Réduction sélective catalytique des NOx par des composés oxygènes

Les moteurs Diesels connaissent un intérêt récent tout particulier car ils rejettent moins de CO2 que les moteurs essences à puissance égale, du fait qu'ils travaillent en "mélange pauvre", i.e. en excès d'oxygène. Ils présentent cependant l'inconvénient de former des NOx (NO et NO2), qui sont des polluants difficilement réductibles en azote en milieu oxydant. L'objectif de cette thèse est de proposer un catalyseur actif en réduction des NOx par l'éthanol (EtOH-SCR) à 200C, qui est la température moyenne d'un échappement de moteur Diesel. Afin de répondre à cette problématique, un catalyseur connu pour être actif à 300C en EtOH-SCR a été choisi : Ag/Al2O3. La première partie de ce manuscrit détaille les modifications apportées au catalyseur de référence (Ag/Al2O3) afin d'élargir sa fenêtre d'activité vers les basses températures. Le support alumine a été modifié par des ajouts de métaux de transition (Mn, Fe, Ti, Zn), puis un second métal a été ajouté en plus de l'argent sur Al2O3 (Ru, Ir, Cu, Co, Gd, In et Sc). Cette partie montre que l'activité des catalyseurs de type Ag/Al2O3 est limitée jusqu'à 300C : le maximum de conversion des NOx en azote (34%) est obtenu avec le catalyseur modifié avec le ruthénium Ag-Ru(0,5%pds)/Al2O3. Les parties suivantes tentent d'expliquer pourquoi l'activité de ces catalyseurs est limitée à basse température. L'éthanol se transforme en acétaldéhyde et éthylène (entre autre) au cours de la réaction de réduction des NOx. Ces deux produits peuvent réagir avec les NOx pour conduire à la formation d'azote, mais la réaction de SCR avec l'acétaldéhyde ne débute qu'à 300C, tandis que celle avec l'éthylène débute à 550C. Seule la réaction entre l'éthanol et NO conduit à la formation d'azote avant 300C. Il est finalement montré que cette réaction est limitée à 150C par l'activation de l'éthanol sur les paires acides-bases de l'alumine, qui apparaissent par déshydratation de Al2O3 à environ 250C. La réaction est ensuite limitée à 250C car les nitrates réagissent difficilement avec l'éthanol pour former N2 avant 300C. Au dessus de 300C, il est montré que la formation d'azote est en compétition avec celle de NH3.Lately, Diesel engines have been extensively studied because they emit lesser CO2 than gasoline engine of equivalent power, since they work in lean condition, i.e. in excess oxygen. However, they produce NOx (NO and NO2), which are pollutants hardly transformed in nitrogen in oxidizing atmosphere. The point of this manuscript is to propose a catalyst active in NOx reduction by ethanol (EtOH-SCR) at 200C, which is the average temperature of Diesel exhaust gas. In order to answer to this problem, a catalyst known to be active at 300C in EtOH-SCR has been chosen: Ag/Al2O3. The first part of this manuscript details the modifications made to the reference catalyst (Ag/Al2O3) in order to broaden its activity window toward low temperature. The alumina support has been modified by adding transition metals (Mn, Fe, Ti, Zn), then a second metal has been added in addition to silver over alumina (Ru, Ir, Cu, Co, Gd, In and Sc). This part shows that the Ag/Al2O3 catalyst activity is limited up to 300C: maximum conversion of NOx to N2 (34%) is obtained with the catalyst modified with ruthenium Ag-Ru(0.5wt%)/Al2O3. The following parts try to explain why the catalysts activity is limited at low temperature. Ethanol is mostly transformed into acetaldehyde and ethylene during the NOx reduction reaction. It has been showed that acetaldehyde and ethylene can react with NOx to yield nitrogen, but the SCR reaction with acetaldehyde begin at 300C, whereas the reaction with ethylene starts at 550C. Only the reaction between ethanol and NO can lead to nitrogen formation below 300C. It is finally showed that this reaction is limited at 150C by the ethanol activation over alumina acid-base pairs, which appears by Al2O3 dehydration at about 250C. The reaction is then limited at 250C because nitrates hardly react with ethanol to yield N2 below 300C. Above 300C, it is showed that nitrogen formation is in competition with NH3 formation.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Ceria doped with praseodymium instead of gadolinium as efficient interlayer for lanthanum nickelate SOFC oxygen electrode

