An electrochemical approach for designing thermochemical bimetallic nitrate hydrogenation catalysts

Abstract

Classically, catalytic promotion in bimetallic catalysts has been ascribed to atomic-scale cooperativity between metal constituents. For catalytic reactions that could involve charge transfer, electron and ion flow may engender bimetallic promotion without atomic-level connectivity. Here we examine this hypothesis in the context of nitrate hydrogenation, a reaction catalysed almost exclusively by bimetallic catalysts. On state-of-the-art PdCu/C, nitrate hydrogenation to nitrite proceeds via electrochemical coupling of hydrogen oxidation and nitrate reduction half-reactions; Pd catalyses the former, while Cu catalyses the latter. Using this mechanistic framework, we predict how different Pd:Cu ratios affect nitrate hydrogenation rates, and rationalize the catalytic activity observed in PtAg/C and Ru/C. Finally, by only promoting the electrochemical hydrogen oxidation reaction with Ni(OH)2, we synthesize PdNi(OH)2Cu/C catalysts with comparable nitrate hydrogenation activity to our best-performing PdCu/C using fivefold less Pd. This work provides an alternative strategy for designing alloy catalysts for thermochemical redox transformations. The mechanism by which bimetallic catalysts can outperform their monometallic counterparts is often unexplained. Now nitrate hydrogenation on bimetallic catalysts is shown to proceed via the electrochemical coupling of hydrogen oxidation and nitrate reduction half-reactions, each of which occurs on one metal component.