Adsorption with Dissociation

In certain cases there is evidence that the process of adsorption is accompanied by the dissociation of the molecule on the surface, as shown in mechanism (10.11) 

Since the process of adsorption can be viewed as a reaction between a gas molecule Aj and two reaction sites, the rate of adsorption will be 

The desorption process involves a bimolecular reaction between two adsorbed atoms, and the rate is 

---

The activation energies for various processes can have a wide variety of values. The chemisorption of hydrogen on nickel, for example, has an activation energy of essentially zero at 20° Kelvin. This is a fast adsorption with dissociation of the hydrogen, and thus is a chemical process with no activation barrier. This would be comparable to some reactions between free radicals in the gas phase there are chemical reactions with no activation barrier. I do not see the conceptual difficulty. I am glad that you raised this question since for many years people... 

The dependence of the rate of reaction on H2, which is a product, in equation (3.59) shows that as its concentration increases the rate of reaction increases and so indicates some kind of autocatalysis (product activation). This study suggested that the rate controlling step is methane adsorption with dissociation on active nickel ... 

Using the notation that is the activity-based equilibrium con.stant for the dissociation reaction, develop expressions for- the amount of molecular hydrogen adsorbed as a function of the equilibrium constants and the hydrogen partial pressure for the two cases (adsorption without dissociation and adsorption with dissociation). How would you discern which process was occurring if you had experimental data on the total hydrogen adsorption as a function of its partial pressure ... 

Even the first observations of damage of this kind in the development of ammonia synthesis showed that the effects of pressurized hydrogen on steel are both chemical and physical (Bosch 1933). The physical effect presumably takes place in different stages. Since molecular hydrogen does not diffuse in the steel, the first step must be surface adsorption with dissociation at elevated temperature. In this process, the metallic wall acts as a catalyst. With pure, dry pressurized hydrogen, for example, dissociation produced by yielding phenomena on the steel surface can supply critical quantities of diffusible hydrogen even at room temperature. 

When a molecule adsorbs to a surface, it can remain intact or it may dissociate. Dissociative chemisorption is conmion for many types of molecules, particularly if all of the electrons in the molecule are tied up so that there are no electrons available for bonding to the surface without dissociation. Often, a molecule will dissociate upon adsorption, and then recombine and desorb intact when the sample is heated. In this case, dissociative chemisorption can be detected with TPD by employing isotopically labelled molecules. If mixing occurs during the adsorption/desorption sequence, it indicates that the mitial adsorption was dissociative. 

Various Langmiiir-Hinshelwood mechanisms were assumed. GO and GO2 were assumed to adsorb on one kind of active site, si, and H2 and H2O on another kind, s2. The H2 adsorbed with dissociation and all participants were assumed to be in adsorptive equilibrium. Some 48 possible controlling mechanisms were examined, each with 7 empirical constants. Variance analysis of the experimental data reduced the number to three possibilities. The rate equations of the three reactions are stated for the mechanisms finally adopted, with the constants correlated by the Arrhenius equation. 

Figure 6.35. Potential energy diagrams for adsorption and dissociation of N2on a Ru(0001) surface and on the same surface with a monoatomic step, as calculated with a density functional theory procedure. [Adapted from S. Dahl, A. Logadottir, R. Egberg, J. Larsen, I. Chorkendorff,...

Figure 3.8 STM images of the same area on Ti02(l 1 0) at 357 K (V, = 1.5 V, /, = 0.1 nA) as a function of time (At = 60s) (a) clean Ti02(l 1 0) with bridging oxygen (BBO) vacancies (b) Ti02 (1 1 0) with a geminate hydroxyl pair formed by adsorption and dissociation of a water molecule. Hv marks the...

One of the most efficient approaches allowing us to investigate in a reasonable time a catalytic cycle on non-periodic materials in combination with reliable DFT functional is a cluster approach. The present study is devoted to the investigation of the effect of the cluster size on the energetic properties of the (p-oxo)(p-hydroxo)di-iron metal active site. As a first step, we have studied the stability of the [Fen(p-0)(p-0H)Fen]+ depending on the A1 position and cluster size. Then, we compared the energetics for the routes involving the first two elementary steps of the N20 decomposition catalytic process i.e. the adsorption and dissociation of one N20 molecule. 

Adsorption of a2 with dissociation, all substances in adsorptive equilibrium. 

P6.04.23. SURFACE REACTION OR ADSORPTION, WITH OR WITHOUT DISSOCIATION. 

The change in the nature of the adsorption with increasing coverage (dissociative followed by associative) has been explained by a statistical consideration of the reaction mechanism shown above120). Associative adsorption is expected to occur at vacant sites for which all adjacent olefin binding sites are occupied by earlier dissociation products (or carbon monoxide, as shown by Fig. 6b), because dissociative adsorption (formation of vinyl and hydride species, followed by hydride migration to another alkene) requires two adjacent vacant sites. 

Adsorption controlled with dissociation k(x2 - Xi/K) [l+KiXi + (K2IKx3) +K3x3y... 

Taking into account the suppression of adsorbed oxygen species by water vapor through a competitive adsorption as verified by TPD, the adsorption of water was proposed to take place at anion vacancies as illustrated in Eq. 20. The re-adsorption (either dissociative or not) may also occur with simultaneous decrease in the sample concentration of oxygen vacancies ... 

---