Development of allotropic noble metal nanoparticles for improved catalysis

Noble metal nanoparticles (NPs) are often used as a catalyst due to their favorable surface-to-volume ratio and their unique properties, such as the surface activity, compared to their bulk counterparts. The influence of NP size and shape on the surface activity has been widely explored, the influence of the crystal phase has however gained much less attention. In recent years, allotropic NPs of various metals have successfully been obtained. Many of these synthesis however yield only very small amounts of material. This Ph.D thesis aimed to generate scalable, colloidal synthesis of allotropic Ag and Ru NPs, and the evaluation of these materials under catalytic conditions.

Chapter 1 summarizes the basic concepts of allotropy and polymorphism in general and specifically applied on noble metal NPs. The synthesis, properties and stability of allotropic Ag and Ru are discussed in detail. Ag and Ru were studied in this work since both metals are known to occur in allotropic structures, functioning as an ideal starting point for the optimization of the synthesis of their allotropic structure. Ag normally occurs in the face-centered cubic (fcc) structure under ambient conditions. The synthesis of allotropic Ag microparticles is not yet fully optimized and usually leads to the formation of mixed structures with only small amounts of hexagonally close-packed (hcp) Ag. Ru normally occurs in the hcp structure under ambient conditions, but can also be synthesized in the allotropic fcc structure. The synthesis of phase pure allotropic Ru NPs is well documented. Finally, the current state of the art regarding the characterization of (allotropic) metal nano- and microparticles is thoroughly discussed to function as a starting point for their further development.

The colloidal synthesis of hcp Ag microparticles is investigated and optimized in Chapter 2. First, an improved evaluation of the obtained crystal phase based on X-ray diffraction (XRD) is developed as the peak assignment reported in literature is ambiguous. This new peak assignment was confirmed via XRD simulations based on our experimental data and the space group of hcp Ag. Next, the colloidal synthesis of hcp Ag was optimized by fine-tuning the synthesis parameters and the addition of surfactants, demonstrating the importance of controlling both the surface energy of the developing NPs and the reaction kinetics in the synthesis of allotropic noble metals. While pure hcp Ag could not be obtained, the improved synthesis yields a hcp fraction of 0.71. Finally, the stability of the newly formed hcp Ag microparticles was studied in various solvents, under increased temperature and under increased N2, O2 and H2 pressures. These results outline catalytic conditions under which hcp Ag could be used as catalyst.

Chapter 3 studies the effect of both regular hcp and allotropic fcc Ru NPs on the photocatalytic activity of TiO2 in the simultaneous oxidation of benzyl alcohol and H2 production. Loading Ru NPs on TiO2 causes the formation of a Schottky barrier at the Ru/TiO2 interface, resulting in an increased charge separation and a retarded charge carrier recombination. Catalytic experiments showed a ca. 4 and ca. 8 fold increased catalytic activity when loading TiO2 with hcp and fcc Ru NPs respectively, compared to bare TiO2. These increased activities are in line with the measured Schottky barrier heights, for hcp and fcc Ru/TiO2, which were respectively 0.3 and 1.2 eV. The increased Schottky barrier of fcc Ru/TiO2, compared hcp Ru/TiO2, is a result of the strongly increased work function of fcc Ru NPs, compared to hcp Ru NPs. The analysis of the Schottky barrier height, the hcp and fcc Ru NP work function, and the catalytic activity are all in agreement with each other.

The properties of fcc and hcp Ru NPs, loaded on TiO2, as thermal catalysts was studied in Chapter 4. The selective hydrogenation of cinnamaldehyde was used as a model reaction since all three of its hydrogenation products, 3-phenylpropanol, 3-phenylpropanal and cinnamyl alcohol, are industrially valuable intermediates. Three different sets of reaction conditions, each resulting in a high conversion and selectivity for one of the three products, were proposed. These reaction conditions were reached by tuning the reaction temperature, the H2 pressure, the solvent and the crystal phase of the Ru NPs. It was demonstrated via DFT simulations that the fcc Ru NPs have a strongly increased work function, compared to the hcp Ru NPs. This increased work function indicates a higher surface stability for the fcc Ru NPs, explaining the decreased catalytic activity of fcc Ru NPs, compared to hcp Ru NPs, in the thermal catalysis of cinnamaldehyde.

In conclusion, my Ph.D. work focused on the development and application of allotropic metal particles as catalyst materials. In this work it was demonstrated that the synthesis of allotropic metal particles is not straightforward and that specific methods should be developed for the synthesis of allotropic structures of different metals, since there is no single method that can be employed for all types of metals. The stability of allotropic Ag and Ru was studied, showing that these materials have a good stability range, but that the limitations of their stability (e.g. temperature, pressure or solvent stability) should be taken into account when selecting the catalytic applications of these materials. Finally, it was shown that different allotropic structures of a material perform different in different catalytic applications, it can therefore be concluded that there is not one better catalyst when comparing metal allotropes, but that the best performing allotrope will strongly dependent on the catalytic reaction at hand.