| AVS 57th International Symposium & Exhibition | |
| Surface Science | Monday Sessions |
| Session SS1-MoA |
| Session: | Nanocluster Reactivity |
| Presenter: | Z.W. Chen, Osaka University, Japan |
| Authors: | Z.W. Chen, Osaka University, Japan W.-J. Chun, International Christian University, Japan K. Fukui, Osaka University, Japan |
| Correspondent: | Click to Email |
Choice of a catalyst precursor sometimes affects the activity and selectivity of the catalyst due to local structures of precursors apart from the thermodynamically favored one. We have studied on precursor dependent structure of Rh/Al2O3 catalysts by using STM and X-ray absorption fine structure (XAFS). Our STM results showed that homogeneous Rh clusters can be randomly dispersed on the Al2O3 thin film (5Å) on NiAl(110) by thermal decomposition of [RhⅡ(OAc)2]2 precursors in vacuum. By statistical analyses of density, height, and diameter of the particles as a function of heat treatment temperature, we concluded that the particle typically included two Rh atoms, hence reflected the precursor composition. In situ STM measurements during heating process also showed that the precursors decomposed around 420 K. The Rh clusters were stable against aggregation up to 800 K. These features were quite different from the typical Rh/Al2O3 catalyst prepared from RhCl3 precursor, where particle size was larger and aggregation easily occurred at such high temperature.
In order to determine the local structure of the clusters and elucidate the precursor dependency on the final structures, we performed XAFS measurements for Rh K-edge at NW-10A station in KEK-IMSS-PF. The Rh/Al2O3 catalysts were prepared from three Rh precursors, [RhⅡ(OAc)2]2, RhⅢ(OAc)3, RhCl3 on γ-Al2O3. The precursor RhⅢ(OAc)3 was selected because of its similar local structure to RhCl3. The as-deposited samples were heated to 500 K, 650 K, 800 K, respectively in vacuum and XAFS were measured after each heat treatment.
In the case of RhCl3, only one peak assignable to Rh-Rh bond appeared after heat treatment above 500 K and its coordination number was increased at higher temperature. It indicated that metallic Rh particles formed just after decomposition of RhCl3, and they aggregated at high temperature. In contrast, in the case of Rh acetate precursors, both of [RhⅡ(OAc)2]2 and RhⅢ(OAc)3, the peak assignable to Rh-O bond was predominant. A weak peak of Rh-Rh bond was also observed in the case of [RhⅡ(OAc)2]2. Curve-fitting results suggested that the particle roughly included two Rh atoms each, which was qualitatively consistent with the STM results. In the case of RhⅢ(OAc)3 precursors, Rh species were dispersed as single atoms after decomposition and they agglomerated at above 650 K. We supposed that Rh-O bond was formed in the process of the acetate-containing precursors decomposition stabilized the small Rh particles on the Al2O3 surface.