Photosynthèse artificielle

Résumé

La photosynthèse artificielle est un procédé chimique qui imite la photosynthèse naturelle des plantes afin de transformer l'énergie solaire en énergie chimique. La photosynthèse naturelle Photosynthèse Définition La photosynthèse existe chez les végétaux chlorophylliens et chez certaines bactéries (cyanophycées). Elle consiste en une suite de réactions biochimiques provoquées par la lumière, le type d'énergie faisant réagir des molécules minérales simples (CO2, H2O…). Ces réactions entraînent la production de molécules organiques glucidiques de masse molaire relativement faible. Certaines de ces molécules sont polymérisées en glucides de masse molaire élevée (amidon pour le stockage, saccharose pour le transport dans d'autres cellules), d’autres enfin s’unissent à des molécules azotées. Le phénomène est caractérisé par une absorption de dioxyde de carbone et par un dégagement d’oxygène en présence de lumière (ou éventuellement de manière retardée

Source officielle

https://fr.wikipedia.org/wiki/Photosynth%C3%A8se_artificielle

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Carbon Dioxide Reduction to Methanol with a Molecular Cobalt-Catalyst-Loaded Porous Carbon Electrode Assisted by a CIGS Photovoltaic Cell**

Etienne Boutin

Conversion of CO2 into valuable compounds, including fuels, with renewable energy sources and sustainable compounds is a challenge addressed by artificial photosynthesis research. In particular, the application of solar assisted electrochemical (EC) processes, in which electrons are furnished by a photovoltaic (PV) cell, is a promising approach. A PV-EC system is described, consisting of a CIGS (copper indium gallium selenide) PV unit linked to a carbon electrode loaded with cobalt phthalocyanine as molecular catalyst, able to achieve the CO2 reduction to CO and then to methanol in aqueous media with limited bias voltage. Using CO as starting material, a partial current density of ca. 0.6 mA cm(-2) for methanol is obtained at a bias voltage corresponding to a low 240 mV overpotential. Remarkably, the liquid fuel production can be sustained for at least 7 h. Under ideal conditions, the CO2-to-CH3OH reaction shows a global Faradaic efficiency of 28 %.

2021

Chargement

Supported vanadia (VOx) belongs to some of the most versatile selective oxidation and reduction catalysts applied for many reactions in the chemical industry and pollution control. Among them, the oxidative dehydrogenation (ODH) of alcohols is particularly attractive since it is one of the main industrial methods of formaldehyde production and a possible route for the transformation of renewable bioethanol to value-added products, such as acetaldehyde and acetic acid. Despite many spectroscopic methods used to investigate the mechanism of this reaction, the details about the redox involvement of VOx species and promoting supports (CeO2 and TiO2) remain debated. In this study, we used an element-specific operando X-ray absorption spectroscopy (XAS) to probe the redox transformations of active sites in vanadia-titania and vanadia-ceria catalysts during ethanol and methanol ODH. By carefully designing the steady-state and transient operando XAS experiments and by applying advanced chemometric methods, we could discriminate specifically and often quantitatively, which role plays each redox couple during the alcohol ODH cycle and uncover hidden structure activity-relationships.Chapter 1 introduces the relevant literature and summarizes the goals and the scope of the thesis.In Chapter 2, we investigate the redox dynamics of active sites in a bilayered VOx/TiOx/SiO2 catalyst (an analog of a widely used VOx/TiO2 catalyst with comparable activity) during ODH of ethanol. Operando time-resolved V and Ti K-edge XAS coupled with a transient experimental strategy quantitatively showed that the formation of acetaldehyde over VOx/TiOx/SiO2 catalyst is kinetically coupled to the formation of a V4+ intermediate, while the formation of V3+ is delayed and its rate is 10-70 times lower. The only very weak redox activity of the Ti4+/Ti3+ couple was detected, which suggests that TiOx species promote the redox activity of VOx indirectly, either via the fast electron transport between VOx species or by changing the electronic structure of VOx species.In Chapter 3, we report on possible difficulties, which can occur during in situ XAS studies of supported vanadia (VOx) catalysts at synchrotron beamlines related to X-ray-induced photochemical reduction of vanadium. We rigorously tested this phenomenon and suggest approaches, which can help to identify and mitigate vanadium photoreduction interfering chemical speciation.In Chapter 4, we focused on the structure-activity relationships in VOx/CeO2 system. We prepared a series of VOx/CeO2 catalysts using high-surface-area CeO2 Using time-resolved operando quick XAS at V K- and Ce L3-edges, we demonstrated that during oxygen cut-off experiments vanadium and cerium change their oxidation states in concert with each other and with the production of acetaldehyde. It suggests that both reducible vanadium and cerium are parts of the same active sites. The maximum VOx/CeO2 activity per mass of catalyst observed at ca. 3 V/nm2 coincides with the maximal amount of cerium atoms, which reversibly change their oxidation state during oxygen cut-off experiments.In Chapter 5, we prove that the mechanistic findings described in Chapter 4 for ethanol ODH over VOx/CeO2 catalysts are also valid for methanol ODH. Moreover, we confirm that the VOx surface density is the main descriptor of the activity for VOx/CeO2 catalysts in alcohol ODH.

