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

Immobilized molecular electrocatalysts\ncan exhibit altered proton-coupled\nelectron transfer (PCET) chemistry compared to their nonimmobilized\ncounterparts. Recent experiments suggested that immobilization of\na cobalt tetraphenylporphyrin (CoTPP) on a graphitic surface causes\nthe rate-limiting PCET step for the hydrogen evolution reaction (HER)\nto proceed through a concerted rather than sequential mechanism. Theoretical\nstudies indicated that protonation of Co(II)TPP is accompanied by\nabstraction of an electron from the graphitic surface to form Co(III)HTPP\nin this concerted PCET mechanism. Herein, we investigate the kinetics\nof this PCET reaction to obtain fundamental mechanistic insights that\nwill guide the design of more effective electrocatalysts. We calculate\nthe rate constant using a vibronically nonadiabatic PCET theory that\ntreats all electrons and the transferring proton quantum mechanically\nand includes hydrogen tunneling effects. The input quantities to the\nPCET rate constant expression are obtained from density functional\ntheory calculations on an explicit atomistic model of the CoTPP, a\nprotonated water cluster, and the graphitic electrode surface. The\ncalculated kinetic isotope effect and transfer coefficients are in\ngood agreement with experimental measurements. These calculations\nshow that hydrogen tunneling is significant in this process and that\nthe PCET rate constant depends on contributions from excited vibronic\nstates. This work provides valuable insights into the PCET process\noccurring at this immobilized electrocatalyst and offers a theoretical\nstrategy that can be applied to related systems.