Reduction of Cu-Promoted Fe Model Catalysts Studied by In Situ Indirect Nanoplasmonic Sensing and X-ray Photoelectron Spectroscopy

The reduction of Cu-promoted Fe model catalysts was investigated using X-ray photoelectron spectroscopy (XPS) and indirect nanoplasmonic sensing (INPS). The catalysts were prepared by evaporation of 1 nm thick Fe particle films onto quartz wafers, followed by deposition of 0, 0.2, or 0.02 nm of Cu (i.e., 0, 2, or 19 wt %) as a promoter. For the XPS measurements, a reaction cell with in vacuo transfer to the measurement chamber was used. The catalysts were first oxidized at 400 °C in Ar/O2, achieving fully oxidized Fe2O3 and CuO. Subsequently, the samples were heated to temperatures between 100 and 400 °C in pure H2, and the resulting change in oxidation state was measured. Fe2O3 was found to be reduced to Fe3O4 at 225 °C and to a mixed state of FeO and metallic Fe at 275 °C. The corresponding temperatures for Cu-promoted Fe catalysts were 100 °C lower. In the absence of FeOx, Cu was reduced to metallic Cu, via Cu2O, at temperatures between 125 and 175 °C. In addition to the XPS measurements, INPS was used to obtain more detailed insight into the reduction process, both in pure H2 and in wet H2, containing 0.8 vol % H2O. These in situ experiments show that the presence of H2O increases the reduction temperature by 36 °C or more, depending on the amount of Cu promoter used, where the catalyst with the industrially relevant 2 wt % promoter material exhibits the smallest increase. The INPS measurements also demonstrate that increasing the amount of Cu promoter decreases the Fe2O3 reduction temperature, in both dry and wet H2. Together, XPS and INPS offer a powerful combination for monitoring the oxidation state of flat model catalysts during pretreatments, an approach that can equally well be used during catalytic reaction conditions.