Speaker
Description
Heterogeneous catalysts are dynamic and become more complex under reaction conditions compared to their pristine states. Reaction induced solid-state chemistry yields in metastable non-stoichiometric phases that extend from the surface to the bulk [1]. This observation is important as these phases may control the activity. Combining operando transmission electron microscopy (TEM) [2], operando scanning electron microscopy (SEM) [3], with analysis of thin cross sections of spent catalysts specifically manufactured by focused ion beam (FIB) milling, we investigate the evolution of catalysts and reaction-added complexities. Here, we illustrate these processes with two important heterogeneous reactions: (i) dry reforming of methane (DRM) on Ni catalysts, and (ii) propane dehydrogenation (PDH) on Pt catalysts.
(i) Ni nanoparticles were investigated during reduction and subsequent DRM reaction by operando TEM and SEM. A phase transition from NiO into Ni$^{0}$ and NPs sintering were observed during activation in H$_{2}$ atmosphere. At DRM conditions (CH$_{4}$/CO$_{2}$, 850°C, 1 bar), syngas with a high amount of water was produced, which led to a rapid structural change corresponding to the formation of NiO and deactivation. Parallel investigation by operando SEM revealed redox transitions at the catalyst. A sharp phase transition between oxidized and reduced states of Ni was detected [3]. This transition separated active and inactive regimes of DRM and is complemented by near ambient pressure X-ray photoelectron spectroscopy. In addition, fluctuating chemical dynamics occurred as self-sustained oscillations. Analysis of a cross-section of the spent catalyst revealed the presence of nm-sized, ill-defined and non-stoichiometric Ni-O phases several atomic layer deep in the catalyst structure. Combined with numerical analysis this data shows that reversible strain induced into the metallic Ni substrate may act as feedback mechanism to switch the catalyst states between partial oxidation of methane- and DRM-like products.
ii) Structural and morphological changes of a Pt catalyst during PDH were studied by operando TEM. A jump in the propylene production has been observed in the temperature regime 438 and 474 °C. Different Pt-C phases have been found. While some particles have been embedded by a graphitic carbon shell, other particles are characterized by the diffusion of carbon into the bulk and appear mainly at or after the jump in the propylene production at 600 °C. High-resolution TEM images revealed that the former particles are still metallic Pt and the latter show the presence of different Pt-C phases, which could also explain why mild regeneration conditions (see also poster AC4.3) have to be applied in order to keep the carbon in the Pt nanoparticles and to maintain their catalytic performance.
In summary, we demonstrate by operando electron microscopy that a catalytic reaction induces solid-state reactions even to simple catalyst systems to form metastable phases that can alter the catalytic outcome.
References
[1] Plodinec M. et al. (2020), ACS Catalysis, 10, 5, 3183–3193.
| Abstract Number (department-wise) | AC 3.2 |
|---|---|
| Department | AC (Schlögl) |
