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.