Bulk properties, of metals

In the past the theoretical model of the metal was constructed according to the above-mentioned rules, taking into account mainly the experimental results of the study of bulk properties (in the very beginning only electrical and heat conductivity were considered as typical properties of the metallic state). This model (one-, two-, or three-dimensional), represented by the electron gas in a constant or periodic potential, where additionally the influence of exchange and correlation has been taken into account, is still used even in the surface studies. This model was particularly successful in explaining the bulk properties of metals. However, the question still persists whether this model is applicable also for the case where the chemical reactivity of the transition metal surface has to be considered. 

In the Introduction the problem of construction of a theoretical model of the metal surface was briefly discussed. If a model that would permit the theoretical description of the chemisorption complex is to be constructed, one must decide which type of the theoretical description of the metal should be used. Two basic approaches exist in the theory of transition metals (48). The first one is based on the assumption that the d-elec-trons are localized either on atoms or in bonds (which is particularly attractive for the discussion of the surface problems). The other is the itinerant approach, based on the collective model of metals (which was particularly successful in explaining the bulk properties of metals). The choice between these two is not easy. Even in contemporary solid state literature the possibility of d-electron localization is still being discussed (49-51). Examples can be found in the literature that discuss the following problems high cohesion energy of transition metals (52), their crystallographic structure (53), magnetic moments of the constituent atoms in alloys (54), optical and photoemission properties (48, 49), and plasma oscillation losses (55). 

The real potential, a , of electrons in metals, as shown in Eqn. 2-4, comprises the electrostatic surface term, - ex, due to the surface dipole and the chemical potential term, M., determined by the bulk property of metal crystals. In general, the electrostatic surface term is greater the greater the valence electron density in metals whereas, the chemical potential term becomes greater the lower the valence electron density in metals. 

Band theory has proved enormously successful for calculation of bulk properties of metals. It is less suitable for the determination of local proper-... 

This topic was reviewed in detail by Che and Bennett [10]. In most of the supported metal catalysts, the size of the metal particles varies in the critical range between 1 to lOnm. The first point addresses the question as to below which particle size the metallic properties are lost how small is a metal [24]. The answer is not so simple and depends on the properties we are looking for. Bulk properties of metal, like the melting temperature, are not reached below 10 nm [24]. 

Despite the limitations of empirical potentials, for the last three decades computer simulations have improved the knowledge on physical properties of metals and alloys. In particular, due to the development of empirical interatomic potentials [18,19], it has become possible to describe by the MD technique a great number of solid properties such as recrystallization, structural relaxation, energetic barriers and mixing [23]. The EAM developed by Foiles et al. [18] has successfully described bulk properties of metal and alloys and some surface relaxation and reconstruction features [17,18,24,25], and the empirical potential developed by Ackland and Vitek has been applied successfully to investigate the structure of the noble metal alloys [19] and the deposition of Cu clusters on Au (111) [26] and of Cu and Au clusters on Cu (001) surfaces [27]. 

As for methanol selective oxidation, the authors did not observe a correlation between the TOF and bulk properties of metal oxides such as enthalpy of formation or temperature of reduction. However, the redox TOF showed a strong inverse trend with the temperature of surface decomposition of adsorbed isopropoxy species (Figure 11.12). 

The change in CPD is often interpreted as the change in surface potential, AF. The work function is mainly comprised of two components the chemical potential, which is a purely bulk property of metal, and the surface potential, which is a surface-sensitive parameter. Protein molecules absorbed on a gold surface have a profound effect on the surface potential and hence on the work function. Consequently, any change in work function caused by the protein molecules on the gold surface usually appear as changes in the surface potential ... 

The quantities jXe, pe >

bulk properties of the metal. The quantities O, and of course F, are surface properties which can vary on a metal surface from one crystallographic plane to the other. Such variations are typically on the order of 0.1 eV but can be as high as 0.5 V. The measured work function , of a polycrystalline metal is an average of the d> values on different crystallographic planes. 

The scientific literature abounds in attempted correlations between the catalytic activities, of a series of catalytic electrode metals and some set of bulk properties, of these metals. Such correlations would help in understanding the essence of catalytic action and will enable a conscious selection of the most efficient catalysts for given electrochemical reactions. 

Most of these studies, mainly in the period 1955 to 1970, have been concerned with cathodic hydrogen evolution. Different parameters characterizing the bulk properties of each metal have been adduced, including physical parameters such as electron work funchon, electrical conductivity, hardness, compressibility, temperature of evaporation, and heat of evaporation, and chemical parameters such as the affinity to hydrogen or oxygen. 

