Praseodymium dioxide

Praseodymium dioxide crystallizes in the fluorite-type structure (space group Fm3m) with four praseodymium atoms and eight oxygen atoms per unit cell. This structure may be visualized easily as an infinite array of coordination cubes (each consisting of a Pr atom at the center with eight O atoms at the corners) stacked so that all cube edges are shared. 

Single crystals of praseodymium dioxide and intermediate terbium oxides were grown by the hydrothermal method [126]. The pressure in the sample autoclave was smoothly raised and lowered by control of heating and cooling of the autoclave. The rare earth oxide and nitric acid sealed in capsules of thin-walled gold tubing. The pressure of the capsules was raised to 165 MPa and isobarically heated to the run temperature (1003 1143 K). Holding at least for two hours to reach equilibrium,... 

An analysis of praseodymium dioxide, also called praseodymium(IV) oxide, in a variety of atmospheres is shown in Fig. 7.8. The measurements were carried out with a Mettler Thermoanalyzer as described in Fig. 7,2. The furnace arrangement was similar to the one shown in the bottom right diagram of Fig. 7.3. The heating rate was 8 K/min, the gas flow was about 10 liter/h at atmospheric pressure, and the sample mass was about 3 g of black Pr02. The thermogravimetry curves of Fig. 7.8 are recalculated in terms of the chemical composition of the praseodymium oxides. 

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... 

Adsorption of Carbon Dioxide and Oxygen on Praseodymium Samples. 

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. 

The Surface Properties of the Praseodymium Compound. Although the efficiency of catalysts in methane conversion has been ascribed to a variety of properties, a number of researchers have demonstrated the importance of basicity of the catalysts employed in this process (14-16). Thus estimates of basicity, such as may be obtained from the adsorption of carbon dioxide, are of some value in characterizing the catalysts. It is obvious that the surface state of a working catalyst at 750°C is different from that at room temperature. However, measurements of the adsorption of carbon dioxide at the latter temperature provide semiquantitative information on sites capable of donating electrons. 

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). 

The lanthanides, unlike the transition metals and the actinides, tend not to form compounds over a range of oxidation states. The +3 oxidation state is characteristic of all of the lanthanides, and the oxide fluorides of formula LnOF (Ln = lanthanide metal) are well known. The less stable oxidation states of + 2 and + 4 are known, but the latter is represented only by the dioxides and tetrafluorides of cerium, praseodymium, and terbium, and no tetravalent oxide fluorides have been reported. 

Diphosphazane dioxide complexes of lanthanides have potential application in the solvent extraction separation of lanthanides. Reaction of lanthanide nitrate with X2P(0)NPr )-P(0)X2(L-L) yields the bis chelate complexes Ln(NC>3)(L-L)2. The structure of the praseodymium complex has been determined by X-ray diffraction and the space group is P32- There are two independent molecules in the unit cell which differ in orientation of the phenyl group. The metal ion is ten-coordinated [264]. 

These elements are usually terpositive, forming salts such as La(N03)g 6H20. Cerium forms also a w ell-defined series of salts in which it is quadripositive. This oxidation state corresponds to its atomic number, 4 greater than that of xenon. Praseodymium, neodymium, and terbium form dioxides, but not quadrivalent salts. 

Cerium, praseodymium, and terbium oxides display homologous series of ordered phases of narrow composition range, disordered phases of wide composition range, and the phenomenon of chemical hysteresis among phases which are structurally related to the fluorite-type dioxides. Hence they must play an essential role in the satisfactory development of a comprehensive theory of the solid state. All the actinide elements form fluorite-related oxides, and the trend from ThOx to CmOx is toward behavior similar to that of the lanthanides already mentioned. The relationships among all these fluorite-related oxides must be recognized and clarified to provide the broad base on which a satisfactory theory can be built. 

Praseodymium(iv). Only a few solid compounds are known, the commonest being the black non-stoichiometric oxide formed on heating Pr111 salts or oxide in air. The oxide system which is often formulated as Pr6On is actually very complicated,49 with five stable phases each containing Pr3 + and Pr4+ between Pr203 and the true dioxide Pr02. 

The higher oxides where the oxygen to metal ratio x in the oxides is in the range of 1.5 to 2.0 are observed for cerium, praseodymium and terbium. These oxides exhibit fluorite-typed dioxides, which do not necessary mean x = 2.0 but usually the x value is slightly smaller than 2.0. Again, the composition of these oxides depends on the temperature, oxygen potential and physical state, besides their history of preparation and treatment [8-11]. 

Because the tripositive ions are the most stable for all the rare earth elements in almost all compounds, the thermochemistry of the solid (crystalline) rare earth sesquioxides dominates this chapter. Some rare earths have divalent or tetravalent states, so the chemistry of solid monoxides and dioxides are included. There are also many nonstoichiometric binary oxides of cerium, praseodymium, and terbium. As much as possible, the thermochemistry of these nonstoichiometric binary oxides is included. The stability, phase diagrams, and structures of ternary and polynary... 

Asami K, Kusakabe K, Ashi N, Ohtsuka Y (1997) Synthesis of ethane and ethylene from methane and carbon dioxide over praseodymium oxide catalysts. App Catal A Gencanl 156 43-56 Avalon (2015a) http //avalonraremetals.com/raie metals/praseodymivnn/... 

Intermediate oxides of Cm that have stoichiometries between the sesquioxide and the dioxide have been reported (Noe and Fuger 1971, Stevenson and Peterson 1975). Oxides with O/M ratios of 1.50 to 1.65,1.714 (Cm.,Oi2), and 1.79 to 2.(K) are the structural groupings (bee, rhombohedral, and fee, respectively) which have been reported. The curium oxygen system bears some resemblance to both the plutonium and praseodymium oxygen systems but additional work is needed on it. 

Among these materials, cerium dioxide (ceria) is of particular importance. Very pure ceria forms a white powder, but more often, it appears pale yellow, and less pure samples can even be brownish. A brownish coloration could be an indicative for the presence of impurities such as praseodymium... 

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