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

Herein, aqueous nitrate (NO ) 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. NO reduction proceeds through nitrite (NO ) and then nitric oxide (NO), before diverging to form either dinitrogen (N) or ammonium (NH ) as final products, with N preferred in potable water treatment but NH preferred for nitrogen recovery. It is shown that the NO and NO starting feedstocks favor NH formation using Ru-rich catalysts, while Pd-rich catalysts favor N formation. Conversely, a NO starting feedstock favors NH 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 NO and NO feedstocks, the thermodynamics of the competing pathways for N-H and N-N formation lead to preferential NH  or N production, respectively, while Ru-rich surfaces are susceptible to poisoning by NO feedstock, which displaces H atoms. This leads to a decrease in overall reduction activity and an increase in selectivity toward N production. Together, these results demonstrate the importance of tailoring both the reaction pathway thermodynamics and initial reactant binding energies to control overall reaction selectivity.