Isotherms for chemisorption

Thus we have an expression relating the current density (/) to the cell potential (V) in terms of the equilibrium surface coverage (0o) and the concentration of oxygen at the surface of the electrode (r. o2o ) To complete the derivation, Langmuir s isotherm for chemisorption is introduced to relate the equilibrium surface coverage to the concentration of the adsorbed species ... 

Figure Bl.26.1. Sorption isotherm for chemisorption of hydrogen on palladium film at 273 K (Stephens S J...

Surface coverage by each species follows the Freundlich isotherm for chemisorption. 

Figure 3.12. Isobar and isotherms for chemisorption of hydrogen on supported platinum cataly-jt5,122,126,135,136 The left-hand part shows the variation in the amount adsorbed (expressed as H/Pts) at a fixed pressure, and the right-hand part the corresponding variations with pressure at fixed temperature (see text for full description).

In considering isotherm models for chemisorption, it is important to remember the types of systems that are involved. As pointed out, conditions are generally such that physical adsorption is not important, nor is multilayer adsorption, in determining the equilibrium state, although the former especially can play a role in the kinetics of chemisorption. 

Plots of an amount of material adsorbed versus pressure at a fixed temperature are known as adsorption isotherms. They are generally classified in the five main categories described by Brunauer and his co-workers (4). In Figure 6.2 adsorbate partial pressures (P) are normalized by dividing by the saturation pressure at the temperature in question (P0). Type I is referred to as Langmuir-type adsorption and is characterized by a monotonic approach to a limiting amount of adsorption, which presumably corresponds to formation of a monolayer. This type of behavior is that expected for chemisorption. 

Fig. 2. (a) Magnetization-volume isotherms for the chemisorption of hydrogen and of benzene on kieselguhr-supported nickel at 150° C (16). (b) Average number of bonds formed by benzene adsorbed on nickel-silica as a function of temperature (17). From J. Amer. Chem. Soc. 79, 4637 (1957) 83, 1033 (1961). Copyright by the American Chemical Society. Reprinted by permission of copyright owner. 

These and other data (10) show that hydrogen chemisorption is operationally of two types Type I chemisorption which is removed by evacuation for 15 min at room temperature, and type II chemisorption which is not removed by evacuation at room temperature even after several hours. The type I chemisorption appears to be independent of the amount of type II chemisorption (compare runs 3 and 5). Figure 2 show s an isotherm for type I adsorption, as defined. This is a typical curve for chemisorption and suggests that type I chemisorption occurs on sites corresponding to roughly 5% of the BET Vm value. (The designation type I and type II chemisorption was chosen in preference to fast and slow because not all of the type II chemisorption is slow. For example, the amount of adsorption in curve 1 of Fig. 1 is 0.154 cm3/gm after 2 min. We would estimate at least one-third of this adsorption is type II. Thus, some type II irreversible chemisorption is quite rapid.)... 

Poisoning is caused by chemisorption of compounds in the process stream these compounds block or modify active sites on the catalyst. The poison may cause changes in the surface morphology of the catalyst, either by surface reconstruction or surface relaxation, or may modify the bond between the metal catalyst and the support. The toxicity of a poison (P) depends upon the enthalpy of adsorption for the poison, and the free energy for the adsorption process, which controls the equilibrium constant for chemisorption of the poison (KP). The fraction of sites blocked by a reversibly adsorbed poison (0P) can be calculated using a Langmuir isotherm (equation 8.4-23a) ... 

Figure 5. Chemisorption isotherms for DHPz at Au and Pt at pH 0 and pH 7. Experimental conditions were as in Figures 1-3.

There is a relationship between yield of the extract and the saturation sorption or imbibition of solvent that is independent of the rank coal or the particular amine solvent (Dryden, 1951). An adsorption isotherm for ethylenediamine vapor on an 82% carbon coal exhibited three main features (1) chemisorption up to 3 to 6% adsorbed, (2) a fairly normal sorption isotherm from the completion of chemisorption up to a relative pressure of at least 0.8, and (3) a steeply rising indefinite region near saturation that corresponded to observable dissolution of the coal. 

In the conceptions underlying Langmuir s adsorption isotherm the heats of adsorption are constant and independent of the amount that is adsorbed. We learned, in Sec. IX, that this does not hold for chemisorption and we shall, therefore, derive an expression based on the logarithmic adsorption isotherm [Eq. (71) or (74)]. 

The apparatus for hydrogen chemisorption was a conventional gas volumetric system which is described by Benson and Boudart [4], Approximately lg of catalyst was used, and the reduction of the samples was carried out in hydrogen, 25 crtr/(min, g cat.), at 519 C for four hours before evacuation. The samples were then cooled and the adsorption experiments performed at 25 °C The amount of chemisorbed hydrogen was determined as the difference between the linear parts of the isotherms for the total and reversible adsorption, extrapolated to zero pressure,... 

