Sustainable Synthesis of Nitrogenated Compounds

Chemical manufacturing, a major contributor to greenhouse gas emissions, must undergo decarbonization to achieve global net zero emissions by the end of 21st century. The synthesis of NH3 via the energy intensive Haber-Bosch process is a significant contributor to greenhouse gas emissions. Electrochemical NH3 synthesis offers a decentralized and green alternative way which can be achieved by two approaches, namely the direct N2 reduction by using water as the proton source and the mediator approach by using Li to indirectly activate N2. Apart from being an energy intensive process, the Haber Bosch process is also responsible for the disruption of the N2 cycle by releasing reactive nitrogen into the environment. NO3- is a significant form of the reactive nitrogen found in agricultural run-off water, industrial effluents, and ammunition waste. This thesis focusses on two key areas: the electrochemical valorization of waste NO3- and the electrochemical reduction of N2 into NH3. Catalyst screening is performed for the selective electrochemical synthesis of NH3 and urea from NO3-. Co is the active catalyst for synthesizing NH3 and Ag is the active catalyst for producing urea. A maximum NH3 FE of 92.37 ± 6.7 % and an NH3 current density (CD) of 565.26 ± 23.56 mA/cm2 is obtained at -0.8 V vs. RHE on the oxide derived Co, and the process can be operated transiently based on the availability of renewable energy for varied NO¬3- concentrations without loss of selectivity. A maximum urea CD of ~-100 mA/cm2 and a urea FE of ~100% is observed at -1.25 V vs. RHE in an Ag gas diffusion electrode (GDE) configuration. A rational approach is developed to design catalysts, electrolytes, and an electrochemical cell for the electrochemical reduction of N2 to NH3 in aqueous media. The effect of operating conditions such as pressure, ethanol concentration, and Li salts are investigated for the Li mediated NH3 synthesis process. A maximum NH3 FE of ~70 % and a maximum NH3 CD of ~-100 mA/cm2 are observed at an applied current density of -150 mA/cm2, when 0.065 ethanol concentration, 20 bar N2 pressure, and 3 M LiBF4 are used

Calcium as a Mediator for the Electrochemical Synthesis of NH3

NH3 is an important commodity chemical to make fertilizers, pharmaceuticals, textile fibers, ammunition, etc. Haber-Bosch process used to make NH3 has a massive carbon footprint associated with it and it is desired to synthesize NH3 in a sustainable manner. Li-mediated NH3 synthesis is a promising approach to make NH3 at ambient conditions and it has been widely investigated. In this letter we explore other mediators beyond Li such as Ca, Mg, Sr, Y, and V. Our DFT results suggest that Ca, and Mg are promising mediators. Hence, we experimentally investigated Ca as a mediator and the proposed process is referred to as the Ca-mediated NH3 synthesis. An NH3 FE of 15.05 ± 2.50 % was obtained from the Ca-mediated NH3 synthesis at 50 bar. This letter serves as the proof of concept for Ca-mediated NH3 synthesis and this work would motivate further research to improve the performance for Li-free ammonia synthesis

Discovery of Ag as an Active and Selective Catalyst for the Electrochemical Synthesis of Urea from NO3- and CO2 with ~100 % Selectivity at -100 mA/cm2 Urea Current Density

The current method to synthesize urea is highly energy intensive and has a massive carbon footprint. Electrochemical synthesis of urea from NO3- and CO2 is an attractive and sustainable way as renewable energy can be used to synthesize green urea at ambient conditions by utilizing the waste NO3- and CO2 from the air or flue gas. In this work, we conduct a thorough catalytic screening on various metal-based catalysts. ~100 % urea Faradaic efficiency and ~-100 mA/cm2 of urea current density is observed at -1.2 V vs. RHE when Ag GDE is used. FTIR analysis further confirms the formation of urea and the presence of *CO intermediates. The excellent kinetics and selectivity towards urea on Ag are explained by a combination of facile first and second C-N bond formation steps and an endergonic (ΔG > 1.5 eV) formamide (HCONH2) formation step from *CONH2 from our DFT studies

Chloride-Promoted High-Rate Ambient Electrooxidation of Methane to Methanol on Patterned Cu–Ti Bimetallic Oxides

The selective electrochemical methane oxidation reaction (MOR) has been challenging due to higher CH4 activation barriers and lower solubility at ambient conditions. Here, we synthesize a conductive gas-diffusion layer patterned with alternating squares of Cu and Ti oxides to achieve highly selective MOR at interfaces consisting of Cu–Ti bimetallic oxides. We observe high MOR faradaic efficiencies of ∼28% at ambient conditions and ∼72% at near-ambient conditions (40 °C) to value-added products such as CH3OH and HCOOH in a Cl–-mediated environment. Density function theory calculations suggest that despite TiO2 exhibiting barrierless thermochemical methane dissociation, an electrochemical oxidation pathway is likely competitive at high overpotentials. The unprecedented current densities of ∼10 mA/cm2 for HCOOH and ∼16 mA/cm2 for CH3OH, along with these insights on ambient MOR, will enable the development of highly efficient electrochemical systems for the utilization of CH4

Chloride-Promoted High-Rate Ambient Electrooxidation of Methane to Methanol on Patterned Cu–Ti Bimetallic Oxides

The selective electrochemical methane oxidation reaction (MOR) has been challenging due to higher CH4 activation barriers and lower solubility at ambient conditions. Here, we synthesize a conductive gas-diffusion layer patterned with alternating squares of Cu and Ti oxides to achieve highly selective MOR at interfaces consisting of Cu–Ti bimetallic oxides. We observe high MOR faradaic efficiencies of ∼28% at ambient conditions and ∼72% at near-ambient conditions (40 °C) to value-added products such as CH3OH and HCOOH in a Cl–-mediated environment. Density function theory calculations suggest that despite TiO2 exhibiting barrierless thermochemical methane dissociation, an electrochemical oxidation pathway is likely competitive at high overpotentials. The unprecedented current densities of ∼10 mA/cm2 for HCOOH and ∼16 mA/cm2 for CH3OH, along with these insights on ambient MOR, will enable the development of highly efficient electrochemical systems for the utilization of CH4