(2hf) Next Generation Catalysis By Microwave, Plasma, and Materials Design

Background and context of the research:

Almost every product that human beings interact with includes catalytic processes in its manufacture, in addition, the world is facing a climate crisis brought on by reliance on fossil energy. Energy systems of the future will likely require a broad mixture of fuels and generation sources. Undoubtedly, much of this energy can be stored by chemical bonds such as C-H and N-H, or others. Batteries and fossil energy will play an oversized role, but both CO₂ utilization, and alternative fuels, such as ammonia present important options for decarbonization and a more circular economy. Novel catalyst systems and reactors will be required for these processes to be developed, as will new catalytic and reaction engineering methods. The proposed research shall be to develop fundamental experimental insights toward developing next-generation catalysts for use in thermal, microwave, and plasma-enhanced reactors. The experimental results, characterization, and reaction methods that are developed can then help theorists expand their simulations, and industrial groups improve the scale-up of these processes.

Overview of research interests:

Heterogeneous catalysis is often viewed as being carried out in a thermal-heated setting with a catalyst that is not consumed by the reaction. Recent research has added more possibilities, such as the Mars-van Krevelen mechanism and new pre-activation and heating methods, such as plasma and microwave irradiation. These methods tend to involve a much more dynamic surface which can present both challenges and opportunities for researchers. Microwave irradiation or plasma treatment of catalytic materials can have enhancing mechanistic effects that change the active species and surface sites on the catalyst which interact with each other, this is similar at least in theory to dynamic processes which require changing temperature and concentrations.

Microwave Sensitive Catalytic Materials:

Microwave heating of metals and catalyst particles rely on different physical mechanisms related to both the magnetic and electrical parts of the radiation and these effects may be engineered to enhance reaction rates and selectivity. Surface polarization may be used to increase reaction rates, as volumetric heating, hot spot formation, and microwave-induced ionic transport may also be exploited by the reaction engineer. The catalyst designer may choose to enhance reaction rates or selectivity by using morphology, particle size, and composition. In addition, the frequencies used may be tuned to polarize surface species such as in Dauenhauer et al.’s catalyst resonance theory ¹. Some applications of microwave catalysis include CO₂ upgrading, ammonia synthesis, dehydroaromatization, and oxidation reactions.

Plasma Catalysis:

A fundamental understanding of plasma-catalyst interaction is currently being investigated. As such, it is an open research field with many opportunities. Of particular interest are surface catalyzed reactions with stable molecules such as CO₂ and N₂. Catalyst design may also be utilized to induce micro-plasma formation in pores as well as to stabilize and direct different reaction mechanisms by stabilizing activated species. The combination of plasma-thermal systems may allow energy requirements to be brought down by pre-activation and generation of highly reactive species.

Research Experience:

My Ph.D. work at West Virginia University under Dr. Jianli Hu was in the development and characterization of chemical looping ammonia synthesis (CLAS) catalytic materials. Ammonia synthesis by Haber-Bosch is responsible for 1% of global energy use per year, 2.5% of global CO₂ emissions per year, and for supporting approximately 4 billion additional persons’ nutritional requirements, CLAS systems may allow ammonia synthesis processes to be decentralized and utilize renewable energy Typically, a material used in the CLAS system is treated with dinitrogen and then subsequently treated with a source of dihydrogen. The nitrogen and subsequent hydrogen treatment are repeated in a continuous loop to synthesize ammonia at atmospheric pressure. CLAS materials which had been previously mentioned in the literature were tested under rigorous cyclic reactions in both traditional tubular thermal fixed bed reactors. Next, these materials were tested in a fixed bed microwave reactor which provided fundamental insights into both the traditional thermal CLAS materials and microwave enhanced CLAS materials. Finally, the work examined the kinetics of common CLAS materials such as Fe, Mn, and CoMo. Principles of reaction engineering and catalysis science were used to determine rate-limiting steps, activation energies, and rate constants for these processes. Additional work was performed on plasma-enhanced chemical looping, identifying gas-phase species, and surface modification. Finally, microwave enhanced CLAS materials were examined to distinguish microwave absorption effects on CLAS reactions as well as dielectric properties of the materials and their relations to particle size.

Publications:

1. Brown, S. W.; Jiang, C.; Wang, Q.; Caiola, A.; Hu, J. Evidence of Ammonia Synthesis by Bulk Diffusion in Cobalt Molybdenum Particles in a CLAS Process. Catalysis Communications 2022, 106438. https://doi.org/10.1016/j.catcom.2022.106438.
2. Hu, J., Brown, S. W., Tiwari, S., Wang, Y., Robinson, B., “Chemical Looping Ammonia Synthesis under Low Pressure” US. Application No.: 63/313,672

References:

(1) Ardagh, M. A.; Shetty, M.; Kuznetsov, A.; Zhang, Q.; Christopher, P.; Vlachos, D. G.; Abdelrahman, O. A.; Dauenhauer, P. J. Catalytic Resonance Theory: Parallel Reaction Pathway Control. Chem. Sci. 2020, 11 (13), 3501–3510. https://doi.org/10.1039/C9SC06140A.

(2) Erisman, J. W.; Sutton, M. A.; Galloway, J.; Kilmont, Z.; Winiwarter, W. How a Century of Ammonia Synthesis Changed the World. Nat. Geosci. 2008, 1, 636–639. https://doi.org/10.1038/ngeo325.

(3) Pfromm, P. H. Towards Sustainable Agriculture: Fossil-Free Ammonia. J Renew. Sustain. Energy 2017, 12.

(4) Appl, M. Ammonia. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2006.