Selective nitrate electrovalorization into ammonia on non-precious metal catalysts for sustainable resourcification

Abstract

The thesis mainly focused on the development of electrocatalysts for proton-coupled electron transfer (PCET) reactions. Firstly, I developed a Cu molecular complex for nitrate electroreduction with high ammonia yield rate in acidic solution, proton kinetics was analyzed and demonstrated significant effect toward nitrate reduction and its competitive hydrogen evolution reaction. Secondly, non-precious metal layered double hydroxides (LDH) were prepared to conduct nitrate or nitrite electroreduction in neutral solution, synergetic effects between Cu and Co can boost nitrate reduction performance together. Thirdly, the LDH was further utilized to conduct C-N bond formation via an electrocatalytic process, a value-added oxime was generated with high Faradaic efficiency. Finally, I constructed a hybrid bilayer membrane (HBM) system for hydrogen peroxide electrooxidation. In Chapter 1, I presented a comprehensive summary on the current understanding of the nitrogen cycle and related strategies to handle waste nitrate. These strategies include industrial nitrate removal methods as well as the emerging electrochemical nitrate reduction technology. In Chapter 2, I provided the general experimental procedures including electrocatalyst preparation, electrochemical measurements, product analyses, and related calculations. In Chapter 3, I addressed the challenges associated to competing hydrogen evolution reaction by tuning Hads adsorption strength on electrode surfaces. CuDAT electrocatalyst was prepared to conduct nitrate reduction in pH 1 with ammonia Faradaic efficiency of 98.9 % and 1.78 mmol h-1 mg-1 yield rate. Kinetic isotope effect (KIE) experiment demonstrated that the ammonia production was correlated to the proton movement. In low nitrate concentrations, hindered proton movement could inhibit hydrogen evolution reaction (HER), thus facilitating nitrate reduction. In high nitrate concentrations, sluggish proton transport restricted the further reduction of intermediate into ammonia. In Chapter 4, I focused on the reaction conditions of nitrate reduction to analyze how anions that are found in real-world wastewater can affect nitrate reduction. Also, synergistic effects between Cu and Co were employed to boost nitrate reduction performance. CuCoAl LDH was synthesized for nitrate reduction to ammonia. Supporting electrolyte showed impact on the nitrate reduction catalytic performance. The synergic effects between Cu and Co were analyzed. Cu lowered nitrate reduction activation barrier and exhibited an enhanced onset potential while suffering from high byproduct nitrite production. Co demonstrated high ammonia selectivity while being limited by high overpotential. The co-incorporation of Cu and Co resulted in excellent nitrate reduction to ammonia with 99.5 % ammonia Faradaic efficiency at a yield rate of 0.22 mol h-1 g-1. In Chapter 5 and 6, I explored nitrate reduction towards unique C-N bond formation via electrochemical means. In Chapter 5, NiFe LDH was designed for nitrite valorization to ammonia with 85 % ammonia Faradaic efficiency at a yield rate of 351 μmol h-1 mgcat-1. Fe-based catalyst showed excellent nitrate reduction activity with increased selectivity toward hydroxylamine byproduct. The introduction of Ni steered the selectivity toward ammonia. The synergistic combination of Ni and Fe together achieve high nitrate reduction activity along with high ammonia selectivity. In Chapter 6, the NiFe LDH was further employed to conduct the co-reduction of nitrite and HCHO, with formaldoxime as the desired product generated at a Faradaic efficiency of 27 % with a yield rate of 30 mmol g-1 h-1. The reaction mechanism was found to involve the condensation of HCHO with NH2OH that was formed from nitrite reduction. 1H and 2D NMR were utilized to probe the origins of protons in the final product. The results showed that protons attached to carbon stemmed from HCHO while the proton on the hydroxyl group came from water. In Chapter 7, I developed a more complicated electrocatalytic platform to alter proton transfer rate. I designed and prepared RuBTA-based SAM and HBM platforms for hydrogen peroxide oxidation. The addition of proton carriers enhanced the reaction current density, suggesting that their capability toward facilitating proton transport rate. Low temperature experiment showed that the proton carriers transferred protons via a “flip flop” diffusion mechanism. These results suggest that proton removal rate can be regulated by introducing lipid and proton carriers to electrocatalytic active sites.