Metal Atoms on MgO Where Are They

We have seen above (Sect. 2.3), how many different defect sites exist even on the surface a simple oxide like MgO. Each of these sites can react in a different way with an adsorbed species and is a potential candidate for the nucleation and growth of metal particles. The complexity of the problem is increased by the fact that the concentration of the defects is usually low making their detection by integral surface sensitive spectroscopies very difficult. A microscopic view of the metal-oxide interface and a detailed analysis of the sites where the deposited metal atoms or clusters are bound become essential in order to rationalize the observed phenomena and to design new materials with known concentrations of a given type of defects. 

For the specific case of isolated Pd atoms, a number of these sites have been investigated theoretically and a classification of the defects in terms of their adhesion properties is possible [32]. We first consider the case of Pd interacting with anion sites, Osc, O4C, or Osc. The binding energy of a Pd atom with these sites increases monotonically from f leV (Osc) to f.5eV (Osc) and consequently the distance of the Pd atoms from the surface decreases. This is connected to the tendency of low-coordinated anions on the MgO 

Although this list is far from being complete, it provides strong evidence that in the initial phases of metal deposition the defect centers play a crucial role in stabilizing the metal atoms and in favoring nucleation and growth. It should be mentioned at this point that the use of CO as a probe molecule to check the adsorption site of the metal atom has proved to be very efficient. Its use will be discussed here with a concrete example. 

An indirect yet powerful tool to monitor the metal adsorption sites is to study CO adsorption. CO is widely used as a probe molecule because of its low reactivity and quite sensitive vibrational frequency Small changes in the substrate electronic structure reflect into measurable changes in CO stretching frequency. The study of CO desorption temperature from a metal covered MgO film and of the IR bands associated to the formation of metal-carbonyl complexes provides an useful tool to identify the surface sites involved in the stabilization of the M-CO species [31]. However, for an atomistic view of the sites involved, it is essential to combine the experimental evidences with the results of ab initio calculations. Recently, this has been done for a series of transition metal atoms, Rh, Pd, and Ag, all belonging to the second transition series but characterized by quite different valence structures (Rh, d Pd, di° Ag, d si) [211,212]. 

Thermal desorption spectra (TDS) of CO for the three metal atoms (Ag, Pd, and Rh) deposited on thin MgO(100) films have been compared with the CO-desorption spectra for the clean thin films. CO desorption at 

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