Praseodymium -oxide

However, reduced ceria is able, alone, to dissociate NO. Martinez-Arias et al. [85] have first investigated by electron paramagnetic resonance (EPR) and FTIR spectroscopies NO reaction on ceria pre-outgassed at different temperatures and showed the role of superoxides differentially coordinated in the formation of hyponitrites species further decomposed into NzO. Later Haneda et al. [86] have demonstrated that reduced ceria and reduced praseodymium oxide dissociate NO even though the presence of a noble metal (Pt) significantly increases the formation of N2 or N20. The main results of this study are summarized in Table 8.9. 

Praseodymium is rarely encountered in everyday life. However, praseodymium oxide combined with zirconium oxide gives rise to a beautiful yellow glazing, which is very popular. 

In conclusion I should like to consider a few of the chemical investigations which might be accomplished in the rare earth field by Mossbauer spectroscopy. The study of nonstoichiometric oxides has been discussed earlier, but there is the problem of finding an appropriate doping nuclide for the praseodymium oxide system. The element most capable of following the changes in oxidation state of the praseodymium is terbium-159, which does have a Mossbauer state, however, with a rather broad resonance (58,0 k.e.v., = 0.13 nsec.). Nevertheless, with a sufiiciently... 

Oxidative Coupling of Methane over Praseodymium Oxide in the Presence and Absence of Tetrachloromethane... 

In this work, we will show that the addition of TCM to the feedstream in the methane conversion process results in the enhancement of the conversion of methane and the selectivity to C2 hydrocarbons on praseodymium oxide primarily as a result of the formation of praseodymium oxychloride, in contrast with the production of carbon oxides on praseodymium oxide in the absence of TCM (8-10). The surface properties of these catalysts are characterized by application of adsorption experiments and X-ray photoelectron spectroscopy (XPS). 

Praseodymium oxide (Pr O ) was obtained from Aldrich and used without further purification. Praseodymium chloride (PrCl3) was prepared from praseodymium chloride hexahydrate (Aldrich 99.9%) by heating at ca. 150 C in air. 

The adsorption of carbon dioxide or oxygen on praseodymium samples was measured by a constant-volume method using a calibrated Pirani vacuum gauge. Praseodymium oxide was heated in oxygen (4 kPa) at 775°C for 1 h, then evacuated at 750°C for 0.5 h just before the measurement. The sample of praseodymium oxychloride was prepared from praseodymium chloride by heating under oxygen flow... 

Methane Conversion. The results for the conversion of methane on praseodymium oxide are shown in Figure 1 and Table I. The major products were carbon monoxide, carbon dioxide, ethylene, and ethane both in the presence and absence of TCM in the feedstream while small amounts of formaldehyde and C3 compounds were detected. Water and hydrogen were also produced. The catalyst produced low methane conversion (ca. 6%) and selectivity to C2+ compounds (ca. 30%) in the absence of TCM in the feedstream. On addition of TCM the conversion of methane after 0.5 h on-stream was increased by almost two-fold (11.9%) and increased still further to 17.2% after 6 h on-stream. The selectivity to C2+ also increased with time on-stream to 43.3% after 6 h on-stream. It is noteworthy that over the 6 h on-stream with TCM present the ratio increased from 1.0 to 2.1. No methyl chloride was... 

Conversion and C2+ selectivity for oxidative coupling of methane over praseodymium oxide in the presence and absence of TCM. 

Adsorption of carbon dioxide or oxygen on the praseodymium samples was carried out in the pressure range of 1-40 Pa to evaluate the number of chemisorption sites on the samples. Praseodymium oxide irreversibly adsorbed 9.5 x 10" mol g of carbon dioxide. The amount of oxygen irreversibly adsorbed on the sample was 15.2 x 10" mol g Carbon dioxide or oxygen was not adsorbed on the samples containing chlorine, i.e., praseodymium chloride and praseodymium oxychloride prepared from the chloride by heating under oxygen flow at 750°C for 1 h. 

