The Stability of Supported Gold Catalysts

Abstract

Gold has supreme cultural and financial value and, in form of nanoparticles smaller than 10 nm, is a unique catalyst for different industrially relevant reactions. Intriguing properties of the gold catalysts have spurred demand in the chemical industry for Au catalysts, the application of which strongly depends on their lifetime and regenerability. In this doctoral dissertation, the selectivity, activity, and stability of supported Au catalysts for different model gas phase hydrogenation and liquid phase oxidation reactions were discussed. The main question was the stability of the catalysts, and the focus was on silica supported Au catalysts. Another goal was to compare the catalytic performance of the Au supported on silica as well as its thermal stability to that of the typically studied Au catalysts like Au on titania. The study was extended to catalytic properties of bimetallic Au-based catalysts as an approach to enhance the catalytic activity and stability. It is shown that the support is a very important factor for the activity and stability of Au catalysts. In the gas phase hydrogenation of butadiene, the Au on silica catalysts were initially three times less active than the Au on titania catalysts, however, they clearly outperformed the Au on titania catalysts within several hours of runtime. Titania supports induced formation of carbonaceous deposits creeping up the Au nanoparticles, and in this way deactivating the active sites. However, silica supports led to much more stable catalysts. Upon treatment at high temperatures, Au nanoparticles on titania grew fast under oxidizing atmospheres, whereas Au nanoparticles on silica were much more stable. In liquid phase oxidation in which major particle growth occurs, the support pore structure can be chosen such that it limits the particle growth. Furthermore, by adding a second metal, the catalytic properties of the Au catalysts were altered but not necessarily improved. Atomic rearrangements in bimetallic nano-catalysts under reaction conditions and their effect on the stability were very important considerations as well. Another main conclusion is that deactivation pathways are different under different reactive conditions. Deactivation due to formation of carbonaceous deposits is the main reason in the studied gas phase hydrogenation reaction. However under oxidizing atmospheres particle growth is the main issue especially at higher temperatures and in liquid phase reactions. Finally, an important lesson is that the deactivation pathway, and hence how the chemical nature or structure of the support and/or composition of the metal nanoparticles can be tweaked, really depends on the specific reaction and conditions.