Carbon nanotubes contain metal impurities which are responsible for the "electrocatalysis" seen at some nanotube-modified electrodes.

Its all done with metals: The title statement is confirmed by comparing the electrocatalytic oxidation of hydrazine at a basal-plane pyrolytic-graphite (BPPG) electrode treated with iron (III) (blue) with that at a multiwalled-carbon-nanotube (MW-CNT)-modified BPPG electrode (red) and at an edge-plane pyrolytic graphite electrode (black; ••••• and -•-• are the responses of iron(III)-treated and MW-CNT-modified BPPG electrode, respectively, in the absence of hydrazine). (Figure Presented) © 2006 Wiley-VCH Verlag GmbH and Co. KGaA