Electrodes chemically modified with molecular active sites are potent catalysts for energy conversion reactions. Such electrodes are typically presumed to operate by the same redox mediation mechanisms as the analogous soluble molecules, with electron transfer and substrate activation in separate elementary steps. Here we uncover solvent-dependent concerted reaction mechanisms for cobalt porphyrins attached to glassy carbon electrodes by flexible aliphatic linkages. In acetonitrile, outer-sphere Co^II/I reduction mediates H₂ evolution in a stepwise sequence. However, in aqueous media, outer-sphere reduction is not observed and H₂ evolution proceeds instead by concerted proton–electron transfer pathways typical of metal surfaces. Consequently, catalysis is not defined by the reduction potential of the parent molecule, but rather by the free energy of hydrogen binding. We attribute these mechanistic changes to electrostatic coupling between the molecule and the surface arising from adsorption. Our results motivate a re-examination of the reaction mechanisms of and design principles for molecularly modified electrodes.

用分子活性位点化学修饰的电极是能量转换反应的有效催化剂。通常假定此类电极通过与类似可溶性分子相同的氧化还原介导机制进行操作，其中电子转移和底物活化在单独的基本步骤中进行。在这里，我们揭示了通过灵活的脂肪键连接到玻碳电极上的钴卟啉的溶剂依赖性协同反应机制。在乙腈中，外层 Co ^II/I还原以逐步顺序介导 H ₂释放。然而，在水性介质中，没有观察到外球体还原，H ₂相反，进化是通过金属表面典型的协同质子-电子转移途径进行的。因此，催化不是由母体分子的还原电位定义的，而是由氢结合的自由能定义的。我们将这些机械变化归因于分子与吸附产生的表面之间的静电耦合。我们的结果促使人们重新审视分子修饰电极的反应机制和设计原理。