Kinetic and structural studies of oxygen availability of the mixed oxides Pr_1–xM_xO_y (M = Ce, Zr)

Abstract

One composition of Pr–Ce mixed oxide and a range of compositions of Pr–Zr mixed oxide were prepared by coprecipitation methods and characterized by x-ray powder diffraction, thermogravimetric analysis, and x-ray photoelectron spectroscopy. Based on phases formed, the PrOy—ZrO2 system in an oxygen-containing atmosphere at moderate temperatures (up to 800–1000 °C) is analogous to that of CeO2–ZrO2. Addition of either Ce or Zr to pure Pr oxide affects both the total amount of oxygen that can be reversibly exchanged between oxide and gas phase and the kinetics of the redox processes. Ce dramatically increases the amount (per Pr atom) and lowers the temperature of exchange, Zr slightly decreases the amount and also lowers the temperature of exchange, and both modifiers speed up the rate. These observations are rationalized in terms of bulk and surface structural features of the mixed oxides.

Origin of the redox deactivation phenomena in modified alumina‐supported Ce/Pr mixed oxide

“…As can be seen in Table 1, the amount of oxygen consumed by lanthanamodified sample (P20C/Al 2 O 3 -La) reduced at 900 • C decreases very significantly, being about 4% of the value at 800 • C. On the other hand, although for P20C/Al 2 O 3 -Si there is also an important loss of OSC at 900 • C, it still retains about 60% of the capacity at 800 • C. Figure 1 shows XPS spectra corresponding to Pr 3d and Ce 3d core levels for the as-prepared samples. In the case of Pr 3d, the absence of the satellite at 968.0 eV, attributed in the literature to Pr 4+ [14] , allows us to ensure that praseodymium is in the trivalent oxidation state. On the contrary, Ce 3d spectra obtained for silicaand lanthana-modified samples showed no evidence of Ce 3+ , and could be attributed to 100% Ce 4+ .…”

“…[11 -13] Although there is less information about praseodymia, some relevant articles can be found, however. [14,15] Ce 3d and Pr 3d spectra provide information about the actual distribution of oxidation states. This knowledge is very useful for the interpretation of the redox deactivation phenomena in the alumina-supported Ce/Pr mixed oxides studied in this paper.…”

Behavior of different soot combustion catalysts under NOx/O2. Importance of the catalyst–soot contact

“…for soot combustion in the absence of NOx, suggesting that when sub-surface/bulk oxygen is taking part in the catalytic process, the mixed oxide is preferred. In fact, Sinev et al [17] demonstrated that solid-gas oxygen transfer was faster for a Ce-Pr mixed oxide than for a pure praseodymia.…”

Oxygen Nonstoichiometry and Defect Chemistry Modeling of Ce[sub 0.8]Pr[sub 0.2]O[sub 2−δ]

“…Some discrepancies exist in the literature on the solubility limit of Pr in CeO 2 . A solubility limit of 30 atom %, as determined by X-ray diffraction ͑XRD͒, was reported by Shuk and Greenblatt 14 and Ftikos et al 15 In contradiction to this, single-phase fluorite samples with Pr doping up to 55 atom % 16 or even 70 atom % were reported by Knauth and Tuller 17 and Rajendran et al 18 The oxygen nonstoichiometry of 20 atom % Pr-doped ceria has been investigated in the high P O 2 regime ͑10 −5 -10 −1 atm͒ in the temperature ranges of 800-950°C ͑for microcrystalline powder͒ 19 and 600-750°C ͑for nanocrystalline powder͒ 20 by coulometric titration ͑CT͒. In both cases, a simple defect model treating the reduction of Pr 4+ to Pr 3+ in an ideal manner ͑i.e., not allowing h O − h O o to vary with the degree of nonstoichiometry͒ accounted well for the determined nonstoichiometry data.…”