Speaker
Description
Smallest alterations in the structure and composition of a solid can have a major impact on the virtues of a catalyst and change its activity, selectivity and productivity. Defects have no translational symmetry, add additional complexity to the structure, and are almost intangible by integral and averaging methods. To date, little is known about how these local structures participate in the catalytic reaction, how they change during the reaction, and what metastable structures are formed during catalysis. Using the chemical electron microscopy approach, the electron microscopy group aims to provide a chemistry-based real structure understanding by identifying the chemistry of local structures and their dynamic relation to catalysis. The investigation strategy includes the development of analysis routines and techniques, local structure imaging at different scales, and visualization of reaction-induced dynamics.
Using open-structured zeolites and transition metal oxides as examples, we have advanced the description of local structures. We quantified, for instance, the local structures of molybdenum and vanadium-based oxides, detected cation displacements and compositional differences between surface and bulk. We also addressed an integral description of local distortion in the bulk and investigated the change of defects during activation and catalytic reaction using identical location imaging with atomic resolution. The results suggest that we are one step closer to disentangling the various contributions to the formation of the intrinsic surface structure, in particular structural flexibility and thermal equilibration. In addition, the observed dynamic nature of defects in the bulk and at the surface is required to enable subtle changes in the band structure of this semiconductor (AC3.1).
To image chemical dynamics and the formation of metastable phases during the reaction, we have implemented operando electron microscopy for both scanning electron microscopy and transmission electron microscopy (TEM) at FHI for thermal gas phase catalysis. Simple catalyst systems such as nickel and platinum catalysts, which were inspected during dry reforming of methane (DRM) and propane dehydrogenation (PDH), respectively, where found to increase their structure and phase complexity. In the former, reversible strain in metallic Ni acts as a mechanism mediator between partial oxidation of methane (POM) and DRM while in the latter, the partial formation of interstitial platinum carbon phases at high propylene production was observed (AC3.2).
We further dealt with the inspection of Cu/ZnO/Al$_{2}$O$_{3}$ catalysts during CO$_{2}$ hydrogenation by means of operando TEM. We observe a temperature dependent reversibility of the ZnO$_{1-x}$ overlayer, which is open at high temperatures and closed at low temperatures. Moreover, we see a direct dependence of the thickness of the ZnO$_{1-x}$ layer from the CO$_{2}$ concentration (AC3.3).
This expertise in the local characterization of solids is pursued in inter- and intradepartmental collaborations and is extended to projects funded by the BMBF (RUSEKU, Ammoref, CatLab), DFG (e-conversion, SPP2080, CRC247), and industry (AmoMax, ChemiTEM, Glovebox TEM, OSEM). In most of these projects the complementarity to the Electronic Structure Group was essential (AC2.3, AC6.2, and AC6.3). The local scale analysis of catalysts under static and working conditions by electron microscopy shows that structural complexity is a general phenomenon in heterogeneous catalysis and may give new momentum to catalyst tailoring.
| Abstract Number (department-wise) | AC 3.0 |
|---|---|
| Department | AC (Schlögl) |
