Characterization of the reductibility of Zr and Pr-doped Ceria
Résumé
1. Introduction
Environmental regulations on NOx emissions of mobile sources are becoming increasingly demanding. The NOx storage-reduction (NSR) catalytic technology is one of the solution lean-burn engines. NSR catalysts work in cyclic gas-composition conditions. During the first step (lean phase), NSR materials store emitted NOx as nitrates until the surface reaches the saturation threshold. A pulse of fuel post-injection then triggers the short second step (rich phase), during which the reducing conditions lead to nitrate decomposition and release as well as subsequent NOx reduction into N2 and surface regeneration. Ceria is an interesting material since it plays a double function first as a high specific surface area support [1] for platinum group metal (PGM) dispersion and secondly as a NOx trap at low temperature until 350°C. Below 200°C, the NOx storage capacity of ceria is higher than that of barium which is commonly used as a storing agent [2]. In addition, ceria can limit the thermal sintering of Pt nanoparticles [3]. This study aims to deeply characterize the impact of the nature of dopant (Zr and Pr) on the redox properties of ceria-based oxides.
2. Experimental/methodology
Different compositions of commercial ceria-based oxides were provided by the Solvay Special Chem Company : CeO2, Ce0.49Zr0.51O2 (CZ)) and Ce0,80Pr0,20O2 (CP20). All samples have been calcined at 800°C for 2 h. In-situ X-Ray Diffraction measurements were carried out in an atmosphere-controlled Anton Paar XRK 900 reactor chamber, either under air, N2 or H2, using a Versatile Panalytical X’Pert Pro MPD Diffractometer equipped with a diffracted beam graphite monochromator (Cu Kα radiation) and a 1-dimensional multistrip detector (X’Celerator). Diffractograms were collected at several temperatures from 25 to 750°C and crystallographic parameters were determined by the Rietveld method. Surface and bulk reducibility was characterized by Temperature-Programmed Reduction (TPR) in H2 from room temperature up to 850°C. Oxygen-Temperature-Programmed Desorption (TPD) experiments were also performed to quantify the surface reactivity of the different oxides toward oxygen. Oxygen was adsorbed at 500°C while the desorption was followed in He from RT up to 800°C with an heating ramp of 10°C/min. In-situ Raman spectroscopy was used to determine structural/electronic defects and surface oxygen species of the samples after reducing/oxidizing treatments. Spectra were recorded with a LabRam HR Raman spectrometer (Horiba-Jobin Yvon) at low temperature after oxidation or reduction at 525°C. Three exciting wavelengths were used (514, 633 and 785 nm) and the laser spot was focused using a ×50 long working distance objective. The in-situ Raman spectra were recorded using a THMS600 cell (Linkam) between -196°C and 525°C under 10 % H2-N2, N2 and 10% O2-N2.
3. Results and discussion
XRD patterns have evidenced the cubic structure (Fm-3m) for CeO2 and CP20 while CZ also
contains a t’ tetragonal phase. The lattice parameters were extracted from in-situ patterns in
air, N2 and H2 as a function of the temperature. The increase of the lattice parameter with
respect to the one measured in air at a given temperature was used to estimate the bulk
reducibility. Figure 1a clearly shows that CP20 is reduced in pure H2 from 300°C, instead of
400°C for CZ and above 700°C for CeO2. These results are in good agreement with TPR
experiments which evidenced that the surface reduction of doped ceria is promoted by the
insertion of Pr. By the same way during O2-TPD experiments, a low O2 desorption peak
below 400°C was only observed on CP20. In addition, the capacity of the ceria oxide to
chemisorb oxygen was strongly improved in the presence of Pr.
Beyond confirming the XRD structural characterization, Raman mappings evidenced a high
homogeneity at the micrometer scale. Furthermore, structural defects and oxygen species
were observed with this technique. In particular, Raman spectra of CP20 oxide highlighted a
defects band at 570 cm-1 (Figure 1B) which can be explained by a resonance effect involving
a particular defect stabilized at the oxidized state. The reduction of cations contained in such
defect modifies its electronic properties inhibiting the resonance effect. Concerning CZ
support, a typical band of Ce3+ was visible after reduction whereas peroxo species were
observed after re-oxydation contrarily to the other solids.
Figure 1. (A) Variation of the fluorite lattice parameter as a function of temperature in pure H2 (B) Evidence of a new
defects band enhanced by Raman Resonance effect for CP20.
4. Conclusion
Structure, surface and bulk properties of three different ceria oxides have been deeply
investigated in both oxidized and reduced states. The role of the dopants, i.e. Pr and Zr, on
the redox properties in cycling lean/rich conditions encountered in NSR processes will be
discussed.
5. References
[1] Z. Say, E.I. Vovk, V.I. Bukhtiyarov, E. Ozensoy, Appl. Catal. B Environ. 142-143 (2013) 89–100.
[2] E. Rohart, V. Bellière-Baca, K. Yokota, V. Harlé, C. Pitois, Top. Catal. 42-43 (2007) 71–75.
[3] Y. Nagai, T. Hirabayashi, K. Dohmae, N. Takagi, T. Minami, H. Shinjoh, S. Matsumoto, J. Catal. 242 (2006) 103–109.