Chemisorption equilibrium

It might be thought that since chemisorption equilibrium was discussed in Section XVIII-3 and chemisorption rates in Section XVIII-4B, the matter of desorption rates is determined by the principle of microscopic reversibility (or, detailed balancing) and, indeed, this principle is used (see Ref. 127 for... 

It is interesting to note that, although the intrinsic rate of desorption is slower than that of adsorption, both rates were found to be sufficiently fast under our experimental conditions so that the adsorption-desorption process on the Pt surface can be assumed to rapidly equilibrate at all times that is, even a ten-fold increase in both the adsorption and desorption rate constants (while keeping their ratio constant) did not significantly change the predicted step responses. With the assumption of chemisorption equilibrium, Equations (1) and (4) can be combined into the form (35)... 

In heterogeneous catalytic hydrogenations suprafacial (as) addition of hydrogen would be expected, as the transfer of hydrogen atoms from the catalyst surface to the reactant is usually assumed. However, in some Pt catalyzed reactions antarafacial (trans) addition of hydrogen is also observed. The ratio of diastereomeric products formed is determined by the chemisorption equilibrium of the surface intermediates and by the relative rates of hydrogen entrance to the different unsaturated carbon sites. Both effects are influenced by steric factors. 

Figure 5.3 shows how the extent of gas adsorption on to a solid surface might vary with temperature at a given pressure. Curve (a) represents physical adsorption equilibrium and curve (b) represents chemisorption equilibrium. The extent of adsorption at temperatures at which the rate of chemisorption is slow, but not negligible, is represented by a non-equilibrium curve, such as (c), the location of which depends on the time allowed for equilibrium. 

The overall quality of the model is excellent, with a coefficient of determination of 0.987 and a relative standard deviation of the error of 14.5 percent. Nonetheless, the values for K, K2, and K3 are jointly confounded with one another and thus represent only one of many families of values for the parameters that would fit the data virtually equally well. This means that inferences that these parameters really represent chemisorption equilibrium constants are unwarranted, but the model is nonetheless useful for its intended purpose. If it had been desirable to do so, additional experiments could have been run to narrow the joint confidence intervals of these parameters. 

The present article deals primarily with the elucidation of the surface nature of common metallic and oxidic catalysts, and with statistical-mechanical investigations of the chemisorption equilibrium on these catalysts. The surface areas of these catalysts as determined by the Brunauer-Emmett-Teller method have been taken into consideration. It was shown that a number of certain metallic catalysts such as nickel, cobalt, and platinum and also oxide catalysts of the spinel type act as an array of homogeneous active sites. There is no reason to believe that a few limited regions of the surfaces of these catalysts, such as corners, edges, lattice defects, etc. are particularly important for their catalytic activity. This conclusion is in accordance with the poisoning experiments of Maxted et al. There is some evidence that the surfaces of these catalysts... 

It was found that chemisorption equilibrium is rapidly attained in most reacting systems through rapid desorption and readsorption. With a few exceptions, chemisorbed molecules can be regarded as immobile since statistical-mechanical calculations of the chemisorption equilibrium agree well with the experiment if two-dimensional translations and rotations of the chemisorbed molecules are assumed to be nonexistent. The chemisorbed state of di- or triatomic molecules can be molecular or atomic, depending on the nature of the adsorbent. For example, the carbon dioxide molecule is chemisorbed with complete dissociation into its three atoms on metallic surfaces, while on oxidic catalysts it is chemisorbed with only partial dissociation. 

It is well-established that hydrogenolysis is a nore demanding reaction than the other reactions. Therefore one should expect the effect of ensemble control by means of chemisorbed sulfur, as observed in sulfur passivated steam reforming. This was demonstrated by Kayes et a1 who observed an optimum sulfur level in terms of catalyst activity and product selectivity. However, rather high contents of sulfur of about 1000 ppm were required. This is not surprising in view of the chemisorption equilibrium HjS/Pt compared to HjS/Ni as illustrated in Table 1. 

Table 5.5 Fast Chemisorption (equilibrium established in a few seconds).

Broadly speaking then, chemisorption equilibrium constants are expected to cluster around the trend line shown in Figure 9.1 and to be limited on the enthalpy side by the strength of the weakest bond in the adsorbing species. Release of more energy than that is likely to lead to bond breakage. 

A zinc-oxide mass containing copper may take care of traces of sulphur passing through the zinc oxide by establishing the chemisorption equilibrium over copper, which is independent of the presence of water (Table 1.18). The prereformer catalyst establishes the H2S/Ni chemisorption equilibrium at a much lower value than H2S/CU (refer to Section 5.4). However, it is expensive to use the prereformer catalyst as a desulphurisation mass. 

This sequence does not include the chemisorption equilibrium for H2S/Ni, Equation (5.14), linked to an exponential decrease in sulphur removal. Equation (5.24), and this may be the reason for the improved rate of regeneration. 

Kj = chemisorption equilibrium constant for component j C = concentration of active sites in the catalyst... 

With the assumption of chemisorption equilibrium invoked here, the surface reaction becomes the rate determining step, and thus in Eq. (5) can be reasonably approximated by the empirical rate expression [Eq. (12)] which was found to be adequate in describing the steady state behavior of CO oxidation under our operating conditions ( ). That is. 

At p(02) = const, the WF initially sharply increases, followed by a constant WF, which corresponds with chemisorption equilibrium (Figure 4.21). This is possible when the lattice... 

Adsorption thermodynamics and chemisorption equilibrium 6.2.1. Experimental results on adsorption equilibrium... 

The Langmuir model is the basic model for chemisorption equilibrium. It rests on a certain number of assumptions that are thus modified to take more complex assumptions into account. We start from the following assumptions ... 

Thermal dehydroxylatlon of the alumina surface taking into account any physical adsorption or chemisorption equilibrium ... 

The acid monolayers adsorb via physical forces [30] however, the interactions between the head group and the surface are very strong [29]. While chemisorption controls the SAMs created from alkylthiols or silanes, it is often preceded by a physical adsorption step [42]. This has been shown quantitatively by FTIR for siloxane polymers chemisorbing to alumina illustrated in Fig. XI-2. The fact that irreversible chemisorption is preceded by physical adsorption explains the utility of equilibrium adsorption models for these processes. 

As also noted in the preceding chapter, it is customary to divide adsorption into two broad classes, namely, physical adsorption and chemisorption. Physical adsorption equilibrium is very rapid in attainment (except when limited by mass transport rates in the gas phase or within a porous adsorbent) and is reversible, the adsorbate being removable without change by lowering the pressure (there may be hysteresis in the case of a porous solid). It is supposed that this type of adsorption occurs as a result of the same type of relatively nonspecific intermolecular forces that are responsible for the condensation of a vapor to a liquid, and in physical adsorption the heat of adsorption should be in the range of heats of condensation. Physical adsorption is usually important only for gases below their critical temperature, that is, for vapors. 

As is made evident in the next section, there is no sharp dividing line between these two types of adsorption, although the extremes are easily distinguishable. It is true that most of the experimental work has tended to cluster at these extremes, but this is more a reflection of practical interests and of human nature than of anything else. At any rate, although this chapter is ostensibly devoted to physical adsorption, much of the material can be applied to chemisorption as well. For the moment, we do assume that the adsorption process is reversible in the sense that equilibrium is reached and that on desorption the adsorbate is recovered unchanged. 

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. 

---