C-C cracking is another side reaction which frequently deactivates pure Pt catalysts. In order to assess the role of C-C cracking in coking for Pt we compared the barrier heights of C-C cracking to C-H activation starting from the same isomer. These are summarized in Table , with relevant barriers shown in Figure . For Pt 4 and Pt 4 C 2 , C-C cracking is only faster than dehydrogenation for for bound acetylene, in agreement with literature. For Pt 4 Ge and Pt 4 GeC 2 , on the other hand, we see that the C-C cracking barrier is always higher than the corresponding C-H activation barrier. The addition of Ge therefore eliminates C-C cracking as a possible side-reaction. Doping with Ge and Sn suppresses these side-reactions for bulk and larger NP systems. Ge therefore reduces the tendency to coke not only by raising barriers for deeper dehydrogenation, but also by preventing coke-forming side-reactions. This leads to a difference in the kinetic accessibility of the Pt 4 C 2 and Pt 4 GeC 2 ensembles. Since there are more feasible C-C cracking pathways for Pt 4 , the isomer with the split C 2 unit is kinetically accessible, and plays a role in the catalytic behavior of Pt 4 C 2 . The Pt 4 GeC 2 isomer with the split C 2 unit is however not accessible due to higher C-C cracking barriers, despite the fact that it is the putative global minimum. Thus, relative populations of isomer in the ensemble may be determined by relative rates of dehydrogenation and C-C cracking, rather than by their relative energies. Given the large activity differences between isomers, detailed knowledge of all possible reactions would be necessary for quantitative predictions of non-equilibrium ensemble behaviors. This is in agreement with work by Peters, which emphasizes the importance of isomer interconversion flux and reaction production leading to deviations of populations from their equilibrium concentrations. Thus reactivity may not be identifiable from equilibrium populations alone. For experimental validation of the electronic origin of the Ge effects on coking and stability, we compare our DFT results to ISS and TPD data for Pt 4 and Pt 4 Ge, before and after varying numbers of ethylene TPD cycles. Each cycle involved first saturating the catalysts with C 2 D 4 at 180 K, then heating to drive desorption of either intact C 2 D 4 or of D 2 , the latter indicating carbon deposition. The TPD data is from previous work, and explored in more detail there. In brief-the as-prepared Pt 4 catalyst has a high T desorption peak for ethylene around 310 K for the first 2 TPD cycles, but which rapidly decreased, indicating rapid loss of strong ethylene binding sites. D 2 desorption also rapidly decreases in successive TPD runs. For Pt 4 Ge, in contrast, the high-T ethylene and D 2 desorption features are stable over repeated runs, with much less D 2 desorption compared to Pt 4 . This reflects higher stability and selectivity against coking of the Pt 4 Ge clusters.