(4cp) Observation and Identification of Catalytic Nickel Nitride Structures for Plasma-Assisted Ammonia Synthesis

To address global challenges for energy and sustainability, my goal is to improve fundamental understanding of materials that can be used as catalysts and energy storage devices. Specifically, I will primarily study the complexity and heterogeneity of catalysts under working conditions. To probe what happens in a working catalyst, I will focus on applying in-situ/operando spectroscopy characterization, especially IR and Raman spectroscopy, and performing temperature programmed experiments on heterogenous catalysts. To further understand the nature of catalyst surfaces, I use quantum simulation methods and new hybrid methods that bridge micro- and macroscopic scales using techniques such as machine learning. With the fundamental understanding obtained by in-situ/operando characterization and DFT calculations, we will be able to rationally design more efficient catalysts to sustainably produce energy and green chemicals. Additionally, the development of novel catalysts will be useful in addressing current environmental challenges, including reducing the concentrations of CO₂ in the atmosphere and upcycling plastic waste.

My Ph.D. study mostly focused on DFT simulations, and I primarily worked on two projects in Prof. S.G. Podkolzin’s laboratory at Stevens Institute of Technology. In the first project, I successfully identified the structures and catalytic activities of molybdenum species (oxide, carbide, and oxycarbide) supported by ZSM-5 zeolite for methane dehydroaromatization by using in-situ/operando characterizations (Raman, IR, UV-Vis, and XAS) and DFT calculations. The complexities of zeolite topology were addressed by hybrid quantum mechanics/molecular mechanics (QM/MM) calculations. In the second project, I worked on biomass conversion using Pt-based bimetallic catalysts. Dispersion-corrected DFT calculations and in-situ characterizations (Raman and XAS) were performed to reveal the structures of bimetallic catalysts under reaction conditions and to understand the complicated reaction pathways of biomass conversion over bimetallic catalysts. A modified doubly nudged elastic band method was developed for transition state calculations, which is a computationally efficient method to determine transition state for complex reaction pathways.

To improve my expertise in in-situ characterization techniques, I joined Prof. B.E. Koel’s laboratory at Princeton University as a postdoctoral researcher, which is one of the leading surface science labs using advanced characterization tools. At Princeton, I applied a similar methodology to study the selective oxidation of primary alcohols over gold catalysts supported by different zeolites (ZSM-5, MOR, and FAU). The nature of gold catalysts was studied by using in-situ DRIFTS and UV-Vis. DFT calculations coupled with machine learning based optimization algorithm were applied to resolve the Al distributions and gold-support interactions on different zeolite frameworks. In the most recent project, I studied plasma-assisted ammonia synthesis. My research focused on the evolution of metal nitride structures under plasma-assisted ammonia synthesis conditions. I designed multiple reaction cells for in-situ characterizations (SEM, Raman, and XAS) under plasma-assisted reaction conditions. Additionally, in-situ/operando XPS and two-photon absorption laser induced fluorescence (TALIF) were performed.

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

Plasma-assisted metal nitride formation (the topic of this presentation)

The synergy between heterogeneous catalysts and nonthermal plasma has shown promising performance for multiple catalytic applications, such as ammonia synthesis and dry reforming of methane. Plasma-assisted catalytic ammonia synthesis from N₂ and H₂ is one of the most studied reactions due to the high demand for ammonia as a feedstock for fertilizer production and as a promising carbon-free energy carrier.

Ni catalyst was found to be more active than the benchmark Ru catalyst for plasma-assisted ammonia synthesis. There have been reports that the accumulation of N-containing species on Ni surfaces (possibly Ni nitrides, Ni_xN_y) served as a contributor to high catalytic activity. However, due to the lack of in-situ studies, the composition and structure of Ni nitride formed in the plasma environment are unknown, and so structure-activity relationships are not established.

