Pendant Proton-Relays Systematically Tune the Rate and Selectivity of Electrocatalytic Ammonia Generation in a Fe-Porphyrin Based Metal-Organic Framework.

Electrocatalytic nitrite reduction (eNO 2 RR) is a promising alternative route to produce ammonia (NH 3 ). Until now, several molecular catalysts have shown capability to homogeneously reduce nitrite to NH 3 , while taking advantage of added secondary-sphere functionalities to direct catalytic performance. Yet, realizing such control over heterogeneous electrocatalytic surfaces remains a challenge. Herein, we demonstrate that heterogenization of a Fe-porphyrin molecular catalyst within a 2D Metal-Organic Framework (MOF), allows efficient eNO 2 RR to NH 3 . On top of that, installation of pendant proton relaying moieties proximal to the catalytic site, resulted in significant improvement in catalytic activity and selectivity. Notably, systematic manipulation of NH 3 faradaic efficiency (up to 90 %) and partial current (5-fold increase) was achieved by varying the proton relay-to-catalyst molar ratio. Electrochemical and spectroscopic analysis show that the proton relays simultaneously aid in generating and stabilizing of reactive Fe-bound NO intermediate. Thus, this concept offers new molecular tools to tune heterogeneous electrocatalytic systems.