Gases adsorbed on the surface

The effects are due to surface band bending induced by charge from the adsorbed molecules. It is known that water acts in this way, with 

The data show that the effect of adsorbed molecules is more complicated than is described above, as there are fast and slow processes with opposite conductance changes (see Fig. 9.13(a)). One possible explanation is that the band bending induces defect states, through the defect equilibration process described in Chapter 6. This might explain the slow decrease of the conductance after the initial 

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For measuring infrared absorption spectra of gases adsorbed on the surfaces of metal single crystals or polycrystalline foils, one uses reflection absorption infrared... 

The top layer of a (111) surface actually has sixfold symmetry, but the rotational symmetry of the top layers together is threefold. Since the near surface region can influence where gases adsorb on the surface and the LEED intensities exhibit threefold rotational symmetry at normal incidence, the (111) surface will be considered to have threefold rotational symmetry. Although most of the adsorption studies have been carried out on fee and bcc crystals, there have been several studies reported on hep crystals. For hep metals the basal or (0001) plane is the surface most frequently studied by LEED investigations and it is the most densely packed plane having threefold rotational symmetry. 

The activation energies and entropies of activation of desorption of gases adsorbed on the surfaces of solids can be determined by measuring the rates of desorption at several constant temperatures. In the temperature-programmed desorption the temperature is usually continuously varied and the rates of desorption are simultaneously influenced by the manner in which the process depends both on time and on temperature. In general, an analysis of the process can be carried out only if the variation of the temperature with time, the heating schedule, is of a simple functional form. At least for simpler systems, two heating schedules fulfill this requirement (1) a linear temperature increase with time, i.e.,... 

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. 

Semiconductor Flammable and other gases Air Two small coda of platinum wire are fixed side by side within a bead of semiconductor material of diameter 2-3 mm. The sample gas flows over the bead and molecules of gas are adsorbed on the surface of the semiconductor—thereby changing the resistance of the semiconductor as measured between the two coib. One coil is also employed as a heater to ensure that the adsorption is reversible. The device is sensitive but has poor selectivity. Dependent on individual design. Flammable gas detector. Inexpensive... 

The above relationship predicts a monotonic increase in p with increasing temperature as shown in Figure 3 for three different gases. Hence, the maximum amount of gas (by the Gibbs definition) adsorbed on the surface of the adsorbent can be attained at a lower pressure by operating close to the critical temperature of the adsorbed gas. Application of even higher pressures then p will result in a large increase in the... 

Another type of reaction is the oxidation of complex ions, for example in the case of aluminum electrolysis. These reactions are rather complicated and occur in several steps. During the first step, the discharge of oxygen ions takes place the oxygen atoms formed are adsorbed on the surface of the carbon anode and molecules of C02 are then obtained. These molecules of C02 can react with the anodic carbon, and a certain proportion of CO may appear. All these gases form bubbles which escape. Usually, the anodic processes have a high overvoltage. 

The components of air are chemisorbed on the fresh titanium and are thus permanently fixed by chemical bonds. Noble gases (helium, argon, etc.) are pumped at a much slower rate, as they are only physically adsorbed on the surface and are held principally by being plastered over by titanium atoms. An ion pump that has pumped a considerable quantity of argon is subject to a condition called argon instability, in which bursts of argon are released at intervals into the system other noble gases show a similar effect. 

Surface-enhanced exciton dissociation models are based on the assumption that the absorption of a photon creates an exciton that diffuses to the surface where it dissociates into a free electron-hole pair, or a free and deeply trapped carrier of opposite sign, through an interaction with a donor or acceptor center associated with the surface. Such a process was first proposed by Lyons (1955). Evidence for this was largely based on early studies of anthracene, where the photogeneration efficiency increases with the absorption coefficient. Further evidence was that gases, particularly O2, adsorbed on the surface of anthracene crystals significantly change the photoconductivity, even for weakly absorbed radiation. 

We may postulate for an adsorbed gas an equation of state that is slightly more complicated than Eq. 3.7 and which takes into account the finite surface occupied by a molecule adsorbed on the surface [18]. The following equation is similar to an equation of state for gases which would take into account the volume occupied by the gas molecules [i.e., P V — b) = RT, with P, pressure]. 

There is a long history of calculations of adsorption potentials for simple gases adsorbed on the exposed low index Miller planes of ionic crystals, especially alkali halides (see the review [26] and references therein). The total interaction potential energy between an adsorbed molecule and the surface of a solid is generally expressed as a sum of dispersion, repulsion, induction, and electrostatic contributions (see, e.g.. Ref. [27]) ... 

The previous part showed that inverse gas chromatography is a very useful tool in the investigation of long-chain aliphatic alcohol monolayers adsorbed on the surface of porous silica gel. Now a simple theoretical model of the adsorbed layer that can be used to analyse the experimental data obtained by inverse gas chromatography is considered. The model is based on the theory of adsorption of simple gases on solid surfaces and, initially restricted to fully localized adsorption [36-38], was extended to treat also long chain molecules [39]. 

In the applications where high pressure of residual gases (nonpermeate stream) is required, membranes are well suited. The transport steps involved in a membrane system are very similar to the transport steps involved in a heterogeneous reaction system where first the reactant should be adsorbed on the surface, then diffused into the pellet, and finally the products are desorbed. The permeating component of the gas stream is sorbed in the membrane, diffused through the membrane, and then desorbed on the other side of the membrane. Fundamental research in the area of development of suitable membranes for different gas processing and petrochemical-related industry applications is being carried out in various academic institutions. 

The sulfur dioxide adsorbed on the surface can be stripped by other gases. Experiments were carried out to determine the effectiveness of... 

A novel method to improve the amount of hydrogen that can be stored in composite cylinders involves cryo-compression of the gas. This depends on the fact that gases are denser at cryogenic temperatures than at ambient temperature. Also, they adsorb more readily on to materials with high surface areas. A medium-pressure composite cylinder (20—40 MPa) is filled with activated carbon as an adsorbent and then enclosed in an insulated jacket of liquid nitrogen (77 K). Compressed hydrogen is introduced into the cylinder where it cools, densifies, and is adsorbed on the surface of the carbon. The resultant storage capacity is several times that of the same cylinder at ambient temperature. 

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