Adsorption of atoms

Lennard-Jones J E and Devonshire A F 1936 Diffraction and seiective adsorption of atoms at crystai surfaces Nature 137 1069... 

In the adsorption of atoms and molecules we need to distinguish between two cases direct or indirect adsorption. The direct adsorption process is one in which the particle collides with the surface and stays at the point of impact as an adsorbed... 

By combining the results of the Newns-Andersons model and the considerations from the tight binding model it is now possible to explain a number of trends in surface reactivity. This has been done extensively by Norskov and coworkers and for a thorough review of this work we refer to B. Hammer and J.K. Norskov, Adv. Catal. 45 (2000) 71. We will discuss the adsorption of atoms and molecules in separate sections. 

In this section, we will investigate the surface structure of the electrode in the potential range before a surface or bulk oxide starts forming, and will restrict ourselves to the adsorption of atomic oxygen only (not OH ) [Jacob and Scheffler, 2007]. Furthermore, in our simulations, we assume a single-crystal Pt(lll) electrode, which will be compared with the experimental CV curve (Fig. 5.9) for poly crystalline Pt. This simplification is motivated by the fact that our interest here is to describe the general behavior of the system only. 

The same Chapter contains results of studies of effects of adsorption of atom particles as well as simplest free radicals on electric conductivity of semiconductor zinc oxide films. 

The combined use of the method of semiconductor sensors and that of molecular beams enabled us to investigate adsorption of atom, molecular and cluster particles of metals on metal oxides. 

This kinetics was observed during adsorption of atom nitrogen [123]. As for the stationary values of electric conductivity a it can be deduced from expressions (2.88) and (2.92), (2.93)... 

These expressions describe the kinetics and the value of response of electric conductivity in oxide adsorbent during adsorption of atomic hydrogen. In general case they are applicable for adsorption of any radical particles of the donor type. However, in each specific case any of the processes of schematics (2.114) controlled by specificity of each system [129] may be a dominant process controlling both the kinetics and the stationary value of electric conductivity. 

N.N. Savvin, Mechanism of Adsorption of Atoms, Radicals and Some Simple Molecules on Metal Oxides According to the Data on Electroconductivity and IR-spectroscopy, PhD (Chemistry) Thesis, Moscow, 1980... 

To resolve the problem applying methods of collimated atom beams, equilibrium vapour as well as radioactive isotopes, the Hall effect and measurement of conductivity in thin layers of semiconductor-adsorbents using adsorption of atoms of silver and sodium as an example the relationship between the number of Ag-atoms adsorbed on a film of zinc oxide and the increase in concentration of current carriers in the film caused by a partial ionization of atoms in adsorbed layer were examined. 

It was shown in a number of works [29] that impurity conductivity of thin zinc oxide films are extremely sensitive to adsorption of atoms of various metals (see Chapters 2 and 3). Using this feature of oxide films, we first employed the sensor method to study evaporation of superstechiometric atoms of metals from metal oxide surfaces, zinc oxide in particular [30]. 

Adsorption of atomic, molecular, and cluster particles on metal oxides... 

It is important to mention that antimony is absolutely passive to molecular hydrogen but highly responsive to adsorption of atomic hydrogen [13]. This properties of amorphous films of antimony with adsorbed atoms of hydrogen make them very convenient to study emission of atom hydrogen due to ordering in antimony films. 

The above conclusion was confirmed by control experiment. It is well known [47] that freshly deposited silver films are very efficient in adsorption of atomic oxygen at room temperature. Therefore, the attachment covered by Ag-film similar to that with a Co304-covering should completely stop the flux of oxygen atoms. (The experiment with the known source of oxygen atoms - hot Pt-filament - confirms the efficiency of above attachment). It occurred [44] that in case of particles leaving the Ag surface such an attachment completely shuts down the flux of particles from the surface of the sample under investigation. 

Recently a novel experimental approach using Schottky diodes with ultra-thin metal films (see Fig. 11) makes direct measurement of reaction-induced hot electrons and holes possible. See for example Refs. 64 and 65. The chemical reaction creates hot charge carriers which travel ballistically from the metal film towards the Schottky interface and are detected as a chemicurrent in the diode. By now, such currents have been observed during adsorption of atomic hydrogen and deuterium on Ag, Cu and Fe surfaces as well as chemisorption of atomic and molecular oxygen, of NO and N02 molecules and of certain hydrocarbons on Ag. Similar results have been found with metal-insulator-metal (MIM) devices, which also show chemi-currents for many exothermic surface reactions.64-68... 

Adsorption of atoms on metals may be classified in four types [Benard, 1983] in terms of their electron levels relative to the electron level of adsorbent metal as described in the following and illustrated in Fig. 5-6 and Fig. 5-7 ... 

In this section we treat the bulk and surface properties of metals relevant to the problems of electrochemical deposition. First, we discuss briefly the bulk and electronic structure of metals and then analyze the surface properties. Surface properties of the greatest interest in electrodeposition are atomic and electronic structure, surface diffusion, and interaction with the metal surface (adsorption) of atoms and molecules in solution. 

As alluded to before, the adsorption of atoms and molecules may also induce segregation in alloys. Upon revisiting the thermodynamic behavior of the improved Cu-Ag alloy catalysts for ethylene epoxidation synthesized by Linic et al, (section 2.1) Piccinin et al. calculated that, while in the absence of oxygen Cu prefers to stay in the subsurface layers, oxygen adsorption causes it to segregate to the surface which then phase-separates into clean Ag(lll) and various Cu surface oxides under typical industrial conditions (Fig. 7). This casts doubt on the active state of the previous Cu-Ag catalysts being a well-mixed surface Ag-Cu alloy. 

Initial reconstruction caused by flame armealing is stopped when the surface is cooled in the atmosphere, though not in water. The rate of transition from unreconstructed to reconstructed surface is determined by the height of the activation barrier [348], especially at the room temperature. Reconstruction may be removed by adsorption of atoms and molecules [349], since unreconstructed, and thus, more open surface, interacts with the adsorbates stronger than does the densely packed surface. Therefore, the removal of reconstructed surface proceeds from the less to the more energetically favored state [348]. Reconstruction coupled with the formation of more dense surface structure may lead to quite a strong increase in the number of surface atoms. For instance, the Au(100)-(1 X 1) Au(100)-(hex) reconstruction is accompanied by the increase in the number of surface atoms by 24%. 

As an introduction to the problem of understanding trends in adsorption energies on metal surfaces, consider the adsorption of atomic oxygen on a range of late transition metal surfaces. Figure 4.1 shows calculated energies as a function of the... 

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