Molecular adsorption on metals

SSIMS has been applied to the study of molecular adsorption on metal surfaces [116], catalytic surfaces [117], surfaces of HF- and NH4F-treated silicon [118], and many others in which surface chemistry is important. 

Recently, Berweger and Raschke considered the polar phonon mode selection rules for TERS for the determination of nanocrystallographic information from solids using TERS as discussed in Sect. 4.2 [93]. This is a very different situation given that the location and orientation of the hot spot relative to the sample and excitation are well characterized and could be extended to the study of molecular adsorption on metals and other surfaces in the future. 

This chapter is organized as follows. First, in sect. 2, we consider the surfaces of metals. In sect. 2.1 we describe the structure of unreconstructed clean metal surfaces and then proceed, in sect. 2.2, to consider the reconstructed surfaces. The surface structure of ordered and disordered metallic alloys is described in sect. 2.3. In sect. 2.4 we describe the surface structures associated with atomic adsorption on metals and in sect. 2.5 we consider molecular adsorption on metals. The structure of semiconductor surfaces is... 

In contrast to these systems of molecular adsorption on metal surfaces, the L-B techniques can be used to fabricate the supported monolayers [40-42]. Although the order of molecules in the L-B films can be quite good, this is potentially a nonequilibrium and difficult procedure, which is largely restricted to polyfunctional molecules bearing only one polar substituent [37]. Moreover, the L-B films on substrates usually have no good thermal stability. Thus, a stable monolayer with a high surface-free energy is difficult to form with this technique [8, 37, 41, 42]. 

Of these, the most extensive use is to identify adsorbed molecules and molecular intermediates on metal single-crystal surfaces. On these well-defined surfaces, a wealth of information can be gained about adlayers, including the nature of the surface chemical bond, molecular structural determination and geometrical orientation, evidence for surface-site specificity, and lateral (adsorbate-adsorbate) interactions. Adsorption and reaction processes in model studies relevant to heterogeneous catalysis, materials science, electrochemistry, and microelectronics device failure and fabrication have been studied by this technique. 

A discussion along this line has been made in regard to the orientation of the hydrogen molecule in the dissociative adsorption on metals 82>. Thus, the interpretation of the function of heterogeneous catalysis on a molecular basis is no longer beyond our reach. The important role of LU MO in the process of polarographic reductions has also been discussed... 

Fig. 14 Schematic depiction of a molecular adsorption on the surface of a metal and corresponding changes in the electronic structure of the molecule, is Fermi level (according to [61])...

Amorphous (rapidly quenched) metals and alloys have been investigated as catalysts (Schlogl, 1985). It has been found that adsorption characteristics of carbon monoxide on metallic glasses are different from those on crystalline materials. For example CO is found to dissociate readily on the surface of Ni76Bi2Si,2 metglass, but it is always molecularly adsorbed on metallic nickel (Prabhakaran Rao, 1985). 

Figure 3.14. Schematic representation of experiment detecting hot electrons created by atomic/ molecular adsorption on a thin metal film. is the Schottky barrier created by the metal/ Si interface. From Ref. [86].

Before a detailed presentation of the ab initio dynamics simulations, first the fundamental difference between atomic and molecular adsorption on the one hand and dissociative adsorption on the other hand has to be addressed. Then I will briefly discuss the question whether quantum or classical methods are appropriate for the simulation of the adsorption dynamics. This section will be followed by a short introduction into the determination of potential energy surfaces from first principles and their continuous representation by some analytical or numerical interpolation schemes. Then the dissociative adsorption and associative desorption of hydrogen at metal and semiconductor surfaces and the molecular trapping of oxygen on platinum will be discussed in some detail. 

Molecular adsorption of CO and NO is relatively strong on many metal surfaces. These adsorbates may undergo both dissociative and molecular adsorption on the same surface depending on the experimental conditions. 

Hydration may modify the reactivity if the metallic sites are not available the acidity of the surface hydroxyl is less than that of the exposed metallic cations, H2O molecular adsorption on hydrated TiO2(H0) surface can result from these weak interactions (H-bonding with the surface hydroxyl groups)[22]Lindan, 1998 223], In this case, there is no direct interaction and the vibration frequencies should be recalculated on another model. Let us note that the basicity of the surface also varies in the case of Ti02(l 10) the lone pairs of a surface hydroxyl are less reactive than those of a naked surface oxygen for MgO it is the reverse the hydrated surface becomes more basic. 

The results presented in the above case are rather general and allow to establish the rule that molecular adsorption on a metal should normally be accompanied by a significant reduction of the work function. When no substantial reduction of work function is seen or when the work function is even observed to increase this can be taken as a strong indication for the presence of chemical interactions, i.e. transfer and chemical bond formation. 

The quantitative analysis of adsorption on metal clusters, as discussed earlier, can be sensitive to the cluster chosen to model the chemisorption site. In a systematic series of studies, we examined the adsorption of various atomic and molecular adsorbates on different metal surfaces. We found that it was important to optimize 1) size, 2) structural configuration, 3) spin state, and... 

The classical example of electrocatalysis is the electochemical evolution of molecular hydrogen from aqeous electrol)d es and the reverse process, the electrol)dic oxidation of molecular hydrogen on metal electrodes. In these reactions, hydrogen atoms are formed as intermediates and remain adsorbed on the electrode. The strength of the adsorption becomes decisive for the rate of the reaction. 

Only a few direct measurements of ionic or molecular adsorption on the surface of a passivated metal exist. As an example, in situ radiotracer studies... 

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