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Purification efficiency of carbon nanotubes (CNTs) by the method of chemical oxidation was considered as a function of position and size of catalyst remains and consequently of the tubes morphology. Oxidation of CNTs by means of both HNO 3 and NaOH treatment efficiently removes small catalyst particles embedded in the tubes top, following "tip-mode" CNTs growth mechanism. Destructive character of the purification can be assumed due to the resulting tiniest tube population increase as a consequence of their body tearing. However, limited purification efficiency was observed in the case of bigger metal particles with variable size and position in CNTs. Bigger particles occur on account of catalyst instability portrayed as small metal particles of active phase migration and merging. The formed agglomerates are not stable in the tubes hollow, but disintegrate leading to different sizes and position of metal particles in the tubes body. Consequently, CNT may be obtained with non-uniform thickness and morphology. The phenomenon is due to liquid-like behaviour of the active phase at reaction temperature (700 • C) which is higher than both Huttig and Tamman temperatures of applied metals. A mechanism is proposed assuming that an isolated bigger part of the mother particle stayed encapsulated inside the tube body inactive for further tube growth, while a smaller fragment of the collapsed particle resided at the tube top acting as a newborn active site. Owing to "replica effect" the tube further grows thinner following the size of the new active site. Consequently CNTs of irregular morphology occur as they resemble metal particles of various sizes following their disintegration. Keywords. CNTs purification; catalyst Fe-Ni/Al 2 O 3 particles encapsulation; active sites disintegration; irregular CNTs morphology.

Carbon nanotubes (CNTs) were synthesized by a catalytic chemical vapor deposition method (CCVD) of ethylene over alumina and silica supported bimetallic catalysts based on Fe, Co and Ni. The catalysts were prepared by a precipitation method, calcined at 600 °C and in situ reduced in hydrogen flow at 700 °C. The CNTs growth was carried out by a flow the mixture of C 2 H 4 and nitrogen over the catalyst powder in a horizontal oven. The structure and morphology of as-synthesized CNTs were characterized using SEM. The as-synthesized nanotubes were purified by acid and basic treatments in order to remove impurities such as amorphous carbon, graphite nanoparticles and metal catalysts. XRD and DTA/TG analyses showed that the amounts of by-products in the purified CNTs samples were reduced significantly. According to the observed results, ethylene is an active carbon source for growing high-density CNTs with high yield but more on alumina-supported catalysts than on their silica-supported counterparts. The last might be explained by SMSI formed in the case of alumina-supported catalysts, resulting in higher active phase dispersion.

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Two mesoporous nanocristalline Pd/SnO2 catalysts were prepared by modified solgel technique differing in the pH conditions (pH = 2 and 9.5) of the synthesis of their supports. Samples achieved different activity and selectivity in water denitration reaction using hydrogen. XPS results of reduced samples indicate a strong interaction between the Pd and the Sn possibly as a result of electron shift from Sn to Pd. The solid solution of Pd2+ and SnO2 is formed by taking O from the surface of the support. In such a way some SnO2-X species may stay onto the surface and be responsible for its pronounced activity