(2di) Analysis and Design of Catalytic Reactions and Materials through Combined Experimental, Kinetic, and Computational Assessments

Catalysis represents a typical confluence of fundamental and practical endeavors that implicate multidiscipline contributions. Accurate analysis and rational design of catalytic processes and materials require systematic, rigorous investigations on the structures and properties of catalysts, mechanisms by which catalytic turnovers occur, as well the challenges encountered in their practice. My research, in the context of CO₂ hydrogenation, alkane hydroisomerization and hydrocracking, and selective catalytic reduction of NOx with NH₃ (NH₃-SCR), has been conducted along these lines on both catalytic surfaces and within porous materials through combined experimental (synthetic, spectroscopic, kinetic) measurements, mathematical and kinetic modeling (mean-field kinetics, reaction-transport formalisms, statistical simulations), and computations including density functional theory (DFT), ab-initio molecular dynamics (AIMD), and machine learning approaches, aiming at establishing fast recycles between experiment-theory and developing quantitative analysis and design strategies of catalytic reactions and materials.

Postdoctoral research:

Catalytic processes towards carbon neutrality and chemical upgrading are two representatives absorbing extensive fundamental and practical interests. Part of my postdoctoral research with Prof. Enrique Iglesia at University of California, Berkeley has been working on CO₂ hydrogenation on dispersed metal nanoparticles. Combining high-pressure kinetic measurements, mathematical treatments for bed-scale axial concentration gradients of intermediates and products, with DFT calculations, we developed mechanistic elementary steps and kinetic formalisms that accurately describe measured rates and selectivities of reverse-water-gas-shift, methanation, and methanol synthesis reactions on an array of metal surfaces with different nanoparticle sizes (1–30 nm) and metal identities (e.g., Ru, Co, Cu). In particular, we derived a steady-state CO pressure, its value predictable a priori using aforementioned kinetic formalisms. Cofeeding this CO pressure (with CO₂ and H₂) leads to 100% selectivity of CO₂ conversion to CH₄ even on a poor methanation catalyst, providing a compelling strategy of practical interest to maximize the CH₄ selectivity in CO₂-H₂ reactions.

CO₂-H₂ reactions occur on metal surfaces (e.g., Ru) nearly saturated with chemisorbed CO (CO*). Such high CO* coverages and consequent intermolecular repulsion within dense CO* adlayers affect stabilities of co-adsorbates and the kinetically-relevant transition states (TS), the extent to which is sensitive to the size of bare-site ensembles and decreases for large ensembles that provide larger "landing" spaces. We developed a general formalism that systematically considers energy penalties incurred in creating ensembles of vicinal bare sites. These energy costs determine the probability of finding such ensembles; the binding properties of each ensemble for the kinetically-relevant TS then complete a reaction probability that is reflected in the rate parameters that determine the overall catalytic turnovers. These formalisms address systematically the extent to which the TS and adsorbates "sense" the repulsion imposed by co-adsorbates based on their space requirements and tendency to interact with the solvating adlayer. This method is not restrictive and remains useful and applicable in general, particularly for crowded catalyst surfaces, the situation typically encountered in practice.

My other postdoctoral project has involved isomerization and β-scission of alkanes mediated by bifunctional catalytic cascades that consist of a metal function for dehydrogenation-hydrogenation and an acid function mediating skeletal isomerization and C-C bond cleavage of alkenes. The metal-acid site proximity is consequential for reactivity and selectivity, especially within voids of molecular dimensions (e.g., zeolites), because of gradients in reactant and product concentrations within acid domains. We developed a diffusion-convection-reaction formalism that rigorously and accurately describes the reaction-transport interplays, and resolve persistent controversies about the chemical or diffusional origins of rate and selectivity consequences of the nanoscale metal-acid proximity. My diffusion-convection-reaction formalism and mathematical framework (and code) are general and support other two parallel (but different) projects in the group.

Graduate research:

NH₃-SCR represents the state-of-the-art technology for controlling NOx emission from stationary and mobile sources. My PhD graduate research with Prof. Xiang Gao and Prof. Kefa Cen at Zhejiang University (ZJU) focused on the mechanistic studies of low-temperature (LT; e.g., 473 K) SCR redox chemistry on industrially-relevant supported vanadia metal oxides and copper exchanged chabazites (Cu-CHA), as well on the development of new catalysts with enhanced LT-SCR performance and resistance to chemical poisoning effects (by sodium and arsenic oxides). In collaboration with Prof. Enrico Tronconi at Politecnico di Milano (PoliMi; I was a visiting student in his group for one year), we for the first time revealed the kinetic relevance of dynamic binuclear Cu^II-sites in the reduction half cycle (Cu^II → Cu^I) of LT-SCR on Cu-CHA zeolites based on converging evidence of probe reactions, operando spectroscopies, transient kinetic measurements and simulations, and first-principles calculations using DFT and AIMD. At ZJU, we elucidated mechanisms of catalyst poisoning by Na and As via systematic characterization of catalyst structures and properties, and developed two new SCR catalysts that show improved resistance to Na and As deactivation than commercial vanadia catalysts, respectively. I also co-supervised a junior PhD student in Prof. Gao's group in building up a database of LT-SCR activity data that covers over 2000 literature reports, based on which we used machine learning approaches to screen and predict catalyst recipes of preeminent LT-SCR activity in a high throughput manner, which were then corroborated experimentally.

Future plans for my own independent research group will continue such combined, quantitative investigations with multitechniques, the complementary nature of which overcomes the bottlenecks when one methodology works well while the other does not. Given my background and skill sets in both experiments and theory, state-of-the-art computational approaches, especially those assisted by artificial intelligence, will also be employed in conjunction with advanced synthetic and characterization protocols to develop quantitative analysis and rational design strategies of catalytic processes and materials of different classes and applications towards, e.g., decarbonization, chemical transformation/upgrading, and clean energy.

Teaching Interests:

The opportunities and interests in teaching and mentoring are a key driving force for me to pursue an academic career. Given my research background, I am prepared to teach any of the core classes in chemical engineering (kinetics, transport, thermodynamics, or reaction engineering), as well as electives in catalysis, emission control, and/or clean energy. I have guest lectured at PoliMi in the course of Catalytic Processes for Energy and Environment. For the mentorship experiences, I have co-supervised three junior PhD and two master students at ZJU, and co-supervised two master students at PoliMi during my visit in Prof. Tronconi's group. I also spent two months as a visiting student for joint research in Prof. De Chen's lab at Norwegian University of Science and Technology; these international experiences let me know how to work with people from different cultural backgrounds and coordinate harmoniously and efficiently as a team, which will contribute to my future teaching, mentoring, and collaboration with diverse students. Further, my research skill sets will allow me to train students not only for mastering different research approaches/techniques, but, most importantly, also for thinking of, analyzing, and solving problems from different perspectives.