(6gf) Nanoengineering Materials with Atomic Specificity for Catalysis and Energy Applications

Authors: 
Choksi, T. S., Stanford University
Research Interests:

Catalysts that transform low carbon feedstocks into energy, fuels, and chemicals are at the core of future sustainable energy solutions. Advances in first principles techniques such as density functional theory (DFT) have, in turn, enabled the in-silico screening of a surprisingly large ensemble of catalytic architectures. Candidate materials are rapidly screened by directly mapping rates and selectivity onto one or two descriptors which are usually energies of atomic adsorbates. This mapping is facilitated by linear scaling relations connecting energies of large reaction intermediates to atomic descriptors. Despite transforming catalyst screening from a primarily Edisonian endeavor to one guided by design principles, current approaches are limited by three key features. First, descriptor-based screening while routinely applied to metallic systems, remains largely elusive for structurally and mechanistically complex materials like oxides, metal/oxide interfaces and two-dimensional materials. Second, despite revealing targeted descriptor values that the best catalysts possess, a direct mapping to the local morphology and composition around a binding site for materials beyond monometallic systems is currently lacking. Finally, catalytic processes are inherently dynamic and do not retain idealized structures under reaction conditions, which is often assumed in screening studies. Addressing these compelling questions will further unlock the promise of computational driven catalyst design.

My graduate research at Purdue University with Prof. Jeffrey Greeley focused on identifying design principles for reducible oxides and bifunctional metal/oxide interfaces. Using methanol oxidation on reducible MoO­3 as a probe reaction, I examined the interplay between catalytic reactions and surface redox processes on reaction kinetics. Building on this analysis, reactivity trends for doped MoO3 were established. Periodic trends uncovered a relationship between electronic and morphological features of defect sites that can ultimately be exploited to rationally engineer reducible oxides. Design principles for bifunctional metal/oxide interfaces were investigated using linear scaling relations. In contrast to metals, alloys, oxides and zeolites, scaling laws at bifunctional interfaces deviate from strict bond order conservation constraints, permitting greater flexibility in engineering bifunctional catalysts. These deviations together with their physical origin are explored using electrostatic interactions at the interface.

Over the past year, my postdoctoral research at Stanford with Dr. Frank Abild-Pedersen within the SUNCAT-Nørskov group has focused on creating a site-specific mapping of adsorption energies to local morphological features of bimetallic alloys. With minimal inputs from DFT, energies of both adsorbates and surface metal atoms are expeditiously predicted with atomic resolution, across a large combinatorial space of structure and composition. Using site specific energies of adsorbates and metal atoms, we can directly screen for active site architectures that not only display high rates/selectivity but are also thermodynamically feasible under reaction conditions.

Future plans for my independent research group will be focused on nanoengineering multicomponent oxides and two-dimensional heterostructures that contain cooperative catalytic functions (acid, base, and redox sites). Targeted catalytic applications will include synthesizing platform chemicals (alkenes, alcohols, and aldehydes) using raw materials derived from both shale gas and biomass-based sources. The first phase of this agenda will identify functional forms explicitly linking energy descriptors to morphological features of active sites. Predicted descriptor values will then be integrated within multiscale models simulating catalytic cycles on inherently dynamics active sites that are characteristic to these materials. These studies will ultimately open pathways towards reverse engineering realistic active site motifs displaying targeted rates and selectivity on fundamentally new classes of catalytic materials.

Teaching Interests:

My teaching interests include core chemical engineering courses like mass and energy balances, thermodynamics, separations, transport phenomena, chemical reaction engineering, and chemical engineering laboratory. Depending on departmental needs, I plan on developing a mezzanine level course on computational materials design and an advanced graduate elective on computational modeling of catalytic processes.

I believe that effective teachers enthusiastically engage students, clearly communicate complex concepts to diverse audiences, emphasize continuous learning, and ultimately inspire the next generation of students. Within an engineering curriculum, it is important to deconstruct complex ideas by building scaffolds, and continuously reinforce linkages between course content, the broader curriculum, and industrial applications. This can be greatly facilitated by proven pedagogical advances like active and blended learning strategies. To mold myself into an effective educator, I enrolled in a course preparing postdocs for faculty careers and I am currently participating in the postdoc teaching certificate program at Stanford. My prior teaching experiences include preparing and delivering lectures in recitations and conducting laboratory sessions in core undergraduate courses of chemical reaction engineering (junior year) and chemical engineering laboratory (senior year). I also served as a guest lecturer in advanced modeling for catalysis studies (advanced graduate elective). I have mentored two high school students (ACS SEED program), one undergraduate honors student, one graduate student, and peer-mentored one postdoc in research projects involving ab initio simulations of heterogeneous catalysts. I strongly endorse STEM outreach programs which tend to propel non-traditional students into STEM fields and I have been involved in several such activities during graduate school.