Pt4Ge/alumina presents a striking contrast. As described in the Methods, the final step in Pt4Ge/alumina preparation involved 750 K heating to desorb residual Cl and hydrogen (as HCl and H2 -Fig. ), and to emphasize the point that the Pt4Ge/alumina samples had already been heated once prior to the initial C2D4 TPD runs, the TPD cycles in Fig. D and E are numbered starting with "2 nd TPD". The C2D4 desorption observed in this 2 nd TPD for Pt4Ge/alumina was quite similar in both intensity and structure to that in the 1 st run on Pt4/alumina, with a lowtemperature component at least partly due to desorption from the Ge/alumina substrate, and a high-temperature component attributed to sites on the Pt4Ge clusters. In contrast, D2 desorption (i.e., carbon deposition) was much weaker for Pt4Ge than in either the 1 st or 2 nd TPD runs for Pt4/alumina, and had a bi-modal temperature dependence, suggesting that what little D2 desorbed, was produced by two processes with different activation energies. The D2 desorption features can, in principle, be fit to extract Ea values, requiring some assumption about the kinetic order of the rate-limiting step. We previously examined D2 TPD from small Ptn/alumina under conditions similar to those here 20 , observing recombinative desorption starting at ~230 K, peaking just below 300 K, and terminating at ~450 K. The fact that the desorption features for D2 generated by C2D4 decomposition do not match the recombinative feature observed in D2 TPD, suggests that some other step in the D2 production pathway is rate-limiting. For simplicity, we assume that this step follows 1 st order kinetics , and further assume a prefactor of 10 15 sec -1 . This crude approximation gives effective (or averaged over multiple accessible pathways) Ea values of ~1.1 V for the 350 K feature, and ~2.5 eV for the 500 K feature, in the range observed for C-H activation in the DFT calculations. During subsequent TPD cycles, there continued to be a small and reducing amount of D2 production, and the C2D4 desorption behavior evolved, but note that when a steady state had been reached after ~15 runs, the C2D4 desorption, particularly at higher temperatures, was substantially higher for Pt4Ge than for Pt4.