The reduction of NO x emissions from diesel vehicles is a major challenge in the exhaust aftertreatment. The SCR-catalyst,
currently used in exhaust gas catalysis, only treats the nitrogen oxides efficiently at temperatures above 200°C. For low
temperatures (≤200°C), the use of a passive NO x adsorber is discussed, which stores the nitrogen oxides at low tempera-
tures and releases NO x again at temperatures above 200°C. As a possible passive NO x adsorber a precious metal loaded
ceria is suggested.
In this work the NO x storage mechanism of pure ceria and precious metal loaded ceria, as well as the stability of formed
NO x species was investigated. For this purpose, characteristic ceria properties were determined by analyzing various ceria
samples with distinct crystal sizes and distinct amounts of defects using optical spectroscopy (Raman, DRIFT, UV-vis), X-
ray diffraction, and N 2 adsorption-desorption. The combination of different methods revealed, that the Raman feature at
around 250 cm
−1
was related to a ceria surface vibration. In literature, the feature had been assigned to a 2TA vibration
of the ceria. Furthermore, the half width of the F 2g Raman feature was not only dependent on the crystal size but also
on the number of ceria bulk defects. Moreover, the amount of defects correlated with the band gap values determined by
UV-vis spectroscopy.
To investigate the NO x storage mechanism focus was put on the use of in situ-Raman spectroscopy. In a first step, the
measurement conditions for Raman spectroscopy were optimized. As the Raman laser caused surface defects on a static
sample, the influence of the laser radiation on the sample was minimized by continuous mixing of the sample using
the fluidized bed method. The continuous mixing resulted in a homogeneous sample temperature and small deviations
between the nominal and the actual temperature. Since accordingly the Raman spectra were averaged over the whole
sample, representative correlations between the temporal evolution of Raman features and changes in the NO x gas phase
concentration could be established.
When using Raman spectroscopy for structural characterization, data interpretation needs to take into account the possi-
ble absorption of radiation by the sample. For example, the reduction of metal oxides may result in new UV-vis absorption
bands, which in turn may affect the Raman spectra. The influence of such changes in UV-Vis absorption on the Raman
features was examined by the combination of UV-vis and Raman spectroscopy. Owing to the reduction of the ceria during
NO 2 storage a new UV-vis absorption band was formed. As the results showed, a change in absorption of about 10% resul-
ted in a decrease in Raman intensity of up to one-third. Therefore, to determine the corrected temporal evolution of the
Raman features, the Raman spectra had to be corrected for absorption effects. Although the absorption correction based
on the Kubelka-Munk theory predicts large intensity changes for small changes in the low-absorption region, it resulted
in the largest deviations among different absorption corrections. On the other hand, the commonly used normalization
by the F 2g phonon mode showed increasing deviations with increasing spectral distance of the investigated feature from
the position of the phonon mode. To reduce these discrepancies, a new wavelength-dependent absorption correction was
introduced. This new wavelength-dependent absorption correction was still useable in the strong-absorption region, as
demonstrated for precious metal loaded ceria.
Ceria is an important NO x storage material often used in combination with barium or zirconium. To elucidate the NO and
NO 2 storage mechanism in ceria in situ-Raman spectroscopy was coupled with simultaneous FT-IR gas-phase analysis.
The Raman spectra revealed not only information about the participation of the ceria surface but also the ceria volume.
Besides the formation of nitrates and nitrites a red shift of the F 2g mode and therefore an increase in the number of oxy-
gen vacancies was observed. Furthermore, for the first time, Raman spectra provided direct evidence of the involvement
of surface oxygen (Ce-O) in NO x storage. This surface oxygen played a key role for NO x storage, as its amount strongly
influenced the NO x storage capacity. A reduction of ceria prior to NO x exposure resulted in a lower NO x storage capacity,
as long as no strong oxidizing agent (e. g. NO 2 ) was present to form new surface oxygen. In the case of NO storage,
new reaction pathways were postulated describing the activation of gas-phase oxygen for ionic nitrite transformation
and formation of new NO x adsorption sites. However, peroxides as activated oxygen species can be excluded, since no
iiicorrelation was found between peroxides and formed nitrate and nitrite species. Based on the results, a mechanism for
NO and NO 2 storage in ceria was formulated. To investigate the stability of formed NO x species on ceria NO x desorpti-
on experiments were performed. During NO x desorption at ≤200°C most nitrites were released. Since during NO/NO 2
storage on reduced ceria predominantly nitrites were formed, 60% of the stored NO x desorbed below 200°C. Besides,
part of the nitrites was transformed to nitrates. On the other hand, the storage of NO 2 in ceria resulted dominantly in
the formation of nitrates, most of wich were released at temperature between 300°C and 400°C. Overall, at a desorption
temperature of 500°C 85–94% of the stored NO x was desorbed.
When ceria was loaded with a precious metal, the amount of surface oxygen decreased. However, for the Pt loaded ceria
the NO x storage capacity increased, whereas the Pd loaded ceria had a similar NO x storage capacity as bare ceria. On
ceria, the precious metals were present as a mixture of metallic and metal oxide particles. However, the composition
and particle sizes were affected by the pretreatment. During NO x storage, the precious metal oxide acted as an oxygen
source and was reduced to its metallic state. It should be noted that the exposure to NO 2 resulted in a faster reduction
of platinum oxide and higher NO x storage capacity than the exposure to NO. Overall, the observed increase of the NO x
storage capacity was not only caused by reduction of the platinum oxide but also by an activation of gas-phase oxygen.
Palladium loaded ceria showed similar yet strongly attenuated effects as platinum loaded ceria. All in all, NO x storage in
precious metal loaded ceria proceeded similarly to NO x storage in ceria, while a synergetic effect of the precious metal
and ceria led to new reaction pathways for NO x storage. During NO x desorption, the loading of ceria with a precious
metal resulted in a higher NO x desorption at temperatures below 300°C. As a result, most of the stored NO x was released
at 200–300°C. The palladium generally destabilized the NO x species. Therefore, compared to pure ceria, a higher amount
of released NO x was observed up to a temperature of 500°C. In contrast, platinum loaded ceria formed both destabilized
and stabilized NO x species resulting in a lower overall amount of desorbed NO x up to a temperature of 500°C. | English |
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