In many catalytic systems, the catalytic performance of a metal supported catalyst would be affected by the size and shape of the metals, and this phenomena is called structure sensitivity. Generally, the structure sensitivity effect is considered being led by a combination of geometric property change and electronic property change of the surface metals. The particle size variation is an effective way to change the surface structure of the supported metal catalyst, leading to different fractions of the active sites exposing on the support that would take effect on catalyzing the reaction. In this project, a series of Ir/MgAl₂O₄ catalysts with different particle sizes that less than 2nm were utilized for ethane and n-butane hydrogenolysis reactions to study the structure sensitivity effect as well as the potential reaction mechanism. The results show that the activity on the catalysts with nanoparticles and mostly single atoms is evidently higher than that with the subnanometer clusters in both reactions, but the selectivity to the target product of ethane is not quite dependent on the particle size in the n-butane hydrogenolysis. After the fundamental analysis, it is proposed that the reaction mechanism of alkanes hydrogenolysis on the single atom catalysts including single active sites is probably distinctive from that generally accepted on the large particles containing multiple active sites from literature. For n-butane hydrogenolysis, the parallel reaction pathway of central C-C bond cleavage is dominant at low temperature or in the low conversion range. As the temperature going up or the conversion increasing at a certain temperature, the parallel reaction pathway of terminal C-C bond cleavage becomes more and more competitive. The series reaction pathway of hydrogenolysis on propane intermediate would always take place, but the level would be drastically enhanced when the conversion keeps increasing in the very high range. The C-C bond cleavage on the ethane product would not easily happen unless the conversion is close to 100%. / M. S. / Shale gas is natural gas trapped in shale rocks. Among all the countries that have abundant shale gas reserves, the US, benefited from advanced extraction technology, has the largest production of it. What’s more, the production rate will keep increasing at least for the coming 20 years, and shale gas will eventually become the largest source for natural gas. After extraction, there is a series of treatments shale gas has to go through before it can be utilized, catalytic reaction of alkanes (molecules found in most fuels) is one of these essential procedures. Although they are among the most important compositions of shale gas, different types of alkanes are difficult to separate and purify through traditional methods like condensation. To overcome this obstacle, this thesis focuses on exploring efficient catalysts to convert the n-butane (a straight chain alkane with 4 carbon atoms) to ethane (alkane with 2 carbon atoms). Two reactions are involved: n-butane hydrogenolysis and ethane hydrogenolysis. Catalysts are some specific materials that can accelerate certain chemical reactions. The catalysts discussed in this thesis are tiny metal (iridium) particles attached to the support material (magnesium aluminate). In this study, the performance of these catalysts with different particle sizes were tested for the above mentioned hydrogenolysis reactions. The results show that changing the particle size of the catalysts considerably affects the rate of these catalytic reactions. The fundamentals of the catalytic system presented in this work can also help the researchers to rationally design the catalysts aiming at higher efficiency and lower cost in the future work.

The catalytic combustion of methane in excess of O2 over Pd catalysts was studied on model catalysts, including polycrystalline palladium foil and palladium single crystals. The kinetics of this reaction could be measured at conditions not accessible to supported catalysts and, thus, the issues of structure sensitivity, mechanism, hysteresis on oxidation, and deactivation could be studied in detail. Methane oxidation on PdO was insensitive to the original metal surface structure which PdO grew from, with turnover rates in the range of 1.3-4.7 s-1 on (111), (100) and (110) single crystals at 160 Torr O2, 16 Torr CH-4, 1 Torr H2O and 598 K. Methane oxidation on Pd metal was also insensitive to the original surface structure, with the turnover rate in the range of 2.0-2.8 s-1 on the three single crystals at 2.3 Torr O2, 0.46 Torr CH4, 0.05 Torr H2O and 973 K. Since there is no support effect and the surface purity could be certified, these turnover rates for this reaction can be used as a benchmark. The turnover rate for methane oxidation was found to decrease 95% when PdO decomposed to Pd metal at 888 K, showing that PdO was more active than Pd metal for methane combustion at this temperature. Water inhibition to the reaction was not observed at a temperature above 813 K on both PdO and Pd metal, while it was observed at 598 K on PdO. The activation energy on PdO was 32 kJ mol-1 in the range of 783-873 K, while it was 125 kJ mol-1 in the range of 568-623 K. The activation energy on Pd metal was 125 kJ mol-1 in the range of 930-980 K. The change of reaction orders and activation energies suggests that the reaction mechanism is a function of temperature and palladium chemical states. We propose that adsorbed water, the most abundant surface intermediate at 598 K, was not present in significant quantities at temperatures above 783 K. This change in surface inhibition by water is the reason for lower activation energy at temperatures above 783 K. Interaction between the catalyst and support, or presence of impurities, is one of the factors for catalyst deactivation. The interaction between oxidized silicon and palladium was investigated on a polycrystalline palladium foil and on supported Pd/SiO2 catalysts. During methane oxidation, oxidized silicon covered the palladium oxide surface as observed by TEM on Pd/SiO2 catalysts and by XPS on palladium foil. On Pd foil, the source of silica was a silicon impurity, common on bulk metal samples. The migration of oxidized silicon onto PdO deactivated the catalysts by blocking the active sites for methane oxidation. Silicon oxide overlayers were also observed covering the Pd surface after reduction of Pd/SiO2 by H2 at 923 K.

