Abatement of an Azo Dye on Structured C-Nafion/Fe-Ion Surfaces by Photo-Fenton Reactions Leading to Carboxylate Intermediates with a Remarkable Biodegradability Increase of the Treated Solution

A novel C-Nafton/Fe-ion structured fabric capable of mediating Orange II decomposition in Fenton-immobilized photoassisted reactions is presented. The catalyst preparation requires the right balance between the amount of the Nafion necessary to protect the C-surface and the minimum encapsulation of the Fe-cluster catalytic sites inside the Nafion to allow the photocatalysis to proceed. The C-Nafion/Fe fabric can be used up to pH 10 under light to photocatalyze the disappearance of Orange II in the presence of H2O2. The photocatalysis mediated by the C-Nafion/Fe-ion fabric increased with the applied light intensity and reaction temperature in the reaction needing an activation energy of 9.8 kcal/mol. This indicates that ion- and radical-molecule reactions take place during Orange II disappearance. The build up and decomposition of intermediate iron complexes under light involves the recycling of Fe2+ and was detected by infrared spectroscopy (FTIR). This observation, along with other experimental results, allows us to suggest a surface mechanism for the dye degradation on the C-Nafion/Fe-ion fabrics. The C-Nafion/Fe-ion fabric in the presence of H2O2 under solar simulated light transforms the totally nonbiodegradable Orange II into a biocompatible material with a very high BOD5/COD value. X-ray photoelectron spectroscopy (XPS) and sputtering by Ar+-ions of the upper surface layer of the C-Nafion/Fe-ion fabric allow us to describe the intervention of the photocatalyst down to the molecular level. Most of the Fe clusters examined by transmission electron microscopy (TEM) showed particle sizes close to 4 nm due to their encapsulation into the Gierke cages of the Nafion thin film observed by scanning electron microscopy (SEM) and optical microscopy (OM).

In situ nanoscale studies of copper-based catalysts for carbon dioxide electroreduction

Karla Banjac

The urgency of climate change demands the simultaneous removal of carbon dioxide from the atmosphere and the transition to renewable energy sources. This aim is realizable through electrochemical reduction of carbon dioxide (CO2RR), which is a promising route for synthesizing renewable carbon-based fuels and chemicals.Among many existing CO2RR catalysts, copper is the only earth-abundant metal that produces energy-rich molecules. Practical application is hindered by its poor product selectivity and modest long-term catalyst stability. To overcome these limitations, an atomistic understanding of the catalytic processes and identification of CO2RR active sites on Cu are needed.Current research implies that the active sites form upon the catalysts' evolution during the reaction. However, fast Cu oxidation hinders their identification in conventional ex situ studies. The following questions remain unanswered: What are the morphology and chemical composition of the active sites? How do they form?This thesis explores the in situ evolution of Cu-based CO2RR catalysts. We study this phenomenon down to the nanoscale through a synergy of in situ microscopy and spectroscopy studies. We employ graphene as a 2D protecting layer on Cu to study the Cu nanoscale structural evolution in situ via electrochemical scanning tunneling microscopy (EC-STM). We use in situ Raman and X-ray photoelectron spectroscopies to study the evolution of the Cu catalysts' oxidation state.First, EC-STM studies reveal the formation of few nanometer-sized cuboids on Cu catalysts as the main structural evolution during CO2RR. We show that this evolution occurs in aqueous electrolytes over a wide range of pH and compositions. Second, we show that metallic Cu is the active phase. Reduction of the Cu oxide, formed either through chemical synthesis or through the exposure of Cu catalysts to air, is unavoidable prior to CO2RR.To understand the nanocuboid formation mechanism, we present a comparative study of the Cu nanocuboids formed upon CO2RR and the Cu2O cubes prepared through electrochemical cycling synthesis. Morphological and chemical composition differences between them allow us to conclude on distinct formation mechanisms. We show that the metallic Cu nanocuboids form during a potential-driven re-organization of atoms in the topmost surface layers.Furthermore, we present EC-STM studies revealing the atomistic details of the Cu nanocuboid formation. By comparing these processes with the multilayer mound growth in Cu homoepitaxy, we show that the nanocuboids are (100) facet multilayers formed upon reduction of Cu oxides. We propose that the nanocuboids are CO2RR active sites, ultimately forming on any Cu catalyst regardless of its initial macroscopic shape and oxidation state.The in situ identification of active sites, presented here, offers a unique framework for the re-interpretation of the existing literature, invites further fundamental theoretical and experimental research, and paves the way for rational catalyst design. Moreover, this thesis further demonstrates the importance of in situ studies across different length scales in understanding key catalytic processes in renewable energy schemes.

EPFL2021

Performance and characterization of Ag-cotton and Ag/TiO2 loaded textiles during the abatement of E. coli

Juan Kiwi, Danièle Laub, César Pulgarin

The bacterial inactivation of Escherichia coli on Ag–cotton textiles were investigated under different experimental conditions with novel Ag-nanoparticles fixed on cotton textiles. The evaluation of the bactericide performance of the Ag–cotton was compared with the one observed for Ag–TiO2–cotton loaded textiles. The procedure used allowed us to prepare highly dispersed Ag-cluster species, 2–4 nm in size, as observed by transmission electron microscopy (TEM). By depth profile analysis, the X-ray photoelectron spectroscopy (XPS) revealed that the Ag profile in the 15 topmost layers remained almost constant before and after E. coli inactivation. The cotton loading was fairly low and of the order of 0.10 wt.% Ag/wt. of cotton. By infrared spectroscopy no modification of the Ag–cotton could be detected before or after E. coli inactivation due to the small absorption coefficient of Ag and the very low metal loading of Ag on the cotton. The E. coli was completely abated on Ag–cotton textiles immediately after the contact took place, due to the strong bacteriostatic effect of the dispersed silver clusters on the cotton surface.

2006

Duality in the Mechanism of Hexagonal ZnO/CuxO Nanowires Inducing Sulfamethazine Degradation under Solar or Visible Light

Juan Kiwi, César Pulgarin, Sami Rtimi, Jiajie Yu

This study presents the first evidence for the photocatalytic performance of ZnO/CuxO hexagonal nanowires leading to sulfamethazine (SMT) degradation. The chemical composition of the nanowires was determined by X-ray fluorescence (XRF). The sample with the composition ZnO/Cux=1.25O led to faster SMT-degradation kinetics. The SMT-degradation kinetics were monitored by high performance liquid chromatography (HPLC). The morphology of the hexagonal nanowires was determined by scanning electron microscopy (SEM) and mapped by EDX. The redox reactions during SMT degradation were followed by X-ray photoelectron spectroscopy (XPS). The interfacial potential between the catalyst surface and SMT was followed in situ under solar and indoor visible light irradiation. SMT-degradation was mediated by reactive oxidative species (ROS). The interfacial charge transfer (IFCT) between ZnO and CuxO is shown to depend on the type of light used (solar or visible light). This later process was found to be iso-energetic due to the potential energy positions of ZnO and CuxO conduction bands (cb). The intervention of surface plasmon resonance (LSPR) species in the SMT degradation is discussed.

2019

In situ nanoscale studies of copper-based catalysts for carbon dioxide electroreduction

Karla Banjac

EPFL2021