Divergent catalytic behaviors of Pt and Pd films in the cathode of a sandwiched type membrane reactor

Proposed schematic representation of NO3− reduction mechanism in the membrane reactor preferentially occurred by the Ag–Pt and Ag–Pd sites. On the cathode surface, Pt particles mainly reduced nitrate ions by heterogeneous electron transfer reaction. Here the Ag acted as electron promoting particles. By contrast, Pd reduced nitrate ions mainly by hydrogenation reactions. Water is splitted on the Pt anode surface generating H+, which is migrated to the cathode surface through the 180μm thick H+ conducting. [Display omitted] ► In the reactor, most active cathode is Ag–Pt–Pd. ► Ag–Pt favored electron transfer reaction. ► Ag–Pd favored hydrogenation reaction. Relative electrochemical and catalytic hydrogenation responses to NO3− reduction by the Ag modified Pt, Pd and Pt–Pd cathodes, deposited on H+ conducting Nafion membrane, have been studied. The electrochemical investigations noticed the maximum reactivity to show by an Ag–Pt electrode exhibiting a reduction current of 24.26mAcm−2 at −1.26V vs. Ag/AgCl reference electrode. Conversely, the catalytic hydrogenation reaction exhibited maximum nitrate reduction rate by an Ag–Pd catalyst having a rate constant of 25.1×10−3min−1. While both electrochemical reactions and catalytic hydrogenation reactions were combined in the electrolyte reactor, the highest reduction efficiency was found to show by an Ag–Pt–Pd cathode assembly, where the reduction current was 20.12mAcm−2 and the rate constant was 84.3×10−3min−1 under controlled current electrolysis condition. In this paper, reasons of such divergent catalytic behaviors have been discussed.