Figure shows TPD data collected for Pt4/alumina and Pt4Ge/alumina samples, both with identical, 0.1 ML-equivalent of Pt deposited as Pt4 clusters. The data shown are from experiments in which 21 sequential TPD runs were done for each sample. For each run, the sample was first exposed to a saturation dose of C2D4 (10 L) at 150 K, and then heated at 3 K/sec to 750 K while mass-spectrometrically monitoring desorption of C2D4 and D2. No signal was observed for desorption of acetylene or other hydrocarbon species, and no additional D2 desorbed in test experiments in which the samples were ramped to higher temperatures, indicating that ethylene either desorbed intact, or dehydrogenated to 2 C(ads) + 2 D2(gas).