Catalytic activation of hydrogen and its sequential reactions with organic moieties are ubiquitous chemistry, found in the hydrogenation of unsaturated hydrocarbon and polar carbonyl moieties and in hydrogenolysis of C-O bonds of oxygenates. Here, we investigate the effects of electronic charges of the reactive hydrogen species, as hydronium ions across diverse reaction microenvironments, comprise of the reacting molecules located within aggregates of solvent mixtures, in the solution or across the solvent-solid interfaces. To investigate the effects of solvent structures on the local acidity of the hydronium ions, we develop a probe reaction, i.e., electrophilic addition of phenol, where its rate constant is highly sensitive to the absolute pH of the hydronium ion. For this reason, it is a reliable kinetic probe for assessing the acidity, a thermodynamic property, of the reactive hydronium ion across diverse solutions, including solvent mixtures. Next, we illustrate how the charges of the reactive hydrogen adatoms on transition metal surfaces would vary with the equilibrium position of the Volmer reaction, which reflects the thermodynamic tendency of hydrogen adatoms to form interfacial protons, by donating their electron into the d-band of the metal. This equilibrium position dictates the relative abundance of hydrogen adatoms and interfacial hydronium ions on the metal surfaces, each with its own catalytic functions, either promoting the hydrogenation or the competitive electrophilic addition of phenol—the formal requires the involvement of H*, whereas the latter the interfacial proton. This work illustrates the connectivity of the electronic charges of reactive hydrogen species, described in terms of acidity, and their different catalytic functions in promoting the electrophilic addition versus the ring saturation catalysis.
