Vapor-phase benzene (Bz) hydrogenation over carbon-supported Pd catalysts has been studied [65, 66]. A 2.1% Pd/C catalyst prepared with a carbon black cleaned in H2 at 1223 K had a surface-weighted Pd crystallite size of 21 nm, giving a Pd dispersion of 5%, based on TEM. The carbon itself had an average mesopore diameter of 25 nm, while the average microspore diameter was 0.9 nm; thus the majority of the Pd resided in the mesopore. The highest activity of this catalyst at 413 K and 50 Torr Bz (Total P = 1 atm, balance H2) was 1:99 μ mole Bz s-1 g -1. The density of the catalyst was 0:60 g cm -3. The catalyst particle size distribution ranged from 10–500 microns (1 m = 10-6 m).
(a) Assuming all the Pd is in the mesopore, are any mass transfer limitations expected based on the W-P criterion?
(b) If, instead, this catalyst had all the Pd in the microspores and it gave this performance, would mass transfer limitations exist? Why?