Consistent with previous DFT-based studies, we note that the calculated diffusion barrier for CH 3 (i.e., E app d = 0.18 eV) is higher than that for CH 2 (0.08), CH (0.12) and C (0.08). However, CH 3 is the most mobile species, where the higher diffusivity is due to a much larger pre-exponential factor for methyl (i.e., e 8.7 )compared to other adsorbates.