In the second TPD runs, the numbers of C2D4 molecules adsorbed into strong binding sites during the initial 150 K dose dropped by roughly half, and continued to drop more slowly in subsequent runs, The obvious question is whether these changes resulted from carbon deposition, from thermal effects such as cluster isomerization or sintering, or from both. Purely thermal effects were probed by experiments such as that presented in Fig. and Table , in which a set of six TPD heat ramps were performed for a Pt4/alumina sample, but with no C2D4 dose prior to the 1 st heat ramp. Thus, at the start of the 2 nd TPD run in this experiment, the sample had been heated to 750 K, but without carbon deposition. The amounts of D2 and high T C2D4 desorption observed when the once-heated sample was given a C2D4 TPD run were 45% and 17% lower, respectively, compared to the amounts during the 1 st TPD run on as-deposited Pt4/alumina, but substantially larger decreases (60% and 30%, respectively)