Associative Adsorption - Dissociative Desorption

This mechanism exists when, for example, a gaseous halide (or oxide) upon adsorption releases a halogen (oxygen) atom and desorbs when regenerated. It successfully describes the TC behavior of the chlorides of Pu, Cr, Ru and Ce, and recently also Bk [115] in gases of various compositions. The first observation of volatilization of plutonium chloride tracer in the atmosphere of chlorine in tracer 

The resulting formula is presented here in the style of Eq. 5.54 to show the change due to the chemical mechanism. So, 

Mechanisms of this type seem to also occur under certain conditions in thermochromatography of platinum metal oxides. In particular, the reaction 

Rather extensive studies have been devoted to the reaction chromatography of volatile hydroxides. They are characteristic for elements of groups six to eight and were experimentally detected for a TAE - seaborgium. Vahle, et al. [113,117] developed the corresponding equations for Mo and W they considered the process  

They attempted its Monte Carlo simulation by the appropriately modified approach described in Sect. 4.3.2. The change replaced ro by an effective value r 1 (much longer than ro). The latter depends on the frequency of collisions of the adsorbed Mo03 with H20 molecules, and so on the concentration of the water vapor (see Eq. 2.7 for a) and entropy change in the reaction. Then the reaction sojourn time is  

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The dissociative adsorption - associative desorption type of reaction also takes place when Ln or An trichlorides are chemically volatilized into the carrier gas containing a rather large concentration of A12C16 vapors. Chromatography evidently proceeds due to the equilibria like ... 

Adsorption requires the O2 molecule to find a pair of adsorption sites and desorption requires two adsorbed G atoms to be adjacent. Hill [15] and Kisliuk [18,19] discuss lattice statistics and the probability of find-Ing pairs of sites in two-dimensional arrays presented by the regular arrangement of surface atoms illustrated in Figure 5.16. Boudart and pjega-Mariadassou [3],and Hayward andTrapnell [13], describe how the probability of finding pairs of sites is used to develop rate expressions on surfaces. When a bimolecular surface reaction occurs, such as dissociative adsorption, associative desorption, or a bimolecular surface reaction, the rate in the forward direction depends on the probability of finding pairs of reaction centers. This probability, in turn, depends... 

Here we shall be concerned with the interaction of inacming diatomic molecules (H-/ 0.) with either types of potential energy wells The molecular InteractJjon (responsible for elastic and direct-inelastic scattering with extremely short residence times of the irpinglng molecules in the potential) and the chemisorptive interaction (leading to dissociative adsorption and associative desorption, reflectively, and associated with H (D) atoms trapped in the chemisorption potential for an appreci le time). 

One of the most significant recent insights in surface chemical dynamics is the idea that the principle of detailed balance may be used to infer the properties of a dissociative adsorption reaction from measurements on an associative desorption reaction.51,52 This means, for example, that the observation of vibrationally-excited desorption products is an indicator that the dissociative adsorption reaction must be vibrationally activated, or vice versa the observation of vibrationally-cold desorption products indicates little vibrational promotion of dissociative adsorption. In this spirit, it is... 

Figure 3.1. Schematic of bond making/breaking process considered in this chapter (a) atomic adsorption/desorption/scattering, (b) molecular adsorption/desorption/scattering, (c) direct dissocia-tion/associative desorption, (d) precursor-mediated dissociation/associative desorption, (e) Langmuir-Hinschelwood chemistry, (f) Eley-Rideal chemistry, (g) photochemistry/femtochemistry, and (h) single molecule chemistry. Solid figures generally represent typical intial states of chemistry and dashed figures the final states of the chemistry.

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... 

They also first proposed the use of detailed balance to relate dissociative adsorption to associative desorption. In hindsight, both the experiments and the PES derived by them to fit the experiments were in significant quantitative error, but this in no way minimizes the major contribution of this early work to the development of reactive gas-surface dynamics. 

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. 

Each of the various processes of adsorption may have desorptions of the reverse forms, for example, dissociative adsorption may have as its reverse, associative desorption. However, the process of chemisorption may not be reversible [ 1.2.2(c)]. Desorption may lead to species other than that adsorbed, for example, ethane dissociatively adsorbed on clean nickel gives little or no ethane upon desorption, 1-butene dissociatively adsorbed to methylallyl and H on zinc oxide gives mainly 2-butenes upon desorption, and some W03 may evaporate from tungsten covered with adsorbed oxygen. 

Hydrogen transport through Pd and Pd-based alloys comprises the next steps [30,31]. The H2 molecules during adsorption are dissociated on top of the metal surface, giving a proton to the interstitial sites and an electron to the metal conduction band (see Section 2.4.2). The second step is the diffusion of atomic H, since the proton will be surrounded by an electron cloud [32], through the bulk of the metal. Finally, an associative desorption process of H2 molecules occurs from the metal surface at the other end of the membrane. 

Mechanistically, the transformation of the N-NDR into an HN-NDR can be explained by the fact that adsorbed halides inhibit the dissociative adsorption of H2O2. The decrease in the reaction current due to the loss of PtOH or the formation of upd-H upon a negative voltage shift is overcompensated by the increase in current density due to the desorption of halide ions. Sustained periodic oscillations appear under potentiostatic as well as galvanostatic conditions in the presence of halides [57] (Fig. 23). The oscillations that are associated with the NDR in the upd-H region were termed oscillations D, those connected to the autocatalytic adsorption of H2O2 oscillations C. 

Before a detailed presentation of the ab initio dynamics simulations, first the fundamental difference between atomic and molecular adsorption on the one hand and dissociative adsorption on the other hand has to be addressed. Then I will briefly discuss the question whether quantum or classical methods are appropriate for the simulation of the adsorption dynamics. This section will be followed by a short introduction into the determination of potential energy surfaces from first principles and their continuous representation by some analytical or numerical interpolation schemes. Then the dissociative adsorption and associative desorption of hydrogen at metal and semiconductor surfaces and the molecular trapping of oxygen on platinum will be discussed in some detail. 

DISSOCIATIVE ADSORPTION AND ASSOCIATIVE DESORPTION OF HYDROGEN AT METAL AND SEMICONDUCTOR SURFACES... 

The extensive surface reconstruction in the presence of N has implications for our discussion of the recombination process, since we must consider whether N2 forms from recombination on the unreconstructed Cu(l 1 1) surface or is formed by decomposition of copper nitride islands. In the latter case N recombination may either leave the local Cu atoms in a metastable (100) arrangement or else recombination might be associated with substantial motion of the Cu atoms as they relax from the nitride adsorption geometry. If N recombination occurs at nitride islands then the dynamics of recombinative desorption will sample a phase space which is completely different to that for dissociation on clean flat Cu terraces, making it impossible to relate these two processes by detailed balance. This is the behaviour of H recombination on Si where the large change in the Si equilibrium geometry induced by H adsorption ensures that the adsorption and desorption processes sample very different channels [13]. 

As mentioned above, some molecules like H2 and O2 can adsorb dissociatively to occupy two surface sites upon adsorption. The reverse reaction is known as associative desorption. In these cases, the rate of adsorption now depends on the number of pairs of available surface sites according to ... 

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