Chemisorption isotherm

When the initial slope of a chemisorption isotherm is not large so that monolayer coverage is approached at conveniently measurable pressures, the gravimetric, volumetric or continuous flow methods can be employed. The gravimetric and volumetric methods for determining this type of isotherm are used in the usual manner, provided that no reaction products contribute to the equilibrium pressure. The only essential difference when measuring chemical and physical adsorption using these techniques, is 

When using the continuous flow method, however, some additional versatility is available in chemisorption measurements. For example, when data is required at an adsorbate pressure of 0.1 atm, a 10 % mixture of adsorbate, mixed with an inert carrier gas, is passed through the apparatus with the sample cooled to a temperature at which no chemisorption can occur. Upon warming the sample to the required temperature, adsorption occurs producing an adsorbate-deficient peak that is calibrated by injecting carrier gas into the flow stream. Equation (15.9) is then used to calculate the quantity adsorbed. This process is repeated for each concentration required. Caution must be exercised to avoid physical adsorption when the sample is cooled to prevent chemisorption. Should this occur, the adsorption peak due to chemisorption can be obscured by the desorption peak of physically bound adsorbate when the sample is heated. 

If two inert gases are not available, an empty cell can be calibrated using pure adsorbate and carrier gas. The void volume can then be calculated as the difference between the empty cell volume and the sample volume, presuming that the density of the sample is accurately known. 

For maximum accuracy those portions of the void volume not heated should be maintained at constant temperature to prevent the void volume from varying during the analysis. 

Often chemisorption produces reaction products which are swept out of the sample cell to the detector. In such cases, to prevent obscuring the adsorption signal, it is necessary to remove the reaction products from the flow stream. Depending upon their nature they can often be removed chemically or by condensation in a cold trap. 

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It would seem better to transform chemisorption isotherms into corresponding site energy distributions in the manner reviewed in Section XVII-14 than to make choices of analytical convenience regarding the f(Q) function. The second procedure tends to give equations whose fit to data is empirical and deductions from which can be spurious. 

As this field is very wide, we will discuss first the gases that can be used to study metal dispersion by selective chemisorption, and then some specific examples of their application. The choice of gases, is, of course, restricted to those that will strongly chemisorb on the metal, but will not physically adsorb on the support. Prior to determining the chemisorption isotherm, the metal must be reduced in flowing hydrogen details are given elsewhere. The isotherm measurement is identical to that used in physical adsorption. 

Whereas determination of chemisorption isotherms, e.g., of hydrogen on metals, is a means for calculating the size of the metallic surface area, our results clearly demonstrate that IR studies on the adsorption of nitrogen and carbon monoxide can give valuable information about the structure of the metal surface. The adsorption of nitrogen enables us to determine the number of B5 sites per unit of metal surface area, not only on nickel, but also on palladium, platinum, and iridium. Once the number of B5 sites is known, it is possible to look for other phenomena that require the presence of these sites. One has already been found, viz, the dissociative chemisorption of carbon dioxide on nickel. 

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 one-point modification of a chemisorption method, which is widely used for measurements of Ac. In this case, only one adsorption point of a chemisorption isotherm is measured, and is compared with only one point on a chemisorption isotherm on a reference material (usually, powder [black] or foil). The identity of the chemisorption properties of the active components in supported and pure form is postulated, but very often does not fulfill, making one-point modification an inaccurate procedure, which can hardly be used in scientific studies. For example, studies of supported Rh catalysts by 02 and CO chemosorption have shown that three different blacks of Rh yield three different results [88], The multipoint comparison of chemisorption isotherms shown that only one black had a chemisorption isotherm that had affinity to the isotherm on a supported metal. 

Woods, R., 1996. Chemisorption of thiols on metal and metal sulphide. In J. O M Bockris, B. E. Conway, R. E. White (eds.). Modem Aspects of Electrochemistry. 29 401 - 453 Woods, R., Young, C. A., Yoon, R. H., 1990. Ethyl xanthate chemisorption isotherms andEh-pH diagrams for the copper/water/xanthate and chalcocite/water/xanthate systems. Inter. J. Miner. Process, 30 17 - 33... 

The asymptotic approach of the quantity adsorbed toward a limiting value indicates that type I isotherms are limited to, at most, a few molecular layers. In the case of chemisorption only one layer can be bonded to the surface and therefore, chemisorption always exhibits a type I isotherm.t Although it is possible to calculate the number of molecules in the monolayer from the type I chemisorption isotherm, some serious... 

Type I isotherms (e.g. ammonia on charcoal at 273 K) show a fairly rapid rise in the amount of adsorption with increasing pressure up to a limiting value. They are referred to as Langmuir-type isotherms and are obtained when adsorption is restricted to a monolayer. Chemisorption isotherms, therefore, approximate to this shape. Type I isotherms have also been found for physical adsorption on solids... 

Practically all experimentally obtained data on chemisorption isotherms may, in fact, be expressed in either Eq. (74) or Eq. (76). 

FIGURE 16 Chemisorption on a metal surface. A, Chemisorption isotherm showing approach to monolayer coverage B, typical data from a pulsed chemisorption technique. 

