Enhanced electrocatalytic nitrate reduction by preferentially-oriented (100) PtRh and PtIr alloys: the hidden treasures of the 'miscibility gap'

Bimetallic alloys of Pt and Rh or Ir were prepared with pulsed laser deposition (PLD) on a well-ordered MgO (100) substrate, leading to epitaxial growth along the [001] plane, as confirmed by surface analysis techniques. The out-of-equilibrium conditions of PLD allowed the exploration of a range of compositions for which phase separation would instead be expected. The electrochemistry of a series of PtRh and PtIr (100) alloys was investigated in 0.5 M H2SO4, showing a surprisingly intense electrocatalytic activity towards the reduction of nitrate for a Pt content of 21-42%. These alloys feature a lower reaction overpotential with respect to Rh and Jr while outperforming the pure metals in terms of reduction current. A detailed analysis of the voltammetric features with respect to alloy composition highlighted a correlation between hydrogen desorption and nitrate reduction activity. In addition, an optimal potential range for nitrate reduction, common to PtRh and air alloys, was observed, corresponding to the potential window in which nitrate adsorption coincides with fast reduction of the key reaction intermediate NOads.