Tokyo, Japan – Chemical reactions at surfaces play a central role in many processes, including catalysis and materials synthesis. It is well known that the outcome of these reactions is influenced by the translational, vibrational, and rotational motions of atoms and molecules. Now, a groundbreaking study shows that electron spin, another fundamental property of matter, also influences how chemical reactions at surfaces unfold.
In an article recently published in Nature Communications, a team of researchers from the Institute of Industrial Science, The University of Tokyo and collaborating institutions reported that controlling the spin orientation of hydrogen atoms dramatically changes the probability of both hydrogen adsorption and hydrogen abstraction reactions on a magnetic nickel surface. The findings establish electron spin as an important degree of freedom in surface chemistry and suggest that external magnetic fields could provide a new way to regulate chemical reaction rates.
In their experiments, the research team fired a spin-polarized beam of hydrogen atoms at a ferromagnetic (i.e., naturally magnetic) nickel surface. An external magnetic field was applied either parallel or perpendicular to the surface, allowing the team to control the orientation of the hydrogen spins. They then measured the amount of hydrogen adsorbed onto the surface under each configuration.
"Our goal is to understand how electron spin influences chemical reactions," says lead author Hirokazu Ueta. "By examining the interaction between hydrogen atoms and a ferromagnetic surface, we can quantitatively assess the role that electron spin plays in these reactions."
The difference was particularly pronounced at low hydrogen coverage. After accounting for background adsorption, the amount of hydrogen adsorbed when the spin was oriented parallel to the nickel surface was more than seven times greater than when it was oriented perpendicular. The difference disappeared on nonmagnetic copper, demonstrating that the effect is associated with the magnetic nickel surface.
"These results reveal that electron spin is an additional degree of freedom that can govern the behavior of atoms during surface reactions," explains senior author Katsuyuki Fukutani. "By manipulating spin with an external magnetic field, we may be able to regulate reactions in ways that are not possible by controlling temperature or other conventional parameters."
The research team also examined whether spin could influence a chemical reaction beyond adsorption. They exposed a deuterium-covered nickel surface to the hydrogen beam, triggering an abstraction reaction that formed hydrogen-deuterium molecules. The reaction proceeded more efficiently when the hydrogen spin was oriented parallel to the surface, and the difference between the two spin configurations increased with the strength of the applied magnetic field.
The team's experiments demonstrate that electron spin, manipulated externally with a magnetic field, is a powerful new lever for controlling surface chemistry. Extending this approach to other materials will be an important step toward practical spin-based control of chemical reactions, from more selective catalysis to new ways of tuning reactivity.
