Submitted by Rosivalda Pereira (mrs.pereira@ufma.br) on 2017-09-15T17:59:28Z No. of bitstreams: 1 AugustoSilva.pdf: 4699819 bytes, checksum: cf18a43cf817a3264880d015853561f6 (MD5) / Made available in DSpace on 2017-09-15T17:59:29Z (GMT). No. of bitstreams: 1 AugustoSilva.pdf: 4699819 bytes, checksum: cf18a43cf817a3264880d015853561f6 (MD5) Previous issue date: 2017-08-16 / In this work the theoretical study of the bimetallic clusters of Au-Ag and Au-Cu was carried out, in which such clusters were determined by the addition of Ag and Cu atoms in pure structures of Au clusters, through the GA-DFT joint methodology. The AuCu and Au-Ag structures were described in a classical way using the Gupta potential via the genetic algorithm. The lower energy structures were optimized via DFT using the PBE functional through the SIESTA 3.2 program package. The most stable clusters were those with 13 atoms for Au-Ag bimetallic clusters, Au-Cu bimetallic clusters stabilized with 13, 15 and 20 atoms. Both clusters have a degree of aromaticity, more pronounced in the AuAg atoms, than in the Au-Cu atoms. The Au-Ag clusters adsorb the oxygen more favorably in end-on or bridge, whereas the bimetallic clusters of Au-Cu adsorb the molecular oxygen generally according to the bridge or double bridge model. The adsorption of oxygen in bimetallic clusters is intrinsically related to the ability of oxygen to perform π backdonation for the metallic atoms in the cluster. / Neste trabalho fora realizado o estudo teórico dos clusters bimetálicos de Au-Ag e Au-Cu, em que tais clusters foram determinados através da adição de átomos de Ag e Cu em estruturas puras de clusters de Au, através da metodologia conjunta GA-DFT. As estruturas de Au-Cu e Au-Ag, foram descritas de forma clássica usando o potencial Gupta via algoritmo genético. As estruturas de mais baixa energia foram otimizadas via DFT usando o funcional PBE através do pacote de programas SIESTA 3.2. Os clusters de maior estabilidade foram os com 13 átomos para clusters bimetálicos de Au-Ag, os clusters bimetálicos de Au-Cu estabilizaram com 13, 15 e 20 átomos. Ambos os clusters possuem um certo grau de aromaticidade, mais pronunciada nos átomos de Au-Ag, que nos átomos de Au-Cu. Os clusters de Au-Ag adsorvem o oxigênio de forma mais favorável em end-on ou ponte, enquanto que os clusters bimetálicos de Au-Cu adsorvem o oxigênio molecular em geral segundo o modelo ponte ou dupla ponte. A adsorção do oxigênio nos clusters bimetálicos está intrinsecamente relacionada à habilidade do oxigênio em realizar retrodoação π para os átomos metálicos.

This thesis investigates the synthesis and activation of highly monodisperse Au25(SR)18 - clusters and bimetallic clusters (AuAg and AuPd) protected with various stabilizers for reduction and hydrogenation catalytic reactions. The first chapter is the introduction chapter, which summarizes the literature involving monolayer protected Au clusters, atomically precise Au clusters, bimetallic clusters, X-ray absorption spectroscopy, research objectives, and organization and scope. The second chapter describes the synthesis of Au25(SR)18 - clusters protected with various thiolate stabilizers for nitrophenol reduction catalysis using NaBH4 as a reducing agent. This chapter also describes the stability of these clusters under reaction conditions using UV-Vis spectroscopy and MALDI mass spectrometry. The third chapter details the synthesis of carboxylic acid-protected Au25 clusters using a NaBH4 purification strategy. Here, the knowledge obtained in the second chapter regarding the exceptional stability of Au25(SR)18 - clusters in the presence of NaBH4 was used to isolate carboxylic acid protected Au25 clusters from a polydisperse reaction mixture. The fourth chapter describes the synthesis and activation of mesoporous carbon supported Au25(SR)18 - clusters for nitrophenol reduction catalysis. Here, thermal removal of thiolate stabilizers led to the enhancement in the catalytic activity at low calcination temperatures; however, at higher calcination temperatures activity dropped as particle sintering was observed. Activation of these clusters on mesoporous carbon support was followed by TEM and X-ray absorption spectroscopy. The fifth chapter describes the thermal and chemical removal of thiolate stabilizers from supported Au25(SC8H9)18 - clusters. Here, the removal of thiolate stabilizers and subsequent growth of Au25 clusters was followed by TEM and EXAFS spectroscopy. The sixth and seventh chapters describe the synthesis of AuPd and AuAg bimetallic clusters using Au25(SR)18 - clusters as precursors and their characterization using UV-Vis spectroscopy, transmission electron microscopy, and X-ray absorption spectroscopy. Here, AuPd bimetallic clusters were thermally and chemically treated, which resulted in the formation of AuPd bimetallic nanoparticles with segregated Pd atoms on the surface. AuPd bimetallic nanoparticles were used for the selective hydrogenation catalysis of allyl alcohol. The last chapter of this thesis includes final conclusions and possible avenues for future work.