Invited Paper AP+AS+MI+NS+SS-WeA9
From Field Ion Microscopy of Tips to 3D Atom Probe Tomography of Real Catalyst Nanoparticles

Wednesday, October 30, 2013, 4:40 pm, Room 203 A

This contribution will address some major achievements made in the application of Field Ion Microscopy (FIM) and 1D/3D Atom-Probe (AP) techniques to study catalysis-related problems. In particular, we shall demonstrate the unique capabilities of FIM to image reaction-induced morphological reshaping of single metal nanoparticles conditioned in the form of tips. As an example, we show how a nearly hemispherical Rh nanoparticle is transformed into a polyhedral morphology in the presence of oxygen gas.

In a second example, we shall inspect the use of 1D AP as a tool to provide a detailed kinetic analysis of adsorption/thermal desorption processes. As an example, measurements of the mean life time of NO molecules adsorbing on ~ 60 atomic sites of a (111) Pt facet will be presented. A quantitative evaluation of the data in terms of activation energies for desorption along with pre-exponential factors becomes possible by temperature variation.

Third, we shall consider the combined approach of FIM and 1D AP in imaging the dynamics of surface reaction processes while mapping the local chemistry during these processes. As an example, we shall present results of the catalytic reaction between oxygen and hydrogen on the surface of a Rh nanoparticle. Moving reaction fronts are followed here by using video techniques. The observed patterns demonstrate a strong non-linearity merging into oscillating reaction behavior between oxygen and hydrogen. 1D atom-probe measurements during oscillations allow distinguishing between oxygen- and hydrogen-covered surface patches. They also indicate the participation of sub-surface oxygen species in a feedback process. The oscillatory behavior has been successfully modeled using theoretical models of non-linear processes along with DFT.

Finally, we show that 3D AP can also be used for a chemical mapping of single nanosized grains of real catalysts. Using focused ion beam techniques, “CoCuMn” real catalyst particles as used for the selective production of 1-alcohols from synthesis gas (CO/H₂), can be conditioned in the form of tips. A 3D AP analysis of a single catalyst grain demonstrated the occurrence of a core-shell structure with Co forming the core. Interestingly, all three metals are found to be present in a 2nm thick shell which is otherwise dominated by large amounts of Cu. Such information is most important when it comes to establish relationships between catalytic activity/selectivity and surface chemical composition.