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

Herein, aqueous nitrate (NO 3 - ) 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 3 - reduction proceeds through nitrite (NO 2 - ) and then nitric oxide (NO), before diverging to form either dinitrogen (N 2 ) or ammonium (NH 4 + ) as final products, with N 2 preferred in potable water treatment but NH 4 + preferred for nitrogen recovery. It is shown that the NO 3 - and NO starting feedstocks favor NH 4 + formation using Ru-rich catalysts, while Pd-rich catalysts favor N 2 formation. Conversely, a NO 2 - starting feedstock favors NH 4 + 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 3 - and NO feedstocks, the thermodynamics of the competing pathways for N-H and N-N formation lead to preferential NH 4 +  or N 2 production, respectively, while Ru-rich surfaces are susceptible to poisoning by NO 2 - feedstock, which displaces H atoms. This leads to a decrease in overall reduction activity and an increase in selectivity toward N 2 production. Together, these results demonstrate the importance of tailoring both the reaction pathway thermodynamics and initial reactant binding energies to control overall reaction selectivity.