New Frontiers in Catalysis Using Single Atoms

Single atom catalysts are exactly what their name suggests: catalysts comprised from one single atom. Catalysts are typically nanoparticles, which range anywhere from tens to thousands of atoms, because the surface area-to-volume ratio is high for nanoparticles.

The surface is where all the chemistry takes place, rendering all the material in the core of the nanoparticle essentially wasted. As the catalyst gets smaller, this ratio gets even better and eventually you are down to one single atom where the entire atom is an active surface for gas conversion without wasted material in the core. The first studies looking at single atom catalysts have been very promising, indicating a highly reactive, selective, and potentially tunable catalyst material with minimal waste.

Representation of single atom catalysts sitting on a support.

Representation of single atom catalysts sitting on a support. Image Credit: Protochips

In order to see single atoms, you need to use a powerful imaging tool like the transmission electron microscope (TEM). While TEMs have proven impressive in their imaging capabilities, they must operate in a high vacuum environment which is completely unlike any environment where a catalyst would be used, making the TEM a controversial tool when doing catalysis research. Think about the catalytic converter in your car engine: it operates at one atmosphere of pressure with air, humidity, and combustion products surrounding it. How can one understand how a catalyst would operate in a catalytic converter if it is studied in a vacuum? Relevant conditions are required for relevant conclusions to be drawn.

Researchers at the University of California at Irvine are the first to observe single atom catalysts under realistic conditions in the TEM using the Atmosphere System from Protochips. The Atmosphere system is designed to enclose the catalyst sample in a hermetically sealed cell that becomes a miniature reactor and can be inserted in the TEM for observation. Once inserted, the user can flow in complex gas mixtures, control relative humidity, and apply heat up to 1,000 °C to see how the catalyst changes over time due to the harsh conditions. This level of understanding is what will lead to the ability to design catalysts that are more efficient, effective, and robust than what is currently available.

Left: The entire Protochips Atmosphere System (latest generation called the Atmosphere Catalysis System) showing custom gas handling system, integrated mass spectrometer, vapor introduction kit, closed-cell holder (inside of the TEM), and software. Right: Close-up of the closed-cell holder: sample is placed between the two MEMS E-chips (shown in a green color) and hermetically sealed via compression from the lid for safe insertion into the TEM.

Left: The entire Protochips Atmosphere System (latest generation called the Atmosphere Catalysis System) showing custom gas handling system, integrated mass spectrometer, vapor introduction kit, closed-cell holder (inside of the TEM), and software. Right: Close-up of the closed-cell holder: sample is placed between the two MEMS E-chips (shown in a green color) and hermetically sealed via compression from the lid for safe insertion into the TEM. Image Credit: Protochips

In this study, single platinum atom catalysts showed 3-5 times more activity when pre-treated under “harsh” reduction conditions as opposed to “mild” reduction conditions and the researchers wanted to gain a better understanding of this phenomenon. The explanation became obvious once they performed the same experiment within the Atmosphere system inside the TEM where they could directly observe the catalyst in each scenario. The “harsh” pre-treatment induced catalyst mobility on the support surface and changed the structure of the support slightly, contributing the increased catalytic activity seen during their original, larger-scale experiments.

In situ AC-STEM characterization of Ptiso/TiO2: a–c, In situ AC-STEM images of Ptiso/TiO2 after 30 min at different annealing conditions: 300 °C,760 torr of O2 for 30 min (a); 250 °C, 760 torr of 5% H2 (balanced with Ar) for 30 min (b); 450 °C, 760 torr of 5% H2 (balanced with Ar) for 30 min (c). The yellow circles identify the same Pt single atom. A false-coloring scale was used in a–c to enhance contrast on the Pt atom. Nat. Mat. Vol. 18, pages746–751 (2019).

In situ AC-STEM characterization of Ptiso/TiO2: a–c, In situ AC-STEM images of Ptiso/TiO2 after 30 min at different annealing conditions: 300 °C,760 torr of O2 for 30 min (a); 250 °C, 760 torr of 5% H2 (balanced with Ar) for 30 min (b); 450 °C, 760 torr of 5% H2 (balanced with Ar) for 30 min (c). The yellow circles identify the same Pt single atom. A false-coloring scale was used in a–c to enhance contrast on the Pt atom. Nat. Mat. Vol. 18, pages746–751 (2019). Image Credit: Protochips

The above in situ TEM results were correlated to XAS spectra to further solidify the conclusions.

XAS of Ptiso/TiO2 catalysts: a, XANES spectra collected at 200 °C immediately following various pretreatments, b. XANES spectra collected under CO oxidation reaction conditions (200 °C, 1% CO, 1% O2, balance He). Corresponding spectra following pretreatment are shown using dotted lines to emphasize changes induced by exposure to reaction conditions. c, EXAFS of catalysts after each pretreatment and under CO oxidation reaction conditions. Nat. Mat. Vol. 18, pages746–751 (2019).

XAS of Ptiso/TiO2 catalysts: a, XANES spectra collected at 200 °C immediately following various pretreatments, b. XANES spectra collected under CO oxidation reaction conditions (200 °C, 1% CO, 1% O2, balance He). Corresponding spectra following pretreatment are shown using dotted lines to emphasize changes induced by exposure to reaction conditions. c, EXAFS of catalysts after each pretreatment and under CO oxidation reaction conditions. Nat. Mat. Vol. 18, pages746–751 (2019). Image Credit: Protochips

With the Protochips Atmosphere system, you have complete quantitative and reproducible control over the gaseous composition, humidity levels, temperature, and pressure introduced to your nanoscale catalyst system, and now you can monitor it all with the integrated mass spectrometer. The Protochips Atmosphere system creates a bridge from desktop, bulk catalyst research into the nanoscale space. With the ability to create the most realistic operating environments within the most powerful imaging tool on the market, even understanding single atom catalysts at the most critical scale is possible.

Read the entire publication here: https://www.nature.com/articles/s41563-019-0349-9

Want to learn more about the most recent generation of the Atmosphere system? Download a brochure here: https://www.protochips.com/gate/test-copy-copy/

Image Credit: Protochips

This information has been sourced, reviewed and adapted from materials provided by Protochips.

For more information on this source, please visit Protochips.

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