Activated adsorption

it is necessary to distinguish between true activated adsorption 

Contrary to the view expressed elsewhere [437], activated adsorption can be distinguished by the dependence of sticking probability on surface or gas temperature, depending on the mechanistic circumstances. Both types of information are useful. If adsorption is trapping-dominated, the Tb dependence is given by the King and Wells [46] model where 

If the s versus N profile for a mobile precursor is typically initially independent of coverage, then a typical profile for the case of activated adsorption follows an exponential decay and s can be written [273, 453] 

In this section, the details of thermal desorption from surfaces will be considered. The rate of the process can be represented in an ideal form by the Polanyi—Wigner equation 

It must be stressed that eqn. (72) represents an ideal desorption process, where both v and Ed are coverage-independent parameters. Unfortunately, very few systems behave in this ideal fashion desorption is the reverse process of adsorption and, as has been described above for adsorption, several properties of the adlayer severely affect the kinetics of the basic desorption process. Thus, in the following sections, the effects on desorption kinetics of surface inhomogeneity, changes in desorption mechanism, precursor states and lateral interactions between adspecies, will be considered. The effects which these parameters have are considerable 

The conception that chemisorption requires an activation energy is 

It now appears clear, however, that the rate of adsorption upon metal surfaces, and charcoal also, and the activation energy of adsorption (if this is the rate-determining factor) depends very much upon the cleanliness of the surfaces, previously adsorbed films slowing down the rate considerably. The very detailed work of Taylor and his school on oxide catalysts, which are porous, at present forms the main support for the theory that the rate of activation either of the adsorbed molecules, or of the surface molecules of the solid with which they combine, is the ratedetermining step in the adsorption. 

The evidence to which the greatest weight was originally attached, in support of the theory that the activation energy is responsible for slowr adsorption, was in outline as follows. In a number of cases adsorption is considerable at very low temperatures, diminishing at first with rising 

1 Some part of the dispute as to the reality of activated adsorption seems to arise from the use of that term nearly synonymously with chemisorption . No one now doubts the existence of chemisorption what is in dispute is the amount of activation energy required for this, and how far it accounts for observed effects, particularly the effect of temperature on the speed, nature, and total amount, of adsorption. 3 Trans. Faraday Soc., 28, 318 (1932). 

With the nickel, at —190 to —180° C., adsorption took place almost instantaneously, and was probably van der Waals at higher temperatures it was slow. Above —100° it was all activated . Rather similar 

If now a linear decrease in Qad with coverage is again assumed and equation 5.64 is used then, with the total surface area, S, being normalized to unity, the equation to integrate is  

In the middle range of coverage, where the pressure is assumed high enough so that BqP 1, but still low enough so that BoPe / 1, this equation simplifies to equation 5.67. If dissociative adsorption is assumed, the derived equation is still identical to equation 5.67 [1]. 

To test these various isotherms to determine which best fits the experimental data, it is most convenient to linearize them. Thus for the different isotherms one gets  

It is worth mentioning a commonly observed chemisorption process, i.e., a relatively slow uptake of adsorbate following a rapid initial uptake, which is described as activated adsorption. The rate of uptake frequently obeys the relationship 

if O is assumed to be relatively invariant and 9 is restricted to be not near unity so that the variation in (1 — 9) can be neglected, then, writing the rate in terms of fractional coverage  

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Kay M, Darling G R, Holloway S, White J A and Bird D M 1995 Steering effects in non-activated adsorption Chem. Phys. Lett 245 311... 

In contrast, physical adsorption is a very rapid process, so the rate is always controlled by mass transfer resistance rather than by the intrinsic adsorption kinetics. However, under certain conditions the combination of a diffiision-controUed process with an adsorption equiUbrium constant that varies according to equation 1 can give the appearance of activated adsorption. 

Activated adsorption of reactants and the desorption of the products on the active centers of the catalyst... 

The sticking coefficient at zero coverage, Sq T), contains the dynamic information about the energy transfer from the adsorbing particle to the sohd which gives rise to its temperature dependence, for instance, an exponential Boltzmann factor for activated adsorption. 

There is a complication in choosing a catalyst for selective reductions of bifunctional molecules, For a function to be reduced, it must undergo an activated adsorption on a catalytic site, and to be reduced selectively it must occupy preferentially most of the active catalyst sites. The rate at which a function is reduced is a product of the rate constant and the fraction of active sites occupied by the adsorbed function. Regardless of how easily a function can be reduced, no reduction of that function will occur if all of the sites are occupied by something else (a poison, solvent, or other function). 

On his return to Princeton after the war, Hugh Taylor organized catalytic research at the Frick Chemical Laboratory. He applied high vacuum technique, liquid air cryoscopy to the study of adsorptive characteristics of catalysts, correlating rates of catalytic reactions and rates of adsorption. He introduced the concept of activated adsorption and defended it against all comers. ... 

Regardless of the exact extent (shorter or longer range) of the interaction of each alkali adatom on a metal surface, there is one important feature of Fig 2.6 which has not attracted attention in the past. This feature is depicted in Fig. 2.6c, obtained by crossploting the data in ref. 26 which shows that the activation energy of desorption, Ed, of the alkali atoms decreases linearly with decreasing work function . For non-activated adsorption this implies a linear decrease in the heat of chemisorption of the alkali atoms AHad (=Ed) with decreasing > ... 

