Hydrogen capacity of neat carbon nitride with specific surface area of 2.9 m2 g-1 was equal to that of activated carbon with 992 m2 g-1 as compared at 313 K under 0.1 MPa of hydrogen. The former linearly increased with specific surface area, and apparent numbers of hydrogen molecules on 1 nm2 of surface area were 20-50, which were close to the ones for neat CNH (carbon nanohorn) samples.
Absorption isotherm for palladium-containing activated carbon consisted of two types of hydrogen sorption. Owing to apparently enhanced sorption by carbon component, hydrogen capacity exceeded the sum of the ones of palladium and activated carbon. Isotherms for palladium-containing CNH were, on the other hand, simple and smooth, which suggests that palladium atoms and carbon atoms were intimately mixed to form some complex. Linear dependence of hydrogen capacity on specific surface area was again observed for palladium-containing CNH, and substantial sorption of hydrogen was determined for samples with high specific surface areas larger than 100 m2 g-1. When carbon nitride was used as the matrix material for palladium modification, the threshold of specific surface area was lowered to 20 m2 g-1 or so.
Similar results were also obtained for nickel-containing activated carbon, CNH and carbon nitride, but the maximum hydrogen capacity, 0.4 mass % under 0.1 MPa of hydrogen, was recorded for palladium-containing CNH which was physically modified through co-evaporation of the component elements. Analysis by calorimetry, IR, ESR etc. is now under progress.