Copper-immobilized platinum electrocatalyst for the effective reduction of nitrate in a low conductive medium: Mechanism, adsorption thermodynamics and stability

The electrocatalytic reduction of NO3- and its intermediate NO2- in neutral medium was performed at a Cu-immobilized Pt surface. The voltammetric investigations showed that the bare Cu electrode has little effect on nitrate reduction reactions (NRR) whereas an enhanced catalytic effect (i.e. a positive shift of the peak potential and an increased reduction current) was observed when Cu particles were immobilized onto Pt surface. At the Cu-Pt electrode surface, the NRR process was observed to occur via a two-step reduction mechanism with a transfer of 2 and 6 electrons in the first and second steps, respectively. Similar results were obtained by chronoamperometric (CA) studies. Closer NRR mechanistic studies at the as prepared Cu-Pt electrode revealed concentration-dependent kinetics with a critical nitrate ion concentration of ca. 0.02 M. Moreover, NRR proceeded via a simple adsorption-desorption mechanism following a Langmuir isotherm with an adsorption Gibbs free-energy of ca. -10.16 kJ mol(-1) (1st step) and ca. -10.05 kJ mol(-1) (2nd step). By means of a Pt vertical bar Nafion vertical bar Cu-Pt type reactor without any supporting electrolyte, bulk electrolysis was performed to identify nitrate reduction products. It was found that after 180 min of electrolysis, 51% of NO3- was converted into NO2- intermediate. This percentage decreased to 30% in CO2 buffered conditions. However, when a tri-metallic Pt-Pd-Cu electrode was employed as a cathode, all of the NO2- produced could be successfully converted into NH3 and N-2. The electrocatalysis of nitrate ion on Cu-Pt electrode surface showed no apparent surface poisoning as confirmed by its stability after excessive CV runs. This was further supported by surface analysis and morphology of the as-prepared catalyst with scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis. (C) 2014 Elsevier B.V. All rights reserved.