Some Further Thoughts on the Hougen-Watson Rate

3 Some Further Thoughts on the Hougen-Watson Rate Equations 

The validity of the approach is also discussed in a paper by Boudart [1986]. Certainly, the nonuniformity of catalytic surfaces, revealed by data on the heats of chemisorption, is a reality, but does this mean that a reaction necessarily senses this nonuniformity — that the reaction is structure sensitive That depends on the reaction itself, but also on the operating conditions. It may be that the reaction requires only one (or perhaps two) metal atoms or actives sites to proceed, but also that the operating conditions lead to a surface which is almost completely covered by species, so that the nonuniformities are no longer felt. In such a case the use of Hougen-Watson rate equations, based on the Langmuir isotherm, is ... not only useful, but it is also correct. In all cases their use provides physical intuition, improvable rate equations and mechanistic insight unattainable through empirical rate laws [Boudart, 1986]. Since then, further support for this point of view has been published. 

The discussion about the need to account for nonuniformity of the active sites is still going on. Kiperman et al. [1989] mention a series of kinetic studies in which it was necessary to formulate the rate equations in terms of nonuniform surfaces. The correspondence on this matter with Boudart [1989] is well worth reading. 

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