Sites, surface

The Langmuir adsorption isotherm can be derived from the following surface reaction  

Many examples of adherence of experimental data representing adsorption of ions at constant pH to Langmuir equation are reported in Tables 4.1 and 4.2. The apparent adsorption capacity (from the plateau level or from the slope of the linearized Langmuir plot, Eq. (5.5) depends on the pH. 

This result is not surprising, namely, the original Langmuir equation was derived for the case of single-gas adsorption, and adsorption from solution occurs in the presence of solvent molecules and of other solutes that are potential competitors for the adsorption sites. A reaction analogous to Eq. (5.2) occurs for each competitor, and the apparent adsorption capacity for the adsorbate of interest is a result of occupation of surface sites by the competitors. The apparent adsorption capacity, i.e. the plateau in the adsorption isotherm (measured in the presence of competitors) does not have specific physical sense. The concentration of surface sites is seldom considered as a fully adjustable parameter, because there are actual atoms or groups of atoms on the surface of the adsorbent responsible for adsorption. The number of these atoms or groups is proportional to the concentration of the adsorbent (solid to liquid ratio). 

The adsorbate-specific can be found in a saturation experiment, i.e. from the course of the adsorption isotherm. In absence of competing species the adsorption plotted as the function of activity of the adsorbate should level out when 

The opinion that the Ns is sample-specific is not generally accepted. In many publications the Ns is treated as a generic property of different samples having the same chemical composition and crystallographic structure. Attempts to derive the Ng from crystallographic data can serve as an example of such approach. For certain crystallographic structure and face, the surface sites are associated with broken bonds and/or coordinatively unsaturated atoms, and their number (per unit of surface area) can be calculated. Naturally, not necessarily all broken bonds and 

These examples may be related specifically to the information categories in Table 1 under the headings of phase structures, surface structure (from atomic to 5(X) nm scale resolution), surface sites (e.g., structure, chemistry, reactivity and defects) adsorption (e.g., distribution, coverage, monolayer versus multilayer. colloidal, molecular fonii and bonding) and surface reactions (e.g., oxidation. dissolution, precipitation and phase transformation). 

It is also noteworthy [62] that the adsorption of water alone on the pyrrhotite or pyrite surfaces does not result in oxidation. The O l.v spectra are then com- 

Similarly detailed information on the chemistry of atoms at. surface sites and on the relationships of such sites to surface reactivity may be obtained for oxide, silicate, aluminosilicate and other minerals using XPS in a.s.sociation with EXAFS and adsorption studies. The reviews by Bancroft and Hyland [481. and Brown et al. [16], provide many examples of the types of information obtainable for these surfaces. 

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The flow can be radial, that is, in or out through a hole in the center of one of the plates [75] the relationship between E and f (Eq. V-46) is independent of geometry. As an example, a streaming potential of 8 mV was measured for 2-cm-radius mica disks (one with a 3-mm exit hole) under an applied pressure of 20 cm H2 on QT M KCl at 21°C [75]. The i potentials of mica measured from the streaming potential correspond well to those obtained from force balance measurements (see Section V-6 and Chapter VI) for some univalent electrolytes however, important discrepancies arise for some monovalent and all multivalent ions. The streaming potential results generally support a single-site dissociation model for mica with Oo, Uff, and at defined by the surface site equilibrium [76]. 

ESD Electron-stimulated (impact) desorption [148, 149] An electron beam (100-200) eV) ejects ions from a surface Surface sites and adsorbed species... 

TPD Temperature-programmed desorption [171, 172] The surface is heated and chemisorbed species desorb at characteristic temperatures Characterization of surface sites and desorption kinetics... 

Fig. XV-2. Adsorption loops in a cubic lattice. The diagrams depict the surface sites as seen from below. (From Ref. 4.)...

Different types of chemisorption sites may be observed, each with a characteristic A value. Several adsorbed states appear to exist for CO chemisorbed on tungsten, as noted. These states of chemisorption probably have to do with different types of chemisorption bonding, maybe involving different types of surface sites. Much of the evidence has come initially from desorption studies, discussed immediately following. 

The atoms on the outennost surface of a solid are not necessarily static, particularly as the surface temperature is raised. There has been much theoretical [12, 13] and experimental work (described below) undertaken to investigate surface self-diffiision. These studies have shown that surfaces actually have dynamic, changing stmetures. For example, atoms can diflfiise along a terrace to or from step edges. When atoms diflfiise across a surface, they may move by hopping from one surface site to the next, or by exchanging places with second layer atoms. 

Figure A3.10.8 Depiction of etching on a Si(lOO) surface, (a) A surface exposed to Br2 as well as electrons, ions and photons. Following etching, the surface either becomes highly anisotropic with deep etch pits (b), or more regular (c), depending on the relative desorption energies for different surface sites [28].

Figure Bl.22.1. Reflection-absorption IR spectra (RAIRS) from palladium flat surfaces in the presence of a 1 X 10 Torr 1 1 NO CO mixture at 200 K. Data are shown here for tluee different surfaces, namely, for Pd (100) (bottom) and Pd(l 11) (middle) single crystals and for palladium particles (about 500 A m diameter) deposited on a 100 A diick Si02 film grown on top of a Mo(l 10) single crystal. These experiments illustrate how RAIRS titration experiments can be used for the identification of specific surface sites in supported catalysts. On Pd(lOO) CO and NO each adsorbs on twofold sites, as indicated by their stretching bands at about 1970 and 1670 cm, respectively. On Pd(l 11), on the other hand, the main IR peaks are seen around 1745 for NO (on-top adsorption) and about 1915 for CO (tlueefold coordination). Using those two spectra as references, the data from the supported Pd system can be analysed to obtain estimates of the relative fractions of (100) and (111) planes exposed in the metal particles [26].