EPFL2022

Chargement

Oxygen evolution reaction catalyzed by first-row transition metal oxides: stability and mechanism

Aliki Moysiadou

Water splitting offers the opportunity for storing solar energy and, thus, producing carbon-neutral and renewable solar fuels. The process, known as artificial photosynthesis, is limited by the electrocatalytic conversion of water into molecular oxygen. Electrocatalysts are essential for reducing energy losses and improving the efficiency of the oxygen evolution reaction. This thesis describes the catalytic properties of first-row transition metal oxides for the OER and provides new insights into the mechanism of the reaction. Decades of research have yielded various transition metal oxides as efficient electrocatalysts for the OER, yet their stability is not well established. Here, a combination of techniques was employed to investigate the stability of unary, and mixed metal oxides; electrochemical activity measurements, electrochemical quartz crystal microbalance, inductively coupled plasma optical emission spectrometry and electrochemical impedance spectroscopy. The change of mass, composition, and activity metrics over time resolved the stability profiles of CoOx, CoFeOx, CoFeNiOx, NiOx, and NiFeOx. According to ICP-OES, the composition of all the five catalysts changed over time, while the CoOx and CoFeOx exhibited the most significant mass loss during an initial period of electrolysis. During a dynamic exchange of metal ions between the catalyst and the electrolyte, cobalt ions dissolve into the electrolyte, and iron from the electrolytic solution incorporates into the catalytic film. Once this dynamic exchange reaches equilibrium, the catalysts are stable. The mechanism of the OER mediated by amorphous cobalt oxyhydroxide was investigated by in situ spectroscopic and electrokinetic experiments. In situ surface-enhanced Raman and X-ray absorption spectroscopy supported a Co(IV)-O species at potentials before the onset of OER. The gradual transformation of the catalyst into a CoO2 concurred with the appearance of a superoxidic species. In situ SERS coupled with 18O and H/D isotope experiments provided evidence for the participation of the superoxidic species in the oxygen evolution reaction. The catalyst exhibits a Tafel slope of 60 mV/dec, a first-order dependence of the catalytic activity, and a proton-coupled electron-transfer pre-equilibrium. These findings propose a new OER mechanism, in which the rate-determining step is the release of oxygen from a Co-superoxo intermediate. Iron incorporation into the cobalt lattice enhances the catalytic activity greatly. Even though there are several studies on the OER mechanism in Co-Fe oxides, many vital questions on the role of iron, the true nature of active sites, and the oxidation states of Co and Fe remain. In situ SERS and XAS indicated the presence of Co(IV)-O species and a Fe valency higher than three at potentials prior to the oxygen evolution. An O-O- band, attributed to the superoxidic species, emerged at potentials lower than the onset of OER. In situ SERS coupled with 18O isotope experiments confirmed that both the lattice oxygen and superoxidic species participate in the oxygen evolution. The Tafel slope is about 37 mV/dec indicating that both the pre-equilibrium and rate-determining step involve a single-electron transfer. These findings support that neither the Co(III)/Co(IV) oxidation nor the O-O bond formation are controlling the rate of the reaction.

EPFL2020

Chargement

Oxygen evolution reaction catalyzed by first-row transition metal oxides: stability and mechanism

Aliki Moysiadou

EPFL2020

EPFL2022