Figure 2. H2 chemisorption isotherms on EUROPT-1 [14] (a) after reduction at 758 K ( >) oxidized before reduction at 758 K...

Figure 3 Hi chemisorption isotherms on EUROPT 1 plotted according to the Langmuir equation [14] (see also Table 1) Symbols as in Fig 2...

Quantitative and qualitative changes in chemisorption of the reactants in methanol synthesis occur as a consequence of the chemical and physical interactions of the components of the copper-zinc oxide binary catalysts. Parris and Klier (43) have found that irreversible chemisorption of carbon monoxide is induced in the copper-zinc oxide catalysts, while pure copper chemisorbs CO only reversibly and pure zinc oxide does not chemisorb this gas at all at ambient temperature. The CO chemisorption isotherms are shown in Fig. 12, and the variations of total CO adsorption at saturation and its irreversible portion with the Cu/ZnO ratio are displayed in Fig. 13. The irreversible portion was defined as one which could not be removed by 10 min pumping at 10"6 Torr at room temperature. The weakly adsorbed CO, given by the difference between the total and irreversible CO adsorption, correlated linearly with the amount of irreversibly chemisorbed oxygen, as demonstrated in Fig. 14. The most straightforward interpretation of this correlation is that both irreversible oxygen and reversible CO adsorb on the copper metal surface. The stoichiometry is approximately C0 0 = 1 2, a ratio obtained for pure copper, over the whole compositional range of the... 

Fig. 12. Carbon monoxide chemisorption isotherms at 25°C on the binary Cu/ZnO catalysts. The labels at the individual isotherms denote the molar composition Cu/ZnO (43).

Chemisorption isotherms generally exhibit a plateau at lower pressures than the micropore filling plateau. This limiting adsorption is due to the completion of a chemically bound monolayer. In our view, these isotherms may be referred to as Langmuir isotherms, even if the mechanism involved may not be strictly in... 

In carbon materials, active surface sites only represent a fraction of the total surface area, called active surface area (ASA). Knowledge of the nature and concentration of the active sites is of paramount importance for a better understanding of the kinetics involved in heterogeneous gas-solid reactions. However, although ASA reveals interesting information about the sample, there is a need for a reliable and standardised method for its estimation. The aim of this work is to compare ASA determination by the usual methods (i.e., gravimetry, TPD) with another method, which is based on pressure measurements in order to perform an oxygen chemisorption isotherm (OCI). The results showed that the OCI method seems to be a valuable and alternative method for ASA determination, as it avoids the main potential source of errors Inherent in the usual methods. 

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. ...

This is a simple second order equation which usually applies to description of dissociative chemisorption from the gas phase on the homogeneous surface. However, Eq. (43a) describes well only an initial part of kinetic chemisorption isotherm for interaction of trimethylchlorosilane on dehydrated pyrogenic silica [113] and mixed alumina-silica and titania-silica [114] surface. At the same time, whole kinetic chemisorption isotherms are described using equations derived for the heterogeneous surface. Thus, high fractional reaction orders in respect to the silica surface OH groups obtained by Hertl and Hair for chemisorption of silanes and siloxanes [106,107] and in other studies [113,114] may be explained by heterogeneity of the oxides surface. 

Heterogeneous surface with the uniform, Gamma—, Beta and sinusoidal distribution functions on rate constants The common integral equation for kinetic chemisorption isotherm on the heterogeneous surface taking into account the distribution function on the apparent rate constants (k) may be written as ... 

Solutions for chemisorption isotherm containing some distribution functions of heterogeneous surface on the rate constants (k), Eq. (53)... 

Figure 5 The six types of International Union for Physical and Applied Chemistry isotherms. The type I isotherm is typical of microporous solids and chemisorption isotherms. Type II is shown by finely divided nonporous solids. Types III and V are typical of vapor adsorption (i.e., water vapor on hydrophobic materials). Types V and VI feature a hysteresis loop generated by the capillary condensation of the adsorbate in the mesopores of the solid. The rare type VI, the step-like isotherm, is shown by nitrogen adsorbed on special carbon.

Dispersion is defined as the fraction of active atoms in surface positions. Higher dispersions are achieved either as monolayers of atomically dispersed material or as very small crystallites. Both exist in practice. In either case, measurement of surface concentration is most easily performed by quantitative probing with selective molecules. There are three common approaches measurement of (I) chemisorption isotherms. (2) reaction titration, and (3) poison titration. 

Dispersion is defined as the ratio of surface atoms to total atoms in the metal crystallites, and it is determined from chemisorption measurements (26,29). A typical hydrogen chemisorption isotherm is shown in Figure 2.4 for a silica-supported ruthenium catalyst containing 5 wt% ruthenium. The quantity H/Ru in the right-hand ordinate of the figure is the ratio of the number of hydrogen atoms adsorbed to the number of ruthenium atoms in the catalyst. The catalyst was treated with a stream of hydrogen in an adsorption cell at 500°C, after which the cell was evacuated and cooled to room temperature for the determination of the isotherm. The adsorption is... 

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