In the solid oxide fuel cell field, the gadolinium doped ceria (GDC) is widely used as interlayer between oxygen electrode and electrolyte materials, as for instance La2NiO4+δ (LNO) and 3 mol.% Y2O3-ZrO2 (TZ3Y), respectively. However, cation interdiffusion occurs between GDC and LNO/TZ3Y during their sintering steps at high temperature, which finally lowers the electrochemical performance of the cathode for the oxygen reduction reaction (ORR). In this paper, an alternative interlayer is proposed, the praseodymium doped ceria (PrDC with 30 at.% Pr). It is shown that reactivity occurs between PrDC and TZ3Y, but the compounds formed seem to favour the ORR. Besides, little reactivity is detected between LNO and PrDC. As a result, the total resistances, i.e. series and polarization resistance obtained with a LNO//PrDC//TZ3Y half-cell from 500 °C to 800 °C are much lower than those obtained with a LNO//GDC//TZ3Y half-cell, sintered in the same conditions. In addition, differences in the shape of the impedance diagrams are observed depending on the interlayer: this feature is discussed throughout the paper

Ceria doped with praseodymium instead of gadolinium as efficient interlayer for lanthanum nickelate SOFC oxygen electrode

In the solid oxide fuel cell field, the gadolinium doped ceria (GDC) is widely used as interlayer between oxygen electrode and electrolyte materials, as for instance La2NiO4+δ (LNO) and 3 mol.% Y2O3-ZrO2 (TZ3Y), respectively. However, cation interdiffusion occurs between GDC and LNO/TZ3Y during their sintering steps at high temperature, which finally lowers the electrochemical performance of the cathode for the oxygen reduction reaction (ORR). In this paper, an alternative interlayer is proposed, the praseodymium doped ceria (PrDC with 30 at.% Pr). It is shown that reactivity occurs between PrDC and TZ3Y, but the compounds formed seem to favour the ORR. Besides, little reactivity is detected between LNO and PrDC. As a result, the total resistances, i.e. series and polarization resistance obtained with a LNO//PrDC//TZ3Y half-cell from 500 °C to 800 °C are much lower than those obtained with a LNO//GDC//TZ3Y half-cell, sintered in the same conditions. In addition, differences in the shape of the impedance diagrams are observed depending on the interlayer: this feature is discussed throughout the paper

An innovative efficient oxygen electrode for SOFC: Pr6O11 infiltrated into Gd-doped ceria backbone

The praseodymium oxide, Pr6O11, is regarded as a potential electrocatalyst for the oxygen reduction reaction. Its mixed conductivity properties are characterized. At 600 °C the oxygen diffusion coefficient value is as high as 3.4 × 10−8 cm2 s−1, and that of the surface exchange coefficient is 5.4 × 10−7 cm s−1, which supposes excellent electrocatalytic properties. The measured electronic conductivity is high enough for using this material as a SOFC cathode. Herein, praseodymium nitrate is infiltrated into Gd doped ceria (GDC) backbone and fired at 600 °C to form a composite oxygen electrode Pr6O11/GDC. Electrochemical measurements show very low polarization resistance, Rp = 0.028 Ω cm2 at 600 °C. A single cell made of a commercial Ni-YSZ/YSZ half cell and of the infiltrated cathode is able to deliver a maximum power density of 825 mW cm−2 at 600 °C. Ageing of this cell for 840 h, at 600 °C and 0.5 A cm−2, shows a degradation rate lower than 1%

La2-xPrxNiO4+δ as suitable cathodes for metal supported SOFCs

Aiming at a tradeoff between the chemical stability of La2NiO4 + δ (LNO) and the high electrochemical performances of Pr2NiO4 + δ (PNO), La2 − xPrxNiO4 + δ mixed nickelates, further referred as LPNO, were studied as possible oxygen electrodes for solid oxide fuel cells (SOFCs). LPNO phases were synthesized using the modified citrate–nitrate route followed by a heat treatment at 1200 °C for 12 h under air. Structural characterizations of those K2NiF4-type compounds show the existence of two solid solutions with orthorhombic structure, namely a La-rich one from x = 0 to 0.5 with Fmmm space group, and a Pr-rich one from x = 1.0 to 2.0 with Bmab space group. The mixed ionic and electronic conducting (MIEC) properties of LPNO phases were investigated through the evolution of the oxygen over-stoichiometry, δ, measured as a function of temperature and pO2, the electrical conductivity, the diffusion and surface exchange coefficients versus x, showing that all compositions exhibit suitable characteristics as cathode materials for SOFCs. In particular, the electrochemical performances measured in symmetrical cells using LPNO materials sintered under low pO2, as requested in metal supported cell, (MSC-conditions) confirmed a decrease in polarization resistance values, Rp, from 0.93 Ω cm2 (LNO) down to 0.15 Ω cm2 (PNO) at 600 °C with increasing x.Performance Et Robustesse pour les cellules d'Electrolyse haute température de Nouvelle génératio

