Modulating Ru-Co bond lengths in Ru1Co single-atom alloys through crystal phase engineering for electrocatalytic nitrate-to-ammonia conversion

Single atom alloys (SAAs) with maximum atomic efficiency and uniform active sites show great promise for heterogeneous catalytic applications. Meanwhile, crystal phase engineering has granered significant interest due to tailored atomic arrangements and coordination environments. However, the crystal phase engineering of SAAs remains challenging owing to high surface energy and complex phase transition dynamics. Herein, Ru1Co SAAs with tunable crystal phases (hexagonal-close-packed (hcp), face-centered-cubic (fcc), and hcp/fcc structure) are successfully synthesized via controlled phase transitions. These SAAs exhibit distinct crystal phase-dependent performance towards nitrate reduction reaction (NO3RR), where hcp-Ru1Co outperforms its counterparts with a NH3 Faradaic efficiency of 96.78% at 0 V vs. reversible hydrogen electrode and long-term stability exceeding 1200 h. Mechanistic investigations reveal that the hcp configurations enables shorter Ru-Co distances, stronger interatomic interactions, and more positive surface potential compared to hcp/fcc-Ru1Co and fcc-Ru1Co, which enhances the NO3− adsorption, reduces the free energy barrier, and suppresses competitive hydrogen evolution. This study reports the synthesis of Ru1Co single atom alloys with tunable crystal phases, revealing that the hexagonal-close-packed phase outperforms others in the nitrate reduction to ammonia due to shorter Ru-Co distances and stronger interatomic interactions, which improve NO3− adsorption.