title: Catalysts Based on 2D Layered Supports : Influence of a Sandwich Confinement on Catalysis and Stability of Nanoparticles
Creators: name: Ament, Kevin
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: nanoparticles
subject: layered materials
subject: nanocomposite
subject: metal-support
subject: catalysis
subject: nanocomposites
subject: 500
subject: 540

description: Within the scope of this thesis, a modular strategy for intercalating nanoparticles between 2D layered materials was successfully developed. In particular, the synthetic layered silicate hectorite (Na0.5Mg2.5Li0.5Si4O10F2, NaHec) and a layered titanate (H1.07TiO1.73O4·H2O, L titanate) were investigated for their potential use as catalyst supports. Both are layered materials with a permanent negative layer charge, neutralized by interlayer cations. Furthermore, when immersed in water, these materials form nematic liquid crystalline phases of sub-nanometer thick single nanosheets. Even at low solid contents and distances of more than 50 nm, the nanosheets retain a co-facial orientation to each other due to their large lateral platelet size. Pre-synthesized nanoparticles carrying a positive surface charge can be added to such a nematic phase, and the large gallery height of adjacent nanosheets allows nanoparticles to diffuse between the nanosheets, whereupon the nanosheets collapse and capture the nanoparticles. This process could be seen as a "quasi-ion exchange," and a special catalyst architecture can be obtained: nanoparticles sandwiched between two sub-nanometer thick oxide sheets. 2D layered materials such as layered silicates have already been applied as catalyst supports before, but the use of natural materials that are very heterogeneous and defective or insufficient synthesis methods hampered access to well-structured catalysts. For this reason, the influence of the confinement on the (catalytic) properties of the sandwiched nanoparticles has not been investigated deeply. In contrast, the "quasi-ion exchange" method described above using a nematic phase of the nanosheets and pre-synthesized nanoparticles leads to well-defined architectures. This thesis demonstrates that the very special sandwich architecture offers great potential regarding increased temperature stability and influencing the electronic structure of nanoparticles, leading to enhanced catalytic activity for CO oxidation and increased H2 sorption capacity. In the first part of the presented work, γ-Fe2O3 nanoparticles were synthesized that were colloidally stable in aqueous dispersion and carried a positive surface charge. These nanoparticles were sandwiched between the nanosheets of a nematic NaHec dispersion. Even though the loading of iron oxide was 64 wt%, a lamellar and mesoporous structure was obtained. The sandwich-like confinement stabilized the nanoparticles and hampered Ostwald ripening up to 700 °C, whereas rapid growth of unsupported nanoparticles was observed already at 400 °C. Additionally, the confinement suppressed the phase transition from γ-Fe2O3 to α-Fe2O3, normally occurring at 300 °C. Changed thermodynamic phase stabilities of nano-sized iron oxide were responsible for this effect. In the second part, the portfolio of nanoparticles that can be intercalated between the nanosheets of a nematic NaHec phase was extended to Pd nanoparticles. The surface potential of these nanoparticles was adjusted by pH that allowed the control of the intercalated loading of nanoparticles. The obtained structure was a potential “bulk version” of literature known model catalysts of flat metal surfaces that were (partially) covered with an ultrathin (&lt; 1 nm) oxide layer. As a test reaction, CO was oxidized to CO2. The sandwiched Pd nanoparticles proved to be more active than the same nanoparticles deposited on the conventional support γ-Al2O3 (temperature of 50 % conversion T50 of 145 °C and 191 °C, respectively). Electronic interactions between nanoparticles and nanosheets rendered the nanoparticle surface slightly positively charged, which was identified as the reason for the enhanced catalytic activity. The electron-deficient Pd nanoparticles were investigated for other possible applications in the third part of the thesis. It is known from the literature that the H2 sorption capacity of Pd nanoparticles is related to the number of holes in the band structure near the Fermi level. H2 pressure-dependent uptake curves and in situ X-ray diffraction demonstrated that sandwiching Pd nanoparticles between nanosheets significantly influenced the sorption behaviour. The maximum storage capacity of H2 was increased by 86 % compared to identic Pd nanoparticles covered with polyvinylpyrrolidone. In the last part, the intercalation strategy using a nematic phase of a 2D layered material was extended to L-titanate. As observed for NaHec, electronic interactions between support and Pd rendered the surface of Pd positively charged. However, the catalytic activity for the oxidation of CO proceeded already below 100 °C (T50 of 86 °C). That implied that L-titanate provided an additional enhancing effect for oxidation catalysis besides electronic influence. Kinetic experiments revealed that L-titanate could donate activated oxygen from its lattice at the support/metal interface (Mars-van Krevelen type mechanism). This bypassed CO poisoning of the noble metal surface at low temperatures. The special sandwich architecture was also of great importance for the enhanced catalytic activity. As the nanoparticles were in touch with the support from two directions, a particularly high interface was generated, making oxygen donation very efficient. The thesis is presented as a cumulative work. The results are discussed in detail in the attached manuscripts.