To examine the pathways that must ultimately lead to carbon deposition, we next performed DFT calculations of ethylene and acetylene binding and dehydrogenation on the thermallyaccessible isomers of the Pt4Ge clusters (Figs. and). Note that adsorbates can bind with different energies on different cluster isomers, which can significantly change isomer populations, with important effects on the mechanism. We found that ethylene predominantly binds to the Pt4Ge clusters (including the most abundant GM) in the π-mode, i.e. with the ethylene π bond coordinated to a single Pt atom, retaining the C sp 2 -hybridization. The barriers for such π-mode-bound ethylene to undergo C-H dissociation are higher than the desorption energies, and thus π-mode-bound ethylene is predicted to mostly desorb intact, rather than undergoing further dehydrogenation, consistent with the small D2 desorption branching observed for the Pt4Ge/alumina catalyst. Importantly, however, one of the thermally-accessible isomers of ethylene-Pt4Ge, LM' (corresponding to LM2 of the bare cluster), binds ethylene in a di-s mode (Fig. ), and in this configuration the barrier to C-H dissociation is significantly lower than in any of the π-mode-bound configurations. LM2 for the bare cluster has P700K of only 0.3% -far too small to account for the amount of D2 desorption observed during TPD. However, the ethylene di-σ bond in LM' is strong, stabilizing LM' and increasing its P700K to ~3%.