Characterizing Active Sites During Catalysis

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).

Single-gas in-situ experimental images, are compared to an image of a particle obtained during measured CO oxidation inside the ETEM.

Experimental (scale bar) and simulated (colorized) images of Ru nanoparticles supported on amorphous silica in different gaseous environments at 200 C. The sets of images on the left show Ru in O2, CO2 and CO environments, while the set on the right is from CO oxidation reaction conditions. Note that the particle from the O2-rich conditions contains a thick surface layer of RuO2. Ex situ experimentation indicated that the catalyst is not very active in oxygen rich atmospheres. This information, combined with the information gained by operando experimentation, informs us the active form of the Ru catalyst is not the one shown in the blue image in the above left, which has a thick surface layer of RuO2. Rather, the active form is something similar to that in the salmon image in the bottom left, which better matches the structure seen in the operando experiment.  Overall, a combination of thorough ex situ, in situ, and operando TEM experimentation allows us to distinguish catalytically significant atomic structures from unreactive spectators, so that we can isolate and characterize the active ones.

Ru Particle

A Ru nanoparticle supported on amorphous silica in a gas environment of 4 Torr stoichiometric mixture of CO and oxygen, at 200°C. Aberration-corrected, atomic-resolution environmental TEM grants us incredible insight into the atomic structure of the surface of the catalyst nanoparticles.