La2NiO4+δ infiltrated into gadolinium doped ceria as novel solid oxide fuel cell cathodes: Electrochemical performance and impedance modelling

This paper is devoted to the study of composite cathodes of La2NiO4+δ infiltrated into a Gd-doped ceria backbone. Porous Gd-doped ceria backbones are screen printed onto yttria-stabilized zirconia or Gd-doped ceria dense electrolytes, and infiltrated with a La and Ni nitrate solution (2:1 stoichiometry ratio). The influence of the preparation parameters on the polarization resistance, such as the concentration of the infiltration solution, the amount of infiltrated phase, the annealing temperature, the thickness of the electrode, and the nature of the electrolyte, is characterized by impedance spectroscopy performed on symmetrical cells. The optimization of these parameters results in a decrease of the polarization resistance down to 0.15 Ω cm2 at 600 °C. Using the Adler-Lane-Steele model, the modelling of the impedance diagrams leads to the determination of the ionic conductivity as well as the surface exchange rate of the infiltrated electrode

Identification and modelling of the oxygen gas diffusion impedance in SOFC porous electrodes: application to Pr2NiO4+δ

An in-depth analysis of the very low frequency impedance arcs observed when measuring efficient solid oxide fuel cell electrodes by electrochemical impedance spectroscopy (EIS) is reported in this paper. The study was carried out on Pr2NiO4+δ//Ce0.8Gd0.2O2- δ//3 mol.% Y2O3-ZrO2 symmetrical half-cell. In the temperature range 500–900 °C, three impedance arcs related to O2 molecular diffusion were distinguished from the EIS measurements. Based on theoretical calculations using the Adler-Lane-Steele (ALS) and Dusty gas models, the arc at highest frequencies was ascribed to the diffusion of O2 in the porous structures of the electrode and collecting gold grid. It obeys the ALS model, i.e. a parallel R//C impedance with a capacitance coming mainly from the solid phase. The second arc at medium frequencies was ascribed to the diffusion of O2 in the porous structure of the ceramic part used to maintain the gold grid. It follows the Dusty gas model, i.e. a Warburg impedance with relaxation time depending on the gas phase properties. Finally, the third one at lowest frequencies was ascribed to the “gas conversion” phenomenon, coming from a difference in the local pO2 above the active sites of the working and counter electrodes. This gas conversion impedance largely increases when clogging the channels of the gas distribution system.Robust Advanced Materials for metal Supported SOF

Oxygen Diffusion and Surface Exchange Coefficients Measurements under High Pressure: Comparative Behaviour of Oxygen Deficient vs. Over-stoichiometric Air Electrode Materials

International audienceMixed ionic electronic conductors (MIECs) oxides are used as electrode materials for solid oxide cell (SOC) application, as they combine high electronic conductivity as well as high oxygen diffusivity and oxygen surface exchange coefficients. The ionic transport properties can be directly determined thanks to the isotopic exchange depth profiling (IEDP) method. To date, the reported measurements have been performed at ambient pressure and below. However, for a higher efficiency of hydrogen production at the system level, it is envisaged to operate the cell between 10 to 60 bar. To characterize the MIEC oxides properties in such conditions, an innovative setup able to operate up to a total pressure of 50 bar and 900 °C has been developed. The main goal of this study was to compare the behaviour of two types of reference materials: the oxygen deficient La-Sr-Fe-Co perovskites, and the overstoechiometric lanthanide nickelates Ln2NiO4+δ (Ln = La, Pr, Nd). Diffusion and surface exchange coefficients obtained under 6.3 bar of oxygen are measured and their evolution discussed in light of the change in oxygen stoichiometries. This analysis allows better understanding of the dependency of the surface exchange coefficient with the oxygen partial pressure