Reduction of a highly dispersed 2.85 wt % Rh/Ti02 catalyst at 473 K after previous calcination at 623 K resulted in EXAFS primary contributions are due to nearest rhodium (average coordination number of 3.1 and distance of 2.67 A) and neiphbors (coordination 2.5 and distance 2.71 A). These oxygen neighbors originated at the metalsupport interface. average rhodium-rhodium coordination number did not change in the SMSI state produced by reducing the catalyst 673 K. However, the average coordination distance contracted by 0.04 A with an accompanying decrease of the Debye-Waller of the Rh-Rh bond of 0.0012 A2. This is due tQ the fact that in the SMSI state the surface of the metal particles not covered with chemisorbed hydrogen. The SMSI state leads to a structural reorganization of the support in the vicinity the rhodium metal particles. This can be concluded from the appearance of a Rh-Ti bond at 3.42 A in the SMSI state with the fact that the average coordination number of the rhodiumsupport oxygen bonds does not increase. Other of rhodium-oxygen bonds could not be detected with EXAFS in this state. Thus, these results provide no evidence coverage of the metal particle by a suboxide of TiOz in the SMSI state.
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Koningsberger et al. (1986) studied this question.