Research Group - Prof. Andreas Rosenauer

Structure and chemical composition have a large influence on the catalytic activity of nanoporous gold (npAu). Sizes of ligaments and pores determine how deep atoms or molecules can penetrate the porous structure during chemical reactions. The density of low-coordinated surface atoms is often considered to be one of the important origins of the high catalytic activity of npAu. Furthermore, the strain state of the metal surfaces leads to chemical properties that are significantly different from those of unstrained surfaces. For this reason, a detailed characterization of the npAu-structure is mandatory in order to understand differences in the catalytic activities of various samples.

It has been found that npAu is not a pure, unsupported catalyst, but must be considered as bimetallic catalytic system. The influence of the less noble residual metal cannot be neglected and a measurement of its distribution within the npAu ligaments is the key for a determination of its contribution to the catalytic activity. Recent results revealed that residual silver in npAu dealloyed from AuAg alloys is distributed inhomogeneously within the porous structure. Residual copper in npAu dealloyed from AuCu alloys is also distributed inhomogeneously but in a completely different manner. As content and distribution of the residual metal within the porous structure can have a strong influence on the material properties, quantitative spatially resolved measurements of the chemical composition are highly required. Finally, due to the large lattice mismatch of gold and copper of 12.8% (compared to 0.2% lattice mismatch of gold and silver) interesting structural differences are expected between npAu dealloyed from both alloys.

In this sub-project information on structure and chemical composition is derived using transmission electron microscopy (TEM). Conventional TEM yields the shape of ligaments projected along the beam direction. From these micrographs information on surface kinks, steps and terraces are obtained. High-angle annular dark field scanning TEM (HAADF-STEM) tomography reveals the 3D structure for characterization of pore size, ligament size, specific surface area and distribution of curvature. Strain is measured from the comparison of diffraction patterns acquired at different positions of the scanned focussed electron beam on the sample using the strain-analysis by nanobeam electron diffraction (SANBED) method.  To investigate the chemical composition energy-dispersive X-ray (EDXS) tomography measurements are carried out.