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 approximate to 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. Randomly alloyed RuPdNPs are used to examine the mechanisms that influence selectivity during aqueous nitrate (NO3-) reduction to form either ammonium (NH4+) or N2. Experimental results starting with either nitrate, nitrite (NO2-), or NO combined with density functional theory (DFT) calculations demonstrate that the end-product selectivity is determined both by the binding energies of the initial reactants and the formation energies of important intermediate species. image