Monolayer of adsorbed gas

The monolayer of adsorbed gas (Vm) and the BET constant (C) are calculated from the slope and intercept ... 

The structure of a surface layer (be it the surface of a pure solid or a monolayer of adsorbed gas) usually differs from that of the underlying substrate. A shorthand notation exists in which the unit mesh of the surface layer is described in terms of the unit mesh of the layer immediately below it. Some examples are illustrated in Figure 5.17. The prefix C indicates that the unit mesh for the surface layer contains centre atom(s) and R indicates that this unit mesh is rotated by the stated angle with respect to the substrate unit mesh. 

From a practical standpoint, metal-metal friction is probably the most intensely studied system. In fact, the study of metal friction is in reality the study of two topics—metal friction and metal oxide friction—because except in very special (and seldom very practical) systems, a truly clean metal-metal contact is never encountered. In the best of situations, a normal metal surface will be covered with at least a monolayer of adsorbed gas molecules or other contaminants. As a result, frictional forces will be less than would be expected in their absence. 

A gas can adsorb on a solid surface either physically (physisorption) or chemically (chemisorption), the latter involving actual bonding at the surface between the gas molecule and the metal as explained in Section 1.2. While the magnitude of the heat of adsorption (defined below) is often used to differentiate physisorption from chemisorption, the most useful way of distinguishing them is the way in which gas molecules cover the surface only a monolayer of adsorbed gas molecules... 

Alternatively, data points can be collected in the decreasing pressure mode . This procedure is usually applied for the quantification of activated adsorption processes (Reuel and Bartholomew, 1984), such as the adsorption of H2. After the pretreatment of the sample (usually after reduction or reaction, and evacuation for a certain period to remove all the adsorbed surface species) the temperature is lowered to the temperature of measurement. First, a known amount of adsorbate gas is added to the reactor. Subsequently, the pressure in the catalyst compartment is lowered stepwise by expansion of the gas into the repeatedly evacuated reference volume. The adsorbed amount of gas can be calculated for each step. From this procedure, the monolayer capacity of the catalyst can be evaluated. 

The surface area of a solid material is important in that it provides information on the available void spaces on the surfaces of a powdered solid [48]. In addition, the dissolution rate of a solid is partially determined by its surface area. The most reproducible measurements of the surface area of a solid are obtained by adsorbing a monolayer of inert gas onto the solid surface at reduced temperature and subsequently desorbing this gas at room temperature. The sorption isotherms obtained in this technique are interpreted using the equations developed by Brunauer, Emmett, and Teller, and therefore the technique is referred to as the B.E.T. method [49]. The surface area is obtained in units of square meters of surface per gram of material. 

In the physisorption process a gas molecule interacts with several atoms at the surface of the solid. Once a monolayer of adsorbate molecules is formed the gaseous molecule interacts with a surface of the liquid or solid adsorbate. Therefore, the binding energy of the second layer of adsorbate molecules is similar to the latent heat of sublimation or vaporization of the adsorbate. 

Specific surface area per gram of solid Contact angle Moles adsorbed per gram Surface concentration Boltzman s constant Relative equilibrium pressure of adsorbate Monolayer volume of adsorbed gas or vapor per gram of solid... 

With monolayer adsorption, we saw how the saturation limit could be related to the specific surface area of the adsorbent. The BET equation permits us to extract from multilayer adsorption data (by means of Equation (77)) the volume of adsorbed gas that would saturate the surface if the adsorption were limited to a monolayer. Therefore Vm may be interpreted in the same manner that the limiting value of the ordinate is handled in the case of monolayer adsorption. Since it is traditional to express both V and Vm in cubic centimeters at STP per gram, we write (see Equation (7.72))... 

The most well-known method for determining the specific surface area of powders is based on a theory of multimolecular adsorption of gases developed by Brunauer, Emmett, and Teller (BET) (1). The BET method involves the determination of the quantity of a gas which, when adsorbed on the surface of the solid, would completely cover the solid with a monolayer of the gas. 

Although a surface in an ambient environment may appear clean, in reality the surface is covered with a layer or layers of adsorbed species. These species may be either physically or chemically adsorbed to the surface, but in both instances prevent the study of a truly clean surface. Under a vacuum environment, such surface contaminants can be removed into the gas phase by sputtering the surface with energetic ions. Once the surface contaminants have been removed, ultrahigh vacuum conditions are required to keep the sample clean. For example, at a pressure of 1 X 10 Torr, a sample receives a flux of approximately 5 X 10 " molecules s cm . Assuming a surface density of 10 atoms cm and a sticking probability of 1 for the adsorption of the gas phase species, the sample would be covered by one monolayer of adsorbed species in seconds. Conversely, by working at pressures below 1 x 10 Torr, a sample can be easily maintained free of contaminants for times on the order of hours, the time frame required for most surface science experiments. 

Fig. 2.7 The yield (from TPD experiments) of gas phase benzene (C Hg), and cyclohexene (C H q) from a monolayer of adsorbed cyclohexyl (C H ) formed at 100 K on Pt(l 11) and the two Sn/Pt(l 11) alloys. Reprinted with permission from [39]. Copyright 1994 American Chemical Society...

Analysis of Heterogeneity. The monolayer analysis consists of three elements an adsorption isotherm equation, a model for heterogeneous surfaces, and an algorithm such as CAEDMON, which uses the first two elements to extract the adsorptive energy distribution and the specific surface from isotherm data. Morrison and Ross developed a virial isotherm equation for a mobile film of adsorbed gas at submonolayer coverage (6) ... 

A linear form of the Langmuir equation can be obtained if in equation 10.4 is replaced by v/vm, where v is the maximum volume of gas adsorbed per unit weight of adsorbent (representing a monolayer of adsorbed molecules, with all S surface sites occupied), and v is the volume of gas (measured at STP ) adsorbed per unit weight of adsorbent at a pressure, P. Equation 10.4 then becomes... 

If an adsorptive is exposed to an adsorbent, the amount of adsorption will increase with time till the amount of adsorption reaches an equilibrium with equilibrium pressure. The temporal response of the amount of adsorption is the adsorption rate. The amount of adsorption is expressed as the amount of adsorbed gas per unit mass of adsorbent wa (mol g 1) or w (g g-1). If an adsorbate is a gas, it may also be expressed by the converted gas volume of the adsorbate va (STP cm3 g-1) under a standard condition (1 atm, 0 °C). Generally, the equilibrium amount adsorbed tends to be proportional to the surface area A, of the adsorbent. Moreover, coverage 0(= tf/tf m) is frequently seen (nam amount of monolayer absorption). The equilibrium amount adsorbed na is a function of pressure P (or concentration c) and temperature T. 

Pulse chemisorption involves exposing the reduced catalyst to a number of pulses of adsorbate gas from a calibrated loop which are passed over the catalyst in a flow of inert carrier at atmospheric pressure. Early studies employed only a single pulse with a recommended volume of ca. twice the volume likely to be consumed.36 The quantity of gas remaining (that is, not adsorbed) from the pulse is measured by TCD and in a typical experiment the first two or three pulses would be completely consumed, while only fractions of the subsequent peaks would be taken up by the sample. Eventually, the area of the peaks detected reaches a constant level indicating that the sample has reached monolayer capacity. The volume of gas adsorbed is calculated from the sum of the areas of peaks fully consumed plus the areas of the partially consumed peaks by equating the area of a peak eluted where no gas was taken up, with the volume of gas involved in... 

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