Langmuir-Hinshelwood mechanism isotherm

The oxidation of propylene oxide on porous polycrystalline Ag films supported on stabilized zirconia was studied in a CSTR at temperatures between 240 and 400°C and atmospheric total pressure. The technique of solid electrolyte potentiometry (SEP) was used to monitor the chemical potential of oxygen adsorbed on the catalyst surface. The steady state kinetic and potentiometric results are consistent with a Langmuir-Hinshelwood mechanism. However over a wide range of temperature and gaseous composition both the reaction rate and the surface oxygen activity were found to exhibit self-sustained isothermal oscillations. The limit cycles can be understood assuming that adsorbed propylene oxide undergoes both oxidation to CO2 and H2O as well as conversion to an adsorbed polymeric residue. A dynamic model based on the above assumption explains qualitatively the experimental observations. 

Mechanistic formulations in this area usually begin from one of two simplified mechanisms, both of which assume the conditions of the Langmuir isotherm. The Langmuir-Hinshelwood mechanism assumes that the rate controlling step is between the two adsorbed species D S and E S, so that the overall mechanism includes steps (ii), (iii) and (vi) in Scheme 9.1. The Eley-Rideal or Rideal mechanism " assumes that the rate controlling step is between a species in solution and an adsorbed species. Thus, if D is the adsorbed species, the mechanism would include steps (ii) and (v) in Scheme 9.1. 

In surface chemistry, adsorption isotherms describe the equiUhrium situation. However, just as in the consideration of the gas-phase chemistry in the interstellar medium, it is the kinetics of surface processes which are more relevant. Two mechanisms for surface-catalysed reactions can be distinguished and are illustrated by the cartoons in Fig. 1.6. In the Eley-Rideal mechanism, it is assumed that reaction occurs when a species (say, A) from the gas-phase impacts on a species (say, B) that is adsorbed on the surface. At significant surface coverage, the rate of reaction will be proportional to the product of the fraction of the surface covered in B (6b) and the pressure (p ) of the species A, which will be proportional to the rate of collisions of A with unit area of the solid surface. An alternative picture is encapsulated in the Langmuir-Hinshelwood mechanism. Here it is assumed that reaction occurs in encounters between species both of which are adsorbed on the surface. Then the rate of reaction will be proportional to the product of the fractions of the surface covered by A and by B that is proportional to 0a b-... 

While transport effects may be eliminated in laboratory reactors, and experiments have shown that certain reactions oscillate under what may be considered isothermal and gradientless conditions, the Langmuir-Hinshelwood mechanism by itself with conventional mass-action kinetics does not give a satisfactory description of them. A number of "extra" features have been added in modeling studies reported in the literature. Among them are an activation energy which depends on the concentration of adsorbed species in one or more of the reaction steps [37, 52 - 54], transition between active and inactive forms of an adsorbed component [7, 17, 55, 56], and periodic switching of the reaction mechanism [16, 18, 40, 57]. 

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... 

The isothermal models can be divided into elementary-step models wherein interaction of the elementary steps (adsorption, desorption, and reaction) can produce oscillations without additional mechanisms, and models with additional, non-Langmuir-Hinshelwood steps, such as phase transitions and oxidation-reduction cycles. The latter models are usually supported by experimental evidence obtained by the methods discussed in Section III. 

These expressions define Langmuir-Hinshelwood isotherms. (Cyril Norman Hinshelwood was a British chemist who won a 1956 Nobel Prize for studying chemical reaction mechanisms.)... 

The similarities of eqs.12-14 to a Langmuir adsorption isotherm have led to inference that the mineralization process takes place on the photocatalyst s surface. Turchi and Ollis [39] have shown that regardless of whether both, one only, or neither of oxidant ( OH) and reductant (pollutant) are adsorbed on the catalyst at the moment of radical attack, the overall rate equation will still follow an apparent Langmuir-Hinshelwood form if there is only one dominant reaction mechanism. 

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