Using TEM to Study Perovskite-Noble Metal Catalysts for Automotive Exhaust Control

Catalytic converters are an important part of an automobile exhaust system. They control and convert highly polluting gases, including CO, hydrocarbons, and NOx, emitted by the engine into harmless gases such as H2O, CO2 and N2.

For many years, catalytic convertors have been produced using noble metals such as Rh, Pd and Pt, but despite their efficiency as catalysts, these metals are very expensive. For several years studies have focused on the development of methods to minimize the materials required for sufficient activity.

The area of the catalyst’s surface where the chemical reactions occur determines its catalytic activity. Engineers and scientists across the world have tried to develop catalyst materials that have an increased surface area to volume ratio, so that the amount of material required and the cost involved can be reduced.

Due to their high surface to volume ratio, nanoparticles possess enhanced catalyst properties over bulk materials and are relatively less expensive. However, some important issues have to be resolved before nanoparticle catalysts can be made available for commercial and industrial use.

Most catalyst materials, particularly those in catalytic converters, operate at approximately 500ºC and above and must operate effectively for many years. At such high temperatures, nanoparticles may become mobile and form larger particles after coalescence.

This may result in a significant reduction of surface area and reduce the performance of the catalyst. The catalytic activity also rely on the support material, as metal nanoparticles can strongly interact with the support.

The interaction of nanoparticles with perovskite materials such as BaCeO3, LaFeO3, and CaTiO3 (CTO) are highly unique and have been the focus of research for many years. Results of studies conducted with bulk measurement methods demonstrate that noble metals dissolve and reform under reduction/oxidation conditions and high temperatures in a self-regenerative process.

This process can prevent excessive particle coalescence to overcome a significant hurdle with nanoparticle catalysts.

Many scientists have explored these materials using bulk techniques, and although highly successful, these techniques do not study the nanoscale behavior. With TEM, samples can be viewed at their atomic scale.

In addition, element identification and crystallographic measurements can also be performed through EELS and EDS. However, as the TEM only analyzes a sample in high temperature vacuum, it was only ex-situ experiments that could be performed in the past.

Because of this, a user has to first analyze a sample, expose it to the appropriate catalyst conditions in a separate reaction chamber, perform the imaging, and repeat the process when necessary. This procedure is not only time consuming, but is also highly expensive. Important information can also be missed during process steps.

The Protochips Atmosphere™ E-cell system provides gas and temperature conditions that closely resemble real life catalyst environment, where the catalyst samples can be exposed. With the device, atomic scale processes can be visualized in real time, enabling the extraction of appropriate critical information from experiments without much additional effort.

Experiment

In situ high-temperature reduction experiments were performed by the Xiaoqing Pan group of scientists from the University of Michigan on a CaTi0.95Rh0.05O3 catalyst sample. From solvent suspension, the powder sample was directly dispersed onto a thermal E-chip or temperature-controlled support.

The Atmosphere software can be used to control a thin, ceramic heating membrane present in the E-chip, to make automatic adjustments to the temperature under different pressures (up to 1 atm) and gas environments. Another window E-chip is placed on top of the thermal E-chip in the TEM holder to create a thin gas cavity which is sealed by the high vacuum of the TEM column. The sample was imaged at 1 atm (760 Torr) forming gas (5% H2 in N2) at 300ºC and 550ºC temperatures, during the experiment. A JEOL 2100F Cs corrected TEM was operated at 200 kV in STEM mode at the University of Michigan’s EMAL facility.

Discussion

The experiments conducted by Katz et al. it is shown that the Rh in CaTi0.95Rh0.05O3 migrates under reducing conditions at increased temperature. Figure 1 confirms these results in situ.

A HAADF STEM image of the sample in 1 atm of forming gas at a temperature of 250ºC is shown in Figure 1a, where the Rh is visible as bright contrast spots. Figure 1b shows the sample after a few minutes at 500ºC, where the Rh has formed into small nanometer sized particles and clusters.

If performed using conventional methods, this experiment can take several hours. Atmosphere has reduced the experiment time to a few minutes and enabled the collection of critical information at appropriate temperatures and pressures. The device also enabled the process to be visualized in real-time.

Figure 1. HAADF STEM images of CaTi0.95Rh0.05O3. The left image shows initial Rh migration and coalescence into nanoparticles and clusters at 250ºC in 1 atm forming gas. The right image shows significant Rh migration at 500ºC in 1 atm forming gas.

When the magnification on an individual CTO nanoparticle on zone axis was increased, the Ca and Ti present in the crystal lattice could be distinguished using Z-contrast (Figure 2a) despite being only an element apart in the periodic table. However, Rh clusters are visible as bright clouds and it is not clear if these are crystalline as they may not be oriented to a zone axis and do not seem to be epitaxial with the CTO.

The alternating Ca and Ti columns are clearly visible in Figure 2b, and due to their higher relative atomic mass the Rh atoms are also clearly identifiable. With the help of high-resolution STEM techniques in tandem with Atmosphere, the noble metal catalyst atoms in the lattice can be monitored in real time during oxidation and reduction experiments. This can help scientists to gain a better insight into the self-regeneration process that occurs at the atomic scale.

Figure 2. STEM HAADF images of CaTi0.95Rh0.05O3 at 500ºC in 1 atm of forming gas. The left image shows Z-contrast indicating the positions of the Ca and Ti atomic columns. The right image is at higher magnification. A line scan indicating pixel intensity shows the positions of Ca and Ti, as well as where Rh atoms appear and show up as brighter contrast due to the higher atomic number.

Applications

Catalyst materials used in the production of automotive applications and in various other industries are important components in the manufacturing of industrial and consumer materials and products. If important issues can be solved, nanoparticle catalysts have a great potential in these processes and applications.

Despite the high vacuum conditions found in the column of the TEM, exceptional results have been obtained from the TEM analysis of nanoparticle catalysts. Atmosphere adds to these results by enabling a user to apply accurate temperatures across a wide pressure range, without causing any disruption to the performance of the most powerful TEMs available in the market.

Being a holder based device, Atmosphere is highly compatible with a majority of the modern TEMs. It can be combined with both existing as well as new instruments without any special modifications.

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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