Characterization of CNT
X-ray diffraction pattern of CNTs are close to graphite. A graphite-like peak (002) is present and measurements of interlayer spacing can be obtained from its position using the Bragg law [2]. Carbon nanotubes are also active in Raman spectroscopy[2]. Most characteristic features are : peak <200 cm−1 which is characteristic of SWNT, frequency depend on tube diameter. The 1340 cm−1 is assigned to residual ill-organized graphite. The 1500 - 1600 cm−1 peak also characteristic of nanotubes. The TEM images are absolute necessary for studying CNTs.
Application of CNT and CNT based catalysts
CNTs are used in several catalytic reactions as catalyst or catalyst supports. In particular liquid phase reactions were studied extensively with MWNT. Higher surface area and mesoporous nature resulted in significant decrease in mass transfer limitations compared to activated carbon.
1. Hydrogenation reactions
This is one of the most studied catalytic reactions both in liquid and gaseous phases. Ni, Rh, Ru supported on CNT were reported to be more active for hydrogenation reactions compared to when supported on activated carbon. Hydrogenation reactions such as hydrogenation of alkenes and α,β – unsaturated aldehyde have been reported for CNT supported catalysts. Ruthenium nanoparticles supported on MWCNTs showed excellent catalytic activity for hydrogenation of aromatic hydrocarbon . The 5wt% Pt/CNT catalyst was reported to be significantly more active than 5wt% Pt/AC for hydrogenation of trans –diphenylethene and trans β-methylstyrene [4]. Rhodium complex grafted on MWCNTs was reported to be very active for cyclohexene hydrogenation [5] while Pd/CNT catalyst was found to be active for benzene hydrogenation [6]. Pt supported on SWCNTs has been found to be active and selective in hydrogenation of prenal (3-methyl-2-butenal) to prenol (3-methyl-2-butenol) [7].
2. Reaction involving CO/H2
CNTs have been investigated for Fischer –Tropsch reactions, methanol and higher alcohol synthesis and hydroformylation reactions. Copper promoted Fe/MWCNT catalyst are active for Fischer-Tropsch synthesis with olefins [8]. Co-Re/Al2O3 deposited on MWCNT by dip coating exhibited an enhancement in Fischer-Tropsch activity than observed with a similar system without CNT arrays [9]. MWCNTs also have been used as promoter for Cu-ZnO-Al2O3 catalysts for methanol synthesis using H2/CO/CO2 [10] The complex [HRh(CO)(PPh3)3] has been grafted onto MCWNTs and used for hydroformylation of propene. Higher conversion and higher regioselectivity toward n-butaldehyde have been reported for CNT supported catalysts compared to that activated carbon or carbon molecular sieve supported catalysts [11]
3. Ammonia synthesis and decomposition
The Ru/C catalyst is studied as an alternative to conventional iron based catalyst for ammonia synthesis at high pressure and temperature. However, Ru/C catalyst is prone to deactivation due to metal sintering, metal leaching or methanation of support. The stability of the catalyst are reported to increase on using CNT as support. Ru-K/MWCNT catalyst has been found to be significantly more active than Ru supported on other carbon supports [12]. The catalytic decomposition of ammonia to generate CO- free hydrogen for fuel cells is receiving increasing attention since the process is more economical than using methanol as hydrogen source. The MWCNT supported ruthenium was found to be more active than MgO, TiO2 or alumina supported Ru [13].