Adsorption activation barrier

In many cases adsorption is not activated ( a = 0) and so the desorption energy is a direct measure of the adsorption heat. As discussed in the previous chapter, however, adsorption is often direct and activated, and desorption methods can be used to determine Ea. Two main types of experiment are used to measure the adsorption activation barrier — angle resolved desorption and time of flight measurements. When desorbing over a net barrier molecules enter the gas phase with excess energy commensurate with the top of the barrier. Since desorption usually involves breaking a surface-molecule bond in the reaction coordinate there is then an enhanced distribution of... 

Fig. 22. The effect of sulphur poisoning on the speed profile of molecules detected by their time of flight. In the absence of S, Lhe distribution is essentially Maxwellian showing desorption with no net adsorption activation barrier, while after poisoning the speed distribution is sharp and indicative of fast molecules desorbing over a net activation barrier to adsorption. From Comsa et al. (19S0).

Some further details are the following. Film nonideality may be allowed for [192]. There may be a chemical activation barrier to the transfer step from monolayer to subsurface solution and hence also for monolayer formation by adsorption from solution [294-296]. Dissolving rates may be determined with the use of the radioactive labeling technique of Section III-6A, although precautions are necessary [297]. 

When an atom or molecule approaches a surface, it feels an attractive force. The interaction potential between the atom or molecule and the surface, which depends on the distance between the molecule and the surface and on the lateral position above the surface, detemiines the strength of this force. The incoming molecule feels this potential, and upon adsorption becomes trapped near the minimum m the well. Often the molecule has to overcome an activation barrier, before adsorption can occur. 

Whereas the adsorption energies of the adsorbed molecules and fragment atoms only slightly change, the activation barriers at step sites are substantially reduced compared to those at the terrace. Different from activation of a-type bonds, activation of tt bonds at different sites proceeds through elementary reaction steps for which there is no relation between reaction energy and activation barrier. The activation barrier for the forward dissociation barrier as weU as for the reverse recombination barrier is reduced for step-edge sites. 

However, the temperature at which a molecule desorbs also reflects how strongly it is bound to the surface [Eq. (12)]. The activation energy in Eq. (12) equals the heat of adsorption provided the adsorption of the molecule occurred without an activation barrier. This condition is usually fulfilled. 

Since the recombination step (c) does not principally differ from a recombination of two H or D atoms to the respective hcmonuclear imole-cule there is no reason to assume a special activation barrier for a H and a D atom to recombine to the HD molecule. Therefore the rate of the HD production is solely determined by the rates of adsorption of H and D, respectively (as long as the reaction is adsorption-controlled, i.e., at hi enou tenperatures), or by the rate of desorption of HD (provided the reaction is desorpticai-oontrolled, i.e., at low temperatures). If wie deal with the first case only we may w/rite ... 

A molecular oxygen state is the most likely to be involved, it would require a barrier of only 67 k.f mol 1 and is exothermic a hydroperoxide state is formed together with NH2(a). When the heats of adsorption of ammonia and oxygen are taken account of, then according to Neurock44,45 there is no apparent activation barrier to N-H activation. 

The rate enhancement for cyclohexane dehydrogenation observed for submonolayer copper deposits may result from changes in the geometric (6) and the electronic (8) properties of the copper overlayer relative to bulk copper. Alternatively, the two metals may catalyze different steps of the reaction cooperatively. For example, dissociative adsorption on bulk copper is unfavorable because of an activation barrier of approximately 5 kcal/mol (33). 

It is important to note that as early as 1931, the density of electronic states in metals, the distribution of electronic states of ions in solution, and the effect of adsorption of species on metal electrode surfaces on activation barriers were adequately taken into account in the seminal Gurney-Butler nonquadratic quantum mechanical treatments, which provide excellent agreement with the observed current-overpotential dependence. 

The rate enhancement observed for submonolayer Cu deposits may relate to an enhanced activity of the strained Cu film for this reaction due to its altered geometric and electronic properties. Alternatively, amechansim whereby the two metals cooperatively catalyze different steps of the reaction may account for the activity promotion. For example, dissociative Hj adsorption on bulk Cu is unfavorable due to an activation barrier of approximately 5 kcal/mol . In the combined Cu/Ru system, Ru may function as an atomic hydrogen source/sink via spillover to/from neighboring Cu. A kinetically controlled spillover of Hj from Ru to Cu, discuss above, is consistent with an observed optimum reaction rate at an intermediate Cu coverage. 

Initial reconstruction caused by flame armealing is stopped when the surface is cooled in the atmosphere, though not in water. The rate of transition from unreconstructed to reconstructed surface is determined by the height of the activation barrier [348], especially at the room temperature. Reconstruction may be removed by adsorption of atoms and molecules [349], since unreconstructed, and thus, more open surface, interacts with the adsorbates stronger than does the densely packed surface. Therefore, the removal of reconstructed surface proceeds from the less to the more energetically favored state [348]. Reconstruction coupled with the formation of more dense surface structure may lead to quite a strong increase in the number of surface atoms. For instance, the Au(100)-(1 X 1) Au(100)-(hex) reconstruction is accompanied by the increase in the number of surface atoms by 24%. 

In the present chapter, we have attempted to illustrate how surface bonding and catalytic activity are closely related. One of the main conclusions is that adsorption energies of the main intermediates in a surface catalyzed reaction is often a very good descriptor of the catalytic activity. The underlying reason is that we find correlations, Brpnsted-Evans-Polanyi relations, between activation barriers and reaction energies for a number of surface reactions. When combined with simple kinetic models such correlations lead to volcano-shaped relationships between catalytic activity and adsorption energies. 

Figure 6.8. Gibbs energy profiles of a proton discharge process resulting in a metal-hydrogen bond formation. The difference in the Gibbs energy of adsorption of hydrogen between metal 1 to metal 2 lowers the activation barrier for the discharge and makes metal 2 the electrocatalytically more favorable (active) electrode material.

The calculations led to predictions of adsorption sites for the nonpolar compounds that are in good agreement with those determined experimentally. The cation site is preferred over the window site. The activation barrier for movement between two cation sites was calculated to be 30 kJ/ mol and that for movement between a cation and a window site 43 kJ/mol. Experimental measurements of activation barriers to diffusion of benzene in faujasites are between 17 and 27 kJ/mol (24). The calculations provide strong support for the mechanism of surface-mediated diffusion for all guest molecules in the limit of infinite dilution and 0 K. The MEPs show that molecules slide along the wall of the supercage, with the plane of the aromatic ring almost parallel to the pore wall. 

Fig. 12. Schematic representation of the effect of adsorption of intermediates on the activation barrier for an electrode reaction. Both initial and final states are taken at the same free energy.

In this chapter, recent results are discussed In which the adsorption of nitric oxide and its Interaction with co-adsorbed carbon monoxide, hydrogen, and Its own dissociation products on the hexagonally close-packed (001) surface of Ru have been characterized using EELS (13,14, 15). The data are interpreted In terms of a site-dependent model for adsorption of molecular NO at 150 K. Competition between co-adsorbed species can be observed directly, and this supports and clarifies the models of adsorption site geometries proposed for the individual adsorbates. Dissociation of one of the molecular states of NO occurs preferentially at temperatures above 150 K, with a coverage-dependent activation barrier. The data are discussed in terms of their relevance to heterogeneous catalytic reduction of NO, and in terms of their relationship to the metal-nitrosyl chemistry of metallic complexes. 

---