Platinum relative adsorption coefficients

Measurements of competitive hydrogenation of butadiene into butenes on palladium and platinum catalysts supported on silica [30] allowed the determination of the relative adsorption coefficients of dienes and olefins. 

Another kinetic method of determining relative adsorption constants does show that toluene is adsorbed more strongly than benzene on the platinum metals. The method determines the effect of the partial pressure of toluene on the rate of hydrogenation of benzene. With the assumption that adsorption is reversible, the ratio of adsorption coefficients, br/bB, is evaluated by the use of equation (37) where U°B and Ub(T) represent the rate of hydrogenation of benzene in the absence of toluene and in its presence at stated partial pressures of toluene (Ft) and benzene (Pb). This method avoids the determination of the relative individual rate constants that is required by the method of Wauquier and Jungers. Xylene inhibits the hydrogenation of alkenes on Pd more effectively than does benzene, which is consistent with the effect of alkyl groups on the basicity of benzene. ... 

In many papers in which the reaction rates or adsorption coefficients of a series of substrates were obtained the effect of structure is discussed only qualitatively. Rilzicka and Cerveny (5) found that the rate of hydrogenation of 1-olefins C6-C17 in ethanol under usual conditions on platinum, palladium and rhodium catalysts decreased monotonically with the chain length. Since in this olefinic series the chain length did not affect (68) the relative adsorptivity of olefins, as demonstrated in Table II, the observed phenomenon may be explained only by the effect exerted by the chain length on the rate constants. Since, moreover, the polar effect of substituents in this series is virtually the same, an explanation can be sought in the steric hindrance of the adsorbed double bond by the free end of the olefin molecule. The trans isomers were adsorbed much more weakly than the respective cis... 

Selectivity control by competitive adsorption also explains how the platinum catalyst works in the Ostwald process, the oxidation of ammonia to nitrogen oxide, used to produce nitrates from ammonia. The low reactivity of platinum causes the rate of ammonia dissociation on platinum to be low. The dissociation of N-H bonds of ammonia is the reverse of the ammonia synthesis discussed in the previous section. Whereas the rate of dissociative NH3 adsorption is relatively small, the initial sticking coefficient for dissociative oxygen adsorption is higher, about 0.01. 

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