Composition and structure of nanoporous Au dealloyed from AuAg and AuCu

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One of the main results in the first period of sub-project “Composition and structure of mono and bimetallic nanoporous foams” of the research unit FOR2213 was the observation of Ag rich clusters within npAu dealloyed from AuAg. This until then unexpected finding gave rise to a variety of questions concerning the understanding of their formation, their occurrence for small residual Ag concentrations and the composition of their surface. It is believed that Ag clusters are a possible reason for the low reproducibility of the catalytic behavior of npAu. Therefore, a part of the present project contributes to a systematic investigation of the aforementioned effects and the development of preparation routes producing a homogenous composition distribution for production of reproducible catalysts.Lattice strain relative to a reference region was measured extensively in npAu ligaments in the first period. It was found that the lattice in partially cylindrical ligaments is contracted along its axis and expanded in radial direction matching theoretical predictions by continuum mechanics. In the second period, we are aiming for a consolidated understanding of strain by quantitative matching of measured absolute strain with strain computed using energy relaxation with empirical potentials based on geometrical models derived from HAADF-STEM measurements. In the second period the portfolio of systems will be extended twofold: First, the role of the less noble element will be further investigated by replacing Ag by Cu in the starting alloys. The distribution of the less noble element is expected to play an important role, since the material system is exhibiting various ordered crystal phases for temperatures below 380°C that impose regions with different compositions. It is also expected that strain effects are significantly larger, since the lattice mismatch between Cu(Ag) and Au is 11.5% (~0%). Second, porous nanoparticles shall bridge the gap between nanoparticles representing smallest length scales of only few nm and npAu foams with ligament sizes of several tens of nanometers. In both cases the composition distribution, strain state and morphology will strongly depend on dealloying parameters such as potential, current and duration and might also change during catalysis. In order to achieve an in-depth understanding of catalytic properties, the characterization of composition, strain and morphology of the nanoporous materials at various stages of its life cycle is an indispensable prerequisite.