Immobilització d’àtoms aïllats i nanopartícules de pd sobre magnetita. Comportament catalític

Resumen

This PhD thesis is demarcated in the field of nanoscience. In the last years, noble metal nanoparticles have attracted the attention of researchers due to their high activity and selectivity in a large number of catalytic reactions. However, metallic nanoparticles have a great tendency to agglomerate in the synthesis as well as during the chemical reactions. For this reason, it is absolutely necessary to immobilize the nanoparticles on a support. Between the large number of possible supports, metal oxides are the most studied and used. In this group the magnetite (Fe3O4) stands out because it can be easily isolated from the reaction media using an external magnet. This minimizes the losing of catalyst and it makes easier its reuse. In this thesis, we have demonstrated that the deposition of palladium on the surface of magnetite nanoparticles, previously functionalized with a terminal phosphine linker, facilitates the formation of small and homogenous palladium nanoparticles. If the palladium amount is conveniently reduced, it is possible to deposit only isolated palladium atoms (SACs). According to the bibliography, SACs are excellent catalysts because all the atoms are potentially able to participate in the reaction. The synthesized catalysts in this study (Pd nanoparticles and Pd SACs) have been investigated in three reactions: 4 nitrophenol reduction, Suzuki Miyaura cross coupling and styrene hydrogenation. We have demonstrated that the palladium nanoparticles and SACs are excellent catalysts. In particular, Pd nanoparticles of 1 nm deposited on magnetite nanoparticles are the best catalyst reported for the 4 nitrophenol reduction. On the other hand, Pd SACs are the most active catalysts reported up to now for the Suzuki Miyaura reaction when 4 bromotoluene and phenylboronic acid are used as reactants. However, the catalyst recycling in this reaction was quite poor. About the styrene hydrogenation, it has been observed that when the palladium particle size is reduced, the catalytic activity rises due to the rising of the surface / volume relation. A really surprising fact is that SACs have shown no activity in this reaction. This result has been attributed to the high positive charge on the palladium. The influence of the linkers on the deposition process of the palladium nanoparticles on the magnetite nanoparticles has been studied too. In this context, the nanoparticles have been immobilized using different linkers equipped with a phosphine or an amine terminal group. Using different catalytic reactions we could see that phosphino terminal linkers lead to better catalytic results than those containing amino functions, which are more used in the bibliography. In order to expand this study, SACs have been used in the reaction of CO2 hydrogenation for first time. This is a really interesting reaction because it converts a pollutant, like the CO2, to higher value molecules like methane or methanol. We observed that the palladium SACs presents a high selectivity toward the formation of terminal alcohols of two or more carbons, especially ethanol. However, the palladium nanoparticles are more selective to the formation of methane and ethane. Nevertheless, when SACs are used at higher temperatures (350 400 °C) they agglomerate forming nanoparticles and, consequently, losing selectivity throw the formation of ethanol. In order to complete this study, palladium SACs were deposited on different supports (CeO2, Al2O3 and ZnO) in order to observe and compare their behavior on the CO2 hydrogenation process. In these assays we could see that the activity and the selectivity were strongly reduced if we compare them with the ones obtained with the magnetite. Here we have demonstrated the enormous influence of the support on the catalytic activity of the SACs. It seems evident that the strong interaction magnetite palladium is a crucial factor in the catalysis.