Original article by See Wee Chee, Juan Manuel Arce-Ramos, Wenqing Li, Alexander Genest & Utkur Mirsaidov. Published in Nature Communications volume 11, Article number: 2133 (2020) .

The catalytic performance of noble metal nanoparticles (NPs) is decided by their surface structure. Hence, understanding the structural dynamics of nanoparticles during catalysis is necessary for the design of improved catalysts that can lead to significant reductions in energy consumption for industrial catalytic processes. Using the DENSsolutions Climate system, scientists from National University of Singapore (NUS) showed that they were able to capture structural changes in Palladium (Pd) NPs during CO oxidation under realistic operating environments and correlate those changes with the NPs’ catalytic activity.

The Pd NPs showed an inactive faceted structure at low temperatures which changed to an active more rounded structure at higher temperatures. This change in NP structure and activity reverses when the temperature is reduced. The reversibility of NP structural transformations has important implications for our understanding of active catalyst structures and reinforces the need for direct operando observations.

This movie was recorded during a temperature ramp from 300 to 500 °C at a rate of 2 °C/s. It clearly shows the change in the NP shape, where the flat facets and sharp corners became more rounded, which occurred concurrently with the change in catalytic activity.

Climate system

For this research, the Climate G+ system was used in combination with the DENSsolutions Gas Analyzer. The system enabled the researchers to attribute the changes in catalytic activity to the observed structural changes, which was further confirmed by thermodynamic calculations. Matching the high-resolution image sequences with outlet gas composition changes helped the authors to understand how the NP structure can influence the availability of active sites on a NP’s surface.

This research exemplifies how the different data streams from the Climate G+ (calorimetry), the Gas Analyzer (partial pressure) and the TEM detector (HR-TEM image) can be correlated into meaningful results: 

TEM detector

HR-TEM images show that the shape of the Pd NP’s in the Nano-Reactor changes from clearly faceted at 400°C to a more rounded shape at 600°C. The Pd NP’s become faceted again after the temperature was dropped to 400°C. The authors were also able to show the correlation between the morphology of the Pd NPs and their activity towards CO oxidation as function of temperature.

Calorimetry

The temperature and microcalorimetry data from the sensitive 4-point probe heater provided additional details. During the temperature ramp from 400°C to 600°C, a spike was seen at 500°C, indicating an exothermic reaction. This exothermic reaction can be interpreted as ignition of the oxidation reaction. After the spike, the Climate system measured a slight drop in heater power which further supported this conclusion. The authors were also able to match this temperature spike with the moment when the structural transitions occurred in the videos during the temperature ramps. Allowing them to correlate the onset of the reaction with the NP structure.

Gas analyzer

This onset of reaction at 500°C was further reflected as a change in gas composition, where the CO:O2 ratio in the gas flow (which was set at 1.6 by the Gas Supply System) clearly dropped and the production of CO2 concurrently increased. After a ramp-down back to 400°C the pressure ratios in the gas flow from the Nano-Reactor were restored back to their original levels, indicating de-activation.
“Our observations imply that the active structure of Pd nanoparticles is not retained outside of active catalytic conversion conditions, which will be important for interpreting results from similar studies of catalysts.

The inline mass spectrometry (Gas Analyzer) was critical for establishing the correlation between nanoparticle and catalyst activity. The low thermal drifts allow us to follow the nanoparticles during heating and cooling ramps.”

Dr. See Wee Chee
Department of Physics and Department of Biological Sciences.
National University of Singapore

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