Speaker
Description
Ammonia synthesis is one of the most important industrially used reactions as it allows to obtain fertilizers, and other chemicals [1]. Since its discovery, several works were conducted in order to understand and improve the active phase for the ammonia synthesis. [2] Here, we study the NH$_{3}$ synthesis on an industrial Fe-based catalyst in different pressure regimes from 0.5-104mbar. We investigate the morphological, electronical and structural properties by operando TEM, XPS, and SEM.
TEM and SEM analyses have shown that the pristine sample consists of a grain-boundary microstructure with the iron oxide wüstite grains linked by various oxide promoters (Al, K, Ca, V, Si). During activation, in situ TEM and SEM imaging have shown the outgrowth of nanoparticles on the surface of the grains. In situ structural analysis indicated that the wüstite particles underwent disproportionation into magnetite, iron and more reduced wüstite. The nanoparticles constituted a seed for the formation of metallic iron platelets. Water and gradual activation of NH$_{3}$ formation were detected during the outgrowth of nanoparticles. This effect is observed only when the catalyst is heated slowly for an extended period of several days in the reaction feed. The full activation of the particles was achieved using the high-pressure quasi in situ TEM. In this study, the sample was analyzed in the TEM before and after reduction treatment at elevated pressure of 10 bar, and using a safe glovebox-transfer. Complex microstructures were observed related to reduction of the crystals. This corresponds to the formation of particles-agglomerates, impure α-Fe, strained particles, layered structures and overlayers of particles.
The chemical state of an active catalyst was characterized by combining in situ XPS/MS experiments in low gas pressures (0.5-2 mbar). Even on a relatively short timescale compared to in situ SEM experiments, it is possible to activate enough sample material in the vicinity of the surface to achieve ammonia synthesis. XPS indicates that the promotor elements remain largely oxidic but can vary in composition which may impact on the reactivity, e.g. by a lower Ca and higher K content. Fe is reduced to its metallic state and adsorbing NH, NH2 and NH3 are detected in the N1s region. Simultaneously, S is diffusing to the surface. Deconvolution of the Fe2p region shows an additional feature, which may be attributed to Fe-S or Fe-N bonds. A potential role of S in the ammonia synthesis is currently under investigation. Combined XPS-MS results also show the critical role of water in the ammonia synthesis which, by H2O stemming from the activation process as well as adsorbing from residual gas, can considerably impede the NH$_{3}$ formation.
References
1. Liu, H. Ammonia Synthesis Catalysts; World Scientific, 2013.
2. Ertl, G.et al. Weiss, M. Journal of Catalysis, 79 (2), 359–377(1983).
| Abstract Number (department-wise) | AC 6.2 |
|---|---|
| Department | AC (Schlögl) |
