Tuning the Selectivity of Nitrate Reduction via Fine Composition Control of RuPdNP Catalysts

Herein, aqueous nitrate (NO₃ ^-) reduction is used to explore composition-selectivity relationships of randomly alloyed ruthenium-palladium nanoparticle catalysts to provide insights into the factors affecting selectivity during this and other industrially relevant catalytic reactions. NO₃ ^- reduction proceeds through nitrite (NO₂ ^-) and then nitric oxide (NO), before diverging to form either dinitrogen (N₂) or ammonium (NH₄ ⁺) as final products, with N₂ preferred in potable water treatment but NH₄ ⁺ preferred for nitrogen recovery. It is shown that the NO₃ ^- and NO starting feedstocks favor NH₄ ⁺ formation using Ru-rich catalysts, while Pd-rich catalysts favor N₂ formation. Conversely, a NO₂ ^- starting feedstock favors NH₄ ⁺ at ≈50 atomic-% Ru and selectivity decreases with higher Ru content. Mechanistic differences have been probed using density functional theory (DFT). Results show that, for NO₃ ^- and NO feedstocks, the thermodynamics of the competing pathways for N-H and N-N formation lead to preferential NH₄ ⁺ or N₂ production, respectively, while Ru-rich surfaces are susceptible to poisoning by NO₂ ^- feedstock, which displaces H atoms. This leads to a decrease in overall reduction activity and an increase in selectivity toward N₂ production. Together, these results demonstrate the importance of tailoring both the reaction pathway thermodynamics and initial reactant binding energies to control overall reaction selectivity.