Indirect Adsorption

The twin problems of cleanliness and structure can now be overcome by the use of single crystals, where both the chemical and physical states of the surface can be monitored using a range of surface spectroscopic techniques. However, single-crystal studies introduce other limitations. In particular the measurements must be carried out under UHV and it is only possible to measure the heats of adsorption indirectly. The most common methods involve either isotherm data and the use of the Clausius-Clapeyron equation or direct analysis of the temperature programmed desorption (TPD) peaks. 

The extensive use of the Young equation (Eq. X-18) reflects its general acceptance. Curiously, however, the equation has never been verified experimentally since surface tensions of solids are rather difficult to measure. While Fowkes and Sawyer [140] claimed verification for liquids on a fluorocarbon polymer, it is not clear that their assumptions are valid. Nucleation studies indicate that the interfacial tension between a solid and its liquid is appreciable (see Section K-3) and may not be ignored. Indirect experimental tests involve comparing the variation of the contact angle with solute concentration with separate adsorption studies [173]. 

The second edition, like the first, is addressed to those workers in academic laboratories or industrial laboratories who are not necessarily specialists in the field of gas adsorption, but whose work is concerned either directly or indirectly with the characterization of finely divided or porous solids. 

It is advantageous to use a low-retentivity carbon to enable the adsorbate to be stripped out easily. When empirical data are not available, the following heat requirements have to be taken into consideration (1) heat to the adsorbent and vessel, (2) heat of adsorption and specific heat of adsorbate leaving the adsorbent, (3) latent and specific heat of water vapor accompanying the adsorbate, (4) heat in condensed, indirect steam, (5) radiation and convection heat losses. 

Another property of importance is the pore volume. It can be measured indirectly from the adsorption and/or desorption isotherms of equilibrium quantities of gas absorbed or desorbed over a range of relative pressures. Pore volume can also be measured by mercury intrusion techniques, whereby a hydrostatic pressure is used to force mercury into the pores to generate a plot of penetration volume versus pres- sure. Since the size of the pore openings is related to the pressure, mercury intrusion techniques provide information on the pore size distribution and the total pore volume. 

Measurements of the adsorption of inhibitors on corroding metals are best carried out using the direct methods of radio-tracer detection and solution depletion measurements . These methods provide unambiguous information on uptake, whereas the corrosion reactions may interfere with the indirect methods of adsorption determination, such as double layer capacity measurements", coulometry", ellipsometry and reflectivity Nevertheless, double layer capacity measurements have been widely used for the determination of inhibitor adsorption on corroding metals, with apparently consistent results, though the interpretation may not be straightforward in some cases. 

Therefore some indirect methods have been worked out to determine the value of ff=0.154,259 In particular (1) salting out of organic compounds from a surface-inactive electrolyte solution, (2) F"" for 1-pentanol or other organic compounds with a high attractive interaction constant a, and (3) dependence of the capacitance minimum on thiourea concentration. It should be noted that indirect estimates based on TU adsorption give... 

This equation has been derived only as a reference for a comparative discussion of data for sd- and d-metals later on. However, the meaning of such a line is that there exists a limit to AX values in the sense that after a given top effect, a further increase in metal-water interaction will not produce higher AX values.6,7 An indirect confirmation of this is given by the observation of a top value in the decrease of 0 upon water adsorption on d-metals from the gas phase.35,36... 

In the case of electrochemically promoted (NEMCA) catalysts we concentrate on the adsorption on the gas-exposed electrode surface and not at the three-phase-boundaries (tpb). The surface area, Ntpb, of the three-phase-boundaries is usually at least a factor of 100 smaller than the gas-exposed catalyst-electrode surface area Nq. Adsorption at the tpb plays an important role in the electrocatalysis at the tpb, which can affect indirectly the NEMCA behaviour of the electrode. But it contributes little directly to the measured catalytic rate and thus can be neglected. Its effect is built in UWr and [Pg.306]

More usually, the participation of adsorbed reactants and intermediates is inferred indirectly from kinetic data (Bockris, 1954). Thus the observation of reactions having a low or zero order with respect to the reactant concentration implies adsorption of the reactant. A reaction scheme... 

In the adsorption of atoms and molecules we need to distinguish between two cases direct or indirect adsorption. The direct adsorption process is one in which the particle collides with the surface and stays at the point of impact as an adsorbed... 

For indirect atomic adsorption, the transition state is that of two-dimensional motion over the surface. 

The reaction coordinate that describes the adsorption process is the vibration between the atom and the surface. Strictly speaking, the adsorbed atom has three vibrational modes, one perpendicular to the surface, corresponding to the reaction coordinate, and two parallel to the surface. Usually the latter two vibrations - also called frustrated translational modes - are very soft, meaning that k T hv. Associative (nondissociative) adsorption furthermore usually occurs without an energy barrier, and we will therefore assume that A = 0. Hence we can now write the transition state expression for the rate of direct adsorption of an atom via this transition state, applying the same method as used above for the indirect adsorption. 

On the basis of entropy changes, why is the direct adsorption of a molecule on a surface site less probable than indirect adsorption through a precursor state ... 

The close similarity between (CO)j species formed, either directly or indirectly, on metal electrodes and the comparable species observed on the same metals following adsorption of CO from the gas phase has already been noted. There is, however, some effect due to the presence of the electrolyte. For example, an Isolated CO molecule in the gas phase, an isolated CO molecule adsorbed onto a Pt surface from the gas phase, and an isolated (CO) g on a Pt electrode at the point of zero charge give rise to bands attributed to C-0 stretching fundamentals at 2143, 2064, and 2030 cm respectively. Thus the metal/... 

The results have been compared with the earlier proposal of a dual-pathway mechanism for Cl oxidation, and, together with previous experimental and theoretical results, summarized in a comprehensive reaction scheme that explicitly includes also the (reversible) exchange between adsorbed species, dissolved product species in the catalyst layer, and similar species in the bulk electrolyte. The traditional dualpathway mechanism, where both the direct and indirect pathways lead to CO2 formation, has beenextended by adding a third pathway that accounts for formation and desorption of incomplete oxidation products. In the mechanistic discussion, we have focused on the role in and contribution to the Ci oxidation process of the formation/desorption and re-adsorption plus further oxidation of incomplete oxidation products. This not only leads to faradaic currents exceeding that for CO2 formation, but may result in additional COad and CO2 formation, via adsorption and oxidation of the incomplete oxidation products. 

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