Monitoring by in situ neutron diffraction of simultaneous dehydration and Ni2+ mobility in partially exchanged NaY zeolites

International audiencePowder neutron diffraction is used to follow structural changes occurring during heat and vacuum assisted dehydration of 3 wt% nickel exchanged NaY. The structures determined from Rietveld refinement confirm the nickel ion displacement from the supercages and sodalite cages towards hexagonal prisms upon water removal. Not only this mobility but also its relationship with the dehydration level is demonstrated, owing to real time quantification of dehydration through changes in the intensity of the incoherent neutron scattering signal. Comparison with TGA data proves that this signal is fully attributable to water molecules adsorbed in the zeolite. STEM/EDX and in situ heating TEM studies provide complementary information on homogeneous Ni distribution (initial state) and on reductive sintering in vacuum at a high temperature

Oscillatory behavior of Pd-Au catalysts in toluene total oxidation

In this work, the activity of bimetallic Pd-Au doped hierarchically structured titania catalysts has been investigated in the total oxidation of toluene. In earlier works, doping titania with group Vb metal oxides ensured an increased catalytic performance in the elimination of VOC molecules. A synergy between gold and palladium loaded at the surface of titania supports provided better performances in VOC oxidation reactions. Therefore, the main focus in this work was to investigate the durability of the prepared catalysts under long time-on-stream periods. Vanadium-doped catalysts showed a stable activity throughout the whole 110 h test, whereas, surprisingly, niobium-doped catalysts presented a cycle-like activity while nevertheless maintaining a high performance in toluene elimination. Operando Diffuse Reflectance Infrared Fourrier Transform spectroscopy (DRIFT) experiments revealed that variations in the presence of OH radicals and the presence of carbonaceous compounds adsorbed at the surface of spent catalysts varies with the occurrence of oscillations. X-ray Photoelectron Spectroscopy (XPS) results show that interactions between the material and the active phase provided extra amounts of mobile oxygen species and participated in easing the reduction of palladium. An enhanced redox reaction scheme is thus obtained and allows the occurrence of the cyclic-like performance of the catalyst

Etude par microscopie électronique en transmission et spectroscopies in situ des catalyseurs Ag/oxyde à porosité organisée (application en oxydation du dichlorométhane)

PARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF

Influence de la basicité de la zéolithe faujasite sur les propriétés du platine dispersé et sur son activité catalytique en oxydation du dichlorométhane

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Rôle de la paire acide-base vis-à-vis de la réactivité de faujasites alcalines Y, X et LSX de basicité variable (réaction modèle de conversion du méthylbutynol et application environnementale en oxydation du dichlorométhane)

Ce travail porte sur l influence de la paire acide-base de Lewis des faujasites Y, X et LSX échangées avec différents cations alcalins, vis à vis de leur réactivité en tant que catalyseurs basiques. En première approche, la réaction modèle de décomposition du méthylbutynol est utilisée pour évaluer leur basicité, menant à la décomposition de cette molécule en acétone et acétylène. Un suivi par spectroscopie infrarouge operando permet de décrire le rôle des paires acide-base et de mettre en évidence l influence majeure du cation alcalin en tant qu acide de Lewis lors d une réaction successive de condensation aldolique de l acétone. Ces catalyseurs montrent par ailleurs une réactivité en oxydation catalytique du dichlorométhane, celle-ci présentant un intérêt environnemental pour la destruction des composés organiques volatils (COV). Additionnés de platine, ces catalyseurs permettent une oxydation totale en CO2. L augmentation de la basicité de ces catalyseurs mène à une plus forte conversion du dichlorométhane. Pour une valeur limite de charge partielle sur l oxygène, la conversion du dichlorométhane est maximale (NaLSX).PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Conversion of xylene over mordenites: an operando infrared spectroscopy study of the effect of Na+

A series of progressively proton-exchanged sodium mordenite samples was studied by operando infrared spectroscopy during xylene conversion. This unique, powerful tool allowed us to characterize the species adsorbed to the surface during catalysis and to follow the evolution of the active sites as the reaction proceeded. We were then able to correlate the surface data with the catalytic results obtained by gas-phase chromatography analysis. When very few hydroxyls are present on site O7 H close to the main-channel walls, the initial isomerization selectivity is very high and the high Na+ content leads to the preferential adsorption of 1,2,4-TMB inside the micropores. When the amount of acidic hydroxyls increases, new sites O2 H are generated at the intersection between main channels and side pockets. The initial disproportionation selectivity then increases roughly. Finally, the last hydroxyls appear on site O9 H located at the end of the side pockets. These constrained hydroxyls are not accessible to the reactant in the reaction conditions, but their presence leads to a global activity increase without any selectivity modification. Acid strength considerations, together with the observation of the “working hydroxyls,” indicate an indirect effect of O9 H on the overall activity, which is probably related to a modification in the void space inside the micropores

