Document Type
Dissertation
Date of Award
12-31-2022
Degree Name
Doctor of Philosophy in Chemical Engineering - (Ph.D.)
Department
Chemical and Materials Engineering
First Advisor
Xianqin Wang
Second Advisor
Xiaoyang Xu
Third Advisor
Joshua Young
Fourth Advisor
Mengqiang Zhao
Fifth Advisor
Zafar Iqbal
Abstract
The urgent climactic crisis currently endangering all life on earth is largely due to the combustion of fossil fuels as an energy source. This work combines physics, chemistry, mathematics and computer science to develop new ways to harvest, store and use sustainable energy. In particular, this work centers on polymeric nitrogen (PN). PN is highly desirable because the energy density is extremely high. This energy density lends PN to applications such as rocket fuel and energetic materials. Recently, the polymeric phase N8 was shown to be highly active for oxygen reduction reactions (ORR); the material is an excellent electrocatalyst for applications such as fuel cells and batteries. This study seeks to contribute to the body of scientific knowledge, acquiring a better understanding of the chemical and physical properties of PN-but there is also the practical goal of furthering sustainable energy technology.
Until recently, very few PN phases have been synthesized, although many have been theoretically predicated. In particular, the crystalline cubic gauche phase and the black phosphorus phase were synthesized using diamond-laser systems from an N2 precursor. The experimental difficulty likely stems from the high pressure and temperature required to break the bonds of N2. In our group, cyclic voltammetry (CV)was used to produce N8-from azide N3- at ambient conditions. This study seeks to explore the possible phases for PN from azide, with a particular focus on N8.
Based on recent experimental work, the synthesis mechanism of N8 is explored in this work. N3* radical acts as the building block to N6 and then to longer chains. CV can provide energy high enough to overcome the activation energy barriers to form N12, but then dissociate into relatively stable N8.
In this synthesis mechanism, the elementary steps, transition states and energy barriers are determined through ab initio density functional theory methods. The information from the theoretical study can guide more effective PN synthesis methods, as yield enhancement occurs through ultra-violet (UV) light exposure. The effects of the substrate on PN synthesis, through changes in electronic structure, are also explored.
The catalytic properties of PN are investigated using theoretical methods. The ORR active phase for N8 is Cs; this phase forms upon O2 adsorption to the PN. For N8, the differences in acidic and alkaline media are articulated to show the importance of *OH desorption in the ORR current density. Similarly, the oxygen evolution reaction (OER) in a Li-air battery is studied. Properties such as the charging potential show better values than Pt.
Based on these chemical and physical insights, PN can be designed as a highly active, green, inexpensive and stable nonmetallic catalyst for ORR and OER. Computational analysis leads to a fundamental understanding of PN properties, and relates experimental phenomena to the material structure. Computational techniques thus enable the development of novel morphologies and compositions, by both explaining experimental observations and predicting new possibilities.
Recommended Citation
Alzaim, Safa, "Polymeric nitrogen: computational catalysis and materials design" (2022). Dissertations. 1914.
https://digitalcommons.njit.edu/dissertations/1914
