Alloying effect-induced electron polarization drives nitrate electroreduction to ammonia

Abstract

Electrocatalytic conversion of nitrate (NO 3 − ) to ammonia (NH 3 ) holds significant potential in the control of nitrogen oxide (NO x ) from stationary sources. However, previous studies on reaction intermediates remain unclear. Here we report that PdCu/Cu 2 O hybrids with mesoporous hollow sphere structure show high selectivity (96.70%) and Faradaic efficiency (94.32%) for NH 3 synthesis from NO 3 − . Detailed characterizations demonstrate that (1) Pd enables electron transfer (Pd 3d → Cu 3d) and causes the polarization of Cu 3d orbitals by forming partial PdCu alloys, which makes Pd electron deficient but offers empty orbits to adsorb NO 3 − , and (2) electron-rich Cu is more conducive to the occurrence of NO 3 − reduction. The mutual confirmation of online differential electrochemical mass spectrometry and density functional theory calculations demonstrates that PdCu alloys block the generation of ∗NOH intermediate and facilitate the formation of ∗N, providing a new mechanism for NH 3 synthesis from NO 3 − reduction reactions. • PdCu/Cu 2 O hybrids with alloying effect for electrocatalytic NH 3 synthesis • Pd causes the polarization of Cu 3d orbitals by forming partial PdCu alloys • PdCu alloys facilitate the formation of ∗N intermediates for better NH 3 synthesis The reduction treatment of NO x by traditional methods is conducted at high temperature (>400°C), and this process requires considerable energy. Our approach is to convert NO x into NO 3 − in solution by an oxidation and leaching absorption process and subsequently effectively convert NO 3 − into value-added NH 3 (a hydrogen source and a fertilizer). Electrocatalytic reduction of NO 3 − to NH 3 holds important potential in the control of NO x . However, previous studies have not been involved in the regulation of intermediates. By depositing Pd species on Cu 2 O, we achieved significantly improved NH 3 synthesis performance. Pd species make Cu 3d orbitals easier to polarize and act as new adsorption sites instead of Cu 2 O. Detailed characterizations demonstrate that highly dispersed PdCu alloys formed by the deposition of Pd 0 on Cu 2 O can tune the original NH 3 synthesis path of Cu 2 O by facilitating the formation of the ∗N intermediate, ultimately leading to a higher selectivity and Faradaic efficiency. PdCu/Cu 2 O hybrids with a mesoporous hollow sphere structure can efficiently synthesize NH 3 . Pd species make Cu 3d orbitals easier to polarize and act as new adsorption sites of NO 3 − instead of Cu 2 O. The highly dispersed PdCu alloys can efficiently facilitate the formation of ∗N and block the generation of the ∗NOH intermediate, leading to a remarkable selectivity toward NH 3 and Faradaic efficiency. This work highlights a promising route to design Cu 2 O hybrids for selective electrocatalytic NH 3 synthesis to control NO x emission.