Nickel Coordination to Lattice Oxygens in Basic LSX, X and Y Sodium Faujasites: A DFT Study

Times Cited: 0 Article English Cited References Count: 79 741swThe location and coordination of nickel cations inside the hexagonal prism of dehydrated sodium faujasites have been investigated as a function of the framework Al content and distribution by density functional theory (DFT) calculations. Three different Si/Al ratios have been considered in order to represent the Na-LSX (Si/Al = 1), Na-X (Si/Al = 1.4), and Na-Y (Si/Al = 2.0) faujasites that have the same framework topology but different basic properties. For each system, the most representative Al repartitions among the framework tetrahedral sites have been taken into account and their related preferred Ni2+ configurations have been established. Upon decrease of the Al content, the Ni2+ coordination changes from a perfect octahedral environment with Ni2+ located in the center of the hexagonal prisms (Ni/Na-LSX) to five 4-fold and four 3-fold coordinations in Na-X and Na-Y faujasites, respectively. The preferred locations, inside the hexagonal prism, are those where Ni2+ can bind the highest number (highest coordination) of basic framework oxygen atoms even though the related Ni-O distances simultaneously increase. All configurations are associated to a significant distortion of the hexagonal prism as compared to the host Na-faujasite cluster which illustrates the zeolite flexibility. While 4-fold coordinated nickel weakly interacts with the framework Y-faujasite, it induces a stronger framework distortion. The monitoring effect of lattice oxygen atoms toward Ni-framework interaction and framework deformation as well as Ni2+ coordination and location inside zeolites are rationalized

A combined experimental and theoretical study of the simultaneous occupation of SIa and SI0 sites in fully dehydrated K-LSX

International audienceIn the case of fully potassium exchanged X and LSX faujasites, it was reported from a single crystal X-ray study on zeolite X and preliminary powder X-ray diffraction (PXRD) study on zeolite LSX that K+ ions can be found concomitantly in both sites I and I' with partial or full occupancy of site I inside the hexagonal prism depending on the Si/Al ratio of the matrix. Such experimental observation of a simultaneous occupancy of adjacent SI and SI' sites in fully exchanged and dehydrated K-LSX was next supported by a theoretical study using the Mott-Littleton approach and a set of formally charged parameters to describe the zeolite. Here, the more appropriate Density Functional Theory (DFT) has been applied to identify the sitting, coordination, environment and properties of the exchanged cations, DFT being indeed recognized as a very successful tool to study the interactions of cations with 2 their environment in zeolites. This approach has been associated to new experimental knowledge gained from argon adsorption measurements and a reinvestigation of the K-LSX structure from Rietveld refinement, to provide an accurate description of not solely the most favorable K+ locations inside the hexagonal prisms of fully exchanged K-LSX, but also of the bonding scheme of all exchanged K+ ions with respect both to each other and to the zeolitic framework. To understand further the origin of the particular sitting of K+ ions in faujasite, calculations have been also performed on Na-LSX, taken as a reference system. Thus, a critical K+- K+ distance was determined from which the charges of potassium cations become equivalent and therefore induces the stability of the system

Application of Arrhenius law to DP and zero-span tensile strength measurements taken on iron gall ink impregnated papers: relevance of artificial ageing protocols

International audienceIron gall inks (IGI) were largely used for writing until the nineteenth century. Under certain circumstances, they provoke a substantial degradation of their cellulosic support.It was shown in a previous works thatc ombination of oxygen and iron largely impacts cellulose chain breaking occurring in acidic conditions (pH 3–4). The present study aims to study the kinetic of this degradation. It assesses the validity of Arrhenius law between 20 and 90°C taking advantage of the fast depolymerization of IGI impregnated papers at room temperature and using two complementary tools: DP measurements and zero-span tensile strength. The first one is sensitive enough to measure degradation at its very beginning, while the second is more appropriate for advanced stage of degradation. Similar activation energies (97± 2 kJ mol-1) were found viaDP and zero-span measurements, and reaction rates of IGI impregnated papers were 1–2 orders of magnitude aboveavailable data related to lignin-free acidic papers. These observations suggest a dominant hydrolytic mechanismthat involves directly or indirectly oxygen and iron