(641g) In-Situ Characterization of Cu-Promoted Fe Model Catalysts by Indirect Nanoplasmonic Sensing and XPS

Fischer-Tropsch (F-T) synthesis and water-gas shift (WGS) reactions are two examples where Fe is used as a catalyst on an industrial scale^{1, 2}. Since the activity of a Fe-catalyst is closely related to the oxidation state of the material, it is desirable to be able to detect oxidation or reduction of the catalyst during operation conditions. We have conducted an in-situ study of a Cu promoted Fe-catalyst by means of an optical spectroscopy technique termed indirect nanoplasmonic sensing (INPS)^{3, 4}, based on the localized surface plasmon resonances (LSPR) in gold nanodiscs. Quartz wafers, optically sensitized with gold nanodiscs embedded in Si₃N₄, serve as sensors, placed in a quartz-tube flow-reactor. A 1 nm layer of Fe was evaporated on-top of the Si₃N₄ surface, serving as the model catalyst and 0.2 or 0.02 nm of Cu was deposited onto the Fe to mimic the presence of a Cu-promoter. The optical transmission spectrum of these Fe/Cu covered sensors is dominated by absorption and scattering caused by the LSP-resonances of the gold discs. However, the LSP-resonances are very sensitive to the chemical environment in their immediate vicinity. Therefore, an oxidation or reduction of the Fe and/or Cu on the sensor surface gives rise to shifts in the LSPR-peak position that can easily be detected via the optical transmission measurements. We use this to study the reduction of the deposited Fe/Cu-oxide during heating in H₂, a frequently applied activation treatment for Fe-catalysts. Furthermore, we investigate the impact on the reduction process when water vapor is present in the gas. We conclude that the Cu-promoter decreases the temperature at which Fe-reduction is initiated and that the addition of water leads to an increase of this temperature as expected. The interpretations of the optical measurements were verified with XPS. These experiments clearly demonstrate the usefulness of the applied INPS-technique for following changes in the oxidation state of a catalyst during use. In addition, although the experiments presented here were performed at near atmospheric pressure and in relatively simple gas mixtures there is nothing in the technique that hinders measurements at elevated pressures or in more complex gas mixtures.