Pr Cations Research Articles

Application of SR Methods for the Study of Nanocomposite Materials for Hydrogen Energy

This work summarizes results of synchrotron radiation (SR) studies of the real/defect structure of nanocrystalline/nanocomposite oxide materials, which determines their functional properties in hydrogen energy field as catalysts and mixed ionic electronic conductors (cathodes and anodes of solid oxide fuel cells, oxygen separation membranes). For nanocrystalline ceria-zirconia mixed oxide prepared via modified Pechini route using ethanol solution of reagents, a high spatial uniformity of cations distribution between domains along with the oxygen sublattice deficiency revealed by full-profile Rietveld refinement of SR diffraction data provide structure disordering enhancing oxygen mobility. For PrNi0.5Co0.5O3–δ – Ce0.9Y0.1O2–δ nanocomposite extensive transfer of Pr cations into fluorite domains generates a new path of fast oxygen diffusion along chains of Pr3+ – Pr4+ cations as directly proved by analysis of the unit cell relaxation after changing pO2 in perfect agreement with data obtained by oxygen isotope heteroexchange.

Nanostructured ceria-praseodymia catalysts for diesel soot combustion

Nanostructured ceria-praseodymia catalysts with different praseodymium contents have been prepared through hydrothermal synthesis to study the effect of Pr as a dopant and the effect of morphology towards soot combustion under “loose” and “tight” soot-catalyst conditions. Samples synthesized through solution combustion synthesis (SCS) have also been prepared as comparative materials. Studies in physicochemical properties of the catalysts have been carried out using complementary techniques. The present work also resorts to soot-TPR as an unconventional method of investigating the ability of solid catalysts to initiate soot oxidation in the absence of bulk oxygen. Ce50Pr50 catalyst (where 50 indicates the atomic percentage of cerium as well as of praseodymium) with mixed structures of nanorods and nanocubes has attained the best catalytic performances, thanks to the high lattice oxygen mobility and the easy reducibility. The insertion of Pr cations to the ceria framework enhances the number of redox sites on the surface, thus generating more oxygen vacancies. As a whole, activity tests in general have proven that despite having relatively low surface areas, ceria-praseodymia nanocubes and nanorods facilitated soot combustion reaction more actively than SCS-based ceria-praseodymia catalysts with larger surface areas. This evidences the beneficial effect of well-defined nanostructures in soot combustion, due to their possession of highly reactive low-index facets (100) and (110). Within SCS-based samples, however, the specific surface area overshadows the importance of praseodymium. This eventually marks the synergistic combination of well-defined nanostructures and praseodymium as a dopant.

Phosphodiester cleavage by trivalent lanthanides in the presence of native cyclodextrins

Testing phosphodiesterase activity of Eu(III) in the presence of native cyclodextrins revealed capacity of β-cyclodextrin (β-CD) to stabilize catalytically active metal hydroxocomplexes in mildly basic solutions. Kinetics of the hydrolysis of bis(4-nitrophenyl) phosphate (BNPP) and transesterification of 2-hydroxypropyl 4-nitrophenyl phosphate (HPNP) as models of DNA and RNA respectively has been studied with La(III), Pr(III), Nd(III), Eu(III), Gd(III) and Dy(III) cations in the presence of β-CD in the range of pH 7.0–9.0. The overall catalytic effect with 2mM lanthanide–β-CD complexes was up to 105 for HPNP and 108 for BNPP at pH 8 demonstrating the highest catalytic activity among so far reported artificial phosphodiesterases. Analysis of concentration and pH-dependences of observed rate constants for different lanthanides showed that active species are binuclear polyhydroxocomplexes of general type [M2(β-CD)(OH)n]6−n with n=3–5. The metal–β-CD and phosphodiester–β-CD interactions were studied by 1H NMR spectroscopy. Mechanistic implications of much higher catalytic efficiency in BNPP hydrolysis as compared to HPNP transesterification are discussed.

Structure of compound Pr5Mo3O16+δ exhibiting mixed electronic—ionic conductivity

The structure of Pr5Mo3O16 + δ single crystals is studied by X-ray diffraction, EDXS microanalysis, transmission microscopy, and XANES spectroscopy. It is found that in the structure Pr and Mo cations mutually replace each other, atomic positions of oxygen are split into several additional positions, and structural voids accommodate interstitial oxygen atoms (which make the main contribution to the conductivity). The disorder of the oxygen sublattice is responsible for the continuity of the framework of the ways of migration of oxygen ions.

Anomalous X-ray diffraction study of Pr-substituted BaCeO3 - δ.

