PNNL

Abstract

This perspective deals with a slice of the vocabulary of heterogeneous catalysis. Michel Boudart, popularized two terms related to the title: “turnover number” [1] and its prerequisite, “dispersion” [2]. The first was borrowed [3] from enzyme catalysis where biologists [4] have long confounded counting the number of turnovers effected by an enzyme catalyst and the rate at which those turnovers occur. Following the pioneering measurements of metallic surface area by Boreskov [5], Boudart devised the second term to describe the surface/bulk ratio of the metal in supported metal catalysts. Likely, he chose to use “dispersion” because particles that are present at low areal density (i.e., that are highly dispersed) frequently exhibit a high exposure: dispersion is the means to the end. Ultimately, he corrected the first term to “turnover rate” [6] and the terminology standards community officially [7] deprecated the second term in favor of “percentage exposure”. The first is in general use by those who study heterogeneous catalysis; the second, however, not so much. Other authors in this issue will describe how to make the corresponding physical measurements; here I deal with why they should be made and extensions to them that may help advance the field of heterogeneous catalysis.