L-H mechanism

Our preliminary experiment using a PBR showed that the order of foe rate of foe CO oxidation over Pt/Al203 was positive with respect to O2 concentration and was negative with respect to CO concentration. This result indicated that the reaction occurs via L-H mechanism with strong adsorption of CO. For this type of reaction, foe surfece coverage of O is suppressed hy CO adsorption, and therefore the rate of CO oxidation is slow in a PBR. On the other hand, adsorption of O onto the surface occurs in foe O2 zone. In other words, in foe countercuirent flow of COad and O2 there exists a reaction fi ont where foe net concentration... 

McCallum and Fletcher, 1976] and single-crystalline [Love and Lipkowski, 1988 Koper et al., 1998 Lebedeva et al., 2002b] Ft were observed. For extended surfaces, the transient shape was explained by the L-H mechanism [Koper et al., 1998] or the L-H mechanism complicated with nucleation and growth of OHads islands [McCallum and Fletcher, 1976 Love and Lipkowski, 1988]. 

Over limited ranges of conditions, power law rate equations often are as adequate as those found by detailed chemical or L-H mechanisms. 

Lee, L-H. "Mechanism of Thermal Degradation of Phenolic Condensation Polymers" in Proceedings of Battelle Sym., on Thermal Stability of Polymers" Dec. 5,6, 1963. 

In several photocatalytic reactions, a linear relation between the rate of photocatalytic reaction and amount of substrate(s) adsorbed on the surface of photocatalyst has been reported.3,1(M2) When the Langmuirian adsorption isotherm was expected, this behavior was sometimes called Langmuir-Hinshelwood (L-H) mechanism even if only a kind of adsorbed substrate was assumed. Strictly speaking, however, this is wrong, because L-H mechanism involves the surface reaction of two kinds of adsorbed species, which is not realized in photocatalytic... 

WCo2 = r = h4Pco00 corresponding to the E-R mechanism is not satisfied. At present the pendulum has swung to the opposite side and most research workers [98] are sure that, over a wide range of the reaction parameters (T = 450-950 K, P = 10-7 to 10 5 Torr), only the adsorption mechanism (L-H) is valid. This belief is based on the data obtained in unsteady-state experiments and using modern physical methods, in particular the molecular beam technique [98, 52, 107]. But a fairly good qualitative description on the basis of the L-H mechanism has been obtained in only a few cases [56, 57] and this description concerns rather limited experimental... 

The applied L-H mechanisms are different. They are associated with different concepts about the character of reactive forms. In accordance with Schwab and Gossner [115], the reaction follows the L-H mechanism with the participation of molecular oxygen. According to Heyne and Tompkins [113], the mechanism includes the steps... 

At the present time, the mechanisms in which the L-H mechanism is completed by the E-R mechanism are widespread. This two-route mechanism was first suggested by Heyne and Tompkins [113]. The same mechanism was offered by Stephens [118], Hori and Schmidt [119], and McCarthy et al. [123]. McCarthy et al. applied the two-route mechanism to interpret the extreme character of the kinetic dependences and the rate self-oscillations. Miniscloux et al. used the kinetic model corresponding to the two-route mechanism to calculate the catalytic reoxidation of CO in exhaust gases. 

In the Langmuir-Hinshelwood (L-H) mechanism for surface-catalyzed reactions, the reaction takes place between two surface-adsorbed species [4,5], As a substitute for concentration, we use surface coverage, and the rate is expressed in this term. We consider that the elementary reaction in the L-H mechanism is the bimolecular surface reaction expressed by the following equations ... 

Even if the L-H mechanism is defined as the reaction of a surface-adsorbed substrate obeying a Langmuir isotherm governing the overall rate, the frequently reported experimental evidence, a reciprocal linear relation between concentration of the substrate in solution and rate of photocatalytic reaction is not always proof of this mechanism. From the linear plot, two parameters are calculated 23). One (often shown as k, not as ks ) is a limiting rate of the reaction at the infinite concentration... 

It has been established (38) that the dominant mechanism of the reaction is the Langmuir-Hinshelwood (L-H) mechanism ... 

The alternative Eley-Rideal mechanism fails to explain all the experimental results. Direct evidence in favor of the L-H mechanism has been obtained... 

Lambert and Comrie (.1.1) investigated the CO + NO reaction on Pt(lll) and (110) surfaces and concluded that the reaction proceeds by a L-H mechanism between Oads and molecular CO ads - NO dissociation is also the prime step in the NO + H2 reaction on a Pt foil at a pressure of 10 mbar (7). NH, was found to be the major product at temperatures lower than 600 K and N2 was the major product at temperatures higher than 600 K when the NO/H2 ratio was —1/5. 

Yamazaki, I., and Piette, L.H. Mechanism of aerobic oxidase reaction catalyzed by peroxidase. Biochem Biophys Acta 77 47-64, 1963. 

Thomsen, M.K., Jacobsen, C. and Skibsted, L.H. Mechanism of initiation of oxidation in mayonnaise enriched with fish oil as studied by electron spin resonance spectroscopy. Eur. FoodRes. Technol. 211, 381-386 (2000). 

Kinetic studies indicate whether a surface reaction-controlled bimolecular reaction proceeds by an L-H or R-E mechanism. Equation 2.24 indicates that in the L-H mechanism -Ra) passes through a maximum when either Pa or Pb is increased while the other is fixed. The decrease in the rate at high Pa or Pb is rationalized by supposing that the more strongly adsorbed reactant displaces other species from the surface as its partial pressure is increased. This type of behavior was observed in the transition metal-catalyzed reaction of cyclopropane with hydrogen, where the strongly adsorbed hydrogen displaced cyclopropane from the surface [20]. In the R-E mechanism, on the other hand, -Ra) tends to become independent of reactant partial pressure when Pa is steadily increased (Eq. 2.25). 

Where, denotes active site. The mechanism of reaction between two adsorbed species on the surface is called as Langmuir-Hinshelword or L-H mechanism which is widely accepted for ammonia synthesis from N2 and H2. 

Pt has been one of the most well characterized materials due to its distinguished chemical properties and outstanding performance in catalytic reactions [32, 33]. Also, CO oxidation is one of the most standard surface chemical reactions. As a standard model to explain the adsorption and dissociation processes of molecules on Pt surface, Langmuir-Hinselwood (L-H) mechanism has been widely accepted [34], In L-H mechanism, chemical reactions take place in most efficient manner when the surface remains as clean metallic state. That is, the most reactive surface is metallic surface. For example, in the case of CO oxidation, oxygen molecnles make contact to metal surface and get dissociated into surface atonfic oxygen. Then, this surface oxygen reacts to CO molecule to form CO2. During this process, surface remains clean and there is no residue remains on the surface. In the past, many stnd-ies of model system under low-pressure experiments have demonstrated the validity of L-H model. 

To understand the dynamics of the hydrogenation process, and validate our proposed L-H mechanism from previous work, the TB-QCMD method was applied to the hydrogenation of CeHe on a Mo-terminated M02C (0001) siuface. By introducing three consecutive hydrogen molecules from the gas phase, we were able to obtain the information on the whole process of the hydrogenation directly [48]. 

Under kinetic conditions the L-H mechanism dominates [19] (see also exercise 1.8). If the energy exchange with the substrate by excitation of phonons and/or... 

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