Coadsorption of NRR and HER intermediates determines the performance of Ru-N4 towards electrocatalytic N2 reduction

Efficiency of the electrochemical N2 reduction reaction (NRR) to ammonia is seriously limited by the competing hydrogen evolution reaction (HER) but our current atomic-scale insight on the factors controlling HER/NRR competition are unknown. Herein we unveil the elementary mechanism, thermodynamics, and kinetics determining the HER/NRR selectivity on the state-of-the-art NRR electrocatalyst, Ru-N4 using constant potential density functional theory calculations (DFT). The calculations show that NRR and HER intermediates coadsorb on the catalyst where HER is greatly suppressed by the NRR intermediates. The first reaction step leading to either *NNH or *H determines the selectivity towards NRR or HER. Our results also demonstrate that an explicitly potential-dependent treatment of reaction kinetics is needed to understand NRR selectivity. We provide crucial insight into the complex NRR/HER competition and the role of non-innocent adsorbates, show the necessity of constant potential DFT calculations, and suggest that interfacial proton donors will improve NRR selectivity.

FeI Intermediates in N2O2 Schiff Base Complexes: Effect of Electronic Character of the Ligand and of the Proton Donor on the Reactivity with Carbon Dioxide

The characterization of competent intermediates of metal complexes, involved in catalytic transformations for the activation of small molecules, is an important target for mechanistic comprehension and catalyst design. Iron complexes deserve particular attention, due to the rich chemistry of iron that allows their application both in oxidation and reduction processes. In particular, iron complexes with tetradentate Schiff base ligands show the possibility to electrochemically generate FeI intermediates, capable of reacting with carbon dioxide. In this work, we investigate the electronic and spectroscopic features of FeI intermediates in five Fe(LN2O2) complexes, and evaluate the electrocatalytic reduction of CO2 in the presence of phenol (PhOH) or trifluoroethanol (TFE) as proton donors. The main findings include: (i) a correlation of the potentials of the FeII/I couples with the electronic character of the LN2O2 ligand and the energy of the metal-to-ligand charge transfer absorption of FeI species (determined by spectroelectrochemistry, SEC-UV/Vis); (ii) the reactivity of FeI species with CO2, as proven by cyclic voltammetry and SEC-UV/Vis; (iii) the identification of Fe(salen) as a competent homogeneous electrocatalyst for CO2 reduction to CO, in the presence of phenol or trifluoroethanol proton donors (an overpotential of 0.91 V, a catalytic rate constant estimated at 5 × 104 s−1, and a turnover number of 4); and (iv) the identification of sudden, ligand-assisted decomposition routes for complexes bearing a ketylacetoneimine pendant, likely associated with the protonation under cathodic conditions of the ligands.

Non-Covalent Assembly of Proton Donors and p-Benzoquinone Anions for Co-Electrocatalytic Reduction of Dioxygen

<p>With weak acids (AH) at high concentrations, potential inversion can occur due to favorable hydrogen-bonding interactions with the intermediate monoanion [BQ(AH)_m]^•–. The solvation shell created by these interactions can mediate a proton-coupled electron transfer at more positive potentials, resulting in an overall two electron reduction ([BQ(AH)_m]^•– + nAH + e^– ⇌ [HBQ(AH)_(m+n)-1(A)]^2–). Here we show that the resultant hydrogen-bonded [HBQ]^– adduct mediates the transfer of electrons and the proton donor 2,2,2-trifluoroethanol (TFEOH) to a Mn-based complex during the electrocatalytic reduction of dioxygen (O₂). The Mn electrocatalyst is selective for H₂O₂ with only TFEOH and O₂ present, however, with BQ present under otherwise analogous conditions, an electrogenerated [HBQ(AH)₄(A)]^2– adduct (where AH = TFEOH) alters product selectivity to 96(±0.5)% H₂O in a co-electrocatalytic fashion. These results suggest that hydrogen-bonded [HBQ]^– dianions can function in an analogous co-electrocatalytic manner to H₂Q.</p>

Non-Covalent Assembly of Proton Donors and p-Benzoquinone Anions for Co-Electrocatalytic Reduction of Dioxygen

<p>With weak acids (AH) at high concentrations, potential inversion can occur due to favorable hydrogen-bonding interactions with the intermediate monoanion [BQ(AH)_m]^•–. The solvation shell created by these interactions can mediate a proton-coupled electron transfer at more positive potentials, resulting in an overall two electron reduction ([BQ(AH)_m]^•– + nAH + e^– ⇌ [HBQ(AH)_(m+n)-1(A)]^2–). Here we show that the resultant hydrogen-bonded [HBQ]^– adduct mediates the transfer of electrons and the proton donor 2,2,2-trifluoroethanol (TFEOH) to a Mn-based complex during the electrocatalytic reduction of dioxygen (O₂). The Mn electrocatalyst is selective for H₂O₂ with only TFEOH and O₂ present, however, with BQ present under otherwise analogous conditions, an electrogenerated [HBQ(AH)₄(A)]^2– adduct (where AH = TFEOH) alters product selectivity to 96(±0.5)% H₂O in a co-electrocatalytic fashion. These results suggest that hydrogen-bonded [HBQ]^– dianions can function in an analogous co-electrocatalytic manner to H₂Q.</p>

Non-Covalent Assembly of Proton Donors and p-Benzoquinone Anions for Co-Electrocatalytic Reduction of Dioxygen

The two-electron and two-proton p-hydroquinone/p-benzoquinone (H2Q/BQ) redox couple has mechanistic parallels to the function of ubiquinone in the electron transport chain. This proton-dependent redox behavior has shown applicability in catalytic...

Multi PCET in Symmetrically Substituted Benzimidazoles

Proton-coupled electron transfer (PCET) reactions depend on the hydrogen-bond connectivity between sites of proton donors and acceptors. The 2-(2’-hydroxyphenyl) benzimidazole (BIP) based systems, which mimic the natural TyrZ-His190 pair of...