The size effect on the stability profile under working conditions is illustrated in Figure using the Pd/TiO2-anatase SAC catalyzed reverse water-gas shift (rWGS) reaction as an example. A progressive, two-fold increase in the rWGS rate per Pd with time-on-stream was observed in the ≤ 0.025 wt% Pd1/TiO2 samples (purple, red, and green curves). Such increases originate from higher intrinsic activity of Pd instead of higher Pd dispersion, as Pd is already atomically dispersed on fresh Pd1/TiO2. In contrast, this was not found in less dispersed Pd/TiO2 at higher loadings. More detailed studies revealed the partial sintering of single Pd atoms (Pd1) into disordered Pd nanoclusters (Pdn, ~ 1 nm) by H2 activation and the redispersion into Pd1 in an oxidative atmosphere. Such dynamic aggregation is responsible for the increasing rate with time-on-stream, and steady-state Pd active sites are similar to the ones formed under H2.