Stœchiométrie | EPFL Graph Search

vignette|Équation de réaction équilibrée illustrant la stœchiométrie de la réaction de combustion du méthane. En chimie, la stœchiométrie, du grec ancien / stoikheîon (« élément ») et / métron (« mesure »), est un calcul qui permet d'analyser les quantités de réactifs et de produits qui sont en jeu au cours d'une réaction chimique. C'est aussi la proportion des éléments dans une formule chimique. Jeremias Benjamin Richter (1762-1807) fut le premier à énoncer les principes de la stœchiométrie, en 1792. Il écrivait alors : Principe Lors d'une réaction chimique on observe une modification des substances présentes : certaines substances sont consommées, on les appelle les « réactifs », d'autres substances sont formées, ce sont les « produits ». À l'échelle microscopique, la réaction chimique est une modification des liaisons entre atomes, par déplacement des électrons : certaines liaisons sont rompues, d'autres sont formées, mais les atomes eux-mêmes sont conservés. C'es

Wastewater treatment processes modelling for anaerobic digestion effluents

Miriel Steudler

The general topic of this research project is the modelling of wastewater treatment processes for anaerobic digestion effluents, which are characterized by high nitrogen concentrations. In order to avoid eutrophication of lakes and rivers caused by nutrient enrichment as it happened in Switzerland a few decades ago, the implementation of efficient nitrogen removal processes for those effluents remains nowadays of importance. Moreover nitrogen removal will even become a pre-requisite for efficient micropollutants techniques to be implemented. The first objective of this master project is the modelling of the biological nitrogen removal process Anammox in the software gPROMS. This process converts ammonium and nitrate (close to 1:1 influent concentration ratio) into dinitrogen and nitrate by Anammox bacteria. In order to obtain an influent with the proper concentration ratio, this process can be coupled to a Sharon reactor operated for partial nitrification (two stages operation). In this study, the Anammox biological developed model has been integrated into a continuous stirred tank reactor (CSTR) combined with a particles separator. The two modeled reactors, Sharon and Anammox, have then been run in series and steady-state results have been obtained. Through a comparison with literature results (lab experience and pilot-plant), the steady-state simulation results in terms of nitrogen removal efficiency and effluent concentrations have been considered as reliable. However, a calibration of the model with real data still needs to be investigated. The second objective of the project is the study of key influencing parameters (pH and temperature) on the operation of the two stages Sharon-Anammox process. A sensitivity analysis showed that temperature and pH are influencing the nitrite to ammonium ratio Sharon effluents and consequently affecting the efficiency of the Anammox reactor; the growth and selection of bacteria in the Sharon reactor are determined by those two parameters (chemostat principle). The optimal range of operation for the Sharon reactor in order to maximize the efficiency of the Anammox reactor are found to be between 306K and 308K and between 7.8 and 8.0 for the pH according to the steady-state simulations; those results are coherent with literature findings and operation of full-scale Sharon reactors. The final objective of the project is the comparison of the Sharon-Anammox process with an activated sludge system in terms of energy consumption and nitrogen removal efficiency. By minimizing in both cases the mmonium effluent concentration and playing with the aeration needs, savings of 75% in benefit of the Sharon-Anammox system have been observed; the order of magnitude obtained by optimization is coherent with the stoichiometry implemented in the model. The biological Anammox model itself will be implemented in other types of reactors models (one stage technologies involving biofilms) and the models predictions will be compared with operational data (at pilot or full-scale). Modelling wastewater treatment processes is a help to better understand the processes themselves, their interactions, their optimal operation conditions both by taking into consideration the energy consumption and the water quality through optimization techniques. The challenge of modelling will however still remain the modelling of bacteria interaction and inhibition, as well as a deep understanding and quantification of various mass transfer processes involved. This will need a close collaboration with expert scientists in order to better understand these phenomena. Modelling is a good support for experimental studies, as it can help to plan efficient experimental campaign by minimizing the required experimentations run.

