关键词: 氮还原, 钯颗粒, 电催化, 法拉第效率

Abstract:

Ammonia (NH₃) plays an essential role in agriculture and modern industries. Electrochemical fixation of nitrogen (N₂) to ammonia (NRR) under ambient conditions with renewable electricity is a promising strategy to replace the industrial Haber-Bosch method. However, it usually suffers from extremely poor ammonia yield and low Faraday efficiency due to the poor electrocatalysts. Therefore, intensive studies have been devoted to developing efficient NRR catalysts till now. Among them, palladium (Pd) can capture protons in the aqueous phase to form stable α-PdH, which balances the competitive adsorption between nitrogen and protons as well as reduces the NRR reaction energy barrier. In addition, carbon-based materials have the characteristics of weak hydrogen adsorption capacity, wide potential window and abundant valence electrons. In this work, graphene composite powder supported palladium particles (PdNPs@GCP) were prepared by chemical reduction under ambient condition via adopting commercial hy-drophobic GCP as carbon carrier for nitrogen reduction reaction. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations results showed that the well-crystallized palladium particles were successfully loaded on the GCP surface, and GCP was conducive to exposure of more active sites. Raman and XPS spectra confirmed the existence of metal-carrier interaction. Benefitting from the specific structure-activity relationship of the PdNPs@GCP, the ammonia yield was 5.2 μg·h^-1·mg^-1 at -0.2 V vs. RHE and Faraday efficiency of 9.77% was achieved at -0.1 V vs. RHE in 0.1 mol·L^-1 Na₂SO₄ under natural conditions. Compared with pure palladium phase and GCP, the NRR activity of PdNPs@GCP was enhanced remarkably. The two-dimensional structure of GCP improved the mass transport efficiency and the hydrophobic surface could inhibit hydrogen evolution reaction through weakening the proton aggregation near the catalyst. Meanwhile, Pd particles on GCP would be favorable for nitrogen adsorption and activation, and the metal-carrier interaction of the catalyst could fine-tune the electronic structure of Pd, optimizing the adsorption and desorption of reaction intermediates to accelerate NRR. Strictly controlled experiments were carried out to eliminate any possible existing internal and external contaminations to confirm the source of the product NH₃. The morphology and component of the catalyst were almost unchanged after suffering a long-term (10 hours) electrochemical test, indicating good stability of PdNPs@GCP. In addition, no byproduct hydrazine (N₂H₄) was detected, proving the excellent NRR selectivity of the catalyst. This work provides a facile strategy for the fabrication of carbon-based composite catalysts, which has a promising prospect in electrochemical ammonia synthesis and other energy transformation field.

Key words: nitrogen reduction, palladium particles, electrocatalysis, Faraday efficiency