HCOOH can be determined by the amount of *H available, and whether the reaction proceeds via a Tafel or Heyrovsky mechanism. While this hypothesis does not explain the preferential stability and ease of formation for HCOO** compared to *COOH, it is possible that CO formation occurs via a different mechanism compared to HCOOH formation. Fig. shows the reaction barriers for the three different pathways for CO vs. HCOOH formation, Eq. 3 (green), Eq. 4 (orange), and Eq. 6 (purple). Eq. 5 does not occur, as the intermediate HCOO** first rearranges itself to *COOH via an energetically unviable step, and then follows pathway 3 (see SI section "Additional Transition State Results"). The barriers are via direct protonation by *H, and via water-assisted H-shuttled hydrogenation ('-S-' in the plot) as described by Nie et al. Fig. shows reaction pathways for the direct protonation mechanism (left) and the water-assisted mechanism (right), along with atomic-scale insets of the initial state (IS), transitition state (TS), and final state (FS). We observe that for the direct protonation results, HCOOH formation has a lower barrier than CO formation on CO (orange bar is smaller than green bar) from monodentate *COOH, with HCOOH formation from HCOO** (purple bar) being even lower. However, if using the water-assisted H-shuttled hydrogenation for *COOH to CO formation (pathway 3 (see SI section "Additional Transition State Results"), but keeping the non-water-assisted pathway results