Depth profile analysis of strong metal-support interactions on {Rh}/{TiO2} model catalysts

Strong metal-support interactions (SMSI) are regarded to be responsible for the change in selectivity and activity of group 8 metal catalysts supported on transition metal oxides. To investigate the nature of interaction between the catalyst components, model systems for {Rh}/{TiO₂} catalysts were prepared with a metal loading of the order of one monolayer on top of several hundred Å of TiO ₂. The samples showed a sufficiently high electrical conductivity to be examined by low-energy ion scattering spectroscopy (ISS) and sputtering as well as by additional surface physical techniques. The model catalysts were annealed at several temperatures up to 823 K and depth profiles of the first atomic layers were measured after every step. It is shown that the rhodium layer is encapsulated by an oxide layer at a substrate temperature of about 750 K, regardless of the presence or absence of hydrogen. The encapsulation cannot be reversed by an oxidation-reduction cycle, which is in agreement with an overall reduction of the free interface energy of the system during the transition into the SMSI state.