In order to understand the promoting mechanism of alkali additives, we have studied potassium overlayers on TiO 2(1 1 0). The surface was prepared by thermal segregation of K from the bulk to the surface above 750-800 K. The bulk diffusion of the Ti/O ions is required for the migration of large K ions inside the lattice. STM and LEIS revealed that segregated potassium forms small clusters of 1-2 nm on the surface, containing also oxygen. These clusters are located preferentially on the one-dimensional defect sites (Ti 2O 3 strings) of the (1 × 1) rutile surface and on the Ti 2O 3 rows of the (1 × n) reconstructed surfaces. According to XPS, the potassium on the surface after segregation at 1000 K is only partially ionized and the Ti 2p region is dominated by the Ti 4+ component. XPS and LEIS provided evidence of a very clear preference for the Rh clusters to grow near 300 K not on the potassium structures but on the potassium-free parts of titania surface. This finding may imply the absence of a direct contact between Rh and K at low Rh coverages. Nevertheless, evaporation of Rh on K/TiO 2(1 1 0) results in more cationic K sites, due to an electron transfer from K to Rh through titania. The decoration and encapsulation of Rh nanoparticles by TiO x proceed also in the presence of potassium. The capping layer does not contain potassium. At large Rh cluster sizes, the wheel-like structure of the cover layer could be identified with a structure found on top of Rh crystallites formed on the K-free titania surface. The presence of potassium stabilized CO on Rh nanoparticles, which is attributed to the indirect charge transfer from potassium structures to rhodium (long range effect).
- Alkali promoter
- Potassium segregation
- Strong metal support interaction
- TiO (1 1 0)
ASJC Scopus subject areas
- Physical and Theoretical Chemistry