Electrocatalytic Nitrous Oxide Reduction Reaction at Sn-Modified Pd–Pt Single Crystalline Electrodes in Acidic Media

Nitrous oxide (N₂O) is a greenhouse and an ozone-depleting gas. Electrocatalytic N₂O reduction reaction (N₂ORR) is known to be catalyzed at noble metal electrodes such as Pd and Pt, and the surface modification of such noble metals with Sn is known to increase the N₂ORR in acidic media. However, the role of Sn at the surface remains unclear. In this work, N₂ORR activity was investigated for single-crystalline Pt, Pd, and Pd–Pt electrodes with the (111) or (100) plane in the presence and absence of Sn at the electrode surface in acidic media. In situ X-ray crystal truncation rod (CTR) measurements of Sn-modified Pt(111) and Pd(111) electrodes revealed the presence of metallic Sn and SnO at their surfaces. The surface Sn modification enhances the N₂ORR activity for Pd–Pt(100) or Pd(100) electrodes but not for the Pt(111), Pd–Pt(111), or Pt(100) electrodes. The Sn-modified 15 atom% Pd–Pt(100) electrode shows higher N₂ORR activity than Sn-modified Pd(100) or Pt(100) electrodes, indicating that the copresence of Pd and Pt at the (100) surface with Sn is important to maximize the N₂ORR activity. Density functional theory (DFT) calculations revealed that the N₂ORR activity of the Sn/Pd–Pt(100) electrode can be attributed to (1) the strong N₂O adsorption capacity of the (100) surface, (2) the reduction of H poisoning by Pd–Pt alloying, and (3) the activation of N₂O molecules by Sn modification. Our model studies using atomically defined single-crystalline electrodes will enable researchers to design and develop practical N₂ORR electrocatalysts from the surface engineering point of view and then contribute to global warming mitigation and stratospheric ozone protection through N₂O removal.