Institute for Catalysis in Energy Processes
The Institute for Catalysis in Energy Processes (ICEP) is a collaborative research effort among faculty in Northwestern University’s Center for Catalysis and Surface Science. While not explicitly supported through ICEP, many of these efforts also benefit from strong and long-standing collaborations with Argonne National Laboratory. The overall scientific goals of ICEP are to understand, design, and synthesize—with molecular-level precision—the catalytic materials that are essential to the large-scale production of fuels and energy-intensive commodity chemicals. With this combination of understanding and enabling synthesis tools, the science undertaken at ICEP promises to lead to new, more active, or more selective materials for many energy-relevant chemical processes.
In 2020, ICEP received $4.5 million of renewal funding from the U.S. Department of Energy Office of Basic Science. The new funding enables ICEP to continue its work along three highly integrated research thrusts. Each thrust has multiple PIs and is highly transdisciplinary, which enables both breakthrough science as well as effective mentoring of associated graduate students and postdoctoral researchers. For this research cycle, the research thrusts are united by an interest in understanding the mechanisms of selective oxidation at atomistic details, and in the role of advanced synthetic capabilities that enable the design of materials to interrogate or harness these mechanisms. The research thrusts are also well-aligned with the current DOE catalysis science priority research directions, specifically to design catalysts beyond the binding site, to understand and control the dynamic evolution of catalysts, and to manipulate reaction networks in complex environments, especially to steer catalytic transformations selectively. Finally, these thrusts are tied together by cross-cutting initiatives on advanced synthesis and characterization tools and on introducing data science approaches in heterogeneous catalysis.
ICEP Research Thrusts
Selective oxidation catalysis encompasses many of the outstanding grand challenges in catalysis, including selective methane oxidation, oxidative dehydrogenation of alkanes, and selective epoxidation using O2. Poor selectivity leads to undesired CO2 and numerous other side products that complicate separations and make many of these reactions commercially infeasible at present. Leveraging years of experience working collaboratively in this field, ICEP will address the challenge of selective catalytic oxidation in the 2020-2023 grant cycle. We believe that many grand challenges in selective oxidation catalysis remain outstanding because of two interconnected reasons: 1) the mechanistic complexity of systems having competing oxidation pathways, and 2) the complex and poorly defined structures of many oxidation catalysts.
The generic reaction scheme shown below conceptually unifies the center research and demarcates the three thrust areas. Like the steps in a catalytic cycle, the thrusts are highly integrated, and many materials, concepts, and investigators are shared between thrusts. In total, they will provide for new insights and technological innovations. Each Thrust will develop new materials that display improved reactivity and help shed light on complex mechanisms. A critical tenant of this proposal is that the relevant catalysts need to be synthesized and characterized with greater chemical and spatial precision than previously possible. Cartoon structures and vague descriptions, to which we still frequently resort, must eventually be replaced with atom-precise constructs.
Thrust 1: Expanded structure-function relationships for complex supported oxides and alloys
Thrust 1 examines reactant activation over sites that dynamically lose and gain O atoms. Reaction between the substrate and the oxidized site is frequently rate-limiting for alkane oxidative dehydrogenation and alcohol oxidative dehydrogenation / coupling, and these reactions will be the foci of this Thrust. Therefore, structure-activity relationships for these catalyst-reaction combinations directly report on these elementary steps. In this Thrust, a major effort will be to synthesize and stabilize a wider range of atomically-precise oxidized sites than have been previously possible. In two broad projects, we will address atomically precise metal nanocluster alloys / intermetallics and atomically precise mixed metal oxide clusters. From this, we expect to develop more powerful synthesis-structure-function relationships and to significantly advance the science underlying C-H and O-H bond activation. In addition, these materials will also be amenable to computational screening and interrogation, something that has been previously challenging for many supported oxides and related structures. Comparing many of these reactions and active sites to similar ones in Thrust 3 will complete the cycle of understanding.
Thrust 2: Studies of O2 activation within complex cycles and at tailored interfaces
Thrust 2 explicitly interrogates reoxidation steps. In many selective oxidations, reoxidation may not be rate-limiting and can thus be less ‘visible’, even to the point of being ignored in many studies. This step is necessarily complex, as it involves coupling the 4e- reduction of O2 to 2e- substrate oxidation steps. Over materials that will also appear in Thrusts 1 and 3, we will ask what active site qualities afford specific structures of hydroperoxy vs. peroxy vs. oxo intermediates, and what are the rates and selectivities of these sites for substrate oxidation. A special focus will be on inverse catalysts where tailored metal oxo clusters will be deposited onto support metal nanoparticles (Project 1) Throughout the Thrust, sites will be interrogated computationally, and also via ‘capture’ experiments (Project 2), where reactions such as alcohol oxidation are coupled with epoxidation or sulfoxidation to selectively react with activated O2 intermediates. Comparing these reactions and active sites to the same ones activated with H2O2 (Thrust 3) will complete the cycle of understanding.
Thrust 3: Non-redox catalysis over isolated metal oxo sites
Thrust 3 addresses reactant activation over oxidized sites that do not require redox to turn over. These sites may be oxide clusters or other structures, but the prototype sites in Thrust 3 will be isolated monometal sites (especially MoOx) on carbon or oxide supports and synthesized through atom-precise means, which also yield computationally tractable structures. Alone, these steps encompass acceptorless dehydrogenation and several mechanistically related reactions (Project 1) and activation of H2O2 to carry out epoxidation or sulfoxidation (Project 2). In the broader context of selective oxidation, these reactions and the sites that carry them out are involved in shunts (short circuits) across the larger catalytic cycle required for the 4e- reduction of O2 and the corresponding reactant oxidation steps. Project 1 corresponds to the case where the active site does not lose O after alkane or alcohol activation, and thus shunts back to the bare oxide through substrate dehydrogenation. The microkinetic reverse of these steps allow new mechanisms of hydrogenolysis, including of environmentally persistent plastics. Similarity, direct H2O2 addition to oxidized sites in Project 2 shunts the oxide directly to hydroperoxide sites that may (or may not) form during O2 activation.