Lean Cu-immobilized Pt and Pd films/-H+ Conducting Membrane Assemblies: Relative Electrocatalytic Nitrate Reduction Activities

Asymmetric reactors having the configuration (Pt|Nafion|M-Cu, M = Pt, Pd) were designed and applied to the electrocatalytic nitrate reduction reaction (NRR) with no supporting electrolyte. The electrochemically-fabricated Cu-M (M = Pt, Pd) catalysts showed a highly-efficient catalytic activity by effectively reducing more than ca. 99% of nitrate ions, under static-reactor conditions, with first-order rate constants of ca. 0.040 min−1 and ca. 0.057 min−1 for Cu-Pt and Cu-Pd, respectively. Similar reactivity trends were observed under flow mode irrespective of flow rates while NRR rates were predominantly controlled by diffusion-limited mass-transfer. A maximum of ca. 93% nitrate removal at Cu-Pd catalyst was achieved within ca. 60 min. The comparatively lower CuO content of ca. 9%, as confirmed by XANES, resulted in a greater number of free NRR active sites inducing the superior catalytic activity of the Cu-Pd catalyst. On the other hand, NRR product selectivity investigations showed that Cu-Pd catalyst was highly ammonia-selective with a maximum of ca. 24% selectivity at a flow rate of 0.1 mL min−1, while Cu-Pt catalyst was much selective to intermediate nitrite ions reaching a maximum of ca. 35% selectivity at the same flow rate.