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

Abstract

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

Oxidation State and Symmetry of Magnesia-Supported Pd13OxNanocatalysts Influence Activation Barriers of CO Oxidation

“…Figure 1 depicts temperature-programmed reaction (TPR) spectra of carbon dioxide produced at sizeselected Pd 13 clusters supported on a magnesia, MgO(001), surface, under two different experimental conditions. In the first (type I) experiment, the Pd 13 cluster samples (as prepared) were exposed to oxygen ( 16 O 2 ) and carbon monoxide ( 13 CO) at 120 K prior to the TPR experiment.…”

“…Through the correlation of first-principles computational modeling with the TPR data, we find that cryogenic O 2 treatment produces highly symmetric Pd 13 O 4 clusters while asymmetric Pd 13 O 6 forms when exposed to O 2 at 370 K. In our simulations, the spectrum of activation energies for reaction steps along the reaction pathways depends on the overall symmetry of the cluster−reactants complex. Highly symmetric structures of the cluster/reactant complex increase the activation energy of the catalytic reaction steps, whereas asymmetric cluster/reactant structures can accelerate the process.…”

“…Auger electron spectroscopy (AES), metastable helium impact electron spectroscopy (MIES), and ultraviolet photoelectron spectroscopy (UPS) were employed to verify the cleanliness and characterize the electronic states of the oxide layer. The Pd 13 clusters were produced using a high frequency (120 Hz) laser vaporization source. 17 The cluster beam was guided by a set of ion optics through differentially pumped vacuum chambers, where the positively charged clusters were deflected by a custom-made, electrostatic ion bender and focused into a quadrupole mass-selecting unit (ABB-Extrel; mass limit 4000 amu).…”

“…17 The cluster beam was guided by a set of ion optics through differentially pumped vacuum chambers, where the positively charged clusters were deflected by a custom-made, electrostatic ion bender and focused into a quadrupole mass-selecting unit (ABB-Extrel; mass limit 4000 amu). The mass-selected Pd 13 cluster beam is further guided by a set of Einzel lenses and finally softlanded onto the MgO surface. Neutralization of the clusters upon deposition either occurs at surface defects or via charge tunneling through the thin films.…”

“…For the first type, the reactivity of the size-selected Pd 13 clusters was measured directly after deposition. In the second type, prior to the reactivity studies the supported clusters were oxidized by annealing them at a given temperature (∼370 K) in an oxygen background of 5 × 10 −7 mbar for 1 min.…”