| A new model of the dynamic structural behavior of supported iron/rhodium bimetallic catalysts was developed, based upon the results from Mossbauer spectroscopy, X-ray diffraction, transmission electron microscopy and differential adsorption microcalorimetry. To summarize, it was determined that two distinctly different metal-phase structures were formed in a graphite-supported Fe/Rh catalyst as a result of different pretreatment steps. Either equilibrium (ordered) bimetallic-alloy crystallites or locally-segregated monometallic crystallites could be produced by different oxidation-reduction sequences. The chemical differences between the surfaces of these distinct metal phases were determined through oxygen adsorption, using an innovative microcalorimeter which was designed and built in this laboratory. The adsorption studies indicated that the behavior of the surface of the supported catalyst particles was also directly dependent upon the pretreatment (gas phase, temperature and time of treatment). In the case of the ordered-alloy phase, the surface of the metal particles consisted on an ordered array of Fe and Rh atoms. In the case of the locally segregated monometallic phases, the surface of the particles was enriched in iron. These results provided insight into the interpretation of past studies and indicated the importance of the many system variables (support material, pretreatment procedure, etc.) on the structure of supported, iron-containing, bimetallic crystallites. The implications of these results and the directions of future studies were also examined. |
