Magdalena Wencka

Abstract:

Intermetallic compounds are known as highly selective, effective and long-lasting new generation catalysts in processes of green hydrogen production via methanol steam reforming and carbon dioxide reduction to methanol and production of plastics on the way of semi-hydrogenation of acetylene. Adsorption properties of surfaces play a key role and are determined predominantly by electronic effects, while geometrical and vibrational properties of the adsorbate-substrate complex provide 'fine-tuning' of the adsorption behavior. Numerous studies investigating the hydrogenation of acetylene and ethylene have lead to the development of the active site isolation concept. For this weakly adsorbed acetylene where the π-bonds are interacting with the surface (π-adsorbed acetylene) will be transformed to ethylene (the desired chemical reaction), while stronger di-σ adsorbed acetylene will undergo undesired full hydrogenation or form carbonaceous deposits that will deactivate the catalyst. Basic idea of the active-site-isolation concept is to surround the catalytically active atoms by atoms of another inactive chemical element, so that the active atoms are isolated by spatial separation. The isolated active sites show preference for the π-adsorbed acetylene, resulting in high selectivity for the semiehydrogenation of acetylene to ethylene, while in the absence of spatial separation (i.e., when many active atoms are close neighbors) the catalyst material is less selective. Intermetallic compounds GaPd, GaPd₂ and Ga₇Pd₃ were demonstrated to show superior catalytic selectivity and stability over the commercial Pd-based catalysts in the semi-hydrogenation of acetylene in a large excess of ethylene, which is an important step in the purification of the ethylene feed for the production of polyethylene. The selectivity of these compounds to ethylene was reported to be between 65 and 75%, which is much higher than that of a commercial Pd/Al₂O₃ supported catalyst, where a selectivity of 15-20% was reported for the 5% Pd/Al₂O₃. High selectivity is considered to result from specific crystalline structures of these compounds, while good stability under the reaction conditions originates from strong covalent chemical bonding in the structure. The metallic character of the Ga-Pd compounds also allows for a substantial electronic density of states (DOS) near the Fermi level, which is a prerequisite for facile activation of di-hydrogen H2 as a reactant. The difference in selectivity between these compounds was found small, with the GaPd₂ selectivity being the highest and that of Ga₇Pd₃ the lowest. I shall present electronic, thermal and magnetic properties of the Ga-Pd phases along orthogonal directions of the structures. By using ⁶⁹Ga and ⁷¹Ga NMR spectroscopy, the electric-fieldgradient (EFG) tensor at the Ga site in the unit cell and the Knight shift, which yields the electronic DOS at the Fermi energy ε_F were determined. Since the catalyst material in a chemical reaction should exhibit as large surface as possible, the nanoparticle morphology of the material is preferred under realistic conditions, but the physical properties of the nanoparticles may differ substantially from those of the bulk. To see the change of electronic properties of the GaPd₂ phase on going from the bulk material to the nanoparticles morphology, the GaPd₂/SiO₂ supported nanoparticles were synthesized and determined their electronic DOS at ε_F from the ⁷¹Ga NMR spin-lattice relaxation rate, which was then compared to the DOS of the bulk. This work complements recent studies of physical properties of the GaPd and InPd intermetallic catalysts.