Adsorption isotherms dissociative

In the meantime, catalysis science was developed (1915-1940) through the efforts of Langmuir (sticking probability, adsorption isotherm, dissociative adsorption, role of monolayers), Emmett (surface area measurements, kinetics of ammonia synthe-... 

Derive the Langmuir adsorption isotherm for the molecular adsorption of CO on a metal with equivalent adsorption sites. Do the same for the dissociative adsorption of H2, and, finally, for the case when CO and H2 adsorb together on the same surface. 

If a molecule dissociates on being adsorbed, the process is referred to as dissociative adsorption. The Langmuir adsorption isotherm for dissociative adsorption, derived ... 

If a homonuclear diatomic molecule (e.g., H2, 02, N2, etc.) is adsorbed with dissociation into atoms, the adsorption isotherms (121) and (123) give 8 as the functions of partial pressure of atoms in gas phase, pat, that would correspond to dissociation equilibrium at the partial pressure of diatomic molecules, p. These values are linked by... 

It is a well-known fact that upon covalent immobilization at the surface, the dissociation constant of the acid-base indicator changes by as much as 3 pK units. This shift clearly illustrates the dramatic effect that the interphase has on the ionization equilibria. It is perhaps the most serious problem with optical sensors that the surface concentration of any species is related to its corresponding bulk activity value through an adsorption isotherm which, with the exception of Henry s law, is a highly nonlinear and variable relationship. It is also known (Davies and Rideal, 1963) that the surface pH is different from the bulk value due to the electrostatic repulsion. 

What functions to describe a distribution of adsorption centers on non-uniform surfaces are needed for calculation of adsorption isotherm, the adsorption and desorption rates of dissociating molecules, for allowing the lateral interaction between the nearest neighbors ... 

It is generally accepted in the literature that the strongly adsorbed H2 found in a H2 adsorption isotherm is dissociatively chemisorbed. However, the nature of the weakly adsorbed H2 is still unclear physisorbed as molecular H2 or chemisorbed as atomic H. Generally, this weakly adsorbed H2 is considered to be chemisorbed. The main argument to call this species chemisorbed rather than physisorbed is the higher energy of adsorption compared to condensation ( 20 and 8 kJ/mole respectively)1. However, no experimental evidence is given yet to decide upon the nature of the weakly adsorbed H8. 

IIA. The Surface Density of Surfactant. The surface density of surfactant is relevant for both the double layer (caused by the surface charge density generated via the dissociation of the surfactant adsorbed at the interface) and the hydration interaction (caused by the ion pair density of the non-dissociated surfactant molecules). The surface density of an anionic surfactant, I, will be related to the saturation surface density, I , via the Frumkin adsorption isotherm... 

Sorption of Cu(tfac)2 on a column depends on the amount of the compound injected, the content of the liquid phase in the bed, the nature of the support and temperature. Substantial sorption of Cu(tfac)2 by glass tubing and glass-wool plugs was observed. It was also shown that sorption of the copper chelate by the bed is partialy reversible . The retention data for Cr(dik)3, Co(dik)3 and Al(dik)3 complexes were measured at various temperatures and various flow rates. The results enable one to select conditions for the GC separation of Cr, Al and Co S-diketonates. Retention of tfac and hfac of various metals on various supports were also studied and were widely used for the determination of the metals. Both adsorption and partition coefficients were found to be functions of the average thickness of the film of the stationary phase . Specific retention volumes, adsorption isotherms, molar heats and entropy of solution were determined from the GC data . The retention of metal chelates on various stationary phases is mainly due to adsorption at the gas-liquid interface. However, the classical equation which describes the retention when mixed mechanisms occur is inappropriate to represent the behavior of such systems. This failure occurs because both adsorption and partition coefficients are functions of the average thickness of the film of the stationary phase. It was pointed out that the main problem is lack of stability under GC conditions. Dissociation of the chelates results in a smaller peak and a build-up of reactive metal ions. An improvement of the method could be achieved by addition of tfaH to the carrier gas of the GC equipped with aTCD" orFID" . ... 

At low concentrations of A2, the fractional surface coverage is proportional to [ 2], which is quite different than the adsorption isotherm derived above for the case without dissociation. In terms of fractional surface coverage, the net rate of dissociative adsorption is expressed by ... 

In some cases, when the first (discharge) step is rate determining but the surface of the electrode is catalytically active for dissociation of the product of the overall reaction, for example, CI2 or H2 at active Pt, the 6 factor in the rate equation for the discharge step [Eq. (79)] can take appreciable values determined by the partial pressure, P, of the reaction product, for example, H2 in the HER, according to an adsorption isotherm of the form. 

In this section we shall derive for later discussion a formula for the adsorption isotherm with special reference to the degree of dissociation of the adsorbate at the catalyst surface. The general expression for an adsorption isotherm of gaseous molecules 5 is given (31) by... 

The reactants from the gas adsorb to bond to active sites on the catalyst surface as molecules or dissociated atoms. The rate of adsorption is proportional to the partial pressure of reactants and to the fraction of uncovered surface sites tf. More than one type of active site can be present. The adsorption isotherms such as the Langmuir isotherm relate the partial pressure of an adsorbed species to its surface coverage, and the form of this relationship is indicative of the type of adsorption process taking place (see, for more details, Masel, Chemical Kinetics and Catalysis, Wiley, 2001). 

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