Hinshelwood versus Eley-rideal mechanism

Sometimes it is given as reaction rate per unit weight or specific surface area of the catalyst, but more instructive is its definition in terms of the turnover frequency (TOP). This is defined as the number of reaction events per active site and unit time and relies on the possibility to evaluate the correct density of active sites. [Pg.109]

Langmuir-Hinshelwood versus Eley-Rideal Mechanism [Pg.109]

Generally, almost all catalytic surface reactions proceed via the LH mechanism. Experimental verification was, for example, obtained in studies in which after adsorption of the reactants A + B, the gas phase is pumped off and the sample temperature continuously increased until the product molecules come off. This procedure is denoted as temperature-programmed reaction spectroscopy (TPRS). In particular, operation of the LH mechanism may be concluded if the relaxation time for product formation is long enough ( 10ps) to reach thermal equilibrium of the adsorbates with the surface. For the CO + O reaction on a Pd (111) surface, this delay time between the adsorption and product formation was determined in molecular beam experiments down to about 10 s, leading to derivation of the action energy for the LH step [15]. [Pg.109]

Reactions involving hot adatoms, such as discussed earlier, clearly represent an intermediate situation between the two limiting cases LH and ER, since the reactants are not in complete thermal equilibrium with the surface (as for LH). Neither does the reaction take place by direct collision from the gas phase (as for ER). Such a kind of intermediate mechanism ( precursor mechanism) has been proposed [16], whereafter one of the reactants is not fully accommodated with the surface but reacts from a state that is vibrationally excited with respect to motion normal to the surface. [Pg.110]

Clear cases for the operation of the genuine ER mechanism are still rather scarce, and experimental evidence is so far restricted to reactions involving the impact of atoms (i.e., energetic species) on the surface. [Pg.110]

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