In this study, the structure sensitivity of hydrogen, oxygen and carbon monoxide adsorption was investigated by changing the metal particle size of Pt/Al2O3 catalysts. 2 % Pt/Al2O3 catalysts were prepared by incipient wetness method / the particle size of the catalysts was manipulated by calcining at different temperatures. The dispersion values for the catalysts calcined in air at 683K, 773K and 823K were measured as 0.62, 0.20 and 0.03 respectively. The differential heats of adsorption of hydrogen, carbon monoxide and oxygen were measured using a SETARAM C80 Tian-Calvet calorimeter. No structure dependency was observed for hydrogen, carbon monoxide or oxygen initial heats of adsorption. The adsorbate:metal stoichiometries at saturation systematically decreased with increasing particle size. Hydrogen chemisorption sites with low and intermediate heats were lost when the particle size increased. On the other hand, oxygen and carbon monoxide initial heats and adsorption site energy distributions did not change appreciably with the metal particle size.

In this study, the effect of Pt particle size on the reaction rate and selectivity of preferential oxidation of CO (PROX) reaction was investigated on Pt/Al2O3. 2% Pt/&amp / #947 / -Al2O3 catalysts were prepared by incipient wetness method / the particle size of the catalysts was modified by calcination temperature and duration. Therefore, the relative amounts of low and high coordination atoms on the metal particle surface can be changed. Over these catalysts, first, the CO oxidation reaction was studied in the absence of hydrogen. The catalyst having the highest dispersion, i.e., lowest metal particle sizes, had the highest activity as indicated by its lowest light-off temperature. On the other hand, the turnover frequencies (TOF) of the catalysts were increasing with decreasing dispersion. The activation energy of the catalysts were also compared and examined: as the particle size increased, the activation energy decreased. In the second part, preferential oxidation of CO reaction in the presence of hydrogen was studied. Both CO conversion and selectivity first increased with increasing reaction temperature, then exhibited a maximum, and finally decreased. Both CO conversion and selectivity did not show any trend for different dispersed catalysts for &amp / #955 / (2PO2/PCO) was 1. In order to reach a definite conclusion about the structure sensitivity of selective CO oxidation, the experiments with different &amp / #955 / &rsquo / s and space times over the same catalysts should be performed.

