Faculty Advisor: Jason Hicks

Elucidating Plasma-Catalytic Interactions for Nitrogen Fixation

Anthropogenic emissions of greenhouse gases such as CO2 from industrial processes pose a significant environmental challenge. One process with a high CO2 footprint is the industrial Haber-Bosch process for ammonia synthesis. This process relies heavily on fossil fuels and is typically conducted at extreme temperatures (~700K) and pressures (~100 atm). This process accounts for 1-2% of global energy consumption and 1.5 % of global CO2 emissions.(1) To this end, alternative pathways to develop environmentally benign and sustainable processes for nitrogen fixation are being sought. Plasma technology provides one such alternative. (2)

Broadly, my work investigates how molecules come together or break apart in plasma—a state of matter composed of charged ions, radicals, and electrons. Traditionally, neutral reactant molecules in the gaseous phase are activated to form products under the addition of thermal energy in the presence or absence of a catalyst. Further, plasma is electrically generated; hence, it is suited for harnessing wind and solar energy resources, making it adept for small-scale modular units and on-site applications. Recent studies have demonstrated that plasma activation of stable molecules can improve reaction rates, especially when coupled with a catalyst. Non-thermal plasma-driven catalysis has been reported to enable the activation of strong chemical bonds, thus facilitating many reaction chemistries at milder conditions.(3) For example, ammonia synthesis has benefited from the application of non-thermal plasma, which reduces the energy-intensive requirements of the conventional synthesis process.(4)

However, the nature of the plasma-catalytic interaction has not been fully described. Results are often Obfuscated by competitive plasma phase reactions, making it difficult to delineate the catalytic contribution to overall product yields. Efficient material design and selection for plasma-catalytic processes would rely on understanding reactions at the metal surfaces.(5) Recent work in the group has employed a carefully designed experimental procedure to sequentially expose the catalyst surface to different reactants. By so doing, the surface reactivity of the metal catalyst can be directly probed. My Prior work has employed this technique for nitrogen hydrogenation to ammonia, where I observed that the hydrogenation ability of the metal catalyst was critical in determining the rate of ammonia production.(6) Motivated by the knowledge gained from my previous hydrogenation studies, future work would be aimed at implementing a similar methodology for other nitrogen fixation pathways, such as oxidation. The oxidative path has the added advantage that both reactants are readily available (in air). However, the process is limited thermodynamically, with only about a 1% conversion observed thermally at temperatures up to 2000 K. This makes the reaction particularly amenable to plasma since plasma has been reported to overcome thermodynamic constraints, producing yields that exceed thermal equilibrium.(7) I will investigate how metal catalyst surfaces interact with nitrogen under plasma-activated oxidation processes and what parameters influence product yield and selectivity.

References

(1) Kyriakou, V.; Garagounis, I.; Vourros, A.; Vasileiou, E.; Stoukides, M. An Electrochemical Haber-Bosch Process. Joule 2020, 4 (1), 142-158. DOI: 10.1016/j.joule.2019.10.006.

(2) Chen, J. G.; Crooks, R. M.; Seefeldt, L. C.; Bren, K. L.; Bullock, R. M.; Darensbourg, M. Y.; Holland, P. L.; Hoffman, B.; Janik, M. J.; Jones, A. K.; et al. Beyond fossil fuel-driven nitrogen transformations. Science 2018, 360 (6391). DOI: 10.1126/science.aar6611.

(3) Mehta, P.; Barboun, P.; Go, D. B.; Hicks, J. C.; Schneider, W. F. Catalysis Enabled by Plasma Activation of Strong Chemical Bonds: A Review. ACS Energy Letters 2019, 4 (5), 1115-1133. DOI: 10.1021/acsenergylett.9b00263.

(4) Mehta, P.; Barboun, P.; Herrera, F. A.; Kim, J.; Rumbach, P.; Go, D. B.; Hicks, J. C.; Schneider, W. F. Overcoming ammonia synthesis scaling relations with plasma-enabled catalysis. Nature Catalysis 2018, 1 (4), 269-275. DOI: 10.1038/s41929-018-0045-1.

(5) Bogaerts, A.; Tu, X.; Whitehead, J. C.; Centi, G.; Lefferts, L.; Guaitella, O.; Azzolina-Jury, F.; Kim, H.-H.; Murphy, A. B.; Schneider, W. F.; et al. The 2020 plasma catalysis roadmap. Journal of Physics D: Applied Physics 2020, 53 (44). DOI: 10.1088/1361-6463/ab9048.

(6) Barboun, P. M.; Otor, H. O.; Ma, H.; Goswami, A.; Schneider, W. F.; Hicks, J. C. Plasma-Catalyst Reactivity Control of Surface Nitrogen Species through Plasma-Temperature-Programmed Hydrogenation to Ammonia. ACS Sustainable Chemistry & Engineering 2022, 10 (48), 15741-15748. DOI: 10.1021/acssuschemeng.2c04217.

(7) Mehta, P.; Barboun, P. M.; Engelmann, Y.; Go, D. B.; Bogaerts, A.; Schneider, W. F.; Hicks, J. C. Plasma-Catalytic Ammonia Synthesis beyond the Equilibrium Limit. ACS Catalysis 2020, 10 (12), 6726-6734. DOI: 10.1021/acscatal.0c00684.

Research Objectives

Objective 1: The proposed study will focus on evaluating the oxidation of plasma-activated nitrogen species on supported metal catalyst surfaces. The first step will involve the synthesis and characterization of selected metal catalysts (such as Pt, Pd, Ag, and Au). The catalysts surface area and site densities would be determined using in-house physisorption and chemisorption units (available in Hick’s lab). At the same time, the crystallinity and particle size analysis would be conducted using the X-ray diffractometer and high-resolution transmission electron microscopy (HR-TEM) in the Molecular Structure facility and Notre Dame Integrated Imaging Facility (NDIIF), respectively. This would form the basis for relating the metallic textural and surface properties to the reactivity in the future part of the study.

Objective 2: After careful synthesis and detailed characterization of the catalytic material. Plasma-activated reactivity analysis will be performed. This would be done by sequential activating nitrogen using plasma before oxidation under thermal conditions. Control experiments in which thermally activated nitrogen would also be evaluated to assess the plasma contribution. These oxidative studies are further complicated by the potential of nitrogen to form different oxides (NO, N2O, and NO2) under plasma stimulation. Here, we will also look at the effect of the catalyst properties (such as particle size and site density) and how they affect the reaction selectivity and product yields. Given the potential for oxidative nitrogen fixation pathways, understanding the influence of these parameters would be pivotal.

Objective 3: Sequential studies have proven that plasma-activated nitrogen species can react over a metal surface, highlighting a catalytic benefit during plasma-assisted nitrogen transformation. However, there are some questions yet to be answered. One such plaguing question is the effect of metal accessibility in sequentially driven plasma studies. How does accessibility affect product yield and selectivity? To answer this question, I will synthesize catalyst materials on glass beads (non-porous support) and compare the product formed to the porous silica-supported material. I will then explore this non-porous material for the sequentially driven nitrogen reduction and oxidation studies, first over non-noble metals like Ni and then noble metals like Pt and Au. Another parameter that could be influenced is the temperature of product formation. The hypothesis is that internal sites would lead to product evolution at a higher temperature than sites located at the external surface of the support material. I will evaluate and compare the temperature of product formation from the non- porous supported catalyst to the silica-supported catalyst material.