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/O 2, achieving fully oxidized Fe 2 O 3 and CuO. Subsequently, the samples were heated to temperatures between 100 and 400 °C in pure H 2, and the resulting change in oxidation state was measured. Fe 2 O 3 was found to be reduced to Fe 3 O 4 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 FeO x, Cu was reduced to metallic Cu, via Cu 2 O, 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 H 2 and in wet H 2, containing 0.8 vol % H 2 O. These in situ experiments show that the presence of H 2 O 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 Fe 2 O 3 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.