2014

Chemical and electrochemical promotion of supported rhodium catalyst

Olena Baranova

The chemical and electrochemical promotion of highly dispersed nanofilm Rh catalysts (dispersion: about 10 %, film thickness: 40 nm) has been investigated for the first time. To this end Rh metal was sputter-deposited, either on a purely ionic conductor (8 % Y2O3-stabilized ZrO2) or on a mixed ionic-electronic conductor (TiO2), the latter being a highly dispersed layer of TiO2 (4 µm) deposited on YSZ. These catalysts are designated as Rh/YSZ and Rh/TiO2/YSZ, respectively. It was established analytically that both in the Rh/YSZ and in the Rh/TiO2/YSZ system, the catalyst films have a nanoparticle-size grain structure. The Rh supported on titania is rather porous, exhibiting a higher dispersion and surface area than Rh on YSZ. Both after reduction (H2, T=400 °C) and after oxidation (O2, T=400 °C), Rh supported on TiO2 was found to be in a more highly reduced state than Rh on YSZ. After reducing treatment, the Rh/TiO2/YSZ samples contain a larger amount of weakly bonded oxygen, which can be attributed to oxygen "backspillover" from the TiO2. A major feature of this research was the electrochemical characterization of the oxygen/Rh/solid electrolyte three-phase boundary by steady-state polarization measurements and by impedance spectroscopy. These are powerful techniques for extracting experimental trends and details that are useful for an understanding of the electrochemical promotion principles. It was shown that the exchange current densities at the Rh/solid electrolyte interface are lower on account of the TiO2 layer. The exchange current densities are more than twice lower at Rh/TiO2(4 µm)/YSZ than at Rh/YSZ, demonstrating that the former interface is much more polarizable. The mechanism of oxygen exchange occurring close to equilibrium (O2/O2- couple) was investigated for the first time at Rh catalyst electrodes interfaced with solid electrolyte. The processes of cathodic oxygen reduction and anodic oxygen evolution are not symmetric, but they are similar in the two systems, Rh/YSZ and Rh/TiO2(4 µm)/YSZ. The cathodic process consists of three steps: dissociative adsorption of oxygen at the gas-exposed Rh surface, atomic oxygen diffusion to the electrochemical reaction sites (ERS), and a two-electron transfer to this oxygen on the ERS. The cathodic process is limited by interfacial diffusion of oxygen atoms from the gas-exposed metal surface to the ERS. The anodic process includes two steps: two-electron transfer reaction, which is the rate-determining step, and oxygen desorption to the gas phase. With data obtained from impedance spectroscopy at the equilibrium potential, it was possible to confirm the reaction scheme proposed. It was demonstrated that Rh nanofilm catalysts interfaced with YSZ or TiO2/YSZ can be electrochemically promoted for the reaction of ethylene oxidation. Small anodic currents cause periodic oscillations in catalytic rate and potential of the Rh/YSZ catalyst, while at Rh/TiO2/YSZ they give rise to a stable and reversible rate enhancement by up to a factor 80. The increase in ethylene oxidation rate is up to 2000 times larger than the electrochemical rate, I/2F, of O2- oxidation. The pronounced electrochemical promotion behavior that has been observed is due to the anodically controlled migration of O2- species from the electrolyte to the Rh/gas interface. At the Rh surface, these species destabilize the formation of rhodium surface oxide (Rh2O3). The existence of backspillover oxygen species has been confirmed by impedance measurements under positive applied potential. Another significant result for heterogeneous catalysis is the finding that thick as well as thin films of Rh/TiO2/YSZ catalyst are open to chemical promotion of ethylene oxidation and partial methane oxidation, ie, they offer a higher catalytic activity and stability than the Rh/YSZ catalysts. The modification of catalytic activity observed for Rh/TiO2/YSZ was attributed to either a "long-range" electronic-type SMSI mechanism or to a self-driven electrochemical promotion mechanism. In both cases, the ultimate cause of promotion are different work functions of catalyst and support. Equilibration of the work functions of two solids in contact induces surface charging, a migration of O2- ions to the catalyst/gas interface, and a weakening of the Rh-O chemisorptive bonds. It facilitates reduction of oxidized surface sites. Another important achievement of the present work was that of exploring and confirming the possibilities of current-assisted activation of Rh/TiO2/YSZ catalysts. In partial methane oxidation using close to stoichiometric gas compositions (CH4 : O2 = 2 : 1) at 550 °C, the inactive (oxidized) Rh/TiO2/YSZ catalyst was successfully activated by applied currents, either positive or negative. This phenomenon is an example of "permanent" electrochemical promotion furnishing a permanent rate enhancement ratio of γ = 11. The activation by negative currents is explained in terms of an electrochemical reduction of rhodium surface oxide, while the activation by positive currents can be explained by the mechanism of electrochemical promotion.

EPFL2005

Atomistic Simulation of Cementitious Systems: An Insight Into Adsorption of Ions and Small Molecules Onto Portlandite and C-S-H Surfaces

Masood Valavi

AbstractCalcium-Silicate-Hydrate (C-S-H) has been studied extensively over the last few decades to gain understanding toward the underlying mechanism of different stages during cement hydration. The variable stoichiometry and nanocrystallinity of C-S-H makes it difficult to characterize experimentally. The second most abundant hydration product of cement is portlandite which has a simpler crystalline structure in comparison to C-S-H and has been used as a training/building system for future simulation of C-S-H bulk and surfaces. In this PhD project we are interested in a better fundamental understanding of the interaction of sulfate and secondary ions (Aluminium, Magnesium,...) with cementitious materials (e.g. portlandite and C-S-H). It has been shown that sulfate considerably affects the morphology of growing portlandite and C-S-H which will affect the physical properties of cement and concrete. A better fundamental understanding of this adsorption or incorporation into bulk structures will improve our understanding of the growth of C-S-H in the presence of such ions. In this project, utilizing molecular dynamics and metadynamics, we will systematically investigate the interaction of ions found in cement pore solutions with portlandite and C-S-H. For this purpose, we will use the Brick model recently developed in our group by Mohamedet al. (1) to create model structures for the C-S-H. We also have started to develop a new force field (ERICA FF1) which constitute different atom types needed for these simulations which are not included in previous cement force fields (Cement FF1 (2) and Cement FF2 (3)â .