Langmuir-Hinshelwood processes

Figure 14) which is equal to the l CO/ CO ratio in the preadsorbed CO. The conclusion is that CO2 is produced by the reaction of adsorbed, rather than gaseous, CO and adsorbed 0 atoms - i.e. the Langmuir-Hinshelwood process. This conclusion has dominated all of the results presented here and appears to be ubiquitous for all transition metals under all conditions that have been studied to date. 

It appears more likely that this product is formed via a Langmuir-Hinshelwood process. To determine whether the mechanism in Fig. 

Transient response experiments have revealed that the formation of N2 and N2O during NO reduction by H2 over Rh proceeds without the intervention of H2 By contrast, the formation of NH3 and H2O involves the reactions of dissociatively chemisorbed H2 with N and 0 atoms, respectively. The results obtained from experiments involving the reduction of adsorbed NO and isotopic substitution of NO for NO can be interpreted on the basis of the reaction mechanism presented in Fig. 11. Key elements of this mechanism are that NO is adsorbed reversibly into a molecular state, that reduction is initiated by the dissociation of molecularly adsorbed NO, and that all products are formed via Langmuir-Hinshelwood process. 

In the case of very strong chemisorption [see Fig. 2(d)], the desorption of molecules is even more activated than the desorption of adatoms. At temperatures not high enough for appreciable desorption of adatoms, the very large value of — AU2 t ensures values of 6 close to unity. The virtual absence of molecular desorption rules out the Langmuir— Hinshelwood process, so that recombination is restricted to the Rideal— Eley mechanism, which is now activated, causing recombination to be... 

Surface reactions can be classified into two generic types. The first includes reactions between two adsorbed species or between an adsorbed species and a vacant site (Langmuir-Hinshelwood processes). For randomly distributed adsorbates on a surface in the absence of adsorbate-adsorbate interactions, the rate of reaction is given by... 

The Langmuir-Hinshelwood picture is essentially that of Fig. XVIII-14. If the process is unimolecular, the species meanders around on the surface until it receives the activation energy to go over to product(s), which then desorb. If the process is bimolecular, two species diffuse around until a reactive encounter occurs. The reaction will be diffusion controlled if it occurs on every encounter (see Ref. 211) the theory of surface diffusional encounters has been treated (see Ref. 212) the subject may also be approached by means of Monte Carlo/molecular dynamics techniques [213]. In the case of activated bimolecular reactions, however, there will in general be many encounters before the reactive one, and the rate law for the surface reaction is generally written by analogy to the mass action law for solutions. That is, for a bimolecular process, the rate is taken to be proportional to the product of the two surface concentrations. It is interesting, however, that essentially the same rate law is obtained if the adsorption is strictly localized and species react only if they happen to adsorb on adjacent sites (note Ref. 214). (The apparent rate law, that is, the rate law in terms of gas pressures, depends on the form of the adsorption isotherm, as discussed in the next section.)... 

All of these rates are measured on surfaces shown to be clean by AES, and this Indicates that these processes occur on surfaces containing only submonolayer coverages of reactant species, exactly the situation required for the Langmuir-Hinshelwood model of surface reactions. 

We can conclude now that one electron returns to the conductivity band during each act of formation of the vacant site to adsorb sensitizer. Because adsorption centers Zr(. ) are not accounted for by (2.81) the energetics of the process does not depend on the manner in which R is closing in A, i.e. on the fact which recombination mechanism (either Langmuire-Hinshelwood or Ili-Ridil) takes place. 

Studies by a number of authors (8.22-27) have shown that H2 adsorbs dissociatively on Rh and that this process is reversible at the temperatures used in the present studies. As noted earlier, the atomic hydrogen formed by this means is believed to be responsible for the formation of NH3 and H2O. Consequently, these products are assumed to be formed by a sequence of Langmuir-Hinshelwood steps. While there is no independent evidence to support this hypothesis for the synthesis of NH3, recent results reported by Thiel et al. (Z7) indicate that the formation of H2O from H2 and adsorbed 0-atoms does proceed via a two step sequence such as that represented by reactions 6 and 7 in Fig. 11. 

