(499h) Chemisorption, Physisorption, and Hysteresis of Hydrogen On Carbon Nanotubes

We used a magnetic suspension
balance, coupled with a residual gas analyzer, to investigate hydrogen
adsorption/desorption on carbon nanotubes (CNTs) for pressures up to 100 bars
at 25 . Such a combination of gravimetric and mass analysis
systems, used for the first time, is capable of overcoming the shortcomings of the
common hydrogen adsorption measurement techniques. The most crucial drawbacks
of the conventional measurement methods are as follows. (i) The inability to
distinguish between hydrogen adsorption and chemisorptions; (ii) the relatively
large errors caused by the leakage of hydrogen out of the system, and (iii) the
difficulty of measuring hydrogen uptake capacity under operating conditions that
differ from those of hydrogen storage systems used in practice.

In the experiments multi-walled carbon
nanotubes (MWCNTs) were exposed to hydrogen, and were subsequently kept under vacuum
for 2 consecutive cycles, in order to study hydrogen adsorption/desorption in
the nanostructured materials. The difference between the first and the second
hydrogenation cycles was in the initial preparation of the MWCNTs. For the
first cycle, the MWCNTs were first degasified at an elevated temperatures (120 ). For the second cycle the MWCNTs were not degasified at
the elevated temperatures and, therefore, hydrogen residue from the first
adsorption/desorption cycle was still adsorbed on the MWCNTs at the beginning
of the second cycle.

The first hydrogenation cycle was
started after pretreatment of the CNT at the elevated temperature. Hydrogen
adsorption kinetics for the cycle indicates that the adsorption equilibrium
occurs within 4 - 5 hours. Careful examination of the recorded adsorption
kinetics revealed the fascinating fact that a considerable part of hydrogen
adsorption in the MWCNTs takes place instantaneously. The measured equilibrium
data revealed some hysteresis for hydrogen desorption in this cycle, which does
not vanish, even when the system's pressure approaches zero. This indicates
that hydrogen adsorption in the first hydrogenation cycle is a combination of
physisorption and chemisorption.

After the first cycle was
finished, the CNTs were exposed to hydrogen for the second time without any pretreatments
at the elevated temperature.  Measurements indicated no hysteresis for during
the second cycle. Therefore, only physisorption took place during the second
cycle. Comparison between the first and the second hydrogen isotherms, together
with the measured hydrogen adsorption kinetics, made it possible to quantitatively
distinguish between the physisorption and chemisorption portions of hydrogen
adsorption in the MWCNTs. According to our data, for pressures less than 20 bars
physisorption and chemisorption are both important, with the former becoming
the dominant mechanism as the pressure increases. To the best of our knowledge,
this is the first time that hydrogen physisorption and chemisorption on carbon
nanotubes have been measured separately.