Proton shuttling flattens the energy landscape of nitrite catalytic reduction

Water molecules can enhance or inhibit hydrogenation reactions depending on the nature of the reactive species and active sites. In metal-catalyzed nitrite (NO₂^–) reduction the presence of protons is essential to complete the reaction in the aqueous phase. By coupling rigorous kinetics studies of nitrite hydrogenation on Pd with kinetic isotope studies and theoretical calculations we have shown that, contrary to previously proposed mechanisms of surface H-insertion on NO*, in aqueous environments the reaction proceeds via H-shuttling in which protons move via the aqueous environment while the electrons reach the NO* through the metal in a concerted fashion. This unique mechanism flattens the energy landscape, which leads to the same apparent activation energy barrier (0.6 eV) for the formation of HNO* and HNOH*. These results are consistent with the hydrogen reaction orders, kinetic isotopic experiments, and micro-kinetic modeling including co-limiting reaction steps for NO* hydrogenation to HNO* and HNOH*. This work provides new insights that will be key in developing more efficient catalysts and processes for catalytic removal of micro-pollutants, such as nitrate and nitrite, in drinking water and more broadly to hydrogenation reactions in aqueous phase.