for a dozen metal catalysts. In addition to vacuum DFT calculations, some or all of these pathways were investigated with different simulation setups, such as: different amounts of *H monolayer (ML), spectator adsorbates, or implicit addition of solvent (see SI section "Additional Transition State Results"). However, we found no clear answer to the discrepancy between simulations and experiments for Ag and Au when considering just surface-bound *H as the proton source. These results showed HCOO** to form more readily than *COOH from CO 2 for CO-producing Ag and Au, and also for HCOOH to form more readily than CO, regardless of the intermediate adsorbate.