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

Journal article, 2015

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 degrees C in Ar/O-2, achieving fully oxidized Fe2O3 and CuO. Subsequently, the samples were heated to temperatures between 100 and 400 degrees C in pure H-2, and the resulting change in oxidation state was measured. Fe2O3 was found to be reduced to Fe3O4 at 225 degrees C and to a mixed state of FeO and metallic Fe at 275 degrees C. The corresponding temperatures for Cu-promoted Fe catalysts were 100 degrees C lower. In the absence of FeOx, Cu was reduced to metallic Cu, via Cu2O, at temperatures between 125 and 175 degrees C. In addition to the XPS measurements, INPS was used to obtain more detailed insight into the reduction process, both in pure H-2 and in wet H-2, containing 0.8 vol % H2O. These in situ experiments show that the presence of H2O increases the reduction temperature by 36 degrees 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 H-2. 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.