The dissociation mechanism of oxygen molecules on the surface of silver revealed
The intricacies of how an oxygen molecule (O2) on a silver surface separates into two oxygen atoms have been determined by RIKEN chemists1. This knowledge will help scientists optimize reactions important to industry.
Many industrial processes use silver surfaces to catalyze the breakdown of oxygen molecules into their constituent atoms. But while this reaction has been widely studied, the exact reaction pathway has remained unknown.
“It’s hard to believe that we still don’t know how oxygen molecules dissociate on a silver surface. The reaction has been exploited in industry, and its mechanism seems simple,” says Minhui Lee from the RIKEN Surface and Interface Science Laboratory. “Observing the reaction at the single-molecule level is key to understanding the details.”
To study the mechanism of dissociation in detail, Lee, Emiko Kazuma and their colleagues, in a team led by Yousoo Kim, used a scanning tunneling microscope (STM) – an instrument with an atomically sharp tip that is brought close enough to a sample to allow electrons to tunnel between the tip and the sample.
Oxygen molecules can absorb onto a silver surface in two configurations. The researchers used STM to study the responsiveness of the two configurations. They then used the STM tip to inject electrons into the molecule by applying positive voltage pulses, or to inject holes with negative voltage pulses. Both processes broke the oxygen-oxygen bond and resulted in the formation of two separate oxygen atoms on the surface.
Studying how the dissociation yield depends on the electron/hole energy sheds light on the excitation channels at work. Two sequential excitations were required to overcome the relatively high reaction barrier, and the molecules were excited to higher-order excited vibrational states before dissociating.
“The feedback pathway is an excitation from the ground state to the higher excitations of the vibrational states, namely the harmonics; in our case, higher than the fifth excited state,” says Lee. “We had to consider the deviation of the system from a harmonic oscillator, a simplification commonly used in quantum physics. It wasn’t easy, but after comparing the experimental data with the calculations, we were able to identify the harmonics involved in the reaction.
The team plans to deepen its work. “In this study, we focused on molecules adsorbed on a metal surface, with the molecular orbitals strongly hybridized with the surface,” concludes Kazuma. “The next step is to study molecules slightly decoupled from the surface in order to understand the influence of the metallic surface on chemical reactions.”