Electrocatalysis plays a vital role in the transition from fossil fuel to renewable energy infrastructure. Bimetallic systems can provide enhanced electrocatalytic activity and/or selectivity due to their altered electronic and/or crystal structures. These two effects are the main parameters responsible for the enhancement of the catalytic properties of multi-metallic systems. In practice, they are often interrelated and can be difficult to distinguish from one another due to random distribution and segregation of the elements in substitutional alloys. With well-defined crystal and electronic structures, intermetallic compounds provide excellent platform materials for a knowledge-based approach aiming for the evaluation and optimization of structural and/or electronic effects in heterogeneous (electro) catalysis. The present PhD thesis focuses on the investigation of the correlations between electronic, geometric and electrocatalytic properties of anode materials in the methanol oxidation reaction (MOR). This is achieved by substitution of indium (three valence electrons) with tin (four valence electrons) in the isostructural series In1-xSnxPd2, which allows for a systematic variation of the total number of electrons per unit cell with a minor variation of the cell parameters. Geometric effects were evaluated by substitution of indium with gallium in the isostructural Ga1-xInxPd2 series, which allows for a systematic variation of the cell parameters (interatomic distances) with the same number of valence electrons per unit cell. By substitution of gallium with tin in the Ga1-xSnxPd2 series, both effects are combined and addressed simultaneously. Single-phase samples of the isostructural series In1-xSnxPd2, Ga1-xInxPd2 and Ga1-xSnxPd2 (0 ≤ x ≤ 1), were synthesized and characterized by metallography, powder X-ray diffraction, and electron microscopy to establish the phase composition and to determine the variation of the lattice parameters with composition. The MOR current densities show a distinct change in slop as the fraction of tin increases in the In1-xSnxPd2 series with a minimum at x = 0.8 which is attributed to the alteration of the electronic properties of the materials. For the GaxIn1-xPd2 series, the MOR current densities show a maximum at x = 0.5 which is attributed to the alteration of the structural properties of the materials. The Ga1-xSnxPd2 series shows two maxima at x = 0.15 and 0.93. The high activity at x = 0.15 and 0.93 is attributed to a synergy of simultaneous alteration of electronic and geometric influences and the catalytic properties. The results contribute to the knowledge-based development of catalytic materials with direct experimental evidence of fine-tuning of electronic and/or geometric influences using isostructural intermetallic compounds as platform materials. This provides a basis of model catalysts for further studies to advance fundamental, as well as applied research in catalysis for the development of a green, sustainable future for the new generations.

Die vorliegende Arbeit beschäftigt sich mit der Entwicklung einer Synthesestrategie einphasiger intermetallischer Verbindungen des Systems Molybdän-Nickel als Volumenmaterial. Anhand dieser Materialien soll aufgeklärt werden, ob geordnete intermetallische Verbindungen eine höhere Aktivität in der elektrokatalytischen Wasserstoffbildung aus Wasser besitzen als nicht geordnete Substitutionslegierungen. Des Weiteren soll der Grund für die veränderte Aktivität identifiziert werden. Dazu wurden die Verbindungen mittels elektrochemischer Charakterisierungsmethoden und zwei verschiedenen Stabilitätstests in der Wasserstoffbildungsreaktion untersucht. Die Ergebnisse legen nahe, dass die Verbindungen unter den Reaktionsbedingungen nicht stabil sind. Bei Molybdängehalten ≤25 At.-% und Potentialen unterhalb des reversiblen Wasserstoffpotentials kann sich jedoch eine passivierende Nickelhydroxidschicht ausbilden, sodass eine fortschreitende Korrosion unter kathodischen Stromflüssen verhindert werden kann. Diese Nickelhydroxidschicht ist jedoch auch für die Desaktivierung der Mo-Ni Materialien verantwortlich. Es konnte weiterhin festgestellt werden, dass die elektrochemisch aktive Oberfläche, in Folge der Auslaugung von Molybdän, proportional zum Molybdängehalt steigt, was sich in vermeintlich höheren Aktivitäten der Mo-Ni Materialien äußert.