Nonadiabatic Proton-Coupled Electron Transfer at a Graphitic Surface Immobilized Cobalt Porphyrin

Immobilized molecular electrocatalysts can exhibit altered proton-coupled electron transfer (PCET) chemistry compared to their nonimmobilized counterparts. Recent experiments suggested that immobilization of a cobalt tetraphenylporphyrin (CoTPP) on a graphitic surface causes the rate-limiting PCET step for the hydrogen evolution reaction (HER) to proceed through a concerted rather than sequential mechanism. Theoretical studies indicated that protonation of Co(II)TPP is accompanied by abstraction of an electron from the graphitic surface to form Co(III)HTPP in this concerted PCET mechanism. Herein, we investigate the kinetics of this PCET reaction to obtain fundamental mechanistic insights that will guide the design of more effective electrocatalysts. We calculate the rate constant using a vibronically nonadiabatic PCET theory that treats all electrons and the transferring proton quantum mechanically and includes hydrogen tunneling effects. The input quantities to the PCET rate constant expression are obtained from density functional theory calculations on an explicit atomistic model of the CoTPP, a protonated water cluster, and the graphitic electrode surface. The calculated kinetic isotope effect and transfer coefficients are in good agreement with experimental measurements. These calculations show that hydrogen tunneling is significant in this process and that the PCET rate constant depends on contributions from excited vibronic states. This work provides valuable insights into the PCET process occurring at this immobilized electrocatalyst and offers a theoretical strategy that can be applied to related systems.