Reaction Between Adsorbates

In the following we consider a surface with adsorbed atoms or molecules that react. We will leave out the details of the internal coordinates of these adsorbed species, but note that their partition functions can be found using the schemes presented above. Let us assume that species A reacts with B to form an adsorbed product AB via an activated complex AB  

Again we assume that the species are distributed randomly over the surface. The equilibrium constant for this system is given by 

The reverse of reaction (173)-(174) is the dissociation of AB on the surface. It is easy to see that had we considered the reaction 

If we assume that both sides of the transition state are in equilibrium with the transition state we find 

Note how the partition function for the transition state vanishes as a result of the equilibrium assumption and that the equilibrium constant is determined, as it should be, by the initial and final states only. This result will prove to be useful when we consider more complex reactions. If several steps are in equilibrium, and we express the overall rate in terms of partition functions, many terms cancel. However, if there is no equilibrium, we can use the above approach to estimate the rate, provided we have sufficient knowledge of the energy levels in the activated complex to determine the relevant partition functions. 

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The CO oxidation occurring in automobile exhaust converters is one of the best understood catalytic reactions, taking place on Pt surfaces by dissociative chemisoriDtion of to give O atoms and chemisoriDtion of CO, which reacts with chemisorbed O to give CO, which is immediately released into the gas phase. Details are evident from STM observations focused on the reaction between adsorbed O and adsorbed CO [12]. 

The rate of reaction between adsorbed species is proportional to their amounts on the surface. 

The extent to which such reactions take place in parallel with the dominant reaction (4.1) is, in general, difficult to quantify as the overall reaction (4.3a) may consist of the elementary step (4.1) followed by reaction between adsorbed CO and adsorbed oxygen on the metal surface ... 

We also assume that adsorbed D and A are in equilibrium with gaseous D and A respectively (pD(g) = Po(ad), pA(g) = pA(ad)) and that product adsorption is relatively weak and their desorption fast, so that the reaction between adsorbed D and A is rate limiting. 

Reaction between adsorbed components on the catalytic surface. 

Figure 7.15. Temperature-programmed SIMS experiment showing the surface reaction between adsorbed C and N atoms to give a surface cyanide species at 475 and 600 K decomposition of CN into C -t N, followed by instantaneous desorption of N2, occurs at...

Catalytic oxidations on the surface of oxidic materials usually proceed according to the Mars-Van Krevelen mechanism [P. Mars and D.W. van Krevelen, Chem. Eng. Sci. 3 (1954) 41], as illustrated in Fig. 9.17 for the case of CO oxidation. Instead of a surface reaction between CO and an adsorbed O atom, CO2 is formed by reaction between adsorbed CO and an O atom from the metal oxide lattice. The vacancy formed is filled in a separate reaction step, involving O2 activation, often on defect sites. 

If only adsorbed complexes take part in the formation of nanoclusters, metal loading, the quantity of nanoclusters formed on the surface, is only proportional to the amount of the adsorption. Hence, the loading is quite small, even if so large amount of complexes is located in solution phase. So, the solute species should be deposited directly onto sites for nanoparticle formation, in order to establish high loading of nanoclusters on the surface. In addition, the resultant nanoclusters are expected smaller and higher dispersed, compared with the particles formed only via surface reaction between adsorbed species, as shown in Figure 3. 

Figure 3. Direct deposition mechanism leading to highly dispersed and nanosized particle formation, compared with reaction between adsorbed species.

A bifunctional catalyst should be able to activate two different reaction steps (methanol and water adsorption and surface reaction between adsorbed species), and so active sites with different properties are necessary. As an example, investigations of possibihty of enhancing activity with regard to methanol electro-oxidation with Pt-Ru-based electrodes are of great interest with regard to improving the electrical efficiency of DMFCs. Several approaches have been considered the effect of Pt-Ru... 

Note in conclusion, that similar problem can he solved at temperatures of the liquid lower than room temperature. At lower temperatures one may expect longer times of establishing stationary electric conductivity of the sensor, in the first place due to lower rate constant of the reaction between adsorbed oxygen and solvent. Consequently, at lower... 

On-Line Mass Spectroscopy as a Tool for the Study of Exchange Reactions between Adsorbates and Bulk Solution Components... 

