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Direct Visualization of Oxygen Storage Dynamics in Pd/CZ Catalyst

As vehicular emissions contribute to poor air quality, governments worldwide are imposing tougher emission regulations for vehicles. This necessitates the advancement of more effective exhaust gas after-treatment systems (i.e., systems that “clean” exhaust gas before releasing it into the atmosphere).

Direct Visualization of Oxygen Storage Dynamics in Pd/CZ Catalyst

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Three-way catalysts (TWCs) or catalytic converters are the most often used method for reducing exhaust emissions from gasoline-powered internal combustion engines. TWCs frequently contain nanoparticles of active metals like platinum (Pt) and palladium (Pd) and oxygen storage materials with a large specific surface area, such as a solid solution of CeO2-ZrO2 (CZ).

These components can catalyze various oxidation and reduction reactions, converting hazardous exhaust from automobile engines to nontoxic gases.

The oxygen stored or expelled from the bulk and surface of the oxygen storage materials affects the durability, precision, and performance of a TWC. To enhance the storage material’s efficacy, it is vital to precisely understand its oxygen movement and dynamics. However, there are no tools that can directly track the oxygen storage process in TWCs.

A group of scientists headed by Assistant Professor Tsuyoshi Nagasawa of Tokyo Institute of Technology (Tokyo Tech) offered a solution to the problem in a recent breakthrough published in Chemical Engineering Journal. Using the isotope quenching approach, the team created a new methodology for direct visualization of the oxygen storage process in Pd/CZ TWCs.

It is difficult to get clarity on the dynamic interactions—such as oxygen adsorption/desorption and surface/bulk diffusion—occurring on TWC surfaces, because they can only be estimated indirectly from the valence change of cerium in CZ, or the oxidation state of the noble metal. However, our method surpasses these problems by incorporating isotope labeling with reaction quenching, which allows us to investigate the oxygen storage processes by tracking the 18O isotope involved in these interactions.

Tsuyoshi Nagasawa, Assistant Professor, Tokyo Institute of Technology

The researchers built a model TWC out of a precious metal, Pd, and a thick CZ substrate stored 18O2 in it at 600 °C before quenching it with two helium gas nozzles enclosed in a water cooling jacket. The 18O distribution on the surface and bulk of Pd/CZ was then analyzed using high-resolution secondary-ion mass spectrometry.

According to the findings, Pd increases the diffusion depth of 18O into CZ bulk as well as its surface concentration. It was also discovered that 18O was selectively adsorbed at the Pd/CZ interface vs. the Pd center, where its concentration was lower. These observations were also supported by density functional theory calculations.

Finally, the researchers computed the local oxygen release/storage rates by comparing the 18O distribution to an oxygen release/storage simulation based on a diffusion equation. They discovered that the local rates were comparable and compatible with traditional oxygen storage capacity measures.

This innovative visualization approach sheds light on the oxygen storage and release mechanisms in metal/oxygen materials systems and can be utilized to further explore and enhance the performance and efficiency of TWCs used in automobile exhaust treatment.

The volatile organic compounds and oxides of nitrogen and carbon commonly produced by combustion engines, if released without treatment, can not only cause breathing-related health issues but can also indirectly impact the acceleration of global warming. With our study, we wanted to contribute towards the world’s mission to achieve better emission practices.

Tsuyoshi Nagasawa, Assistant Professor, Tokyo Institute of Technology

Journal Reference:

Nagasawa, T., et al. (2022) Visualization of oxygen storage process in Pd/CeO2-ZrO2 three-way catalyst based on isotope quenching technique. Chemical Engineering Journal. doi.org/10.1016/j.cej.2022.139937.

Source: https://www.titech.ac.jp/english

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