(2gy) Spectroscopic and Computational Study of Catalytic Nickel Nitride Structures for Plasma-Assisted Ammonia Synthesis

My goal is to make advances in molecular simulation and advanced spectroscopic characterization to address the fundamental and applied challenges to resolve the current global challenges related to energy and environment. Specifically, I focus on determining the structure of catalysts under in situ conditions and the surface reaction mechanism for better utilizing traditional and sustainable feedstocks to produce fuels and chemicals. To understand the nature of catalyst surfaces, I use existing quantum simulation methods and new hybrid methods that bridge micro- and macroscopic scales using the techniques such as machine learning. In situ spectroscopy and steady state kinetic testing are utilized to validate the models and verify computational predictions. With the fundamental understanding of how reactions proceed on catalyst surfaces, we can better design catalysts and chemical engineering processes to selective produce desired products.

I have experience with combining density functional theory (DFT) simulation and in situ characterization on multiple heterogeneous catalytic processes. For example, I successfully identified the structures and catalytic activities of molybdenum species (Mo oxide, carbide, and oxycarbide) supported on ZSM-5 zeolite for methane dehydroaromatization reaction by using DFT simulations and in situ characterizations (IR, Raman, ultraviolet–visible spectroscopy (UV-Vis), and X-ray absorption spectroscopy (XAS)). Selective oxidation of primary alcohols over zeolites (ZSM-5, MOR, and FAU) supported gold catalysts were successfully studied using the same methodology. The complexities of Al distributions and gold-support interactions on different zeolite frameworks were successfully addressed by hybrid quantum mechanics/molecular mechanics (QM/MM) calculations. I also worked on biomass conversion to fuels over bimetallic Pt-based catalysts. Dispersion-corrected density DFT calculations and in situ characterizations (Raman and XAS) were performed to reveal the complicated reaction mechanism of biomass conversion over bimetallic catalysts. Recently, I studied the structure and catalytic activity of nickel nitride for plasma-assisted ammonia synthesis. I designed multiple reaction cells for in situ scanning electron microscope (SEM), IR, Raman, and XAS characterizations under plasma-assisted reaction conditions. I also used facilities at the Princeton Plasma Physics Laboratory (PPPL) for other advanced in situ characterization, such as two-photon laser-induced fluorescence (TALIF), surface frequency generation spectroscopy (SFG), electron paramagnetic resonance (EPR), and Kelvin probe diagnostic. DFT calculations were performed to interpret the in situ spectra and predict the catalytic activity. Beyond catalysis, I performed DFT calculations to predict the properties of a wide range of materials. For example, I calculated the band gaps of reduced graphene oxide (rGO) with different oxygen-containing functional groups. Based on calculations, epoxide groups are unique for allowing to tune the band gap, while other oxygen-containing functional groups are not effective. Tuning the band gap of rGO with epoxide functional groups is a highly promising alternative to known inorganic and organic semiconductor materials. I also calculated the cation (Li⁺, Na⁺, and K⁺) interaction with hydrophilic polydopamine (PDA) via cation-Ï€ and cation-σ interactions. The fundamental understandings are useful in designing PDA-based hydrogels for water purification.

Based on my research interests and experiences stated above, I have established two specific directions that I will focus on in my future independent research.

1. Plasma-assisted ammonia synthesis over metal nitride (the topic of this presentation)

The synergy between heterogeneous catalysts and excited gas-phase species generated by nonthermal plasma (NTP) has been studied and has shown promising performance for multiple catalytic applications, such as ammonia synthesis, steam reforming of hydrocarbons, and dry reforming of methane. Among these reactions, plasma-assisted catalytic ammonia synthesis from N₂ and H₂ is one of the mostly studied processes due to the high demand for ammonia as a feedstock for fertilizer production and as a promising energy carrier. In addition, ammonia synthesis is a good model reaction for plasma-assisted catalysis, due to the extensive knowledge about thermal catalytic ammonia synthesis and the relative simplicity of the reaction without appreciable byproducts.

In order to optimize catalysts for plasma-assisted catalytic ammonia synthesis, experimental and computational efforts have been undertaken to understand the relationship between the plasma parameters and catalytic activities on different metal catalysts. Despite these efforts, primarily due to the lack of in situ spectroscopic studies, the nature of catalyst surfaces under plasma-assisted reaction conditions are not well understood and fundamental structure-activity relationships are missing. In my current and future studies, I perform in situ/ex situ spectroscopic studies, reaction kinetic testing, and computational studies of the structures and catalytic activity of Ni nitride for plasma-assisted ammonia synthesis.