A series of iron, cobalt and nickel transition metal phosphides was synthesized by means of temperature-programmed reduction (TPR) of the corresponding phosphates. The same materials, Fe₂P, CoP and Ni₂P, were also prepared on a silica (SiO₂) support. The phase purity of these catalysts was established by x-ray diffraction (XRD), and the surface properties were determined by N₂ BET specific surface area (Sg) measurements and CO chemisorption. The activities of the silica-supported catalysts were tested in a three-phase trickle bed reactor for the simultaneous hydrodenitrogenation (HDN) of quinoline and hydrodesulfurization (HDS) of dibenzothiophene using a model liquid feed at realistic conditions (30 atm, 370 °C). The reactivity studies showed that the nickel phosphide (Ni₂P/SiO₂) was the most active of the catalysts. Compared with a commercial Ni-Mo-S/g-Al₂O₃ catalyst at the same conditions, Ni₂P/silica had a substantially higher HDS activity (100 % vs. 76 %) and HDN activity (82 % vs. 38 %). Because of their good hydrotreating activity, an extensive study of the preparation of silica supported nickel phosphides, Ni₂P/SiO₂, was carried out. The parameters investigated were the phosphorus content and the weight loading of the active phase. The most active composition was found to have a starting synthesis Ni/P ratio close to 1/2, and the best loading of this sample on silica was observed to be 18 wt.%. Extended x-ray absorption fine structure (EXAFS) and x-ray absorption near edge spectroscopy (XANES) measurements were employed to determine the structures of the supported samples. The main phase before and after reaction was found to be Ni₂P, but some sulfur was found to be retained after reaction. A comprehensive scrutiny of the HDN reaction mechanism was also made over the Ni₂P/SiO₂ sample (Ni/P = 1/2) by comparing the HDN activity of a series of piperidine derivatives of different structure. It was found that piperidine adsorption involved an a-H activation and nitrogen removal proceeded mainly by means of a b-H activation though an elimination (E2) mechanism. The relative elimination rates depended on the type and number of b-hydrogen atoms. Elimination of b-H atoms attached to tertiary carbon atoms occurred faster than those attached to secondary carbon atoms. Also, the greater the number of the b-H atoms, the higher the elimination rates. The nature of the adsorbed intermediates was probed by Fourier transform infrared spectroscopy (FTIR) and temperature-programmed desorption (TPD) of the probe molecule, ethylamine. This measurement allowed the determination of the likely steps in the hydrodenitrogenation reaction. / Ph. D.

In many catalytic systems, the activity and selectivity of supported metal catalysts or extended metal surface catalysts would be affected by the metal surface structure, and this phenomenon is called structure sensitivity. Generally, structure sensitivity is led by the change of geometric and electronic properties of the metal on the surface. The variation of metal nuclearity and metal-support interactions are effective ways to change the geometric and electronic properties of the supported metal catalyst, leading to different types of the active sites exposing on the support that would take effect on catalyzing the reaction. In this work, a series of supported Ir catalysts (on MgAl2O4 and SiO2) with different particle sizes less than 3 nm were utilized for hydrogenolysis of n-butane and ethane to study the structure sensitivity as well as the potential reaction pathways. The results indicate that the activity of n-butane hydrogenolysis increases as Ir particle size increases in the small particle size range (0.7–1.4 nm) and then drops when the Ir particle size further increases and the Ir single atoms might be inactive for hydrogenolysis after the post-reaction analysis. The selectivity of n-butane hydrogenolysis is dominated by central and one terminal C–C bond cleavage on the n-butane molecules at low temperature range. The selectivity to central C–C bond cleavage is highly dependent on the size of Ir and increases with a decrease in particle size down to ~1.4 nm but remains constant with further decrease in size. The hydrogenolysis of ethane shows a similar trend in the small size range but the activity is much lower than n-butane, which supports the low level of series reaction pathway in the case of n-butane hydrogenolysis. In addition to Ir nuclearity, the effect of electronic properties was also studied on another series of Ir catalysts supported on ZnAl2O4, in which zinc replace the magnesium within the same spinel structure. The characterization results including HAADF-STEM and volumetric CO chemisorption show the difference of Ir nuclearity in the subnanometer regime and nanoparticles (~1.4 nm), while XPS and DRIFTS indicate the difference of electronic properties from metal-support interaction on the two Ir catalysts with the same nuclearity but reduced at different temperatures. Acetylene hydrogenation is structure sensitive on Ir/ZnAl2O4 catalysts and the activity and selectivity are mainly determined by Ir nuclearity instead of the difference in electronic properties. The Ir single atoms and subnanometer clusters are more selective to the target product of C2H4 but less active than large Ir nanoparticles as there might be more π-bonded adsorption than di-σ bonded adsorption for C2H2 on the Ir single atoms and subnanometer clusters. / Doctor of Philosophy / The supported metal catalyst is a kind of effective substance that could help increase the reaction rate when being properly utilized in the reaction. From the industry point of view, the best thing is to maximize the catalyst productivity and minimize the expense so that the economic benefit could be magnified. The catalyst effectiveness in a certain reaction might be different when the surface structure of the catalyst varies. Usually, only the fraction of the surface metals could take effect. As the particle size of the catalyst decreases, the fraction of the surface atoms that contain active sites drastically changes, leading to a different catalytic performance and probably lower cost with improved efficiency for metal utilization. Therefore, it is very significant for the researchers to study the reaction structure sensitivity on the same series of catalysts with different particle sizes. Also, by understanding the reaction mechanism and fundamentals of the catalytic system, it would be possible for the researchers to rationally design the catalysts aiming at higher efficiency and lower cost. In this work, the reaction of hydrogenolysis that cleaves the C–C bonds within the alkanes molecules was studied on the supported Ir catalysts (Ir/MgAl2O4 and Ir/SiO2) with different particle sizes ranging from mostly single atoms, subnanometer clusters to nanoparticles. For n-butane hydrogenolysis, it is found that the selectivity to the target product of ethane is weakly dependent on particle size when smaller than 1.4 nm but decreases as the size further increases. Meantime, the activity is highest on the catalyst with surface-average particle size of 1.4 nm. Therefore, Ir size of ~1.4 nm is optimum for activity and selectivity to ethane. The series of Ir/ZnAl2O4 catalysts was tested for structure sensitivity by another probe reaction, semi-hydrogenation of acetylene. The adsorbed acetylene molecules could be hydrogenated by adding two hydrogen to form the adsorbed ethylene before desorption or further hydrogenation to form ethane. Our results show the Ir single atoms and subnanometer clusters are more selective to the target product of ethylene but less active than the large nanoparticles. With the understanding of structure sensitivity, researchers are able to rationally design the catalysts based on their necessity for certain reactions.

