Because vehicular pollutants contribute to poor air quality, governments all over the world are imposing stricter emission standards for automobiles. This necessitates the development of more efficient exhaust gas after-treatment systems (systems that “clean” exhaust gas before it is released into the atmosphere).
Three-way catalysts (TWCs) or catalytic converters are the most commonly used method for treating exhaust emissions from gasoline-powered internal combustion engines. TWCs frequently contain nanoparticles of active metals such as platinum (Pt) and palladium (Pd) and oxygen storage materials with a high specific surface area, such as a solid solution of CeO2-ZrO2 (CZ). These components can catalyze a variety of oxidation and reduction reactions, converting harmful exhaust from automobile engines to harmless gases.
The durability, precision, and performance of a TWC are affected by factors such as the amount of oxygen stored or removed from the bulk and surface of the oxygen storage materials. To improve storage material efficiency, it is necessary to clearly understand oxygen transport and dynamics. Unfortunately, there are no techniques that allow direct tracking of the oxygen storage process in TWCs.
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.
Prof. Nagasawa
However, in a recent study published in Chemical Engineering Journal, a team of researchers led by Assistant Professor Tsuyoshi Nagasawa of Tokyo Institute of Technology (Tokyo Tech) presented a solution to the problem. Using the isotope quenching technique, the team developed a novel technique for direct visualization of the oxygen storage process in Pd/CZ TWCs.
Prof Nagasawa explains, “It is difficult to get clarity on the dynamic interactions—such as oxygen adsorption/desorption and surface/bulk diffusion—occuring 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.”
The team prepared a model TWC consisting of a precious metal, Pd, and a dense CZ substrate, stored 18O2 in it at 600 °C, and then quenched the catalyst using two helium gas nozzles covered in a water cooling jacket. They then used high-resolution secondary-ion mass spectrometry to analyze the 18O distribution on the surface and bulk of Pd/CZ.
The results indicated that Pd improves the diffusion depth of 18O into CZ bulk, as well as its surface concentration. It further revealed that 18O was preferentially adsorbed at the Pd/CZ interface as compared to the Pd center, where its concentration was lower. Density functional theory calculations also agreed with these observations.
Finally, the team calculated the local oxygen release/storage rates by comparing 18O distribution and an oxygen release/storage simulation using a diffusion equation. They found that the local rates were comparable and consistent with conventional oxygen storage capacity measurements.
This new visualization process provides useful insights into the oxygen storage and release mechanisms in metal/oxygen materials systems and can be used to further investigate and improve the performance and efficiency of TWCs used for 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,” concludes Prof. Nagasawa.
