Pd/PdO Electrocatalysts Boost Their Intrinsic Nitrogen Reduction Reaction Activity and Selectivity via Controllably Modulating the Oxygen Level.

Abstract

The electrocatalytic nitrogen reduction reaction (eNRR) is regarded as promising sustainable ammonia (NH3) production alternative to the industrial Haber-Bosch process. However, the current electrocatalytic systems still exhibit a grand challenge to simultaneously boost their eNRR activity and selectivity under ambient conditions. Herein, we construct Pd/PdO electrocatalysts with a controlled oxygen level by a facile electrochemical deposition approach at different gas atmospheres. Theoretical calculation results indicate that the introduction of an oxygen atom into a pure Pd catalyst would modulate the electron density of the Pd/PdO heterojunction and thus influence the adsorption energy for nitrogen and hydrogen. The calculation results and experiments show that the Pd/PdO heterojunction with a moderate oxygen level (O-M) exhibits optimal eNRR performance with a high NH3 yield of 11.0 μg h-1 mgcat-1 and a large Faraday efficiency (FE) of 22.2% at 0.03 V (vs RHE) in a 0.1 M KOH electrolyte. The moderate affinity of Pd to N in the Pd/PdO heterojunction and the inhibition of the hydrogen evolution reaction (HER) can facilitate the breaking of the triple bond of N2 and promote the protonation of N, which is confirmed by ex situ X-ray photoelectron spectroscopy (XPS) and in situ Raman spectroscopy. In situ Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) calculations further disclose that the O-M catalysts prefer the distal association pathway during the eNRR process. This work opens a new way to construct heterostructures by controlling the oxygen level in other electrochemical fields.