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Towards a better catalyst

Platform optimizes the design of new, tunable catalytic systems

By Leah Burrows, SEAS Communications

(CAMBRIDGE, Mass.) —In the late 1700s, a Scottish chemist named Elizabeth Fulhame discovered that certain chemical reactions occurred only in the presence of water and that, at the end of those reactions, the amount of water was not depleted. Fulhame was the first scientist to demonstrate the power of a catalyst — a material that can speed up a chemical reaction without being consumed by it.

The Wyss Institute is developing a new type of coating for catalytic converters that, inspired by the nanoscale structure of a butterfly’s wing, can dramatically reduce the cost and improve the performance of air purification technologies, making them more accessible to all. Credit: Wyss Institute at Harvard University

Two hundred years later, catalysts have become one of the engines of modern life. The chemical industry relies on catalysts for 90% of its processes — everything from refining oil, turning petroleum into plastic, and producing fertilizers food and medicine, to scrubbing the air of noxious pollutants emitted from cars and factories.

Designing catalytic systems for such a broad range of applications is a big challenge. Catalysts need to be integrated into systems spanning a wide range of sizes, shapes, and material compositions, and control a variety of chemical reactions under vastly different conditions. In addition, most specialized catalysts rely on rare and expensive metals such as platinum, palladium, and rhodium supported on high-surface-area metal or metal oxide matrices.

Now, a team of researchers from the Wyss Institute at Harvard University and the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) has developed and tested a new approach to optimize the design of tunable catalytic systems.

The research, led by Joanna Aizenberg, a Core Faculty member of the Wyss Institute, is described in a series of papers published in Advanced Materials, Advanced Functional Materials, and Chemistry – A European Journal. Aizenberg is also the Amy Smith Berylson Professor of Materials Science and Professor of Chemistry & Chemical Biology.

Nature has had billions of years of R&D to perfect the design of catalytic systems.

Tanya Shirman

One of the biggest challenges in developing effective catalysts is designing the nanostructured porous solids on and in which reactions take place. For a long time, Aizenberg’s research has focused on studying complex natural micro and nanostructured materials — such as those in iridescent opals or in butterfly wings — and unraveling the ways biology controls the chemistry and morphology of its nanoscale building blocks. Inspired by natural processes, the team of researchers developed a methodology to create perfect, highly ordered, opal-like micromaterials for a wide range of catalytic and photocatalytic reactions.

To create these structures, the researchers introduced a co-assembly method in which tiny, spherical particles and matrix precursors are deposited simultaneously from a single mixture to produce defect-free films over centimeter scales. The researchers demonstrated this process with widely-used catalytic materials, including titania, alumina and zirconia, incorporating various mono- and multi-metallic nanoparticles.

“Expanding this methodology to non-biological crystalline materials will result in microscale architectures with enhanced photonic, electronic, and catalytic properties,” said co-author Tanya Shirman, a Technology Development Fellow at the Wyss Institute and postdoctoral fellow at SEAS.

In the design of the catalytic particles themselves, the researchers also turned to nature, using bio-catalysts, such as enzymes, for inspiration. In biological systems, the nanoscale catalytic materials attach to larger entities such as proteins and cells, which self-organize to form larger networks of precisely designed catalytic sites.

Towards a better catalyst
The new technology is inspired by the nanostructure of a butterfly’s wing, which is porous, repels water, and refracts light. Credit: Shutterstock/Ondrej Prosicky

“Nature has had billions of years of R&D to perfect the design of catalytic systems,” said Tanya Shirman. “As a result, they are incredibly efficient and enable the coordination and fine-tuning of sophisticated reactions through optimal positioning of the catalytic complexes.”

The researchers mimicked the hierarchical architecture of natural catalysts by developing a highly modular platform that builds complex catalysts from organic colloids and inorganic catalytic nanoparticles. The team can control everything from the composition, size, and placement of the catalytic nanoparticles to the colloid size, shape, and connectivity, and the overall shape and patterns of the network. The resulting catalytic systems use significantly lower amounts of precious metals than existing catalysts.

“Precious metal is a very limited resource,” said co-author Elijah Shirman, a postdoctoral fellow at the Wyss Institute and SEAS. “By optimizing the design and minimizing the amount of precious metals used in catalytic systems, we can create more sustainable catalysts in general and use catalytic materials in ways that are not currently affordable.”

The method is relatively simple: First, the catalytic nanoparticles attach to the colloids through various kinds of chemical and physical bonding. Coated with nanoparticles, the colloids are next placed into a matrix precursor solution and allowed to self-assemble into the desired pattern, which can be controlled by confining the assembly within a certain shape. Lastly, the colloids are removed so that a structured network that is decorated with nanoparticles partially embedded inside the matrix is formed. This hierarchical porous architecture with firmly attached catalytic sites maximizes the surface area for the catalytic reaction and enhances the robustness of the catalyst.

“Our synthetic platform allows us to take the components of the assembly and form a fully interconnected, highly ordered porous microarchitecture, in which catalytic nanoparticles are uniquely incorporated,” said Tanya Shirman. “This provides exceptional mechanical, thermal, and chemical stability as well as high surface area and full accessibility to diffusing reactants.”

Towards a better catalyst
The butterfly-wing-inspired architecture allows precious metal catalysts (white) to be strategically placed on the porous scaffold (gray) so that the catalytic reaction is much more efficient and cost-effective. Credit: Wyss Institute at Harvard University

“The technology developed in my lab is particularly promising for bridging the gap between state-of-the-art R&D and real-world applications,” said Aizenberg.  “Due to its modular design and tunability, this framework can be used in various fields from the synthesis of important chemical products, to pollution abatement. Our results clearly show that we are now able to create better catalysts, use less precious metal and improve known catalytic processes.”

This technology is now being validated and developed for commercialization by the Wyss Institute.

Aizenberg’s team is currently focusing on developing next-generation catalysts for a number of applications – from clean air technologies and catalytic converters to advanced electrodes for catalytic fuel cells – hoping to test their designs soon in real-world systems.

The team recently received second place in Harvard’s President’s Innovation Challenge, which identifies and promotes promising technology enterprises that have the potential for significant societal and environmental impacts.

The research was co-authored by Cynthia Friend, Ph.D., who is the Theodore William Richards Professor of Chemistry and Professor of Materials Science at SEAS; Anna V. Shneidman, Ph.D., a Postdoctoral Fellow in the Aizenberg group; Alison Grinthal, a Research Scientist in the Aizenberg group; Katherine R. Phillips, a graduate student in the Aizenberg group; former Aizenberg group students Hayley Whelan, Eli Bulger, Marcus Abramovitch, Jatin Patil, and Rochelle Nevarez; Theresa M. Kay, a former Research Fellow in the Aizenberg group; Postdoctoral Researchers in the Friend group Judith Lattimer, Ph.D., Mathilde Luneau, Ph.D., and Christian Reece, Ph.D.; Michael Aizenberg, Ph.D., Senior Staff Scientist at the Wyss Institute; and Robert Madix, Ph.D., a Senior Research Fellow at SEAS.

This work was supported by the National Science Foundation Designing Materials to Revolutionize and Engineer our Future program, the Integrated Mesoscale Architectures for Sustainable Catalysis, and an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

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