An Electrochemical Approach for Designing Thermochemical Bimetallic Nitrate Hydrogenation Catalysts

Bimetallic alloy catalysts can achieve a diverse range of reactivity inaccessible to pure metals. Classically, catalytic promotion in alloy catalysts has been ascribed to atomic scale cooperativity between the metal constituents. For catalytic reactions that could involve charge transfer to solution, electron flow across a conductive support can be coupled to ionic flow through solution to permit, in principle, bimetallic promotion even in the complete absence of atomic level connectivity between metal constituents. We examine this hypothesis in the context of nitrate hydrogenation, a reaction that is catalyzed almost exclusively by bimetallic catalysts. Using the state-of-the-art PdCu/C catalyst, we show that the overall nitrate hydrogenation reaction proceeds via the electrochemical coupling of the hydrogen oxidation and nitrate reduction half-reactions. Studies of each metal in isolation reveal that Pd exclusively catalyzes the former, while Cu exclusively catalyzes the latter. These findings suggest that nitrate hydrogenation on PdCu alloys proceeds via galvanic coupling of complementary half-reactions on Pd and Cu, obviating the need for atomic scale cooperativity. Based on this mechanistic insight, we design two new nitrate hydrogenation catalysts, RuCu and RuAg, that operate via the same principle with comparable activity to PdCu. This work enables new strategies for the design of bimetallic catalysis for thermochemical transformations, by pairing metals with disparate electrochemical reactivity.