Syngas adsorption on precipitated iron catalysts reduced by H 2, syngas or CO and on those used for high-pressure FT synthesis by in situ diffuse reflectance FTIR spectroscopy

Abstract

The adsorption properties of precipitated Fe catalysts reduced by H 2, syngas or CO and those used for FT synthesis from CO hydrogenation were studied by in situ diffuse reflectance FTIR using high-pressure syngas as probe molecules. High-pressure syngas adsorption on the H 2-reduced sample gives rise to two weak infrared bands at 2013 and 2033 cm −1, revealing the presence of metallic iron species on the surface. On syngas- and CO-reduced iron samples, the adsorption of syngas gives rise to two new bands at 1999 and 2021 cm −1, while the bands at 2013 and 2033 cm −1 are present as well. The bands at 1999 and 2021 cm −1 may be assigned to CO adsorption on iron carbide species. After the syngas-reduced sample is treated in a H 2 flow, the bands at 1999 and 2021 cm −1 disappear in the spectra of syngas adsorption. After the H 2-reduced sample is treated in a syngas flow, syngas adsorption gives rise to four bands at 1999, 2013, 2021 and 2033 cm −1, the same as that on the syngas-reduced sample. These results suggest that the metallic iron and iron carbide species can be transformed into each other under some conditions. FT synthesis on the reduced samples was performed at 250°C and 1.2 MPa and in situ monitored by infrared spectroscopy. The spectra recorded show that, after the reaction is performed for about 30 min, the bands at 1999 and 2021 cm −1 appear and grow with time, indicating that carburization of the metallic iron species has occurred. High-pressure syngas adsorption at room temperature on the samples after performing the FT synthesis for 3 h shows that the bands at 1999 and 2021 cm −1 become very much stronger, suggesting that large amounts of iron carbides have been formed. The following thermal desorption of the adsorbed species in a helium flow at atmospheric pressure indicates that, for H 2-reduced sample, the interaction of the adsorbed CO with iron carbides is strong. For syngas- and CO-reduced samples, however, the interaction is rather weak. The adsorption features of syngas on the H 2-reduced sample are different from those on the syngas- and CO-reduced samples. So the fine iron carbide particles formed during the reduction in syngas or in CO have played an important role in morphological reactions of the iron catalyst during FT synthesis. These fine particles may have inhibited the aggregation of the iron carbide particles formed during the reaction for FT synthesis.