substrate. Attaching empty porphyrin molecules to substrates introduces a powerful technique in molecular engineering, since empty porphyrin cycles can be metallized with various metal centers that can act as electrocatalysts for different electrochemical reactions. In summary, this thesis provides multiple engineering aspects, which are useful in designing molecular components on metal electrodes for energy conversion. Such hybrid catalyst surfaces are capable of improving the catalytic activity and allow to study the mechanism of catalytic reactions. Implementing organic components provides new routes for tailoring novel materials, which deliver new insight into the complex field of electrocatalysis.

EPFL2020

Catalyst design for C-O bond hydrogenolysis of renewable materials.

Antoine Philippe Van Muyden

Anthropogenic carbon dioxide emissions leading to climate change require to use of renewable carbon sources such as CO2 and biomass which differ from fossil resources by having a higher number of oxygen atoms. Therefore, catalytic C-O bond cleavage will play a pivotal role in their conversion into carbon neutral fuels, materials and chemicals. This thesis will focus on the most challenging substrate for selective C-O bond hydrogenolysis, diaryl ether present in lignin (i.e. one of the components of biomass), summarise the state of research and improve the comprehension in the characteristics a catalyst requires to selectively cleave these bonds without altering other functionalities. We showed that the modification in the electronic state and the resulting polarity between two different metals present in a bimetallic nanoparticle favour the selectivity towards hydrogenolysis of a polar C-O bonds over aromatic ring hydrogenation. In addition, we demon-strate that single metal sites cannot hydrogenate an aromatic ring and hence are selective. Final-ly, we applied our knowledge in C-O bond cleavage in CO2 conversion using propylene carbonate as a relay molecule to produce propylene glycol and methane.

EPFL2020

Amorphous Molybdenum Sulfide Films as Hydrogen Evolution Catalysts in Water

Daniel Andreas Merki

EPFL2012

Molecular Engineering of Electrode Surfaces for Electrocatalysis

Patrick Eduard Alexa

EPFL2020