Some methods construct the thermochemical network using only the species that are part of this mechanism and derive a set of internally consistent ∆ f H. However, in other methods, it is necessary to introduce a set of additional gas-phase or adsorbed species to determine ∆ f H of the target species using hypothetical reactions and not the elementary reactions in the mechanism. These approaches connect the target to the existing global thermochemical network. In Table , we list all gas-phase species (either part of the mechanism or in a hypothetical reaction) and their ∆ f H taken from the ATcT database (version 1.130 ). We also assume that Pt(111) represents bulk Pt, which is why ∆ f H Pt(111) is always 0 kJ mol -1 . Technically, assuming that Pt(111) is bulk Pt is not in agreement with the IUPAC standard states since there is an enthalpy of formation to form Pt(111) from bulk Pt (proportional to the surface energy of Pt(111)). However, assuming the active site as a reference is the standard procedure in determining adsorption enthalpies from experiments or theory . We note that in cases where multiple active sites are present, additional consideration is required to determine whether the difference in energies between active sites should be accounted for or not. Having multiple active sites with ∆ f H of zero is not thermodynamically consistent but may be appropriate if the relative concentrations of active sites are known or if surfaces are not assumed to be in equilibrium.