Fig. 4: COOH to CO reaction pathways and transition states at different potential.
a The minimum energy path (MEP) for cis-COOH to CO conversion at −0.53 V applied potential (0.8 charge) on Ni–N4 site. We use a total of eight images (00–07) for this conversion process. During the reaction of cis-COOH with water, the OC–OH bond length increases from 1.38 to 1.86 Å with a transition state (TS) at image 03. Eventually it forms CO by breaking OC–OH bond. The arrow indicates the forward direction of the reaction via MEP. b The free energy barrier for the cis-COOH to CO at −0.53 V applied potential via MEP. Image 03 (TS) shows the maximum energy barrier of 7.02 kcal mol−1 at −0.53 V applied potential for the conversion of cis-COOH to CO on Ni–N4 site. c The transition state (TS) between reactant cis-COOH and product CO changes with applied potential. Staring from product, at zero applied potential the product (P0V) CO is 3.57 Å (OC–OH) distance away from neighboring water while the O–H bond distance in water is 0.98 Å. The transition state at zero (TS0V) applied potential shows a geometry similar to the product. At −0.53 V applied potential, the OC–OH distance at the transition state (TS−0.53V) decreases while the O-H bond increases. With a more negative applied potential of −0.8 V, the OC–OH bond of the transition state (TS−0.8V) becomes very early, closer to the reactant at same potential. The reactant at −0.8 V (R−0.8V) has OC–OH = 1.38 Å and O–H = 1.86 Å. The arrow indicates the reactant to product direction. d The relation of distance between OC–OH to the fractional charge in the TS as a function of applied potential. e The change of distance between O-H and the fractional charge in the TS as a function of applied potential. The red circle represents the TS at different applied potentials. (Gray color: entire surface represents implicit solvation along the whole reaction pathways, brown: carbon, blue: nitrogen, green: nickel, red: oxygen, off white: hydrogen atom).
