Nitrate reduction pathways on Cu single crystal surfaces: Effect of oxide and Cl−

The origin of different nitrate reduction activity between the (100), (111), and (110) faces of Cu is examined using vibrational spectroscopy and calculations. Shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS) reveals a suite of intermediates from the nitrate reduction process on Cu(100), Cu(111), and Cu(110) including NO2− and HNO. All three faces show similar intermediates, suggesting the same mechanism is operative on all of them. Critical to the reduction pathway on the bare Cu surfaces is the reduction of nitrate to nitrite concomitant with partial oxidation of the Cu surface. This priming action facilitates nitrate reduction and reduces overpotentials, particularly on the Cu(111) and Cu(110) faces, which are more susceptible to oxidation. Decoration of the surfaces with Cl− suppresses nitrate reduction, resulting in higher overpotentials and lower current density. NH3 is observed by SHINERS as a direct nitrate reduction product in the presence of Cl−, rather than NOx species observed on the bare Cu surfaces, indicating a reaction pathway unique from the bare, undecorated surface.