Une perte auditive neurosensorielle est généralement associée à uneréduction de l’intelligibilité de la parole, et ce tout particulièrement dans le bruit.Les contributions respectives d’une réduction de l'audibilité et de déficitssupraliminaires sont encore débattues.L'objectif principal de cette thèse était d'évaluer l'effet spécifique desdéficits supraliminaires sur l’intelligibilité de la parole. L'effet de l'audibilité étaitcontrôlé en mesurant l’intelligibilité de signaux de parole sans signification filtrésdans les régions basses et moyennes fréquences au sein desquelles la détection desons purs était normale chez des auditeurs malentendants présentant par ailleursune perte auditive en hautes fréquences. Dans ces régions fréquentielles oùl’audibilité est supposée normale, des déficits d'intelligibilité de la parole légers àsévères ont été observés dans le silence comme dans le bruit chez les auditeursmalentendants. Les déficits étaient similaires dans les bruits masquantstationnaires et fluctuants. Ces résultats démontrent l’influence des déficitsauditifs supraliminaires sur l’intelligibilité de la parole.Le second objectif de cette thèse était d'étudier l'origine de ces déficitssupraliminaires. Les résultats indiquent qu’une réduction de la sélectivitéfréquentielle cochléaire ne peut pas expliquer entièrement les déficitsd’intelligibilité de la parole des auditeurs malentendants. L'influence de lasensibilité à la structure temporelle fine reste incertaine / Speech intelligibility is reduced for listeners with sensorineural hearingloss, especially for speech in noise. The extent to which this reduction is due toreduced audibility or to supra-threshold deficits is still debated.The main goal of this PhD work was to investigate the specific influenceof supra-threshold deficits on speech intelligibility. The effect of audibility wascontrolled for by measuring speech intelligibility for hearing-impaired listenersusing nonsense speech signals filtered in low- and mid-frequency regions wherepure-tone sensitivity was normal. Hearing-impaired listeners with hearing loss inhigh-frequency regions showed mild to severe intelligibility deficits for speechboth in quiet and in noise in these frequency regions of normal audibility. Similardeficits were obtained for speech in steady and fluctuating masking noises. Thisprovides additional evidence that speech intelligibility may be strongly influencedby supra-threshold auditory deficits.The second aim of this PhD work was to investigate the origin of thesesupra-threshold deficits. Results showed that reduced frequency selectivity cannotentirely explain the speech intelligibility deficits of the hearing-impaired listeners.The influence of temporal fine structure sensitivity remained unclear

Ce travail se concentre sur le développement de systèmes catalytiques capable d’oxyder complètement le méthane à basse température. Le sujet principal concerne la conception d'une nouvelle génération de catalyseurs à base de palladium qui sont résistants aux composés soufrés et à l'eau.<p>Notre objectif a été atteint grâce à l'utilisation d'un support oxyde mixte produit par sol-gel. En effet, nos catalyseurs de palladium supporté sur un oxyde de silicium dopé au titane se sont révélés être résistants à l’empoisonnement au soufre et présentent des performances élevées pour la conversion du méthane.<p>En variant les quantités de TiO2, il a été montré que les performances atteignent un maximum pour une composition en masse de 10% TiO2. Les analyses structurelles et de surface ont montré que nos supports mixtes contiennent des liens Ti-O-Si. Nous pensons que ces liens sont responsables de l’activité accrue du catalyseur.<p>Par ailleurs, les catalyseurs contenant du titane présentent une tolérance supérieure vis-à-vis du SO2 lorsque celui-ci est ajouté aux réactifs ou que le catalyseur est exposé à une atmosphère de SO2 pur à 350°C pendant 15 heures. Nous avons mis en évidence par XPS que les sites Ti-O-Si sont également responsables de cette tolérance aux composés soufrés. Ceci est accompli par l'insertion du SO2 dans le support qui forme des liens soit Ti-O-SOx•••Si soit Si-O-SOx•••Ti. L’analyse XPS a également montré que sur le long terme, l’exposition au SO2 conduit à la formation d’une couche de PdSO4 de 18 à 20 Å. Étonnamment, les catalyseurs sont capables de récupérer entièrement leur activité initiale après ce traitement. Cette régénération se produit grâce à un mécanisme concerté avec le méthane permettant la décomposition totale du PdSO4. Par ailleurs, des études en présence d'eau ont montré que ces propriétés restent inchangées.