Researchers Demonstrate Enzyme-Like Heterogeneous Catalysis for the First Time

Imagine Swiss Alps without tunnels! Anyone attempting to travel through them would have to ascend and descend the hills and go zigzag around the ranges.

Considerably more time and energy are saved to travel through a tunnel than to ascend a hill. This is analogous to how catalysts operate: they accelerate the chemical reactions by decreasing the energy needed to achieve the desired physical state. In industrial manufacturing processes, heterogeneous catalysis—which usually involves the use of solid catalysts kept in a liquid or gas reaction mixture—finds several promising applications. As heterogeneous catalysts are in a different phase, they can be easily separated from a reaction mixture. Thus, the catalysts can be efficiently recovered and recycled, being relatively eco-friendly. Furthermore, they show very stable activity even under adverse reaction conditions. Although it has these benefits, heterogeneous catalysis has been regarded to enable less interaction and controllability when compared to homogeneous catalysis because of little understanding of its reaction process.

Scientists at the Center for Nanoparticle Research (headed by Director Taeghwan HYEON) within the Institute for Basic Science (IBS) together with Professor Ki Tae NAM at Seoul National University and Professor Hyungjun KIM at KAIST for the first time showed enzyme-like heterogeneous catalysis. They created a highly active heterogeneous TiO₂ photocatalyst including several single copper atoms. They used this catalyst for the photocatalytic hydrogen production and discovered that the catalyst is as active as the most active and costly Pt-TiO₂ catalyst.

The scientists were dedicated to modeling the catalyst structure analogous to the most efficient and reactive catalysts, which are biological enzymes. Enzymes consist of catalytically active metal atoms and neighboring proteins that work very closely to maintain their feedbacks going backward and forward. Due to this cooperative internal communications, enzymes can rapidly modify their structure to make it optimally suitable for desired reactions (usually called the induced-fit model.) During the adaptation, enzymes erratically return to their original shapes and turn reformed.

For the first time, we found that an enzyme-like reversible and cooperative activation process occurs even in heterogeneous catalysts. This is an unprecedented platform that merges advantages of both heterogeneous catalysts and biological enzymes. While featuring the robust stability of heterogeneous catalysts, cooperative and reversible characteristics of enzymes adds significant controllability, which in the end brings high activity for hydrogen (the most efficient and ideal fuel) production from photocatalytic water splitting reaction.

Professor Taeghwan Hyeon, Director, Center for Nanoparticle Research

Biological enzymes have been regarded as a key model for creating artificial catalysts. They have been successfully employed in developing homogeneous catalysts for a variety of reactions. Even then, there had been no account on industrially significant heterogeneous catalysts with these enzyme-like characteristics because of the dearth of atomic-level understanding of heterogeneous catalyst. This new research shows that heterogeneous catalysts can function as enzymes, verifying the basic principle that cooperative interplay between atomic catalysts and adjacent environment considerably impacts the overall material properties and catalytic activity.

The scientists produced an enzyme-like heterogeneous catalyst by incorporating theoretical simulations and nanomaterial synthesis technologies. They enclosed a round-shaped TiO₂ substrate with single-atom coppers. They covered TiO₂ and copper atoms together. The copper single atoms were successfully stabilized completely on titanium sites by subsequent baking. It was important for this research to develop site-specific single atom catalysts, as this single-atom structure directly imitates the structure of enzymes (composed of single-atom metallic ions and neighboring proteins).

Fascinatingly, the produced site-specific single-atom Cu/TiO₂ catalysts went through distinctive photoactivation process. TiO₂ excites an electron by the absorption of light. The excited electron is shifted to a single copper atom via simple oxidation state change. The transport of an electron, in turn, changes back neighboring TiO₂ structures (similar to the induced-fit model of enzyme). This active state subsequently returns to the first resting state since an electron is transferred back to the TiO₂ from a metal atom.

Indeed, this interactive and reversible mechanism was verified with the white Cu/TiO₂ quickly becoming black under light illumination, and back to initial white color when purged with air. Due to these enzymatic characteristics, single-atom Cu/TiO₂ catalyst converted more than 40% of light energy into H₂, a remarkably high catalytic activity, which is as active as the most active and expensive Pt-TiO₂ photocatalyst. (Hydrogen is recognized to be the most efficient and suitable fuel as it only produces water as the byproduct).