I will perform in-situ/operando spectroscopic studies and reaction kinetic testing to identify the structures and catalytic activity of Ni nitride for plasma-assisted ammonia synthesis. An in-situ Raman cell was developed to probe the formation of Ni nitride. The composition of Ni nitride as well as the oxidation state of Ni under plasma-assisted reaction conditions will be further studied by in-situ AP-XPS and XAS. In addition, in-situ DRIFTS will be used to study the intermediates formed when the Ni catalyst was exposed to N₂- and N₂/H₂-plasmas.

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, new information on Ni nitride will be useful in the development of catalysts for a wide range of reactions, including N-containing polymer upcycling and biomass processing. Our study also demonstrates the possibility of catalyst modification by treatment using nonthermal plasma, which will be useful in catalyst activation and materials synthesis.

Plasma-assisted catalytic upgrading of polymers

Global plastics production has reached a rate of more than 400 million metric tons per year. However, only 20% of discarded plastic is recycled, and conventional recycling methods are insufficient to address the growing accumulation of plastic waste. Plasma-assisted catalytic upcycling of polymers using heterogeneous catalysts is a new technology that could address the current challenges. However, suitable computational and spectroscopic methods are currently not available and are urgently needed for plasma-assisted catalytic polymer upcycling.

I have experience with coupling zeolite supported catalysts and nonthermal plasma for multiple reactions. I intend to transfer the methodology to plasma-assisted catalytic polymer upcycling. Without catalysts, the excited species generated by plasma could deconstruct polymer very efficiently but not selectively. With zeolite catalysts, the selectivity can be controlled, since the small molecules can diffuse inside the zeolite pores while the excited species generated by plasma cannot. The strategy is intended to protect the products and limit over-cracking to gases by plasma, leading to changes in catalytic selectivity for polymer upcycling

Due to the complexity of polymeric system, I will start with long-chain alkane model compounds. QM/MM calculations will be applied to understand the metal-support, plasma-metal, plasma-polymer, polymer-metal, and polymer-support interactions. The influence of zeolite acidity, topology, and sites proximity in determining catalytic acidity and product selectvities will be evaluated. As a long-term plan, I will develop a batch plasma-assisted reactor system equipped with in-situ/operando ATR probe to study the molecular structure of polymers under upcycling reaction conditions.

Plasma-assisted selective oxidation

H₂O₂ is widely used as an oxidant under mild conditions for industrial processes and fundamental research. Plasma-liquid interaction systems are promising for chemical synthesis, and H₂O₂ can be directly formed from water at plasma-water interfaces. Therefore, I propose to couple nonthermal plasma and heterogeneous catalysis in direct H₂O₂ synthesis using H₂O and O₂, which is a green and sustainable approach compared to the current industrial processes. H₂O₂ produced by plasma-assisted process can be used as an in-situ oxidant for selective oxidation of methane to methanol.

In my work, I will systematically study the effect of catalysts, specifically Au-based bimetallic catalysts, as well as plasma properties for direct H₂O₂ synthesis and methane oxidation using plasma. In-situ UV-Vis and Raman will be applied to understand reactant adsorption and decomposition over catalysts with and without plasma. Optical emission spectroscopy (OES) and laser diagnostic will be performed to identify the formation of radicals and other possible intermediates in the liquid phase. The laser diagnostic experiments, such as TALIF, can be performed at Princeton Plasma Physics Laboratory (PPPL) via Princeton Collaborative Low Temperature Plasma Research Facility (PCRF) user program.

Teaching Interests

I have worked as a teaching assistant for 6 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 the courses and engage the students in my lectures in a chemical engineering program.

As an instructor, I consider my goal to be helping students better understand the foundational principles 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 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. To help students stay engaged with the courses, I will make lectures and handouts with examples of how the concepts are being used in practical applications. I will arrange for one or more lectures per course concerning my own laboratory research to broaden students’ hands-on experiences with solving real scientific problems using the knowledge they are learning in the class.

I will develop community-engaged learning in my lecture, and I will encourage the students to communicate with each other. Other than courses, I will encourage students to present their research at local conferences. In my experience, presenting their research on complicated scientific problems can greatly help the students understand their projects and increase their knowledge. These “storytelling” skills will be a great benefit for the students’ future career.