Zero-order reactions heterogenously catalyzed

Reactions in which liquid or gas-phase reactions are catalyzed by solid catalysts (heterogeneous catalysis) often exhibit kinetics that are zero order in the reactant species. Explain why this might be so. 

Experimental studies have shown that not only oxidation reactions, but nearly all mineral dissolution reactions in nature, can be interpreted as a heterogeneous surface rate catalysis. As discussed earlier these reactions are usually stirring-independent and zero-order with respect to the products of dissolution (Fig. 9.4). The functionality of the dissolution reaction with respect to pH, and the fact that activation energies are significantly lower than the strength of metal oxide bonds (Table 9.4), suggests that the surface reactions controlling dissolution are catalyzed. 

The reactivity of the compound in a heterogeneously catalyzed reaction depends on the rate constant of the surface reaction and on the adsorption coefficient. If the kinetic equation (3) is reduced to a zero-order equation with respect to the concentration of the initial compound, the rate constant... 

A higher form of interpretation of the effect of solvents on the rate of heterogeneously catalyzed reactions was represented by the Langmuir-Hinshelwood kinetics (7), in the form published by Hougen and Watson (2), where the effect of the solvent on the reaction course was characterized by the adsorption term in the kinetic equation. In catalytic hydrogenations in the liquid state kinetic equations of the Hougen-Watson type very frequently degrade to equations of pseudo-zero order with respect to the concentration of the substrate (the catalyst surface is saturated with the substrate), so that such an interpretation is not possible. At the same time, of course, also in these cases the solvent may considerably affect the reaction. As is shown below, this influence is very adequately described by relations of the LFER type. 

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