High-resolution electron microscopy investigation of metal-support interactions in Rh/TiO2

High-resolution electron microscopy (HREM), combined with digital image processing and computer simulation techniques, has been used to carry out a microstructural characterization of some Rh/TiO2 catalysts reduced at 473, 773 or 823 K. A sample resulting from the reduction of the catalyst at 773 K, reoxidized with flowing O-2 at 673 K and finally reduced at 473 K was also investigated. In contrast to earlier literature reports, the HREM images recorded here suggest the existence of some definite structural relationships between both the rutile and anatase titania microcrystals and the rhodium particles grown on them. Though some micrographs have been interpreted in terms of slightly reduced Magneli phases, our HREM study suggests that the major microstructural feature of the high temperature (773 and 823 K) reduced samples is the formation of an amorphous reduced titania phase, associated with which metal decoration occurs. Some metal sintering is also observed. The reoxidation treatment, although inducing reversal of the decoration process leads to larger metal particles than those of the starting low-temperature reduced catalysts. Note, also, that twinned metal particles are more often observed in the catalyst resulting from the mild reduction of the reoxidized sample.