In‐situ
HREM
observation and adsorbed gas imaging of ceramics supported fine metal particle catalysts under reaction gas atmosphere
Authors: Arakawa Yuuki , Tanaka Hiromochi , Muto Shunsuke , Kishita Keisuke , Sakurabayashi Yasumori , Hirata Hirohito 
Published: December 2016

Three‐way catalysts used for purifying automotive exhaust gas usually take a form of noble metal (platinum group metals) fine particles supported on a heat‐resistant metal oxide, which converts NO
x
, CO, HC to N
2
, CO
2
, H
2
O. Incessant efforts have been made to improve the catalytic performance because the governmental regulations for exhausted gas emissions has been continuously updated in a more strict direction, while the price of those rare metals are unstable, because of the uneven distribution of the resources. Technical issues of compatibility between the high catalytic activity and material costs by reducing rare metals or by development of alternative catalysts without them have not yet been fully solved. It is thus important to design an appropriate catalyst without relying on a rule of thumb by unraveling the mechanism of catalytic activities. In the present study we observed in situ catalytic reactions of fine metal particles under gas atmosphere at atomic level resolution and also tried to visualize the catalytic active site by imaging gas molecules adsorbed on the particle surface, using electron energy‐loss spectroscopy (EELS).

We selected Rh/ZrO
2
, Pt/ZrO
2
system, where the particle‐supporting ceramics interaction is suppose2d to be relatively weak, to examine the structural changes and adsorption behaviors of reactive gas molecules. The sample powders were dispersed in an organic solvent and a drop of the solution was painted by a brush on a tungsten wire of the single‐tilt specimen heating holder. The observation was done between RT and 600 degrees Celsius under vacuum, under O
2
, NO or CO atmosphere (diluted by Ne), using the Reaction Science Ultra‐High Voltage S/TEM, JEM1000K RS of Nagoya University, operated at 1000 kV, equipped with a GIF quantum equivalent post‐column EELS and with a differential pumping environmental cell which allows us to introduce a gas to the specimen chamber up to the pressure of 10,000 Pa [1]. Dynamical structural changes were observed at the HRTEM mode and energy‐filtered TEM spectral imaging (EFTEM‐SI) was applied to the plasmon‐loss region. A multivariate analysis method was applied to the obtained datacubes to isolate the spectral features specific to volume and surface plasmon of the metal particles and gas molecules [2].

The reference plasmon spectra of Rh
2
O
3
, ZrO
2
and L
2,3
ELNES of the gas molecules to identify the resolved spectra from the SI datacubes are shown in Fig. 1. Before observing catalytic reactions, the sample was heated up to remove the organic solvent, though it should be noted that fast heating brought about surface precipitates covering Rh particles under O
2
atmosphere, which was found to come from ZrO
2
support. This phenomenon was already reported as the strong metal support interaction (SMSI) [3], as shown in Fig.2. Under a mixture gas atmosphere of NO and CO at 300 degrees Celsius carbon layers covering the surfaces of ZrO
2
support and partly Rh particles were observed, presumably due to the reduction of CO, though it is unclear whether it is caused by a catalytic reaction. On the other hand, it was observed in case of the Pt/ZrO
2
system that reversible solution/precipitation of Pt particles into/from the ZrO
2
support by heating/cooling between RT and 300 degrees Celsius, which could be also SMSI.

Considering that the strong core‐loss spectra of the gas molecules appear in the low‐loss region, we attempted to detect and image adsorbed gas molecules on metallic particles, using the EFTEM‐SI technique and multivariate analysis. Unfortunately no gas molecule‐related spectrum was found at Rh/ZrO
2
under O
2
atmosphere because of SMSI, whereas under a mixture gas atmosphere of NO and CO the spectral component having a peak around 12 eV (Component 3 in Fig. 3), different from that of ZrO
2
, Rh, or Rh
2
O
3
was separated, which is presumably originated from NO and/or CO, as shown in Fig. 3. We have also conducted the EFTEM‐SI experiments under the conditions where SMSI is carefully avoided. Detailed results and argument are presented in the poster.