Postdoc Christian F. Elkjær, Atomic Force Analyses, R&D, Haldor Topsøe A/S

Structural dynamics in copper catalysts

The size, shape and surface structure of nanoparticles affect their catalytic properties in ways that are difficult to predict. The as-prepared state of the nanoparticles is, however, not guaranteed as it tends to adapt to the surrounding gas environment. It is therefore important to characterize the surface structure and dynamics of nanoparticles at the atomic-scale under exposure to reaction conditions (in situ) in order to help establish structure-functionality relationships in catalysis.

Here, I will focus on in situ transmission electron microscopy (TEM) of Cu catalysts with a particular focus on methanol synthesis, for which the industrial catalyst is comprised of Cu, ZnO and Al₂O₃.  Whereas Cu alone is active, the methanol production is significantly enhanced by the presence of ZnO. The role of the Cu-ZnO interaction and the nature of the catalytic active surface site has been subject of much research^1,2 and is still debated.

To allow detailed studies of the Cu-ZnO interplay, it is often beneficial to turn to model systems with reduced complexity compared to the high surface area catalyst. Recently, we have developed such a model system consisting of Cu nanoparticles in intimate contact with small ZnO crystallites. Using these, oxidation and reduction dynamics of the binary Cu-ZnO system was studied by XPS and in situ TEM in a parallel approach³. These studies also indicated that reduced Zn remains in the surface of Cu. Similar observations were found in the past using EELS⁴. The combination of these observations and complementary spectroscopy data and activity tests allowed the formulation for a coherent model for the methanol synthesis⁵ for which the catalytic active phase is a Cu-Zn surface alloy and for which this phase is determined by the Cu and ZnO particle sizes.

In order to improve the control over Cu and ZnO particle sizes, we employed in situ TEM to study the activation of Cu-based catalysts. In a first attempt we studied growth of Cu on SiO₂, by the reduction of a homogeneous precursor consisting of Cu phyllosilicate platelets⁶. By monitoring the individual nanoparticles over time, quantitative information about the nucleation time and size evolution of the Cu nanoparticles were obtained from TEM image series. By comparing the quantitative TEM data of the process with kinetic models, it was found that the growth process was best characterized as an autocatalytic reaction with either diffusion- or reaction-limited growth of the nanoparticles. Moreover, the results indicated that the Cu particle size was related to the size of the precursor particles and that tuning the dimensions of the phyllosilicate could provide control over the distribution of Cu particle sizes. 

Host: Associate professor Jeppe Vang Lauritsen, iNANO & Dept. of Physics and Astronomy, Aarhus University