An exaggerated emphasis on heats of chemisorption has probably been harmful in the proper understanding of the role of chemisorption in surface phenomena. Thus the marked nonuniformity of all surfaces with respect to heats of chemisorption has led to rather elaborate treatments where models of surface heterogeneity (statistical distribution of energy sites) or, less successfully, specific forces of interaction between adsorbed species have been invoked to explain the non-Langmuirian adsorption isotherms. For instance the Frumkin isotherm can be obtained with a linear variation of heats of adsorption with coverage, and the Freundlich isotherm is attributed to an exponential variation of heats of adsorption. 

In the present work we examine the microporosity of a TSLS complex formed from synthetic imogolite and natural montmorillonite. Nitrogen adsorption and desorption isotherms are reported and analyzed in terms of microporous volume and surface area. Also, the adsorption isotherm for an organic adsorbate, m-xylene, is reported. Preliminary FTIR results for the chemisorption of pyridine and catalytic studies of the dealkylation of cumene suggest that TSLS complexes are promising microporous acids for shape selective chemical conversions. 

Fig. 2 shows equilibrium isotherm for adsorption of glutathione on GhF. The experimental equilibrium isotherm for adsorption of glutathione is independent of the initial concentration of glutathione. Glutathione may be adsorbed by chemisorption and the equilibrium isotherm may be expressed by the Langmuir equation ... 

Before examining the NMR, it is useful to consider some adsorption and desorption studies of this standard catalyst. Hydrogen chemisorption data have been modeled by a Langmuir isotherm for pressures between 1 and 80 Torr (Fig. 26a) (80) but also by a Temkin isotherm at pressures between 0.1 and 100 Torr (Fig. 26b) (48). The Langmuir isotherms in Fig. 26a give H/Pt ratios for monolayer adsorption of 1.17 (circles) and 1.06 (triangles). The break point in the Temkin isotherm of Fig. 26b corresponds to H/Pt = 0.82 (48). (The part of the isotherm below 0.1 Torr has not often... 

Fig. 26. Volumetric hydrogen chemisorption isotherms for EuroPt-1. The three sets of data arc independent, (a) The two lines are for slightly different pretreatments the circles provide the more representative data. The lines represent Langmuir isotherms (b) the line represents a Temkin isotherm. In the common pressure range, both fits are equally plausible, [(a) Reproduced with permission from Bond and Lou (80) (b) reproduced with permission from Ohosters et al. (48). Copy.right 1996 Royal Society of Chemistry. ...

The relationship of 0 or In 0 to V follows from the adsorption isotherm that applies to the chemisorption of the intermediate a Langmuir relation does not always apply, as we indicate in Section X on reaction order of elec-trocatalytic processes. For example, the Temkin isotherm for the condition 0.1 < 0 < 0.9, in the form... 

The Nature of Weak Chemisorption on Pd.—Several workers have investigated adsorption and absorption of H on Pd in order to determine the surface areas of both unsupported and supportedpowders. Lynch and Flanagan have presented adsorption isotherms for weak chemisorption on Pd. At 273 K the isotherm shows no tendency to saturate and the excess sorption is >0.34 monolayers. 

We attempted to measure an adsorption isotherm for the adsorption of ethylene at 25° on a chromia activated at 337°. At 2, 8, 18, 28, 36, and 48 torr, the weight rapidly reached a steady state at 60 torr, the weight very slowly increased at 90 torr it increased rather rapidly. In 12 minutes at 90 torr, 0.5 rnolecules/100 of extra ethylene adsorbed. Adsorption from about 25 to 50 torr is linear with pressure. If one extrapolates the linear region to zero pressure as we did with carbon monoxide, one computes a chemisorption of 0.6 molecules/100 A. Most of the ethylene adsorbed at 90 torr is not removable by helium flushing at either 25 or 100°. 

For chemisorption study, the adsorption isotherm (first isotherm) was obtained in each case by plotting the amount of O2 adsorbed at 300 °C against the O2 equilibrium pressure, Peq, up to 250 mm Hg. The resorption isotherm (second isotherm) was measured after outgassing for one hour at the same temperature. The total chemisorption values were taken as the difference between the first and the second isotherms. To minimize adsorption on the support and maximize adsorption on the supported complex, the total chemisorption values on pure support were subtracted from the total chemisorption producing the so called net adsorption values (a ) on the supported complex as expressed in m mol O2 g" CoPc. These values were used to calculate the degrees of dispersion (D), the chemisorption stoichiometries and specific surface areas of supported CoPc phase [18,19],... 

All chemisorption experiments were single point measurements at 8.10 Pa. By measuring the adsorption isotherms from 10 -10 Pa for a few catalysts, it was checked that a relative comparison of the thus obtained chemisorption values was as justifiable as any other method based on other measuring points, or on extrapolation of measuring points to zero pressure, as advocated by Benson and Boudart (24). No corrections were made for chemisorption on the bare supports as such, because this was found negligible. 

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