Single-walled carbon nanotubes (SWNTs) offer the prospect of both new fundamental science and useful (nano)technological applications 1 . High yields (70-90%) of SWNTs close-packed in bundles can be produced by laser ablation of carbon targets 2 . The electric-arc technique used to generate fullerenes and multiwalled nanotubes is cheaper and easier to implement, but previously has led to only low yields of SWNTs 3,4 . Here we show that this technique can generate large quantities of SWNTs with similar characteristics to those obtained by laser ablation. This suggests that the (still unknown) growth mechanism for SWNTs must be independent of the details of the technique used to make them. The ready availability of large amounts of SWNTs, meanwhile, should make them much more accessible for further study.In our electric arc-discharge apparatus 5 , the arc is generated between two electrodes in a reactor under a helium atmosphere (660 mbar). The cathode was a graphite rod (16 mm diameter, 40 mm long) and the anode was also a graphite rod (6 mm diameter, 100 mm long) in which a hole (3.5 mm diameter, 40 mm deep) had been drilled and filled with a mixture of a metallic catalyst and graphite powder. The arc discharge was created by a current of 100 A; a voltage drop of 30 V between the electrodes was maintained by continuously translating the anode to keep a constant distance (ϳ3 mm) between it and the cathode. Typical synthesis times were ϳ2 min. As the catalyst we used mixtures such as Ni-Co, Co-Y or Ni-Y in various atomic percentages; these are known to yield a series of interesting carbon nanostructures 6 . The mixture used by Guo et al. 7 during their laser ablation process (Co and Ni, both at 0.6 at.%) did not produce a good yield of nanotubes in our case. However, we found that a mixture of 1 at.% Y and 4.2 at.% Ni gave the best results. In this case we observed (in a total carbon mass of 2 g): (1) large quantities of rubbery soot condensed on the chamber walls; (2) web-like structures between the cathode and the reactor walls (no webs when either Y or Ni were absent); (3) a cylindrical deposit at the cathode's end; and (4) a small 'collar' (ϳ20% of the total mass) around the cathode deposit, as a black, very light and porous but free-standing material.Within all these products it was possible to observe by scanning electron microscopy (SEM; using a JEOL JSM 6300F instrument) filament-like structures that are more or less dense, depending on where in the reactor they were deposited. The 'collar' deposit was densest; the soot was the least dense. A characteristic SEM image of the collar deposit (Fig. 1) shows large amounts of entangled carbon filaments, homogeneously distributed over large areas (here at least a few square millimetres) and with diameters ranging from 10 to 20 nm. The average length between two entanglement points is several micrometres; we could not identify any filament ends. From several SEM images we estimate the yield of these filaments (with respect to the total volume of the solid material...

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Several samples of palladium-loaded single-wall carbon nanotubes and palladium-loaded MAXSORB activated carbon were prepared by means of the reaction of the raw carbon support with Pd2(dba)3.CHCl3. When carbon nanotubes were used as the support, the palladium content in the samples reached 13-31 wt % and fine particles of 5-7 nm average size were obtained. In the case of the samples with MAXSORB as the support, the palladium content was higher (30-50 wt %) and the particle size larger (32-42 nm) than in the nanotube samples. At 1 atm and room temperature, the hydrogen capacity of the palladium-loaded samples exceeds 0.1 wt % and is much higher than the capacity of the raw carbon supports (less than 0.01 wt %). The maximum hydrogen capacity at 1 atm and room temperature was found to be 0.5 wt %. A maximum hydrogen capacity of 0.7 wt % was obtained at 90 bar in a palladium-loaded MAXSORB sample, while the capacities for the raw carbon nanotubes and MAXSORB at the same pressure were 0.21 and 0.42 wt %, respectively. At low pressure, it was observed that the H/Pd atomic ratios in the palladium-loaded samples were always higher than in the bulk Pd. The spillover effect is considered as a possible cause of the high H/Pd atomic ratios. On the other hand, the effect of the pressure increase on the spillover was observed to be very low at high pressure and room temperature.