This work deals with the structural and catalytic properties of well-defined CeOx/Pt(111) and CeOx/Pd(111)-O inverse model catalysts. The studies were performed by X-ray photoelectron spectroscopy (XPS), Auger-electron spectroscopy (AES), scanning tunneling microscopy (STM) and quadrupole massspectrometry (QMS). For these purposes CeOx-submonolayers were deposited on the fcc(111) metal surfaces under various conditions. The analysis of the surface morphology revealed a difference in the growth of the resulting CeOx nanoformations on these substrates. On Pt(111) the CeOx was growing exclusively two-dimensional in the used parameter regime. In contrast a three-dimensional growth of the CeOx nanoislands were observed on the with subsurface oxygen saturated Pd(111)-O system. The Pd(111) surface was saturated with subsurface oxygen before evaporation because longer oxygen exposures leads to a creation of this special type of surface oxide, which can be created during the evaporation process as well. The kinetic studies exhibited a pronounced catalytic promotion of the CO oxidation reaction on both catalytic active substrates caused by the CeOx nanoformations. This observed effect depends on the CeOx coverage. The catalytic promotion was ascribed to enhanced sticking coefficients of the reactants and explaned by a combined spillover-active-border concept on the atomic scale. Kinetic studies in the bistabiltity region of the reaction on Pt(111) have shown first ever two independent bistabilties which was explained by an existence of an intermediate CO phase. The presence of CeOx nanoislands (Î˜ < 1) shifts the corresponding kinetic phasediagram towards higher CO pressures und lower temperatures. The presence of subsurface oxygen caused a significant decrease of the catalytic activity of Pd(111) which is overlapping with the observed catalytic promotion in the CeOx/Pd(111)-O system. During in situ XPS studies of the oxidation state of cerium inside the nanometric CeOx islands could be shown that the oxidation degree depends on the reaction conditions. Using a CeOx/Cu(111)-Modellsystem, which is inactive for the catalytic CO oxidation reaction, it could be shown that the substrate got a major role for the redox abilities of the CeOx nano-formations. Furthermore it was shown that the reduction for the nanometric CeOx cluster occurs via CO-spillover from the fcc(111) metal surface to the oxidic islands. In contrast to this, the oxidation of the CeOx islands goes via direct interaction of the oxygen at the gas-CeOx interface.
CO oxidation, heterogeneous catalysis, chemical surface reaction, Platinum, Palladium, cerium oxide, metal-oxide interface, redo