Electrochemical reduction of NO_x species at the interface of nanostructured Pd and PdCu catalysts in alkaline conditions

Electrochemical reduction of NO_x species such as nitrates (NO₃^{−), nitrites (NO₂₋₎}, nitric oxide (NO), nitrogen dioxide (NO₂) and their mixtures, was studied at the interface of palladium (Pd) and palladium-copper (PdCu) nanoparticles supported on carbon Vulcan (C). The electro-catalysts were synthesized by impregnation route with a low noble metal content of 5% wt. Pd and 2.5% wt. Pd for the mono and bi-metallic electrocatalyst, respectively. It was found by XRD analysis the formation of a solid solution in the bi-metallic catalyst and the TEM analysis suggest that the incorporation of copper decreases the particle size from 12 to 3 nm in comparison with its counterpart free of copper. Also, XPS technique verify the presence of Pd and Cu species in their metallic-oxidation states. Linear sweep and cyclic voltammetry techniques were used for the evaluation of the electrochemical NO_x reduction, using alkaline solutions of NO₂⁻ or NO₃⁻ saturated with NO₂ (synthesized in-situ) and NO (from commercial source). The results showed that the catalytic-activity at the current versus potential (i-E) characteristics improves significantly due to the presence of copper (as also demonstrated by CO-stripping-electrochemical active surface area calculations), inhibiting the process associated with the hydrogen evolution reaction. It is also noted in this work that the reduction faradic-current is c.a. 6 times higher at saturated solutions with NO and NO₂. The NO_x species were reduced mainly to nitrogen, ammonia and hydrazine as confirmed using on-line differential electrochemical mass spectrometry (DEMS) during steady-state experiments.