In addition to the assumptions implicit in the use of the Langmuir isotherm the following assumption is applicable to all Hougen-Watson models the reaction involves at least one species chemisorbed on the catalyst surface. If reaction takes place between two adsorbed species, they must be adsorbed on neighboring sites in order for reaction to occur. The probability of reaction between adsorbed A and adsorbed B is assumed to be proportional to the product of the fractions of the sites occupied by each species (0A9B). Similar considerations apply to termolecular reactions occurring on the surface. 

Case II. Irreversible Bimolecular Reaction Between Adsorbed Species on the Same Type of Site... 

Case V. Reversible Reactions Between Adsorbed Species (Change in Number of Moles on Reaction)... 

Figure 2.8 The surface reaction between adsorbed carbon monoxide and hydrogen to methane over rhodium catalysts occurs at lower temperatures in the presence of a vanadium oxide promoter, which is known to enhance the rate of CO dissociation (from Koerts el al. 113]). 

The controlling mechanism is surface reaction between adsorbed ethylene and adsorbed HCl. Find the rate equation. 

The rate of a surface catalyzed reaction, A2+B = C+D+E, is determined by the reaction between adsorbed A and gas phase B. Substance A2 dissociates upon adsorption. The Arrhenius equation applies to all constants. Find the temperature at which the initial rate is a maximum. 

The reaction, CO + Cl2 = > C0CI2, in the presence of activated carbon catalyst was studied at several temperatures with the results shown Potter Baron, CEP 47, 473, 1951). The controlling mechanism is believed to be reaction between adsorbed CO and Cl2, but the amount of adsorbed CO is relatively small. Find the Arrhenius constants of all the terms. 

Diffusion of A to the surface and surface reaction between adsorbed A and B occur simultaneously. Adsorption of A is relatively small but a component C also is adsorbed. 

The kinetics of CO oxidation from HClOi, solutions on the (100), (111) and (311) single crystal planes of platinum has been investigated. Electrochemical oxidation of CO involves a surface reaction between adsorbed CO molecules and a surface oxide of Pt. To determine the rate of this reaction the electrode was first covered by a monolayer of CO and subsequently exposed to anodic potentials at which Pt oxide is formed. Under these conditions the rate of CO oxidation is controlled by the rate of nucleation and growth of the oxide islands in the CO monolayer. By combination of the single and double potential step techniques the rates of the nucleation and the island growth have been determined independently. The results show that the rate of the two processes significantly depend on the crystallography of the Pt surfaces. 

It is known that CO oxidation involves a surface reaction between adsorbed CO molecules and an adsorbed oxygen species, which we shall refer to as a surface oxide, but which may be an adsorbed oxygen... 

The rate determining step is a reaction between adsorbed CHsOad or CHsOHad and adsorbed OHad or H20ad- ... 

The Langmuir-Hinshelwood reaction between adsorbed CO and O atoms is well established as the dominant reaction mechanism for conditions where CO is the primary surface species . This mechanism has been confirmed by numerous UHV studies of the coadsorption of the transient kinetic... 

The kinetic data indicate that the rate equation of DPM could be expressed as a surface reaction between adsorbed DPM and dissociatively adsorbed hydrogen as mentioned above. Applying this result to the asym DAM, and taking the above discussions into account, the reaction scheme of asym DAM could be drawn as in Fig. 3. 

The kinetic studies of the hydrogenolysis of DPM indicate that both the DPM and hydrogen are adsorbed on the same kind of active sites on the catalyst. Also, the rate-determining step of the hydrogenolysis is a surface reaction between adsorbed DPM and dissociatively adsorbed hydrogen. When the rate equation for DPM is applied to asym DAMs, their reactivities can be satisfactorily explained, and it is suggested that the product selectivity is proportional to the ratio of the adsorption equilibrium constants of the two aryl groups. 

This may be explained by the bifunctional theory of electrocatalysis developed by Watanabe and Motoo [14], according to which Pt activates the dissociative chemisorption of methanol to CO, whereas Ru activates and dissociates water molecules, leading to adsorbed hydroxyl species, OH. A surface oxidation reaction between adsorbed CO and adsorbed OH becomes the rate-determining step. The reaction mechanism can be written as follows [15] ... 

Equation 13.3 implies that the reaction between adsorbed NH3 and gas-phase NO is rate determining and is also consistent with the fact that the reaction is virtually... 

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