How to observe the oxidation of magnesia-supported Pd clusters by scanning tunnelling microscopy

The structure of adsorbed bare and oxidised Pd-9 clusters on a MgO(001) bilayer supported by an Ag(001) surface are studied by density functional theory (DFT). Results for the bare cluster are compared with corresponding calculations of Pd9 on a free-standing MgO(001) trilayer - a system that already reproduces the adsorption properties of thick MgO films. A 3D double trigonal antiprism is compared to a 2D fragment of a Pd(111) plane. In contrast to the findings for gold clusters, the Pd-9 3D ground state on a thick MgO film is still energetically favourable on a thin metal supported MgO film - despite the substantial charge transfer from the metal support to the 2D cluster. Simulated scanning tunnelling microscopy images predict that the oxide formation should be easily detectable for negative bias voltages, since the highest occupied molecular orbitals of both the pristine as well as the oxidised clusters possess a strong Pd(d) contribution localised on the Pd atoms. At positive bias, the nodal structure of delocalised unoccupied orbitals sometimes masks the nuclear positions. [GRAPHICS]