Advanced Reduction of Nitrogen-Oxyanions Using Precious Metal-Based Model Catalysts
Wong, Michael S
Doctor of Philosophy
Nitrate (NO3−) is a contaminant detected globally in surface water and underground aquifers. Nitrate pollution occurs due to the overuse of nitrogen-rich agriculture fertilizers, wastewater discharge, and contaminant leaching from landfills. This anion, in addition to its partially reduced form, nitrite (NO2−), can cause adverse health effects in humans including methemoglobinemia (blue baby syndrome), and is a suspected carcinogen. Pd-based catalytic reduction of nitrate and nitrite to nontoxic dinitrogen has emerged as an advanced treatment technology for drinking water decontamination. The primary goal of this work is to better understand the catalytic mechanisms of nitrate reduction using structure-controlled model palladium (Pd)-based catalysts to catalytically remove NO3−/NO2− from drinking water. The effects of surface coverages of metal promoter, catalyst support, and other promoting metals were explored. This work provides new insights into the reaction mechanism, and the design of catalysts with enhanced activity and selectivity in addition to deactivation resistance in model drinking water. Bimetallic Pd-based catalysts have been found to be promising for treating NO3−/NO2− contaminated waters. Those containing indium (In) are unusually active, but the mechanistic explanation for catalyst performance remains largely unproven. Different surface coverages of In deposited on Pd nanoparticles (NPs) (“In-on-Pd NPs”) exhibited room-temperature nitrate catalytic reduction activity that varies with a volcano-shape dependence on In surface coverage. The most active catalyst had an In surface coverage of 40%, whereas monometallic Pd NPs and In2O3 have nondetectable activity for nitrate reduction. X-ray absorption spectroscopy (XAS) results indicated that In is oxidized in the as-synthesized catalyst; reduces to zerovalent metal in the presence of H2, and reoxidizes following exposure to NO3−. Density functional theory (DFT) simulations from collaborators suggested that sub-monolayer coverage amounts of metallic In provide strong binding sites for nitrate adsorption and lower the activation barrier for the nitrate-to-nitrite reduction step. This improved understanding of the In active site expands the prospects of improved denitrification using metal-on-metal catalysts. The use of magnetic iron oxide (Fe3O4) support was also used to explore the recyclability and reusability of Pd-In nanoparticles. Magnetic catalysts offer the possibility of rapidly eliminating NO3−, without generating a secondary waste stream, and easily reusing for multiple reactions. In order to evaluate the function of Fe3O4 magnetic core, a four-component catalyst (Pd-In/nFe3O4@SiO2) was synthesized and NO3− reduction reaction was conducted in both clean water and simulated drinking water (SDW). The magnetically recoverable bimetallic Pd-In material exhibits excellent chemical stability, reusability, and high nitrate removal efficiency. The Pd-In/Fe3O4@SiO2 contains nanocrystalline magnetite with a silica shell upon which indium-decorated palladium nanoparticles were attached. The SiO2 shell slowed down iron leaching from Fe3O4 and the bimetallic nano-domains showed nitrate reduction activity in deionized (DI) water without obvious deactivation through multiple recovery and reuse cycles. This magnetically responsive reusable catalyst, which retained activity in simulated drinking water, can serve as a design basis for materials to degrade other oxyanion water contaminants. Lastly, the promotional effect of gold in trimetallic InPdAu was explored for nitrate hydrogenation. A range of mixed alloy PdAu nanoparticles (NPs) were synthesized with varying Pd:Au atomic ratios (90:10 to 10:90), before depositing submonolayer amounts of In metal. The resulting series of In-on-PdAu NPs especially Pd-rich samples had higher activity than In-on-Pd NPs for nitrate hydrogenation, due to optimized electronic and ensemble effects between Pd and Au that resulted in acceleration of the intermediate reduction of the overall hydrogenation reaction. The Au-rich NPs had lower activity, likely due to over-dilution of Pd surface that resulted in unfavorable hydrogen and nitrate/nitrite binding energies. In-on-PdAu generally showed higher N2 selectivity than In-on-Pd, respectively. In-decorated mixed PdAu alloy structure further enhances nitrate reduction performance and expands the prospects of improved denitrification using metal-on-metal catalysts. In summary, Pd-based catalysts can be tailored for enhanced activity, selectivity, longevity and reusability, and catalytic treatment holds the promise for advanced nitrogen-oxyanions treatment.
nitrogen-oxyanions; catalytic reduction; precious metal