Activation energy of chemisorption

Adsorption. This step depends on the possible interaction between molecules and the catalyst surface. When the reactants reach the active sites, they chemisorb on adjacent active sites. The chemisorption may be dissociative and the adjacent active sites may be of the same or different origin. The chemisorbed species react and the kinetics generally follow an exponential dependence on temperature, exp( EfRT), where E3 is the activation energy of chemisorption. 

Oxygen chemisorption proceeds very rapidly on clean surfaces of most metals. At room temperature the sticking probability ranges 0.1 to 1 and is close to unity for many metals. This corresponds to a very small value of the activation energy of chemisorption. 

The thermodynamics of these elements is quite different between the oxygenated and the hydrogenated compounds. Between CO and a competitive adsorption could occur as the sticking coefficients are similar. Furthermore, the catalytic surfaces could be very rapidly equilibrated under H2 or as their activation energies of chemisorption are small. Not only the thermodynamics controls the reaction, but also the structure of the catalyst. 

B. M. W. Trapnell Liverpool University) Ni, Co, and Fe are relatively inactive in most saturated hydrocarbon exchange reactions. This may be because of the activation energy of chemisorption being high on ferromagnetic metals. At 0° evaporated films of these metals chemisorb no CH4 and very little C2H6, whereas on all other metals I have studied (W, Mo, Ta, Cr, Rh, Pd, Ti) coverages up to 30 % may be achieved. 

Thus far we have examined only the process of chemisorption, in the sense of it being the first step on the road to chemical reaction on a surface. If we summarize to this point, chemisorption is a chemical interaction between the adsorbate and the surface. The heats and activation energies of chemisorption are typical of those of a chemical reaction, and that is exactly what it is a chemical reaction, albeit three-dimensional on one side of the arrow and two-dimensional on the other side. The activation energies are such that the species involved have sufficient energy to cross the activation energy barrier at temperature levels that are experimentally accessible and of practical importance. 

Fig. 9. The activation energy of chemisorption of hydrogen by carbon as a fimction...

At 1 > OH > 0-5 (subregion 11a, Figure 3.21) the activation energy of chemisorption, a, is close to zero. At OH < 0.5 (subregion Ilb, Figure 3.21) chemisorption proceeds very slowly at room temperature, but the rate of reaction increases sharply with an increase in the reaction temperature to 100°C. The physical adsorption of water on... 

The measurement of rates and activation energy of chemisorption is very important for catalytic mechanism and its applied research. It is clear that when the adsorption is the reaction determining step of catalytic reaction, the rate of adsorption process will determine the rate of the reaction and activity of catalysts. Therefore, activation energy and heat of adsorption can reflect the nature of the active site and distinguish the t3rpe of active site imder certain conditions. 

It was noted in Section XVII-1 that chemisorption may become slow at low temperatures so that even though it is favored thermodynamically, the only process actually observed may be that of physical adsorption. Such slowness implies an activation energy for chemisorption, and the nature of this effect has been much discussed. 

Fig. XVIII-13. Activation energies of adsorption and desorption and heat of chemisorption for nitrogen on a single promoted, intensively reduced iron catalyst Q is calculated from Q = Edes - ads- (From Ref. 130.)...

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 > ... 

This linear variation in catalytic activation energy with potential and work function is quite noteworthy and, as we will see in the next sections and in Chapters 5 and 6, is intimately linked to the corresponding linear variation of heats of chemisorption with potential and work function. More specifically we will see that the linear decrease in the activation energies of ethylene and methane oxidation is due to the concomitant linear decrease in the heat of chemisorption of oxygen with increasing catalyst potential and work function. 

Works [40, 91] surveyed y versus temperature for deactivation of 02( Aj ) on quartz at 350- 900 K. The obtained temperature dependencies were in the Arrhenius form with the activation energy of 18.5kJ/mole. A conclusion was drawn up about the chemisorption mechanism of singlet oxygen deactivation on quartz surface. A similar inference was arrived at by the authors of work [92] relative to 02( A ) deactivation (on a surface of oxygen-annealed gold). 

Fig. 1. Decrease of activation energy in dissociative chemisorption by van der Waals adsorption (Bj), tr complex adsorption (E ), and resonance effects (Ei) Ej is activation energy of homogeneous reaction.

There is no distinct threshold between physical adsorption, i.e., reversible adsorption with small activation energy of desorption, chemisorption with a significant activation energy of desorption, and formation of surface compounds with a high activation energy for... 

The explanation for the apparent correlation between catalytic activity and electron affinity of metals cannot be as simple as that which has been advanced for the homogeneous catalysts. This is because chemisorption on metals (unlike the splitting of hydrogen by metal ions in solution ) is an exothermic process and, hence, as shown earlier, catalytic activity depends not only on a low activation energy of adsorption but also on a low heat of adsorption. The interpretation applied earlier to homogeneous catalysts can account for an inverse dependence of Ea on the work function, but does not suggest any obvious reason why Q should show a similar dependence. 

Fig. 7. Potential energy diagram for van der Waals (aa) and chemisorbed hydrogen (66) [J. E. Lennard Jones, Trana. Faraday Soc. 28, 333 (1932)]. aQ and A5 represent heats of chemisorption and van der Waals adsorption. aF represents activation energy for chemisorption.

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