Title: Understanding the Functionality of Zeolite-Stabilized Single-Atom Catalysts

Chair:

Dr. Giannis Mpourmpakis, Department of Chemical and Petroleum Engineering, University of Pittsburgh

Committee:

Dr. Götz Veser, Chemical & Petroleum Engineering, University of Pittsburgh

 Dr. Christopher E. Wilmer, Chemical & Petroleum Engineering, University of Pittsburgh

Dr. Jeffrey D. Rimer, Chemical & Biomolecular Engineering, University of Houston

Abstract: Sustainable development of fuels and chemicals has been recognized as an inevitable paradigm for the future societies. Towards sustainability, there are two important steppingstone chemicals: methane (CH4) and ammonia (NH3). The former is the largest component of natural gas, which is a cleaner energy source and chemical feedstock compared to other fossil fuels. The latter is considered as one of the most important substances in hydrogen economy, as a stable, energy efficient and economic hydrogen carrier. Developing active catalysts for activation of these small molecules is crucial for achieving sustainability, where breaking the stable C-H and N-H bonds is the most challenging part due to the stable nature of these bonds. Noble metal-based catalysts (e.g., Pt, Rh and Ru) have exhibited high performance, but their industrial application is limited by their scarcity. Reducing the metal particle catalyst size improves their utilization efficiency, which can be maximized at the atomic scale. This has given rise to a new class of materials, the single-atom catalysts (SACs). Further, with the decrease in particle size, a metal catalyst loses its metallic properties and obtains molecule-like properties, opening new avenues for chemical catalysis. Zeolites are promising support materials for SACs, since atomically incorporating metals into their micropores is experimentally feasible via ion-exchange. Furthermore, specific pore structures and Brønsted/Lewis acid properties can introduce synergistic effects for enhancing catalytic performance.

This proposal will focus on understanding the nature of single metal atom centers stabilized in zeolite cages, such as the Ru-exchanged 13X zeolites (Ru-13X), for activation of small molecules (CH4 and NH3) by using computational chemistry methods. As a first step, Density Functional Theory (DFT) calculations will be carried out to unravel the high experimentally observed catalytic activity of Ru-13X towards NH3 decomposition. The focus will lie on determining the reaction pathway and understanding the role of Ru oxidation state in the first N-H activation step. Next, multiple metals other than Ru (e.g., Rh, Pd, and Fe) will be explored as active centers for the X-H bond (X = C or N) dissociation of NH3 and CH4 and a predictive catalytic activity model will be constructed. These molecules are key in energy and chemical conversion strategies and their combined investigation can reveal synergistic trends given their isoelectronic configuration. Lastly, complete reaction pathways of NH3 decomposition and non-oxidative CH4 coupling will be determined on the active metal centers identified from the second step. These DFT-calculated reaction energy profiles will feed microkinetic models to obtain a detailed understanding of the reaction mechanism and calculate reaction rates. Overall, the proposed work will shed light on the nature of highly active zeolite-stabilized SACs, rationalizing experimental observations, as well as guiding further efforts for developing highly active catalysts for CH4 and NH3 activation.