Design of Robust Heterogeneous Catalysts for Sustainable Chemistry

Robust heterogeneous catalysts based on polymer derived non-oxide ceramics (SiC/SiCN) with integrated late transition metal (Ni, Pd) nanoparticles (NPs) were designed, characterized and tested for their catalytic potential. For the decoration of SiCN support with metal NPs, a molecular approach was applied which makes the use of amido metal complexes for the chemical modification of the preceramic polymer (commercially known as HTT 1800, Clariant Advanced Materials GmbH, Sulzbach, Germany) by the transfer of metal to its nitrogen functions. The modified polymer was transformed to an amorphous SiCN material containing metal NPs by controlled pyrolysis under an inert atmosphere. By following aforementioned approach, Cu@SiCN materials (non-porous) have already been developed in our research group. In present thesis, attempts were made to fabricate porous SiCN/SiC materials with good accessibility of metal (Ni and Pd) NPs. Firstly, the synthesis of palladium containg SiCN materials by the pyrolysis of palladium modified polysilazane was attempted. It was found that during polymer to ceramic transformation, palladium reacts with silicon to form intermetallic palladium silicide particles. The materials were characterized (TGA, FT-IR, NMR, PXRD and TEM) and applied in catalytic hydrogenation reactions. The activity of the catalysts was found low because of the silicidation of metal and non-porous SiCN support. The issues of the low porosity of SiCN ceramics and the formation of metal silicides were addressed in the synthesis of Ni@SiCN materials. Controlled pyroylsis of Ni modified polysilazane at 600°C provided microporous Ni@SiCN materials, whereas at 1100°C the formation of nickel silicides in non-porous SiCN matrix was observed. Ni@SiCN materials were characterized by solid state NMR, FT-IR, PXRD, TGA, N2-physisorption and TEM. Hyperpolarized 129Xe NMR was used to study the effect of nickel loading and annealing time on pore structure. The microporous materials showed good thermal stability and activity in selective hydrogenation of aryl acetylenes but as polymer to ceramic conversion was not complete at 600°C, the materials possessed low hydrothermal stability at higher temperatures due to the hydrolysis of Si-N bonds. In order to replace Si-N bonds with more stable Si-C bonds, a commercial polycarbosilane was used to fabricate SiC materials with integrated Ni NPs by the pyrolysis of nanostructured polycarbosilane-block-polyethylene (PCS-b-PE) polymer. PCS-b-PE was synthesized by the reaction of PCS with PE-OH catalyzed by an amido nickel complex followed by its nanostructuring via microphase separation technique. The length of PE block affected the type of pores generated in SiC material and the synthesis of highly porous materials with micro-, meso- and hierarchical porosity was achieved. Ni@SiC materials were characterized by TGA, PXRD, N2-physisorption and TEM. Hierarchically porous Ni@SiC selectively cleaves aromatic C-O bond of aryl ethers in water and was found a reusable catalyst. Lastly, controlled pyrolysis of palladium modified polysilzane provided nanoporous SiCN materials with very small and highly accessible Pd NPs. This shows the role of SiCN support whose nitrogen atoms have a stabilizing effect on small metal nanoparticles. Moreover, SiCN support ineracts nicely with the NPs leading to the prevention of their leaching. In general, NPs have agglomeration tendencies with increasing metal loading but in the case of Pd@SiCN, size of metal NPs remained small even with a palladium loading of 14 wt%. Pulse titration with hydrogen showed high accessibility of Pd. Pd@SiCN materials showed better activity than commercial Pd/C catalyst in oxidation of alcohols in the absence of air or oxygen with the evolution of hydrogen. M@SiCN/SiC materials may find applications in sustainable production of chemicals and fuels from renewable sources, for instance, from lignocellulosic biomass.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2013

Author dc:creator

• Zaheer, Muhammad

Contributors dc:contributor

• Kempe, Rhett

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/110/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:110