Hu and Ruckenstein s results (130) showed that on the reduced nickel-containing catalyst, the reaction took place by a Langmuir-Hinshelwood mechanism involving adsorbed CH4 and oxygen species. Furthermore, they indicated that a slow dynamic redox process consisting of lattice oxygen formation and its reduction by carbon species was at least partly responsible for the CO formation. 

When chemisorption is involved, or when some additional surface chemical reaction occurs, the process is more complicated. The most common combinations of surface mechanisms have been expressed in the Langmuir-Hinshelwood relationships 36). Since the adsorption process results in the net transfer of molecules from the gas to the adsorbed phase, it is accompanied by a bulk flow of fluid which keeps the total pressure constant. The effect is small and usually neglected. As adsorption proceeds, diffusing molecules may be denied access to parts of the internal surface because the pore system becomes blocked at critical points with condensate. Complex as the situation may be in theory,... 

The amount of TiO was varied, and the assembly was tested for its photocata-lytic activity using degradation of 2,4-xylidine as test reaction probe. The decrease in the concentration of xylidine and the corresponding increase in oxalate concentration were monitored through HPLC. A pseudo-first order kinetics was ob served in the photodegradation process based on the Langmuir-Hinshelwood mechanism. An inverse correlation was observed between the kinetic rate constant and the obtained Light-Induced Optoacoustic Spectroscopy (LIOAS) frequency maxima. 

Liquid phase hydrogenation catalyzed by Pd/C is a heterogeneous reaction occurring at the interface between the solid catalyst and the liquid. In our one-pot process, the hydrogenation was initiated after aldehyde A and the Schiff s base reached equilibrium conditions (A B). There are three catalytic reactions A => D, B => C, and C => E, that occur simultaneously on the catalyst surface. Selectivity and catalytic activity are influenced by the ability to transfer reactants to the active sites and the optimum hydrogen-to-reactant surface coverage. The Langmuir-Hinshelwood kinetic approach is coupled with the quasi-equilibrium and the two-step cycle concepts to model the reaction scheme (1,2,3). Both A and B are adsorbed initially on the surface of the catalyst. Expressions for the elementary surface reactions may be written as follows ... 

The electrooxidation of CO to C02 is, similar to its electroless counterpart in gas-phase catalysis, one of the most widely studied electrochemical reaction processes [131,152]. It is generally assumed that the electrooxidation of adsorbed CO proceeds primarily via a Langmuir-Hinshelwood type mechanism involving either adsorbed water molecules or, at higher electrode potentials, adsorbed surface hydroxides (see Figure 6.25) according to... 

Non-linearities arising from non-reactive interactions between adsorbed species will not be our main concern. In this section we return to variations of the Langmuir-Hinshelwood model, so the adsorption and desorption processes are not dependent on the surface coverage. We are now interested in establishing which properties of the chemical reaction step (12.2) may lead to multiplicity of stationary states. In particular we will investigate situations where the reaction step requires the involvement of additional vacant sites. Thus the reaction step can be represented in the general form... 

The Langmuir—Hinshelwood rate equation (16) used by several authors for other types of catalyst was interpreted by Tanabe and Nitta [290] on the basis of their results with NiS04. The authors assume that the surface reaction of adsorbed ethylene and water molecules, which was found to be the slowest process, may be written in greater detail as... 

Interactions 2 and 4 represent a Rideal mechanism and Interactions 2a and 4a a Langmuir-Hinshelwood mechanism. However, to form C03"(adS), by Interaction 1 in Mechanism I, CO must first be adsorbed since Interaction 1 is a fast process, the adsorption of CO would be the slow step of Mechanism I, and the kinetics of the reaction would depend on pco- However, it has been shown (8, 28) that the reaction is zero order with respect to CO, and therefore the adsorption of CO and its conversion to COa udsi are faster processes than Interaction 2 which is the rate-limiting step and hence may be written in the form of 2a (Langmuir-Hinshelwood mechanism). 

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