(4lt) Catalyst Discovery and Reaction Engineering By Coupling Chemical Reactions across Phase Boundaries

Decarbonizing the chemical industry requires electrifying energy-intensive chemical processes that are powered by fossil fuels and a transition to chemical building blocks from renewable sources. My research initiatives will contribute to this transformative goal by developing the theory and applications behind innovative chemical processes that leverage unconventional and renewable material inputs and chemical driving forces.

My research group will couple chemical reactions across phase boundaries to design chemical processes with unprecedented reactivity and product selectivity. In one thrust, high-frequency ultrasound will be used as a low-carbon electricity-powered energy source to generate free radicals from cavitating bubbles as powerful chemical initiators for diverse chemical transformations in aqueous solution. The other thrust will discover new bifunctional heterogeneous catalysts by synergistically coupling complementary reactions on distinct active sites separated by an intervening fluid phase.

These endeavours will integrate first principles calculations (density-functional theory), multi-scale modelling techniques (e.g. micro-kinetic modelling, kinetic Monte Carlo simulations, continuum-level transport models), and experimental kinetic measurements, to develop fundamental insights into and applications of these multi-phase processes. This combination of theoretical, computational, and experimental tools will enable the invention, simulation, and validation of sustainable chemical processes.

Teaching Interests

My education and training as a chemical engineer with expertise in both experimental and computational methods has prepared me to teach the core courses in the chemical engineering curriculum. Among these, I am most enthusiastic to teach reaction kinetics and statistical/classical thermodynamics at the graduate and undergraduate levels, an elective on decarbonizing the chemical industry at the undergraduate level, and an elective on modelling chemical processes from molecular to continuum scales at the graduate level.

During my PhD training at UC Berkeley, I gained extensive experience in instructing undergraduate courses with over 100 enrolled students. At UC Berkeley, I was a Graduate Student Instructor for courses in the undergraduate curriculum, including: Introduction to Chemical Engineering Design (1^st year), Transport Processes (2^nd year), and Chemical Engineering Laboratory (3^rd year). There, I prepared lecture and discussion materials, presented these materials, created exams and homework assignments, provided additional instruction at office hours, and offered additional mentorship to those students struggling in the course.

In my experience, chemical engineering principles are best explained to a diverse group of students by combining multiple pedagogical approaches. I plan to present the theories underpinning complex chemical phenomena along with physical demonstrations and computer simulations to build intuition beyond what is gleaned from equations. I also intend to integrate music into my lessons by “sonifying” datasets derived from chemical engineering problems. For example, I will incorporate melodies derived from reaction networks using kinetic Monte Carlo, as I have done in a tutorial I’ve distributed online¹.

I am also passionate about science outreach and research dissemination. As a faculty member, I will continue my efforts of developing beginner-level instructional tutorials on tools and methods used in my research². I will also further develop the aforementioned sonification tools to share research findings through music to peak the interests of audiences beyond specialists in the field.

Furthermore, I am passionate about incorporating the tenets of wellbeing into my approach to interactions with mentees and students. This is exemplified in my participation as a contributing member in the Wellbeing Community of Practice at Nanyang Technological University and certifications in Psychological First Aid and Foundation of Care and Support. With this training, I will build and sustain a safe, inclusive, and diverse research group and classroom for students to thrive.

References:

1. https://ari-fischer.github.io/site/kinetics/tutorials/2023/09/26/sonifyi...
2. https://ari-fischer.github.io/site/

Research Background

My PhD studies with Professor Enrique Iglesia at the University of California, Berkeley (UC Berkeley) addressed long-standing controversies surrounding the mechanisms of arene hydrogenation reactions on metal catalysts and the role that metal-oxide scaffolds (e.g. Al₂O₃, MgO, and TiO₂) play as promoters. Toluene hydrogenation on Pt surfaces was used to probe the mechanisms behind hydrogenating aromatic rings on metal surfaces. These studies combined rigorous experimental kinetic measurements and mechanistic modelling, thermodynamic calculations of toluene-derived surface species using density-functional theory, and thermodynamic descriptions of densely covered metal surfaces using lattice statistics. These techniques were used to develop a unified mechanism-based kinetic model for toluene hydrogenation that was capable of describing kinetic trends across a wide range in temperature (333-533 K). This became possible only after recognizing that toluene and its partially-hydrogenated derivatives (e.g., methylcyclohexene isomers) take up spaces of different sizes on the Pt surface, and after developing a formalism capable of quantitatively capturing such differences. In doing so, I addressed the mechanistic underpinnings of an unusual kinetic behaviour for arene hydrogenation reactions—that rates become negatively correlated with temperature beyond a certain temperature threshold—which eluded mechanistic explanation since first reported before 1950.

The improved understanding of mechanisms for toluene hydrogenation on Pt surfaces shed light on possible synergies with metal oxides promoters. I discovered that partially hydrogenated species derived from toluene (e.g. methylcyclohexene isomers) formed at low concentrations (below 1 Pascal) dictated by the kinetics of toluene-H₂ reactions at Pt-surfaces. While toluene is unreactive at Al₂O₃ surfaces, toluene derived cyclo-alkenes and cyclo-alkadienes are hydrogenated by H₂ adsorbed at surface Lewis-acid-base pairs (vicinal Al-O). I unified the newfound hydrogenation activity of Al₂O₃ surfaces with a mechanistic picture for toluene-H₂ reactions at Pt surfaces through a quantitative reaction-transport formalism to describe rate enhancements conferred by the Al₂O₃. These mechanistic insights and the quantitative framework I have developed enable new avenues to improve catalyst performance by leveraging bifunctional synergies, with particular relevance to the hydrogenation-dehydrogenation processes central to emerging applications of organic molecules as substrates for H₂ storage and transport.

My postdoctoral research with Asst. Professor Tej S. Choksi at Nanyang Technological University (NTU) and the Cambridge Centre for Advanced Research and Education in Singapore (CARES) is elucidating the mechanisms behind ultrasound-driven organic chemistry in aqueous solutions. Ultrasound irradiation forms hydroxyl radicals (•OH) in aqueous solution by driving the homolysis of water within cavitating gas bubbles. I have developed a quantitative, mechanism-based framework to predict the reactions rates and products formed from these •OH initiators in solution. These studies rely on micro-kinetic modelling, density-functional theory based-descriptions of reactivity, and cheminformatics for efficiently treating networks with dozens of reactions. While ultrasound is commonly associated with the total oxidation of organic molecules, I’ve shown that ultrasound-derived •OH radicals can be harnessed to selectively oxidize the aldehyde functions in glyoxal to carboxylic acids. I’ve also shown that these radical initiators fragment aromatic rings to form valuable dicarbonyl products with diverse chemical applications. These mechanistic insights open new possibilities for sustainable chemistry using renewable electricity to power ultrasound sources and water as a solvent