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.

L’objectif de cette étude portait sur la mise en œuvre de réacteur catalytique membranaire pour une application dans le traitement d’effluents liquides contaminés par des polluants organiques. Des phases catalytiques ont été déposées au sein des structures poreuses par différentes techniques afin de bien maîtriser la localisation des phases actives. L’optimisation des conditions opératoires a ensuite été réalisée. Ces matériaux sont actifs pour l’oxydation de polluants présents dans les effluents liquides et la configuration en mode contacteur a permis d’accroître l’efficacité et la stabilité des phases catalytiques pour ces réactions de dégradation grâce à un meilleur contact entre les réactifs et les sites actifs. / The aim of this study was to evaluate catalytic membrane reactor for wet oxidation efficiencies of pollutants in waste water. In a first part, we have prepared catalytic membrane using several techniques of deposition in order to well control the position of the active phase in the porous structure. After optimisation of the experimental parameters, the study of pollutant degradation has showed that catalytic membrane reactor, in contactor configuration present highest efficiency than conventional reactor due to optimized contacts between reactants and active sites.

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.

京都大学 / 新制・課程博士 / 博士(人間・環境学) / 甲第24711号 / 人博第1084号 / 新制||人||254(附属図書館) / 2022||人博||1084(吉田南総合図書館) / 京都大学大学院人間・環境学研究科相関環境学専攻 / (主査)教授 吉田 寿雄, 教授 田部 勢津久, 教授 中村 敏浩, 教授 田中 庸裕 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DFAM