Speaker
Description
Defects influence the performance of a heterogeneous catalyst and occur locally in any solid. These local structures are characterized by deviations in the geometric and electronic surrounding of the solid compared to the averaged bulk structure. They influence the local chemical properties and the appearance of the working structure, and, thus, the performance of the catalyst. In order to identify potential precursors that may form the active sites and to facilitate catalyst design, a detailed knowledge of the pristine structure is required.[1] Here we exemplarily reveal the local structures of open-structured selective oxidation catalysts, such as orthorhombic (Mo,V)O$_{x}$ (also referred to as M1) and titanium silicalite (TS-1).
M1-type oxides are promising catalysts for the selective (amm)oxidation of light alkanes. Their performance depends on the composition. In order to better understand their structure-activity correlation, we have quantified the local structures of pristine ternary (Mo,V)O$_{x}$ [3] and visualized cations displacements in quinary (Mo,V,Te,Nb)O$_{x}$ [4] by (scanning) transmission electron microscopy ((S)TEM), have highlighted the local distortions in the bulk by pair distribution function analysis (PDF) and have revealed compositional gradients from surface to bulk by electron energy loss spectroscopy (EELS) [5]. We have further tracked the structural changes induced by the reaction media by quasi in situ (S)TEM and identical location imaging. The results indicate that we are on the one hand one step closer to disentangle the various contributions that lead to the formation of the intrinsic surface structure, in particular structural flexibility and thermal equilibration, and on the other hand suggest that the dynamic nature of defects in the bulk and at the surface is needed to allow for subtle modifications in the band structure of this semiconductor.
TS-1 has found application as a catalyst in the industrial synthesis of propylene oxide via the hydrogen peroxide mediated direct epoxidation of propylene. Due to its bidirectional and interconnected sinusoidal and straight channel structure, TS-1 zeolite exhibits a variety of internal and external surfaces. While historically the active component has been described as a single substituted, tetrahedrally coordinated Ti in the siliceous framework, using a combination of $^{17}$O-NMR [6] and Ti K-edge XANES, we were able to show the presence of molecular TiO$_{x}$ domains and correlate them with the catalytic activity. In order to probe the location of these molecular species we employ Aloof-EELS measurement. Our recent results reveal a structural complexity within the TS-1 system that may require additional consideration when exploring the origin of its function.
References
1. Schlögl, R., In Modern heterogeneous oxidation catalysis : design, reactions and characterization, Mizuno, N., Ed. Wiley-VCH: Weinheim, 2009; pp 1 - 42.
3. L. Masliuk, M. Heggen, J. Noack, F. Girgsdies, A. Trunschke, K. Hermann, M.G. Willinger, R. Schlögl, T. Lunkenbein The Journal of Physical Chemistry C, 2017, 121, 24093-24103.
4. T. Lunkenbein, L. Masliuk, M. Plodinec, G. Algara-Siller, S. Jung, M. Jastak, P. Kube, A. Trunschke, R. Schlögl Nanoscale 2020, 12, 6759-6766.
5. L. Masliuk, F.-P. Schmidt, W. Hetaba, M. Plodinec, G. Auffermann, K. Hermann, D. Teschner, F. Girgsdies, A. Trunschke, R. Schlögl, T. Lunkenbein The Journal of Physical Chemistry C 2020, 124, 23069–23077.
6. C.P. Gordon, H. Engler, A.S. Tragl, M. Plodinec, T. Lunkenbein, A. Berkessel, J.H. Teles, A.-N. Parvulescu, C. Coperet Nature 2020, 508, 708–713.
| Abstract Number (department-wise) | AC 3.1 |
|---|---|
| Department | AC (Schlögl) |