In view of the potential-work function equivalence of solid state electrochemistry (Eq. 4.30 or 5.18) and of the fact that for non-activated adsorption, AEd>Pt=0=A AHo,pt, where AHo.pt is the enthalpy of chemisorption of O on Pt, these equations can also be written as ... 

In the same spirit DFT studies on peroxo-complexes in titanosilicalite-1 catalyst were performed [3]. This topic was selected since Ti-containing porous silicates exhibited excellent catalytic activities in the oxidation of various organic compounds in the presence of hydrogen peroxide under mild conditions. Catalytic reactions include epoxidation of alkenes, oxidation of alkanes, alcohols, amines, hydroxylation of aromatics, and ammoximation of ketones. The studies comprised detailed analysis of the activated adsorption of hydrogen peroxide with... 

How can one determine the activation energy for an activated adsorption process ... 

To improve accuracy, usually data are collected at various pressures, followed by the extrapolation of the adsorbed amount of gas to zero pressure. In commercial equipment this is often done in the so-called increasing pressure mode by the stepwise injection of small amounts of gas. Note that these methods can only be used easily for non-activated adsorption (Reuel and Bartholomew, 1984), e.g. for CO chemisorption. 

Alternatively, data points can be collected in the decreasing pressure mode . This procedure is usually applied for the quantification of activated adsorption processes (Reuel and Bartholomew, 1984), such as the adsorption of H2. After the pretreatment of the sample (usually after reduction or reaction, and evacuation for a certain period to remove all the adsorbed surface species) the temperature is lowered to the temperature of measurement. First, a known amount of adsorbate gas is added to the reactor. Subsequently, the pressure in the catalyst compartment is lowered stepwise by expansion of the gas into the repeatedly evacuated reference volume. The adsorbed amount of gas can be calculated for each step. From this procedure, the monolayer capacity of the catalyst can be evaluated. 

Replacement of gas by the nonpolar, e.g., hydrocarbon phase (or oil phase) is used to modify the interactions between molecules in a spread film of investigated long-chain substances [6,15,17,18]. The nonpolar solvent-water interface possesses the advantage over that between gas and water, that the cohesion (i.e., interactions between adsorbed molecules due to dipole and van der Waals forces) is negligible. Thus, at the oil-water interfaces behavior of adsorbates is much closer to ideal, but quantitative interpretation may be uncertain, in particular for the higher chains which are predominantly dissolved in the oil phase to an unknown activity. Adsorption of dipolar substances at the w/a and w/o interfaces changes surface tension and modifies the surface potential of water [15] ... 

There is further emphasis on adsorption isotherms, the nature of the adsorption process, with measurements of heats of adsorption providing evidence for different adsorption processes - physical adsorption and activated adsorption -and surface mobility. We see the emergence of physics-based experimental methods for the study of adsorption, with Becker at Bell Telephone Laboratories applying thermionic emission methods and work function changes for alkali metal adsorption on tungsten. 

In the pharmaceutical industry, surface area is becoming more important in the characterization of materials during development, formulation, and manufacturing. The surface area of a solid material provides information about the void spaces on the surfaces of individual particles or aggregates of particles [5], This becomes important because factors such as chemical activity, adsorption, dissolution, and bioavailability of the drug may depend on the surface on the solid [3,5]. Handling properties of materials, such as flowability of a powder, can also be related to particle size and surface area [4],... 

The development of modem separation techniques has affected the purification procedures employed for D-galacturonanases. Fractional precipitation with ammonium sulfate and with organic solvents are now used only in combination with new separation techniques. To separate fractions having D-galacturonanase activity, adsorption to pectate or calcium pectate gel has been used in several instances.157-207... 

Substituting (54) into (55), we obtain a kinetic equation typical of activated adsorption (with an activation energy M x), although in our case there may be no potential barrier near the surface. This is a result of the increase in the number of adsorption centers on heating. In the usual theories of... 

Adsorption. Adsorption refers to the adsorption of gas on the gas-solid interface only. It may be van der Waal s adsorption or chemisorption, the latter including activated adsorption. 

Activated Adsorption. Activated adsorption—that is, adsorption with a measurable rate of adsorption and a measurable temperature coefficient of rate of adsorption—is a type of chemisorption which is, for instance, found in the adsorption of nitrogen on certain metals at elevated temperatures. The difficulties of deciding whether or not true van der Waal s adsorption exists in cases where the heats of adsorption exceed considerably the heats of condensation will become apparent later in the text. 

Before leaving the nickel experiments, it may be well to refer to the experiments on hydrogen adsorption variously reported in the literature. As an example, the work of Maxted and Hassid (13) had as its main objective the measurement of the slow activated adsorption of hydrogen on reduced nickel oxide catalysts. It has been proved by the foregoing that the slow adsorption is actually absorption. When plotting their data as isobars, as was done in Fig. 9, the similarity between these isobars and those obtained with sintered nickel films is evident. 

The adsorption (chemisorption) of hydrogen on clean metal surfaces is almost always accompanied by absorption of hydrogen into the interior of the structure. This absorption is a slow activated process and has in the past been mistaken for activated adsorption of hydrogen on the surface. 

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