Elucidating the relationship between catalyst structure and catalytic activity remains one of the foremost challenges for the heterogeneous catalysis community. It is intuitively accepted that catalysts are effective because there exists some place on the catalyst surface, called the active site, where the activation energy for the rate-limiting step is reduced substantially. However, in reactions involving gas-solid interactions on nanostructured supports, the distinction between catalytically significant atomic structures and unreactive spectators is obscured by the variety of surface states that exist under reaction conditions. The ultimate goal for this project is to use atomic-resolution imaging and spectroscopy to identify and characterize the active sites on a heterogeneous catalyst.
Operando transmission electron microscopy (TEM) is an emerging technique with the potential to simultaneously observe the atomic structure of catalytic surfaces and measure the activity of the catalyst nanoparticles. The advanced technique requires an aberration-corrected environmental TEM equipped with instrumentation that can measure the gas composition within the environmental chamber. One such microscope is the FEI Titan 80-300kV ETEM at ASU. Our group has developed and employed the operando TEM technique to study the active form of a Ru catalyst for CO oxidation (see figure below). The active form of the catalyst was highly disputed in the literature, as researchers speculated upon whether pure Ru, RuO2, or Ru with a thin surface layer of RuO2 was the most active form. Operando TEM experiments by our group provided direct evidence for very thin layers of RuO2 forming on the surface of Ru nanoparticles during CO oxidation (see second figure below).