<p>L'état du palladium a également été étudié et nous a permis de prouver qu’une activation/stabilisation du catalyseur est nécessaire. Celle-ci est réalisée en présence des réactifs par de légères modifications chimiques du support et de la phase de palladium. En effet, l'augmentation de l'activité du catalyseur a été corrélée avec une augmentation des quantités de Ti3+ et Pd0. La présence de palladium métallique dans le catalyseur semble être l'élément clé dans l'activation des liaisons C-H.<p>Enfin, nous avons étudié l'influence de la taille/la dispersion des particules de palladium sur la vitesse de réaction. L'utilisation de synthèses en phase liquide nous a permis de produire des solutions colloïdales de particules de palladium avec des tailles contrôlées. Cette étude a révélé que la combustion du méthane est une réaction sensible à la structure. Néanmoins, un meilleur contrôle de la forme des nanoparticules devrait être réalisé pour déterminer les facteurs structurels influençant la réaction./ The present work focuses on the development of highly efficient catalytic systems able to completely oxidize methane at low temperature in order to comply with modern environmental legislation. The main subject concerns the design of a new generation of palladium-based catalysts that are sulfur and water resistant. <p>Our goal was achieved through the use of a mixed oxide support produced by sol-gel. In fact, palladium-supported on titanium-doped silica catalysts have proven to be sulfur tolerant and exhibit high performances for the methane conversion. <p>Varying the amounts of TiO2 showed that the performance reached an optimum for a 10 wt.% TiO2 loading. According to the structural and surface analyses, the mixed oxides contained Ti-O-Si linkages, believed to be responsible for the better activity as compared to PdO supported on pure oxides. <p>Moreover, the titania-containing catalysts exhibited a superior tolerance towards SO2 when either adding it to the reactants or feeding it as a pure pretreatment atmosphere at 350°C (15 hour on stream). We evidenced using XPS that the Ti-O-Si sites are also responsible for the higher sulfur tolerance of the catalysts by the insertion of SO2 in the support forming either Ti-O-SOx•••Si or Si-O-SOx•••Ti. XPS analyses also evidenced that the long-term SO2-treatment leads to the formation of PdSO4 with a thickness of 18 to 20 Å. However, the catalysts can entirely recover their initial activity after this treatment. This regeneration was proven to be occurring through a concerted mechanism with methane leading to the total decomposition of PdSO4. Moreover, studies in presence of water showed that these properties remained unchanged.<p>The state of the palladium was also investigated and allowed us to evidence that an activation/stabilization of the catalyst is necessary. This is achieved in presence of the reactants by slight and subtle changes in both the support and the palladium phase. The increase of the catalyst activity was correlated with an increase of Ti3+ and Pd0 fractions. The presence of metallic palladium in the catalyst seems to be the key element in the activation of the C-H bonds. <p>Finally, we have studied the influence of the size/dispersion of the palladium particles on the reaction rate. The use of wet-chemistry synthesis allowed us to produce colloidal solutions of palladium with controlled particles sizes. This study revealed that the methane combustion is a structure sensitive or demanding reaction. Nevertheless, a better control of the shape of the nanoparticles should be achieved to determine the structural factor influencing the reaction.<p> / Doctorat en Sciences / info:eu-repo/semantics/nonPublished