Rh-Based Catalysis Chemistry of Aqueous-Phase Nitrogen Treatment Reactions
Over the past century, the increase in human population has driven a need for the increased fixation of atmospheric nitrogen to produce nitrogen-based fertilizers and munitions. These anthropogenic processes have led to significant pollution of freshwater by inorganic nitrogen species (e.g. NO3-, NO2-, NH4+). Nitrate (NO3-) and nitrite (NO2-) consumption is toxic to humans, and ammonia/ammonium (NH3/NH4+) is toxic to aquatic life. Both the oxidized and reduced species contribute towards eutrophication and soil acidification. Catalytic denitrification has emerged as a promising technology for the remediation of nitrogen-contaminated waters. The primary goal of this work is to investigate rhodium (Rh)-based catalysts for the remediation of inorganic nitrogen contaminants in water. This study addresses the effects of pH and salinity on the catalytic mechanism of nitrate/nitrite hydrogenation. A new support material for Rh was developed and a bifunctional reaction mechanism was found for ammonia oxidation. This work provides new insights into nitrogen conversion mechanisms in water over Rh, and insights into the design of active and selective catalysts for a desired treatment application. The NO2- reduction properties of alumina-supported Rh were investigated using Pd as a benchmark, where the bulk solution pH was varied to probe the effect of reaction conditions on the catalytic chemistry. Pd expectedly showed high reduction activity and high N2 selectivity at low pH, and near inactivity at high pH. Rh, while inactive at low pH, showed moderate activity and high NH4+ selectivity at high pH. Hydrazine (N2H4) was also detected as a reaction intermediate when NH4+ was formed. Density functional theory (DFT) simulations of the reactions performed by collaborators revealed new insights into the reaction mechanism at different pH, which were corroborated by in aqua surface-enhanced Raman spectroscopy (SERS). These results update the common view that only Pd-based catalysts are effective for NO2- reduction and suggest unexplored avenues for nitrogen chemistry. The effect of chloride on the NO3- hydrogenation activity and selectivity was evaluated for carbon-supported rhodium and palladium catalysts. In the absence of buffer, chloride monotonically increases the overall catalytic activity of In-Rh/C. In-Pd/C showed a volcano-shaped dependence on the nitrate reduction rate showing lower activity in 2M NaCl than DI water. CO-SERS analysis of chemisorption on model substrates suggested that suggests that NaCl has an electronic effect on both metals which decreases the N-O bond strength of the crucial NO* surface intermediate. The In-Rh/C catalyst also demonstrated high activity for nitrate reduction towards ammonia in simulated ion-exchange brines implying it may be an option for nutrient recovery from these streams. A new Rh supported on a semi-crystalline niobium oxide support was developed to activate aqueous ammonium at room temperature and atmospheric pressure, converting into gas-phase nitrogen species (N2, NO, N2O) and trace amounts of NO3- using only O2. Raman spectroscopy correlated the chemical treatment of the support prior to metal addition to the generation of terminal Nb=O species, which are crucial for catalytic activity. Isotopic experiments reveal a bifunctional mechanism, where Nb binds and converts ammonium to ammonia, which decomposes on Rh. While further optimization is needed to drive selectivity towards N2, Rh supported on modified niobium may be the basis of a new generation of catalysts that can oxidize NH4+ under mild conditions. In addition to nitrogen contaminant degradation for clean water applications, I also investigated Zr-based metal-organic framework (MOF) materials for the adsorptive treatment of perfluorooctane sulfonate (PFOS) contaminated water. In this work, the sorption properties of UiO-66 metal-organic frameworks (MOFs) with varying defect were studied for the removal of PFOS. Large pore defects (~16 and ~20 Å) within the framework were critical to increasing the capacity due to higher internal surface area and an increased number of coordinatively unsaturated Zr sites to bind PFOS head groups. The enhanced PFOS adsorptive properties of UiO-66 highlight the advantage of structurally defective MOFs as a water treatment approach towards environmental sustainability.
rhodium, catalytic denitrification, catalyst, surface-enhanced raman spectroscopy, nitrate, nitrite, ammonium, ammonia
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