Activation of the C-H bond is important for hydrocarbon catalysis with applications in energy technology such as production of synthetic fuels. Subnanometer clusters such as Pt-4 show great promise for catalytic activities potentially much greater than monolith. Using density functional theory, we investigated C-H bond breaking, an important step of the conversion reaction of methane to liquid fuels, on Pt and Pd subnanometer clusters with graphene and carbon nanotubes as supports. Our results show that CNT supports can be selected by size and chirality to provide stable support for subnanometer Pt and Pd clusters and tailor their catalytic activity. (C) 2012 Elsevier B. V. All rights reserved.