Electrochemical Nitrate Reduction to Ammonium by Electropolymerized Molecular Catalyst Films
Creators: name: Askari, Maiko
Publisher: name: My University
Year: 2024
Subjects: subject: nitrate reduction
subject: electrocatalysis
subject: Chemistry
subject: Science

description: Electrochemical reduction of nitrate to ammonia is a promising way to convert a ubiquitous waste product into value added chemicals. By using molecular catalysts, catalysts can be tuned synthetically to modulate activity and selectivity for small molecule transformations. One downside, however, is the relative low activity of molecular catalysts compared to heterogenous counterparts, and the challenges of designing soluble molecular catalysts especially for reduction of nitrates, where many molecular catalysts are not stable in aqueous solution. By immobilizing molecular catalysts onto a support, a hybrid system with synthetically tunable catalyst sites and higher rates can be designed with comparable activity to solid state catalysts for nitrate to ammonia. In this dissertation I will discuss a novel polymeric chromium catalyst that can reduce nitrate to ammonia with rates and selectivity comparable to state-of-the-art solid-state electrocatalysts, understanding the loading dependence activity of this polymeric system to nitrate reduction activity, and finally integrate this catalyst into a low-temperature carbon capture device that operates on reversible nitrate reduction chemistry. Chapters 1 provides background information for this thesis. Chapter 1 includes a discussion of mechanisms and strategies used for designing molecular catalysts for nitrate and nitrite reduction. In Chapter 2, I will discuss the novel electropolymerized chromium terpyridine (p-TPTCrCl3) films for nitrate reduction. This catalyst system is capable of nitrate reduction with mass activity rates comparable to solid-state heterogenous catalysts for nitrate reduction at mild potentials. I found that the p-TPTCrCl3 system follows a proposed cascade mechanism, in which it can reduce intermediates sequentially to ammonium, also evidenced by response to changes in mass transport seen in stir-rate dependence. In Chapter 3, I expanded upon the understanding of the system discussed in Chapter 2, especially with the per-catalyst site activity for nitrate reduction turnover. I studied the impact of polymerization method and catalyst loading on electrochemical properties as well as film morphology. I found that film loading has a strong effect with low current densities for films deposited by potentiodynamic deposition resulting in rough films with high precipitate seen by SEM. For the potentiostatic films, lower coverage films had much higher per-site turnovers compared to the system studied in Ch 2. In Chapter 4, in collaboration with the Carlos Morales-Guio group at UCLA, I integrated the p-TPTCrCl3 system into a low temperature CO2 capture cell based on the reversible nitrate/ammonia redox processes. CO2 is captured by an ammonia-based mixed salt process generating captured CO2 in the form of ammonium carbamate. At the anode the captured CO2 is oxidized to release CO2 generating NO3- from oxidized ammonia, and at the cathode, nitrate is reduced back to ammonia by cathodes including p-TPTCrCl3 deposited on a porous carbon support to regenerate the ammonia. I found the operating mechanism is a pH swing where the pH at the anode swings to low pH, shifting the equilibrium to release CO2. Finally, in Chapter 5, I discuss future directions I believe this work should take, and some preliminary data.