Measurements of hysteresis in the rate of CO oxidation over a
Pt-alumina catalyst by Oh er al. (1979) cover a 30-fold change in the particle diameter. These measurements offer a unique opportunity to test intraparticle diffusion criteria, or more specifically to "calibrate" the numerical value used in the criterion against the dramatic phenomenon of hysteresis. Oh er al. showed that hysteresis resulted from strong coupling of CO oxidation kinetics to intraparticle mass transport. To date, the various criteria for identifying transport interference with catalytic kinetics have had insufficient testing. For example, in a recent technical note in this Journal, Hudgins (198 1) demonstrated that a widely quoted criterion for the interference of external mass transport fails to predict experimental observations of interference. Our experience is that the intraparticle mass transfer criteria such as the Thiele modulus for specific rate models or Hudgins' criterion (Hudgins, 1968) are more satisfactory. Criteria for intraparticle diffusion interference within a catalyst particle have been stated in various forms (Butt and Weekman, 1974). For brevity, we will consider only Hudgins' criterion set. It is related closely to the Thiele modulus criterion. A generalized criterion has been given by Hudgins {1968) as follows: