Specialized iNANO lecture: Structural dynamics in copper catalysts
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iNANO AUD (1593-012), Gustav Wieds Vej 14, 8000 Aarhus C
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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 research1,2 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.
- P.L. Hansen, J.B. Wagner, S. Helveg, J.R. Rostrup-Nielsen, B.S. Clausen, H. Topsoe. Atom-resolved imaging of dynamic shape changes in supported copper nanocrystals. Science 295, 2053-2055 (2002).
- M. Behrens, F. Studt, I. Kasatkin, S. Kühl, M. Hävecker, F. Abild-Pedersen, S. Zander, F. Girgsdies, P. Kurr, B.L. Kniep, M. Tovar, R.W. Fischer, J.K Nørskov, R. Schlögl. Science 336, 893 (2012).
- C. Holse, C.F. Elkjær, A. Nierhoff, J. Sehested, I. Chorkendorff, S. Helveg, J.H. Nielsen. Dynamic Behavior of CuZn Nanoparticles under Oxidizing and Reducing Conditions. The Journal of Physical Chemistry C 119, 2804-2812 (2015). In situ electron energy loss spectroscopy studies of gas-dependent metal-support interactions in Cu/ZnO catalysts. J. Phys. Chem. B 107, 7753-7758 (2003).
- J.B. Wagner, P.L. Hansen, A.M. Molenbroek, H. Topsoe, B.S. Clausen, S. Helveg.
- S. Kuld, M. Thorhauge, H. Falsig, C.F. Elkjær, S. Helveg, I. Chorkendorff, J. Sehested. Quantifying the Promotion of Cu catalysts by ZnO for methanol synthesis. Science 352, 969-974 (2016).
- R. van den Berg, C.F. Elkjær, C.J. Gommes, I. Chorkendorff, J. Sehested, P.E. de Jongh, K.P. de Jong, S. Helveg. Revealing the formation of copper nanoparticles from a homogeneous solid precursor by electron microscopy. Journal of the American Chemical Society 138, 3433-3442 (2016).
Host: Associate professor Jeppe Vang Lauritsen, iNANO & Dept. of Physics and Astronomy, Aarhus University