Metal films, adsorption

Madey and co-workers followed the reduction of titanium with XPS during the deposition of metal overlayers on TiOi [87]. This shows the reduction of surface TiOj molecules on adsorption of reactive metals. Film growth is readily monitored by the disappearance of the XPS signal from the underlying surface [88, 89]. This approach can be applied to polymer surfaces [90] and to determine the thickness of polymer layers on metals [91]. Because it is often used for chemical analysis, the method is sometimes referred to as electron spectroscopy for chemical analysis (ESCA). Since x-rays are very penetrating, a grazing incidence angle is often used to emphasize the contribution from the surface atoms. 

Wu M-C, Estrada C A, Corneille J S and Goodman D W 1996 Model surface studies of metal oxides adsorption of water and methanol on ultrathin MgO films on Mo(IOO) J. Chem. Phys. 96 3892... 

Turbadar T 1959 Complete adsorption of light by thin metal films Proc. Phys. See. 73 40-4... 

In this article, we will discuss the use of physical adsorption to determine the total surface areas of finely divided powders or solids, e.g., clay, carbon black, silica, inorganic pigments, polymers, alumina, and so forth. The use of chemisorption is confined to the measurements of metal surface areas of finely divided metals, such as powders, evaporated metal films, and those found in supported metal catalysts. 

We consider the adsorption of a single molecule, j, on a metal film M. The film is deposited on a solid electrolyte, e.g. YSZ, or is partly covered by a promoter, or simply has a significant coverage of adsorbed reactants and products on its surface, so that we may consider (Chapter 5) that an effective double layer is present at the the metal-gas interface (Fig. 6.15). 

Zellner MB, Goda AM, Skoplyak O, Barteau MA, Chen JG. 2005. Trends in the adsorption and decomposition of hydrogen and ethylene on monolayer metal films A combined DFT and experimental study. Surf Sci 583 281-296. 

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... 

The conclusions from this work were (i) that the mechanism that operates is of wide applicability, (ii) that exchange proceeds by either the dissociative chemisorption of benzene or by the dissociation of benzene which has previously been associatively chemisorbed, and (iii) that M values of about 2 indicate that further dissociation of surface-area measurements. Surface areas of metal films determined by the chemisorption of hydrogen, oxygen, carbon monoxide, or by physical adsorption of krypton or of xenon concur... 

Table 4. Initial (A) and integral (B) enthalpy of adsorption of CO on polycrystalline evaporated metal films (kJ mol-1) (Ref.31))...

The enthalpy of adsorption of hydrogen onto evaporated metal films, (M-H) has been measured for a few metals (Table 21b). Attempts112 have been made to calculate 2 (M-H) = 2 (M-H) — Z)(H-H), where if (M-H), the enthalpy of the metal-hydrogen bond, is given approximately by the equation due to Pauling. 

The Adsorption of Hydrogen on Metal Films Partially Covered with... 

In this section a method for the direct calorimetric determination of heats of adsorption on evaporated metal films is described and results for the heals of adsorption of hydrogen on nickel, iron, and tungsten are reported. In all cases the heats of adsorption decrease with the fraction of surface covered in a mode that can satisfactorily be explained by interaction of adsorbed atoms. A criterion for mobility of the adsorbed atoms is developed... 

While the surface of clean metal films appears to be homogeneous with regard to heat of adsorption and surface coverage (the latter within the limits of size of different crystallographic sites), the rate of hydrogenation of ethylene is markedly dependent on the crystal parameter. 

Another gas easy to measure is CO. This does not mean that better agreement has already been achieved here. One group of authors (67) did not find any systematic variation in the CO heat of adsorption with alloy composition of evaporated metal films other authors (65) found a moderate... 

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