The Structure of a Rhodium/Titania Catalyst in the Strong Metal-Support Interaction State as Determined by EXAFS

Abstract

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