SYNTHESIS AND STUDY OF GOLD CATALYSTS IN CO OXIDATION AND SELECTIVE OXIDATION OF GLUCOSE

Abstract

In gold catalysis the performance of a supported catalyst is affected by the size, the distribution of the gold nanoparticles, the type and the structure of the support material and the different types of additives. The knowledge obtained about the behavior of gold as catalyst is not consistent and complete. Based on the need for better understanding the activity – structure relationship of nanostructured gold, which leads us to improve the catalysts performance, modified supported gold catalysts were developed for CO oxidation and glucose oxidation reactions in this work. First, unmodified gold catalysts of two different gold particle sizes supported on reducible (TiO2, CeO2) and irreducible (SiO2) oxides were prepared and their catalytic behaviors were compared in CO and glucose oxidation (Chapter 5.1). A correlation was found between the temperature required for 50% CO conversion and the glucose oxidation reaction rate, the activity order of the catalysts was reverse in the two reactions. This result revealed that the known support and size effect in CO oxidation is not valid for glucose oxidation. In the latter reaction weak metal – support interaction is favored, the silica supported catalysts showed higher activity than the ceria or titania supported ones. The higher activity of the larger gold particles found in glucose oxidation is explained by the different surface geometry needs for the larger glucose molecule compared to CO. In CO oxidation further development of the most active Au/CeO2 catalysts was carried out. For this purpose Au/SiO2 catalysts modified by different amount of CeO2 in a special way, when nanosize Au decorated with CeO2 patches were prepared (Chapter 5.2). High resolution transmission electron microscopy (HRTEM) measurements revealed thin, nanosize CeO2 moieties over gold already at extremely low 0.04wt% CeO2 loading, which decreased the temperature of 50% CO conversion by 280°C compared to Au/SiO2 reference. 0.16wt% CeO2 was enough to approach the activity of the Au/CeO2 reference sample. At 0.6wt% CeO2 content the catalyst greatly exceeded the activity of the pure Au/CeO2 used as reference. Further increase of the CeO2 content above 0.6wt% did not change the activity significantly. HRTEM proved that up to this concentration ceria is attached onto gold surface and further increase in Ce-loading caused CeO2 spread over the support surface as well. Strong interaction of Ce species with stabilizer ligands located around Au is suggested as the reason for CeO2 localization on gold. For the development of the most active Au/SiO2 catalysts in selective glucose oxidation the addition of silver to Au nanoparticles was decided. Addition of a second metal to gold resulted synergetic activity increase in many oxidation reactions. AgAu bimetallic nanoparticles also have shown increased activity in glucose oxidation but supported AgAu nanoparticles were firstly applied in this work. SiO2 supported AgAu bimetallic catalysts were prepared by sol adsorption method with 10/90, 20/80, 33/67 and 50/50 Ag/Au molar ratios (Chapter 5.3). UV-visible spectroscopy and HRTEM proved that the reduction of HAuCl4 in Ag sol resulted in alloyed AgAu colloid particles and their alloyed structure remained after calcination and reduction treatment. The AuAg bimetallic effect and its dependence on the Ag/Au molar ratio were studied in glucose oxidation. Synergistic activity increase was observed compared to the Au/SiO2 reference sample in case of the bimetallic samples up to Ag/Au=33/67 molar ratio. Maximum activity was reached at Ag/Au=20/80. Oxidation/reduction pretreatment slightly affected the activity of the catalysts; however the sequence of the samples remained the same. A reaction mechanism was proposed for glucose oxidation over our silica supported silver – gold catalysts, which is consistent with our experimental results and based on previous studies for alcohol oxidation and glucose oxidation on gold catalysts. The higher activity of the bimetallic samples is suggested to be caused by the improved O2 activating ability provided by Ag sites. The further increase of Ag loading above the optimal concentration may dilute or cover the Au to such an extent that the number of gold ensembles necessary for glucose activation decreases deteriorating the activity. Characterization of the parent monometallic Ag sol by HRTEM and Selected Area Electron Diffraction (SAED) showed the coexistence of the commonly known face centered cubic crystal phase of Ag nanospheres with the rarely observed hexagonal 4H–Ag structure in the same concentration. This hexagonal polytype of Ag has been observed, to date, only in nanocrystalline and continuous films or nanorods; in nanospherical form it was reported, to the best of my knowledge, for the first time.