In the case of the praseodymium oxychloride produced from praseodymium oxide by the reaction in the presence of TCM (white-green portion), an O Is peak at 529.3 eV with a shoulder at 531 eV was observed. The peak shifted to 528.9 eV after xenon ion-sputtering for 0.5 min. The peak of Pr 3d was similar to that for praseodymium chloride, that is, the main peak was observed at 932.9 eV with a shoulder at 928 eV which was more clearly defined than that in the spectrum for praseodymium chloride. The peak shifted to 932.5 eV after the sputtering and the shoulder at 928 eV intensified as seen in the spectra for praseodymium chloride. The peak of Cl 2p was present at 198.8 eV and the position of the peak did not change after the sputtering. 

There were two peaks at 528.9 and 531.1 eV in the spectrum for O Is of the praseodymium oxychloride taken from the inlet portion of the reactor after the reaction in the absence of TCM (the sample originated from praseodymium chloride heated at 750°C for 1 h). The peak at 531.1 eV disappeared after xenon-ion sputtering while the main peak was present at 529.1 eV. The spectra for Pr 3d before and after xenon-ion sputtering were similar to those for praseodymium oxychloride which originated from praseodymium oxide. Although the Cl 2p spectra for the... 

Figure 5 XPS bands of praseodymium oxide (a) C Is (b) after sputtering (c) O Is (d) after sputtering (e) Pr 3d (f) after sputtering.

No adsorption of carbon dioxide or oxygen was observed on either praseodymium chloride or oxychloride. This finding is consistent with the XPS results. The main peaks at 529 eV in the spectra for praseodymium oxychloride samples are also attributed to the lattice oxygen of the oxychloride while the peaks at 531 eV are assignable to O Is for praseodymium oxide, suggesting that the surfaces of the oxychloride samples are partially oxidized to praseodymium oxide. The 3d binding energy of 933 eV for praseodymium in the chloride and oxychloride implies that the valence of praseodymium is 3+, while the shoulder at 928 eV could be attributed to metallic praseodymium (77). 

Although generalization of the present results to other catalyst systems is tempting, the evidence currently available is undoubtedly insufficient to justify such an extrapolation. It is clear, however, from both the present and previous work in this laboratory, that the chlorine atoms interact with the surface of the catalyst. There is also strong evidence to support the contention that the effect produced by the introduction of TCM into the feedstream can be primarily attributed to a modification of the catalyst surface and not to a gas phase process. The present work demonstrates that, at least with praseodymium oxide, the oxychloride is produced on addition of TCM and further, the oxychloride is largely responsible for the beneficial effects. Since the enhancements observed with TCM have been shown (2, 4-7) to be related to the nature of the catalyst, it is conceivable that these effects, while dependent on the formation of the oxychloride, are also a function of the thermodynamic stability of the oxychloride. Further work is in progress. 

Praseodymium oxide (PrgOn) [12036-32-7] M 1021.4. Dissolved in acid, ppted as the oxalate and... 

In 1966 Hyde et al., showed that the tensimetric isobaric data for the praseodymium oxides indicated a stable phase with a composition PrOi.sis at oxygen partial pressures of 30-205 torr in the temperature range 420-450 °C. This composition has not been prepared as a single phase because it easily decomposes to the e-phase, PrOi.778, during heating from the a-phase, PrOi.833, or on cooling down from PrOi.80- The eutectoid reaction of PrOi.sis always occurs and thus this phase co-exists with the other phases (Hyde et al., 1966). 

PrOi 833. One of the most dramatic features of the entire praseodymium oxide system is the phase Pr0On, which has a narrow range of composition over wide variation of temperature and pressure. This fact accounts for the observance of this phase when samples of heated oxides are cooled slowly in air. 

In traditional areas such as ceramic tiles, praseodymium oxide in a zirconium silicate matrix is used as a yellow stain. Cerium oxide is used as an opacifier or to give esthetic effects in the glaze. Other pigment colours, such as orange with yttrium oxide and light purple with neodymium oxide can be prepared. The compositions of these pigments are given in Table 12.19. 

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