90
H. Kong et al.
2.5 9 1017 molecules s-1 (m2-ASA)-1 (ASA—active sur-
face area) was measured at ambient temperatures and
pressures, regardless of the nature of the carbon material
under investigation. The ASA was described in terms of
atoms located at edge positions on the graphite basal plane
and was determined from the amount of oxygen able to
chemisorb at these sites. The H2-TPD investigation by
Zhou et al. [20] revealed that hydrogen adsorption on the
CNTs can occur at ambient temperature and pressure, with
adsorption of most (99%) of H2 being reversible. It was
also found that the desorbed product was almost exclu-
sively gaseous hydrogen at temperatures lower than 723 K
and involved CH4, C2H4 and C2H2, in addition to H2, at
temperatures of 773 K and above, implying that H2
adsorption on the CNTs may be in the two forms: asso-
ciative (molecular state) and dissociative (atomic state).
This has also been evidenced by the following-up LRS
characterization of H2/CNTs adsorption system [22].
It is clear that the high reactivity, especially the high
specific reaction rate, of HDA of toluene and tetralin over
the CNT-supported catalyst is closely related to the pecu-
liar properties of the CNTs as support, especially their
excellent performance for adsorption/activation of H2 (see
Fig. 7d). Based on the above H2-TPD results, we suggest
that there exists a considerably larger amount of reversibly
adsorbed H-species on the CNT-supported Pt catalyst
under the reaction conditions for the HDA of toluene and
tetralin. This would generate a surface micro-environment
with high stationary-state concentration of H-adspecies at
the surface of the functioning catalyst, especially at the Ptx0
active-sites, since the highly conductive CNTs may pro-
mote hydrogen spill-over from the CNTs to the Pt0x active-
sites, and thus be favorable to increasing the rate of the
HDA reaction of toluene and tetralin. This is very similar
to the cases in synthesis of methanol from CO hydroge-
nation over the CNT-promoted Cu–ZnO–Al2O3 [23, 24] or
from CO2 hydrogenation over CNT-promoted Pd–ZnO
[25]. In contrast, due to the lacking of the graphene-like
structure and surface consisted of sp2-C, AC may simply
act as the catalyst support with low capability of adsorbing
H2 (see Fig. 7e) and thus rather limited promoter effect.
active Pt0-species closely associated with HDA of toluene
and tetralin. On the other hand, it also markedly improves
the capability of catalyst for adsorbing and activating H2
(one of the reactants). Both these factors are favorable to
increasing the rate of HDA reaction of toluene or tetralin.
Acknowledgments The authors are grateful for the financial sup-
ports from National Basic Research (‘‘973’’) Project (No.
2009CB939804) of China.
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4 Concluding Remarks
In the developed CNT-supported Pt catalysts for HDA of
toluene and tetralin, the CNTs played dual roles as a cat-
alyst support and a promoter. The CNT-supported/pro-
moted Pt-catalyst displays excellent performance for
highly effective HDA of toluene or tetralin. Using the
CNTs in place of AC as the catalyst support resulted in an
increase of the molar percentage, in the total Pt-amount at
surface of the functioning catalyst, of the catalytically
123