The Mechanisms of Reactions at Solid Surfaces

The role of a surface in initiating chain carriers which propagate primarily in the homogeneous phase has already been discussed it is a relatively minor branch of catalytic behavior. Reactions which proceed entirely at the surface of a catalyst appear to be much more common. This latter class is distinguished from the former in showing a direct proportionality between rate of reaction and amount of catalyst, other parameters being held constant. When such a relation docs not obtain experimentally, one is faced with either a mixed mechanism or a diffusion-controlled rate, and the mechanistic interpretation of the rate data becomes extremely difficult. 

Where the rate is first-order in catalyst,an interpretation of the rate data is possible but always subject to uncertainties in our knowledge of the sorption isotherms of all species involved in the mechanism. Let us consider, for example, the decomposition of a single molecular species on a catalyst surface. An example might be the dehydration of alcohols on the surfaces of metal oxides such as alumina gel at 300 C to give an olefin plus water. This endothermic reaction, which is thermodynamically favored in the gas phase by an entropy increase of about 36 e.u., can be written as 

The dehydration reaction is usually accompanied by the thermodynamically less favorable side reaction of dehydrogenation 

At low pressures and temperatures in the range of 200 to 400°C it is found that the rate of dehydration is first-order in alcohol and slower than 

Transition metals are good selective catalysts for the dehydrogenation. However, in studies of the competitive rates in the cases of CaHsOH and HCOOH, G. M. Schwab and E. Schwab-Agallides, ibid. 71, 1806 (1949), have demonstrated that the physical state of the catalyst as well as its chemical structure could affect the relative amounts of the two competing reactions. 

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