Chemisorption of gases on metals

TOMPKINS, F.C., Chemisorption of Gases on Metals, Academic Press (1978)... 

Books. M. W. Roberts, Chemistry of the Metal-Gas Interface , Oxford University Press, Oxford, 1978 F. C. Tompkins, Chemisorption of Gases on Metals , Academic Press, London, 1978 Experimental Methods in Catalysis Research , ed. R. B. Anderson and P. T. Dawson, Academic Press, London, 1976 Chemistry and Physics of Solid Surfaces , ed. R. Vanselow and S. Y. Yong, CRC Press, Cleveland, Ohio, 1977 Advances in Characterisation of Metal and Polymer Surfaces , ed. L. H. Lee, Academic Press, New York, 1976 K. Tamaru, Dynamic Heterogeneous Catalysis , Academic Press, London, 1978 The Solid-Vacuum Interface , ed. A. van Oostrom and M. J. Sparnay, Surface Sci., 1977, 64 Electron Spectroscopy , ed. C. R. Brundle and A. D. Baker, Academic Press, New York, 1977, Vol. 1 Auger Electron Spectroscopy (Bibliography 1925—1975) , compiled by D. T. Hawkins, Plenum, New York, 1977. 

Tompkins (1978) concentrates on the fundamental and experimental aspects of the chemisorption of gases on metals. The book covers techniques for the preparation and maintenance of clean metal surfaces, the basic principles of the adsorption process, thermal accommodation and molecular beam scattering, desorption phenomena, adsorption isotherms, heats of chemisorption, thermodynamics of chemisorption, statistical thermodynamics of adsorption, electronic theory of metals, electronic theory of metal surfaces, perturbation of surface electronic properties by chemisorption, low energy electron diffraction (LEED), infra-red spectroscopy of chemisorbed molecules, field emmission microscopy, field ion microscopy, mobility of species, electron impact auger spectroscopy. X-ray and ultra-violet photoelectron spectroscopy, ion neutralization spectroscopy, electron energy loss spectroscopy, appearance potential spectroscopy, electronic properties of adsorbed layers. 

B.E.Nieuwenhuys, "Chemisorption of Gases on Metal Films", P.Wissmann,ed. Chapter 8, Elsevier, Amsterdam (1987)... 

The chemisorption of gases on metals has been the subject of particularly intensive investigations, and the available data allow the catal5hic properties of metals to be explained well. Experimentally determined, quahtative orders of catalytic effectiveness are often found in the literature. For example, for the adsorption of hydrocarbons ... 

Tanaka K, Tamaru K. A general rule in chemisorption of gases on metals. J Catal. 1963 2 366. 

Thus the electronic structure of the metals is decisive for their catalytic activity. The transition metals, with their partially filled d orbitals, are particularly good catalysts. These orbitals are responsible for the covalent binding of gases on metal surfaces in chemisorption and catalysis. Whereas transition metals have one or more unpaired d electrons in the outer electron shell, the weakly chemisorbing main group elements have only s or p electrons. It is postulated that unpaired d electrons are necessary to hold the chemisorbed molecules in a weakly bound state, from which they can then be transferred into a strongly bound state. 

There have been many attempts to relate bulk electronic properties of semiconductor oxides with their catalytic activity. The electronic theory of catalysis of metal oxides developed by Hauffe (1966), Wolkenstein (1960) and others (Krylov, 1970) is base d on the idea that chemisorption of gases like CO and N2O on semiconductor oxides is associated with electron-transfer, which results in a change in the electron transport properties of the solid oxide. For example, during CO oxidation on ZnO a correlation between change in charge-carrier concentration and reaction rate has been found (Cohn Prater, 1966). 

We picture the chemisorption of some gases on metals thus ... 

For a surface-catalyzed reaction to occur, chemical bonds must be involved, and so our interest is primarily with chemisorption. Again, some general classifications of various metals for chemisorption of gases are possible, as shown in Table 2.1-3 from Coughlin [26], and similar properties are involved. Note that the transition elements of the periodic table are frequently involved, and this appears to be based on the electronic nature of their d-orbitals. 

Chemisorption of gases over solids is often used for metallic surface area and dispersion of particles on supported catalysts analyses. Table 10.2 presents some properties comparing the physical and chemical adsorptions. 

The work function of a solid is also sensitive to the presence of adsorbates. In fact, in virtually all cases of adsorption the work function of the substrate either increases or decreases the change being due to a modification of the surface dipole layer. The formation of a chemisorption bond is associated with a partial electron transfer between substrate and adsorbate and the work function will change. Two extreme cases are (i) the adsorbate may only be polarized by the attractive interaction with the surface giving rise to the build up of a dipole layer, as in the physisorption of rare gases on metal surfaces and (ii) the adsorbate may be ionized by the substrate, as in the case of alkali metal adsorption on transition metal surfaces. If the adsorbate is polarized with the negative pole toward the vacuum the consequent electric fields will cause an increase in work function. Conversely, if the positive pole is toward the vacuum then the work function of the substrate will decrease. 

The ammonia formafion reaction is involved by many factors. As a surface-catalysed reaction, the adsorption of the gaseous reactants, especially nitrogen, on the catalyst surface or into the structure of the catalyst is concerned. There are a few metal catalysts that can induce chemisorption of N2 on their surfaces. A classification of metals according to their abilities in chemisorption on various gases is presented in Table 18.1. 

As this field is very wide, we will discuss first the gases that can be used to study metal dispersion by selective chemisorption, and then some specific examples of their application. The choice of gases, is, of course, restricted to those that will strongly chemisorb on the metal, but will not physically adsorb on the support. Prior to determining the chemisorption isotherm, the metal must be reduced in flowing hydrogen details are given elsewhere. The isotherm measurement is identical to that used in physical adsorption. 

Some data on the adsorption stoichiometry of various gases on relevant transition metals have been collected in Table 3.7, which illustrates the usefulness of certain molecules for catalyst characterization by chemisorption. Note that Cu as active phase can be measured well with N2O and CO, but not with H2. It is not wise to determine Ni dispersion with CO, due to the possibility of carbonyl formation Ni carbonyls are volatile and poisonous. Note that in Table 3.7, for Rh the H/Me ratio is size dependent. This phenomenon is not restricted to Rh it is common in the chemisorption of metals. 

Concurrent stream of the development of nanomaterials for solid-state hydrogen storage comes from century-old studies of porous materials for absorption of gasses, among them porous carbon phases, better known as activated carbon. Absorption of gases in those materials follows different principles from just discussed absorption in metals. Instead of chemisorption of gas into the crystalline structure of metals, it undergoes physisorption on crystalline surfaces and in the porous structure formed by crystals. The gases have also been known to be phy-sisorbed on fine carbon fibers. 

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