In the case of chemisoriDtion this is the most exothennic process and the strong molecule substrate interaction results in an anchoring of the headgroup at a certain surface site via a chemical bond. This bond can be covalent, covalent with a polar part or purely ionic. As a result of the exothennic interaction between the headgroup and the substrate, the molecules try to occupy each available surface site. Molecules that are already at the surface are pushed together during this process. Therefore, even for chemisorbed species, a certain surface mobility has to be anticipated before the molecules finally anchor. Otherwise the evolution of ordered stmctures could not be explained. 

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

Type 2 tlie inliibiting species takes part in tlie redox reaction, i.e. it is able to react at eitlier catliodic or anodic surface sites to electroplate, precipitate or electropolymerize. Depending on its activation potential, tlie inliibitor affects tlie polarization curve by lowering tlie anodic or catliodic Tafel slope. 

Henry s law corresponds physically to the situation in which the adsorbed phase is so dilute that there is neither competition for surface sites nor any significant interaction between adsorbed molecules. At higher concentrations both of these effects become important and the form of the isotherm becomes more complex. The isotherms have been classified into five different types (9) (Eig. 4). Isotherms for a microporous adsorbent are generally of type I the more complex forms are associated with multilayer adsorption and capillary condensation. 

Equation 6 shows that the adsorption of component 1 at a partial pressureis reduced in the presence of component 2 as a result of competition for the available surface sites. There ate only a few systems for which this expression (with 5 1 = q 2 = 5 ) provides an accurate quantitative representation, but it provides useful quaUtative or semiquantitative guidance for many systems. In particular, it has the correct asymptotic behavior and provides expHcit recognition of the effect of competitive adsorption. For example, if component 2 is either strongly adsorbed or present at much higher concentration than component 1, the isotherm for component 1 is reduced to a simple linear form in which the apparent Henry s law constant depends onp. ... 

Other species present in the medium can chemisorb strongly (and irreversibly) on surface sites, eg, HPO , F , and NO , displacing some... 

It can be seen from these two factors, ie, particle charge and van der Waals forces, that the charge must be reduced or the double layer must be compressed to aUow the particles to approach each other closely enough so that the van der Waals forces can hold them together. There are two approaches to the accomplishment of this goal reaction of the charged surface sites with an opposite charge on an insoluble material and neutralization of... 

The catalysts with the simplest compositions are pure metals, and the metals that have the simplest and most uniform surface stmctures are single crystals. Researchers have done many experiments with metal single crystals in ultrahigh vacuum chambers so that unimpeded beams of particles and radiation can be used to probe them. These surface science experiments have led to fundamental understanding of the stmctures of simple adsorbed species, such as CO, H, and small hydrocarbons, and the mechanisms of their reactions (42) they indicate that catalytic activity is often sensitive to small changes in surface stmcture. For example, paraffin hydrogenolysis reactions take place rapidly on steps and kinks of platinum surfaces but only very slowly on flat planes however, hydrogenation of olefins takes place at approximately the same rate on each kind of surface site. 

Lateral interactions between the adsorbed molecules can affect dramatically the strength of surface sites. Coadsorption of weak acids with basic test molecules reveal the effect of induced Bronsted acidity, when in the presence of SO, or NO, protonation of such bases as NH, pyridine or 2,6-dimethylpyridine occurs on silanol groups that never manifest any Bronsted acidity. This suggests explanation of promotive action of gaseous acids in the reactions catalyzed by Bronsted sites. Just the same, presence of adsorbed bases leads to the increase of surface basicity, which can be detected by adsorption of CHF. ... 

For a radionuclide to be an effective oceanic tracer, various criteria that link the tracer to a specihc process or element must be met. Foremost, the environmental behavior of the tracer must closely match that of the target constituent. Particle affinity, or the scavenging capability of a radionuclide to an organic or inorganic surface site i.e. distribution coefficient, Kf, is one such vital characteristic. The half-life of a tracer is another characteristic that must also coincide well with the timescale of interest. This section provides a brief review of the role of various surface sites in relation to chemical scavenging and tracer applications. 

Turning to non-metallic catalysts, photoluminescence studies of alkaline-earth oxides in dre near-ultra-violet region show excitation of electrons corresponding to duee types of surface sites for the oxide ions which dominate the surface sUmcture. These sites can be described as having different cation co-ordination, which is normally six in the bulk, depending on the surface location. Ions on a flat surface have a co-ordination number of 5 (denoted 5c), those on the edges 4 (4c), and dre kiirk sites have co-ordination number 3 (3c). The latter can be expected to have higher chemical reactivity than 4c and 5c sites, as was postulated for dre evaporation mechanism. 

Here n indicates an active surface site, and X— indicates die species X adsorbed on an active site. The first reaction allows for the possibility that methane may occupy more than one active site on adsorption. The dik d and fourth reactions show die observed retarding effects of steam and hydrogen... 

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