(446d) Pd-Catalyzed Electrohydrogenation of Dinitrogen to Ammonia

Understanding new mechanisms for low-temperature N₂ activation is essential for facilitating NH₃ synthesis under mild conditions. Electrochemical reduction of N₂ to NH₃ has recently received considerable attention, which can enable sustainable, distributed production of NH₃ when powered by solar- or wind-generated electricity. However, this process typically has a low activity and selectivity for the N₂ reduction reaction (NRR) due to the barrier for N₂ activation and the competing hydrogen evolution reaction (HER) in aqueous electrolyte. Here we present our recent study of an electrohydrogenation mechanism for ambient N₂ fixation over Pd nanoparticle catalysts, which can form Pd hydride and promote hydrogenation reactions via hydride transfer. Using a neutral phosphate buffer solution electrolyte that can largely suppress the undesired HER, we obtained a relatively high yield rate and a Faradaic efficiency of 8.2% for NH₃ production at 0.1 V vs the reversible hydrogen electrode over ligand-free Pd particles of around 6 nm in size. In operando X-ray absorption spectroscopy (XAS) study confirmed the formation of α-phase Pd hydride during the reaction. The NRR activity of Pd at low overpotentials outperforms Au and Pt nanoparticles, which is attributed to a unique electrohydrogenation mechanism that involves an electrochemical formation of α-phase PdH_x and a Grotthuss-like hydride transfer on α-PdH_x for N₂ hydrogenation to form *N₂H, thus to lower the energy barrier for the rate-limiting step of NRR. We will also discuss about structure-activity relationships for NRR on Pd catalysts. Interestingly, when we tried to control the Pd nanoparticle size using Oleylamine (OAm) or Polyvinylpyrrolidine (PVP) surfactants, we found that the Pd nanoparticles with residue surfactants showed negligible activity for NRR, which may indicate that N₂ adsorption and reaction on metal catalysts might be very sensitive to surface cleanness or molecular environment. Thus, fundamental studies of NRR on well-controlled clean and model catalysts are essential to understand the NRR catalysis.