The discovery of a catalytic route to the growth of single-layer carbon nanotubes suggests that it may be possible to produce these materials with better selectivity and in higher yield. Increasing the production efficiency is essential for characterization and application of these materials. We have discovered several catalyst promoters, in particular S, Bi, and Pb, that greatly enhance the single-layer carbon nanotube yield, and extend the distribution of nanotube diameters to much larger sizes (> 3 nm). Co crystallites encapsulated in graphitic polyhedra also form abundantly when S, Bi, or W is present. Understanding these catalytic process is of substantial scientific and technological importance.