The slow up-take of hydrogen on platinized carbon was due to activated diffusion of adsorbed hydrogen atoms away from platinum centers which chemisorbed molecular hydrogen rapidly. The phenomenon was studied both with hydrogen and deuterium, between 300 and 392C, from 30 to 60cm of pressure on carbon samples containing 0.2 and 1% platinum by weight.

Surface Diffusion of Hydrogen on Carbon. Robell, A.J., Ballou, E.V., Boudart, M.: Journal of Physical Chemistry, 1964, 68[10], 2748-53