关键词: 合成氨, 氮还原, 铂, 钛酸锶钡

Abstract:

Ammonia is an important industrial raw material and a potential green energy. Using renewable energy to convert nitrogen into ammonia under ambient condition is an attractive method. However, the development of efficient photoelectrochemical ammonia synthesis catalysts remains a challenge. Perovskite such as BaSrTiO₃ (BST) is a good photocatalytic material. However, BST is active under ultraviolet light and has a high recombination rate of photogenerated electron-hole pairs. By dispersing precious metals, it can effectively regulate the absorption of sunlight by BST. In this work, we used a two-step method to prepare BST. The H₂PtCl₆·6H₂O solution was dispersed on the BST, and then followed by calcination in a tube furnace to obtain Pt@BaSrTiO₃ (Pt@BST). X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were utilized to analyze the structures, morphologies, and surface chemical composition of the synthesized materials. Results showed that the well-crystallized Pt particles were successfully loaded onto the BST surface, and Pt and BST interacted to produce a metal-semiconductor heterojunction, improving the performance of N₂ reduction. The N₂ adsorption and desorption isotherms showed that the increase in the specific surface area helped the catalyst to adsorb N₂, and the contact area with H₂O also increased, which promotes the occurrence of NRR and thus produces more NH₃. UV-Vis and PL spectroscopic techniques were used to characterize and analyze optical properties of the obtained catalyst. It is indicated that decoration of Pt reduces the band gap of the catalyst and increases the visible light absorption range, in addition, further enhances the charge separation and transfer, inhibits the recombination of electron-hole pairs, and improves the efficiency of charge separation. The performances of BST and Pt@BST for photoelectric catalytic synthesis of ammonia under ambient condition were studied. The yield of ammonia first increased and then decreased with the increase of Pt content. When the Pt content was 4wt%, the yield was the highest. The results showed that the ammonia yield of Pt@BST was 26.57 × 10^-8 mol·h^-1·mg^-1 and Faraday efficiency (FE) was 5.43% at -0.3 V (vs. RHE) in 0.1 mol·L^-1 Na₂SO₄ under natural conditions, suggesting that the ammonia yield of Pt@BST was twice that of pure BST (13.12 × 10^-8 mol·h^-1·mg^-1). We conducted control experiments of ¹⁵N₂ isotope and Ar in order to eliminate internal and external environmental pollution. Confirming that the detected NH₃ was produced exclusively via nitrogen reduction reaction. After recycling the test six times at -0.3 V (vs. RHE), both FE and ammonia yield rate showed a slight variation, indicating the high stability of Pt@BST during N₂ reduction process. This work provides a simple strategy for further designing the preparation of noble metal modified perovskite catalysts, and has promising application prospects in ammonia synthesis under ambient condition.

Key words: ammonia synthesis, nitrogen reduction, Pt, BaSrTiO₃