Atomically imaging single atom catalysts and their behaviors by scanning tunneling microscopy

Understanding the mechanism of single-atom catalysis is essential to design and refine systems for improved catalytic performance. However, given the complex structure and large variety of single-atom catalysts (SACs), characterizing the single-atom catalytically active sites is extremely tricky and challenging. Over the past decade, although still far from satisfactory, scanning tunneling microscopy (STM) has helped provide numerous fundamental insights to understand single-atom catalysis. In this review, we summarize how STM enables atomically precise imaging of SACs including their geometric and electronic structures and their behaviors in the activation of absorbed small molecules, and how the combination of STM and other techniques helps to reveal charge states, charge transfers, dynamic reaction processes, and reaction mechanisms in single-atom catalysis. Finally, the future expectations on STM in three-dimensional, spatial and temporal imaging, and operando characterization are proposed. We believe that the combination of STM and single-atom catalysis is attractive and will further flourish heterogeneous catalysis research.