Fig. 4. First-principles-based microkinetic modeling bridges theory and experiment.

a–c Computed activity, the apparent activation energy ($E_{\mathrm{a}}$), and most abundant intermediates (insets; superscript = H-Pd coordination number) of HD exchange reaction on dilute Pd ensembles. The activity increases in the order of Pd monomers, trimers, and dimers. At 50 °C, trimers have a higher $E_{\mathrm{a}}$ (0.52 eV) compared to monomers and dimers (0.20–0.25 eV). d Pd speciation (at.%) within the surface (blue) and subsurface/bulk (light and dark orange) responsible for the observed coordination numbers in the initial state (S0), and after pretreatments A (S1), B (S2), and C (S3). On the basis of theoretical modeling, surfaces of S0/S1 are constrained to trimers, and either monomers or dimers for S2 and S3 each. a–d Pd atoms are green and Au atoms are yellow.