Adsorption and Sticking

The rate of adsorption of a gas on a surface is determined by the rate of collision between the gas and the surface and by the sticking coefficient  

In equilibrium, the rate of adsorption equals the rate of desorption. 

Concepts of Modem Catalysis and Kinetics. I. ChorkendorfF, J. W. Niemantsverdriet Copyright 2003 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30574-2 

To describe the adsorption, we need to know the sticking coefficient. As discussed in Chapter 3, it can conveniently be expressed in the Arrhenius form  

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For all three surfaces 1 ML is defined as one CO molecule per (1 x 1) unit cell. The surface atomic densities - or number of unit cells per cm - for Pt 211), Pt 311, andPt 411 are0.53x 10, 0.78x 10 and0.31 x 10 atomscm", respectively. Adsorption of CO on Pt 211 appears to proceed in three distinct stages, with an initial heat of adsorption and sticking probability of 185 kJ/mol and 0.76, respectively [4]. 

With its application to the three-way exhaust catalyst, the energetics of CO adsorption on Rh 100 have been studied using SCAC, the first such studies on a surface plane of rhodium [11], Figure 9.5 shows the coverage-dependent heat of adsorption and sticking probability one monolayer is defined as one CO molecule per (1 x 1) unit cell. The heat of adsorption plot shows two distinct regimes the first extending... 

Mention was made in Section XVIII-2E of programmed desorption this technique gives specific information about both the adsorption and the desorption of specific molecular states, at least when applied to single-crystal surfaces. The kinetic theory involved is essentially that used in Section XVI-3A. It will be recalled that the adsorption rate was there taken to be simply the rate at which molecules from the gas phase would strike a site area times the fraction of unoccupied sites. If the adsorption is activated, the fraction of molecules hitting and sticking that can proceed to a chemisorbed state is given by exp(-E /RT). The adsorption rate constant of Eq. XVII-13 becomes... 

Frequently, adsorption proceeds via a mobile precursor, in which the adsorbate diffuses over the surface in a physisorbed state before finding a free site. In such cases the rate of adsorption and the sticking coefficient are constant until a relatively high coverage is reached, after which the sticking probability declines rapidly. If the precursor resides only on empty surface sites it is called an intrinsic precursor, while if it exits on already occupied sites it is called extrinsic. Here we simply note such effects, without further discussion. 

Having estimated the sticking coefficient of nitrogen on the Fe(lll) surface above, we now consider the desorption of nitrogen, for which the kinetic parameters are readily derived from a TPD experiment. Combining adsorption and desorption enables us to calculate the equilibrium constant of dissociative nitrogen adsorption from... 

The primary process initiating dust surface chemistry is the collision of a molecule from the ISM with the surface. The sticking probability is a measure of how often molecules will stick to the dust surface but this depends on the collision energy, the temperature of the grain surface and the nature of the chemical surface itself. The silicate surface is highly polar, at least for a grain of sand on Earth, and should attract polar molecules as well as atoms. The adsorption process can also be reversed, resulting in thermal desorption, both as the reverse of adsorption and by new molecules as the product of surface reactions. 

Adsorption and reaction of NO2. In contrast to the lack of reactivity of CO2 on the clean Au(lll) surface, NO2 is molecularly chemisorbed via its two oxygen atoms on clean Au(lll) at temperatures of 175 K and below to form a 0,0 -nitrito surface chelate with C2v symmetry (12). The NO2 sticking... 

This section introduces the principal experimental methods used to study the dynamics of bond making/breaking at surfaces. The aim is to measure atomic/molecular adsorption, dissociation, scattering or desorption probabilities with as much experimental resolution as possible. For example, the most detailed description of dissociation of a diatomic molecule at a surface would involve measurements of the dependence of the dissociation probability (sticking coefficient) S on various experimentally controllable variables, e.g., S 0 , v, J, M, Ts). In a similar manner, detailed measurements of the associative desorption flux Df may yield Df (Ef, 6f, v, 7, M, Ts) where Ef is the produced molecular translational energy, 6f is the angle of desorption from the surface and v, J and M are the quantum numbers for the associatively desorbed molecule. Since dissociative adsorption and... 

Since both adsorption and desorption experiments have been performed, it is possible to determine if the experiments satisfy detailed balance and probe the same phase space. They do not, probably because sticking at low , is dominated by dissociation at the steps while associative desorption at higher 0N principally measures desorption from the terraces [244]. There is also ambiguity as to whether energy loss to the lattice and e-h pairs is the same in the two different types of experiments. 

In this section will be described the experimental procedures which measure the rate of adsorption and the sticking probability the experimental results will be given and will be interpreted in terms of an energy-level diagram and an activation energy and finally the bearing of these results on catalysis will be discussed. 

In the literature it has frequently been reported that when gases are adsorbed on metals, the first part is adsorbed instantaneously and later parts are adsorbed much more slowly. This slow adsorption has been ascribed to pores or capillaries or to an approximate balance between adsorption and evaporation. Only rarely has the analysis mentioned small sticking probabilities as the chief cause. The present work shows... 

Equations similar to eqns. (5), (6) and (8) were obtained by Zhdanov [104] to describe the monomolecular adsorption and associative desorption and Eley-Rideal s bimolecular reaction. He examined the dependence of the rate constants of these processes on the surface coverages and discussed various approximations applied previously to describe the effect of lateral interaction of adsorbed molecules on the desorption rate constant. He also considered the effect of the lateral interaction on the pre-exponential factor of the rate constants for various processes, and in terms of the "precursor state model, the effect of ordering the adsorbed molecules on the sticking coefficient and the rate constant of monomolecular desorption. 

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