Toward the sustainable production of ammonia by integration of electrochemistry with light energy and nanostructured catalysts

Nixon, Rachel

Permalink

https://hdl.handle.net/2142/129745 Copy

Description

Title

Toward the sustainable production of ammonia by integration of electrochemistry with light energy and nanostructured catalysts

Author(s)

Nixon, Rachel

Issue Date

2025-04-29

Director of Research (if dissertation) or Advisor (if thesis)

Jain, Prashant K.

Doctoral Committee Chair(s)

Jain, Prashant K.

Committee Member(s)

• Rodríguez-López, Joaquín
• Olshansky, Lisa
• Su, Xiao

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• ammonia synthesis
• nitrate reduction
• plasmon-assisted electrochemistry
• waste upconversion
• nanostructured catalysts

Abstract

As a key component of fertilizers, refrigerants, and explosives, ammonia is a commodity chemical of pivotal importance in the global economy. With its relatively easy storage and higher volumetric energy density than liquid hydrogen, liquid ammonia also holds value as a zero-carbon fuel, further adding to its market demand. The Haber-Bosch process outcompetes geological and biological processes to meet this global demand for ammonia; however, this process faces limitations in terms of sustainability. It utilizes hydrogen gas sourced from steam methane reforming and requires energy-intensive extreme temperature and pressure conditions leading to sizeable greenhouse gas emissions. Biological, electrochemical, and photochemical routes of converting nitrogen gas to ammonia under ambient conditions have emerged as more sustainable alternatives to the Haber-Bosch process but suffer from low yield rates due to the stability of the dinitrogen bond. In recent years, there has been growing interest in developing methods to convert nitrate, a common pollutant in agricultural and industrial wastewater streams, to ammonia. Although the abundance of nitrate pollution is likely not high enough to provide the high volumes of ammonia supplied by the Haber-Bosch process, the opportunity to transform the nitrate, which poses threats to both the environment and human health, to the value-added product of ammonia could incentivize environmental remediation efforts. Nitrate reduction therefore represents a means of closing the nitrogen cycle and a more sustainable route toward ammonia synthesis when conducted electrochemically under ambient conditions and when using an abundant resource such as water as the proton source rather than hydrogen gas. Unfortunately, many recent reports of electrocatalysts for nitrate reduction involve low selectivity for ammonia, low ammonia synthesis rates, poor catalyst stability, or cumbersome catalyst fabrication processes. Also, few reports demonstrate reduction of nitrate from samples with environmentally-relevant conditions, namely low nitrate concentrations on the order of 1 mM. The work in this dissertation addresses many of these limitations and advances the field of nitrate upconversion by integrating innovative approaches such as plasmonics, bimetallic nanostructured catalysts, and plasma conversion with electrochemistry. Chapter 2 describes a new approach toward enhancing electrochemical reactions in which light energy is used to stimulate electrocatalysis by exploiting a phenomenon known as plasmonic excitation. Electrocatalyst materials such as coinage metal nanostructures absorb light in the visible region and upon absorbing the radiation, their conduction band electrons collectively oscillate and activate the plasmonic excitation. This excitation then decays to produce several effects that act synergistically with the applied bias to catalyze reactions involving molecules adsorbed on the nanostructure surfaces. The strong resonance of the plasmonic excitation can lead to enhanced electromagnetic fields around the nanostructure, which can potentially activate adsorbates. Plasmon-induced energetic charge carriers (i.e., electrons with energies above the Fermi level and energetic holes below the Fermi level) can facilitate reduction and oxidation processes. Finally, localized heating around the nanostructure can cause Arrhenius enhancement of reaction rate. We applied this plasmon-assisted electrochemistry approach to the conversion of nitrate to ammonia, as described in Chapter 3. At a potential of –0.3 V vs. the reversible hydrogen electrode (RHE), nitrate is converted to ammonia on an electrode comprised of gold nanoparticles coated on a glassy carbon electrode (GCE); however, the conversion is sluggish. By plasmonic excitation of the electrode, a 15× larger rate of ammonia electrosynthesis is accomplished at this potential. We further confirmed that this enhancement was non-thermal in nature and due to the action of energetic carriers generated by plasmonic excitation. This resolution was accomplished by measuring the local temperature of the electrode surface under plasmonic excitation and performing dark control experiments at that measured temperature. In Chapter 4, we address the challenge of converting low concentrations of nitrate to ammonia by fabricating a new bimetallic catalyst consisting of Au and Fe. By electrodepositing Fe onto Au nanoparticles, we obtained a AuFe catalyst that possesses nanoscale features and exhibits a higher mass-specific activity of ammonia electrosynthesis than both the original Au nanoparticles and a commercial Fe foil. This bimetallic composition also provides a more positive nitrate reduction onset potential than the Au nanoparticle catalyst and a higher selectivity for ammonia and lower selectivity for the undesirable nitrite byproduct than the Fe foil. This bimetallic nanostructure also was shown to catalyze the conversion of nitrate at concentration levels found in polluted river water, demonstrating practicality for environmental remediation purposes. Finally, to enable ammonia synthesis from abundant nitrogen gas in the atmosphere, we apply the plasmon-assisted electrochemical approach to the reduction of nitrite derived from plasma-induced oxidation of air, as described in Chapter 5. Nitrite solutions produced by plasma-induced reactions in mixtures of nitrogen and oxygen as well as air were subjected to plasmon-assisted electroreduction and successfully converted to ammonia. Each of these methods therefore represents a route toward sustainable upconversion of waste to ammonia.

Graduation Semester

2025-05

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/129745

Copyright and License Information

Copyright 2025 Rachel Nixon

Owning Collections