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

Abstract 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.