In-situ X-ray diffraction activation study on an Fe/TiO_2 pre-catalyst

摘要

This study focuses on the use of in situ powder X-ray diffraction (PXRD) and quantitative phase analysis using the Rietveld method to monitor the structural properties of a titania-supported iron (10% Fe/TiO_2) pre-catalyst during calcination (oxidation) and activation (reduction) in the temperature range 25-900℃. The TiO_2 oxidation study revealed an increase in anatase particle size before the anatase to rutile phase transformation, lending credibility to the bridging mechanism proposed by Kim et al. [(2007), Mater. Sci. Forum, 534-536, 65-68]. Pre-catalyst oxidation experiments allowed for the determination of a suitable calcination temperature (450℃) of the pre-catalyst in terms of maximum hematite concentration and appropriate particle size. These experiments also confirmed that the anatase to rutile phase transformation occurred at higher temperatures after Fe addition and that anatase was the sole donor of Ti~(4+) ions, which are known to migrate into hematite (Gennari et al., 1998), during the formation of pseudobrookite (Fe_2TiO_5) at temperatures above 690℃. Using the results from the oxidation experiments, two pre-catalyst samples were calcined at different temperatures; one to represent the preferred case and one to represent a case where the pre-catalyst had been excessively heated. Samples of the excessively heated catalysts were exposed to different reducing gas atmospheres (5, 10 and 100% H_2/N_2) and heated in the in situ PXRD reactor, so that diffraction data could be collected during the activation process. The results show that reduction with gases containing low concentrations of H_2 (5 and 10%) led to the formation of ilmenite (FeTiO_3) and we were able to show that both anatase and rutile are consumed in the reaction. Higher concentrations of H_2 led to the formation of magnetite (Fe_3O_4) and metallic iron (Fe~0). We also noted a decrease in the anatase to rutile transformation temperature under reducing atmospheres when compared with the pre-catalyst heat-treatment experiment. A reduced calcination temperature prior to reduction allowed more facile Fe reduction.