Fig. 5: Calculated free energy profiles of the transformations of 1-butene, 1: isomerization (to the left) vs. hydrogenation (to the right).

Lowest energy pathways only. For easy comparability of isomerization and hydrogenation, we start at the center with 1-butene 1 in the gas phase that adsorbs on Pd(111) (red), Pd(100) (orange), Pd(110) (blue), and Pd(211) (green). The isomerization of 1 to 2-butene, 2, starts with a dehydrogenation of 1 to 1-buten-3-yl 5 and co-adsorbed H*. As reference of this transformation, we use the free energy value of 1-butene 1 in the gas phase at 373 K, 100 kPa; the partial pressure values were P_1-butene = 5 mbar and P_H2 = 3.7 mbar with a standard surface concentration of θ° = 0.5. For the hydrogenation of 1 to butane 9, we use 1-butene 1 and H₂ as free energy reference. This transformation starts with a hydrogenation to 2-butyl 8; see Fig. 1. In the final step of each reaction path, the product, 2-butene 2 or butane 9, respectively, is desorbed into the gas phase. The free energy of 9* is shifted, by −40 kJ mol⁻¹ from the DFT model results. This empirical correction reflects the challenging nature of modeling such processes by the DFT methodology applied; for details, see the text. This correction is inspired by experimental data on the typical entropy loss during the adsorption, from the gas, of saturated hydrocarbons on late transition metal surfaces.