ISS allows us to track changes induced by TPD cycles in in the surface layer of the catalysts, which can be compared to the estimated total number of strong ethylene binding sites per clusters, estimated prior to each TPD cycle (details in SI). For Pt 4/ Al 2 O 3 , the "as-prepared" Pt ISS intensity reflects a catalyst where essentially all the Pt atoms are in the He + -accessible surface layer. The Pt intensities were attenuated, relative to those for the as-prepared catalyst, by factors of 15%, 45%, and 70%, respectively, after 1, 6, and 21 TPD runs, reflecting shadowing/blocking of He + scattering by bound C atoms. Note, however, that after 6 TPD runs, 6 to 8 C atoms per Pt 4 cluster were estimated to be present, but more than half of the Pt atoms remained He + -accessible, whereas saturating the Pt clusters with CO molecules resulted in much larger (˜80%) attenuation of Pt ISS signals. This supports our results showing restructuring of Pt 4 upon the addition of C 2 , which has also been seen experimentally on the surfaces of larger NPs. The number of ethylene molecules adsorbed in strong binding sites decreases more rapidly after each TPD run, by factors of 48%, 68% and 80%, respectively, after 1, 6 and 21 runs. The decline in strong ethylene binding sites is partly offset by an increase in weak binding sites,. Thus, the combination of ISS and TPD suggests that while there is some steric site blocking by carbon deposition, the larger effect on ethylene binding is electronic, decreasing the binding energies such that few strong binding sites remain. Because these strong binding sites are responsible for the dehydrogenation activity, the catalyst deactivates.