Ni nitride (Ni_xN_y) formation was indicated by the results of temperature-programmed desorption (TPD), in situ Raman, and ex situ high-resolution X-ray photoelectron spectroscopy (HR-XPS) after N₂- and N₂/H₂-plasma treatment. H₂-plasma treatment resulted in metallic Ni without surface nitride. The catalysts with preformed Ni nitride were initially more active than those with metallic Ni. DFT calculations will be performed to interpret the in situ Raman spectra and identify the actual structure of Ni nitride under plasma-assisted reaction conditions.

Identification of the role of Ni nitride will be useful in the development of improved catalysts for plasma-assisted catalytic ammonia synthesis. Other than ammonia synthesis, better understanding of the formation and reactivity of metal nitride will be helpful in improving catalysts for a wide range of reactions that require transformation of nitrogen-containing molecules, including nitrogen-rich biomass processing and selective catalytic reduction reactions. Beyond catalysis, metal nitride formation due to plasma treatment could open opportunities in designing and synthesizing new nitride materials as electromagnetic radiation absorbers, energy storage devices, and photovoltaic devices.

2. Catalytic hydrocracking of polymers over zeolite supported metal catalysts

Global plastics production has reached a rate of more than 400 million metric tons per year, and plastic production is projected to increase to 1100 million metric tons per year in 2050. However, only 20% of discarded plastic is recycled, and conventional recycling methods are insufficient to address the growing accumulation of plastic waste. These accumulated plastic wastes have created a global environmental challenge, and the development of more efficient recycling/upcycling processes is needed. Chemical upcycling of polymers using heterogeneous catalysts is a promising technology to address the current challenges. Extensive experimental efforts are made to find the optimal catalytic system for polymer upcycling. However, suitable computational and spectroscopic methods are currently not available and are urgently needed for catalytic polymer upcycling.

In my previous studies, I am able to build efficient computational models for methane activation and primary alcohol oxidation over zeolite supported catalysts. I intend to transfer the knowledge and methodology to catalytic polymer upcycling over zeolite supported catalysts. Due to the complexity of polymeric system, I will start with long-chain alkane model compounds hydrocracking over zeolite supported catalysts to obtain insight into the reaction mechanism. QM/MM calculations will be applied to understand the metal-support, polymer-metal, and polymer-support interactions. With my well-establised computational method, the influence of zeolite acidity, topology, and sites proximity in determining catalytic acidity and product selectvities could be evaluated. The molecular structure of polymers, such as degree of branching, and the interaction between polymer and catalyst will be evaluated by in situ IR. As a long-term plan, I will develop a novel batch reactor system equipped with in situ/operando ATR or Raman probe to study the molecular structure of polymers under high-pressure hydrocracking reaction conditions. To better simulate the polymeric structure, machine learning approaches will combine with DFT calculations to provides a good compromise between computational accuracy and cost.

In summary, my research will combine existing computational and experimental tools, and develop a novel in situ/operando probe to study reaction mechanisms of polymer upcycling. With the fundamental understanding, process design of polymer upcycling including reactor design, economic analysis, and life cycle assessment could be performed more efficiently.

Teaching Interests

I have worked as a teaching assistant for 5.5 years at the Stevens Institute of Technology, and I have taught almost all the undergraduate core courses for the chemical engineering program at Stevens. I have also audited some undergraduate core courses in the chemical engineering program at Princeton University. Based on my experience, I am confident in my ability to clearly deliver the principles for these courses and engage the students in my lectures in a chemical engineering program.

As an instructor, I consider my goal should be to help students better understand the foundational principles of our discipline, and develop critical and creative thinking skills. To train the next generation of engineers and scientists, my teaching philosophy contains two major parts: (a) research and industrial-application based learning and (b) teamwork-engaged and project-oriented learning.

Based on my experience, students will be more successful if they understand how to use the fundamental principles in problem solving and efficiently communicate. Thus, I will minimize asking students to memorize concepts and equations without understanding. To help students stay engaged with their courses, I will make lectures and handouts with examples of how the concepts are being used in practical applications. In addition, I will encourage students to present their understandings on complicated scientific problems periodically. This “story-telling” skill will be a great benefit for students’ future career.