Structure of Adsorbed Organometallic Rhodium: Model Single Atom Catalysts

We have determined the structure of a complex rhodium carbonyl chloride [$\mathrm{Rh}(\mathrm{CO}{)}_2\mathrm{Cl}$] molecule adsorbed on the ${\mathrm{TiO}}_2(110)$ surface by the normal incidence x-ray standing wave technique. The data show that the technique is applicable to reducible oxide systems and that the dominant adsorbed species is undissociated with Rh binding atop bridging oxygen and to the Cl found close to the fivefold coordinated Ti ions in the surface. A minority geminal dicarbonyl species, where Rh-Cl bond scission has occurred, is found bridging the bridging oxygen ions forming a high-symmetry site.

Figure 1

(a) Rhodium, (b) carbon, and (c) chlorine NIXSW responses from as deposited saturation coverage of $\mathrm{Rh}(\mathrm{CO}{)}_2\mathrm{Cl}$ under CO. All graphs plotted normalized intensity versus photon energy in eV.

Figure 2

(a) The proposed adsorbed local structure of the $\mathrm{Rh}(\mathrm{CO}{)}_2\mathrm{Cl}$ molecule majority species with Rh bound to a single bridging row oxygen and (b) the minority geminal dicarbonyl species in square planar geometry bridging the bridging row oxygen. C-Rh bond lengths are assumed to be 1.9 Å and are shown in positions defined by the experimentally determined heights above the surface. Surface oxygen and titanium ions are large (red) and small (gray) spheres in the surface, respectively, while chlorine (yellow) is the largest ion; rhodium (gray) is the central ion which is bound to two (black) carbon atoms in both images. All radii are scaled to the appropriate element and ionic state; oxygen atoms in carbonyl groups are not shown for clarity.