Several additional conclusions concerning the nature of the chemisorbed layer can be drawn from the Hall effect measurements (33, 34) The chemisorbed species, together with the surface metal atoms, represent complexes analogical to the ordinary chemical compounds and, consequently, one might expect that the metal atoms involved in these complexes will contribute to lesser extent or not at all to the bulk properties of the metal. Then we should speak about the demetallized surface layer (41). When the Hall voltage was measured as a function of the evaporated film thickness... 

In the frame of the itinerant model, the surface is represented by a potential barrier of various origins and shapes, in most cases treated as onedimensional problem (e.g., 56-60), without taking into account the potential variation in the plane of the surface3 [with the exception of (61) where this effect is qualitatively discussed in connection with the field ionization probability]. Obviously, the nonlocalized model is suitable and often used for the theoretical interpretation of the changes of the bulk properties of the metals caused by the surface effects (the changes of the electrical resistance, magnetic properties, galvanomagnetic effects, etc.). 

It has also to be remembered that the band model is a theory of the bulk properties of the metal (magnetism, electrical conductivity, specific heat, etc.), whereas chemisorption and catalysis depend upon the formation of bonds between surface metal atoms and the adsorbed species. Hence, modern theories of chemisorption have tended to concentrate on the formation of bonds with localized orbitals on surface metal atoms. Recently, the directional properties of the orbitals emerging at the surface, as discussed by Dowden (102) and Bond (103) on the basis of the Good-enough model, have been used to interpret the chemisorption behavior of different crystal faces (104, 105). A more elaborate theoretical treatment of the chemisorption process by Grimley (106) envisages the formation of a surface compound with localized metal orbitals, and in this case a weak interaction is allowed with the electrons in the metal. 

The chemistry of metal oxides can be understood only when their crystal structure is understood. Knowledge of the geometric structure is thus a prerequisite to understanding the properties of metal oxides. The bulk structure of polycrystalline solids can usually be determined by x-ray... 

Association and mobilities are related in a complex way to the bulk properties of the solvent and solute. These properties include the charge density and distribution on the ions and the Lewis base properties, the strength and nature of the solvent molecule dipole, the hydrogen-bonding capability, and the intermolecular structure of the solvent. Some correlations can be made on the basis of mobility and association trends in series such as the halides and alkali metals within a single solvent others can be drawn between solvents for a given ion. It appears that conductance measurements provide a clear measure of the sum of ion-solvent interactions, but that other techniques must be used in conjunction with conductance if assessments of individual contributions from specific factors are to be made. 

Structure and properties of deposits. These can be understood and interpreted on the basis of a variety of surface and bulk analytic techniques and methods that reveal electrical, magnetic, and physical properties of metals and alloys. 

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. 

Fine metal particles have received much attention in recent years from the viewpoints of chemical, physical, and biological interests (1-4). They are one of the most promising advanced materials. Compared with metal oxide or metal salts, metals have the highest electric and thermal conductivity, considerably higher weight and melting point, and usually excellent catalytic properties. These properties of metals cannot be replaced by other materials. Thus, even after the rapid growth of plastic, bulk metals keep their important position as one of the most common raw materials. 

Correlations between catalytic activity and a variety of bulk properties of semiconductors have been reported (i) the average band gap of III-V and II-VI semiconductors and activity towards hydrogenation of isopropanol (ii) enthalpy of oxides and their activity towards oxidation of propylene and (iii) number of d-electrons (and crystal field stabilization energy) or 3rf-metal oxides and their activity towards N2O decomposition. The last correlation, due to Dowden (1972), is important since it provides a connection between heterogeneous catalysis and coordination chemistry of transition-metal compounds. A correlation between the catalytic activity of transition-metal sulphides towards hydrodesulphurization of aromatic compounds and the position of the transition metal in the periodic table has been made by Whittingham ... 

Table 5.1. Adsorption properties of metal monolayers on metal substrates. The clean substrate properties are also given for comparison. Substrates are ordered by lattice type (fee, bcc, hep, cubic, diamond and rhombic). The structures, nearest neighbor distances and heats of vaporization refer to the bulk material of the substrate or the adsorbate. VD, ID and S stand for vapor deposition, ion beam deposition and surface segregation, respectively. TD, WF and TED stand for thermal desorption, work function measurements and transmission electron diffraction, respectively...

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