The effect of Pr doping on the crystal structure and site occupancy was studied for the nominally synthesized BaCe1 - xPrxO3 - δ (x = 0, 0.2, 0.4, 0.6 and 0.8) perovskites using anomalous X-ray powder diffraction (AXRD) data and Rietveld analysis. Crystal structure parameters were accurately determined using 10,000 eV photons, and the Pr occupancy was refined using data collected with 5962 eV photons, close to the Pr LIII absorption edge. BaCe1 - xPrxO3 - δ crystallizes in the Pnma (No. 62) space group for all x values. Pr cations are mainly located at the Ce sites (perovskites B site), but a small fraction of them increasingly substitute some of the Ba ions at the A site as Pr content increases. The Pr doping introduces electronic defects (Pr(+3)/Pr(+4)) and oxygen vacancies needed for H2O incorporation and H-ionic conductivity. A decrease in the orthorhombic distortion would produce the opposite effects on the electronic and ionic mobility. The electronic mobility should increase due to an improvement in the overlap of the (Ce/Pr)4f-O2p orbital, while the proton mobility should decrease as a consequence of a larger hopping distance.

Role of Pr cations and the low temperature transition in Pr0.50Sr0.50CoO3: A comparison to Pr0.50Ca0.50CoO3

The low temperature properties of Pr0.50Sr0.50CoO3 are studied in comparison to Pr0.50Ca0.50CoO3, which is known to exhibit a Pr3+/Pr4+ valence shift below TMI~80K. The evolution of the Pr–O network strongly differs in the Sr and Ca compounds, but both have in common a prominent and sudden contraction of some Pr–O bonds. The structural phase transition that accompanies the so-called magnetocrystalline anisotropy transition at TS~120K in Pr0.50Sr0.50CoO3 brings about a notable contraction of some Pr–O2 bonds. The observations give support to the active participation of Pr-4f electrons at the intriguing magnetostructural transition (TS) in Pr0.50Sr0.50CoO3. In addition, we have confirmed that (in absence of praseodymium) the magnetostructural transition is suppressed in (Nd2/3La1/3)0.50Sr0.50CoO3, a perovskite with the same average cationic size at the A-site than Pr0.50Sr0.50CoO3.

Ce-Pr mixed oxides as active supports for Water-gas Shift reaction: Experimental and density functional theory characterization

Experimental measurements by several techniques and density functional theory (DFT) calculations are combined to characterize Ce-Pr mixed oxides and deeply understand the influence of Pr dopant in their geometric and electronic structure, reducibility and catalytic behavior concerning the Water-gas Shift reaction (WGSR). Samples with nominal Pr dopant content of 0, 5, 15 and 50at% (atomic percentage) are synthesized via urea decomposition method and subsequent calcination in air at 450°C. X-ray diffraction characterization shows an increase of ceria lattice parameter for Pr loadings lower than 15at%, which is consistent with a solid solution formation. X-ray photoelectron spectra indicate a surface enrichment in Pr cations and a diminution of Ce3+ surface concentration as Pr content increases. DFT calculations confirm the preferential reduction of Pr cations due to a single oxygen vacancy formation in Ce-Pr mixed oxides, and also a significant decrease in the energy needed for this O removal compare to that in pure CeO2. Both experimental measurements and DFT calculations evidence that the addition of Pr dopant to ceria promotes oxygen vacancies formation, redox properties and oxygen storage capacity (OSC). Even more, the same promoting effect of Pr in the WGSR intrinsic catalytic activity of Ce-Pr mixed oxides is observed.

IR spectroscopy of rare-earth aluminum borates RAl3(BO3)4 (R = Y, Pr-Yb)

Rare-earth aluminum borates RAl3(BO3)4 (R = Y, Nd-Yb) obtained by spontaneous high-temperature flux crystallization form two polytype modifications described by the space groups R32 (D 3 7 ) and C2/c (C 2 6 ). They differ in the symmetry versions in mutual arrangement of layers. These borates have been investigated by mid- and far-IR spectroscopy in combination with factor-group analysis of the vibrations of BO 3 3− ions; translational motions of Al3+, R3+ (Y, Nd-Yb), and BO 3 3− ions; and BO 3 3− rotations. Rare-earth aluminum borates are assigned to different space groups according to their IR spectra. Borates with large rare-earth Nd and Pr cations are crystallized into the space group C2/c (C 2 6 ), while borates with smaller (Y, Sm-Yb) ions are crystallized into the space group R32 (D 3 7 ). The rhombohedral structure of the latter compounds includes monoclinically ordered domains, as is evidenced by the presence of monoclinic-phase bands in the IR spectrum. NdAl3(BO3)4 and SmAl3(BO3)4 can form both monoclinic and rhombohedral polytypes. The IR spectrum of the monoclinic SmAl3(BO3)4 phase contains bands due to the rhombohedral phase, while the IR spectrum of NdAl3(BO3)4 (which is described by the space group R32) contains bands due to the monoclinic polytype. The presence of domains with another arrangement of layers in the polytype structure is a characteristic sign that corresponds to the order-disorder theory; this theory explains the structure of polytypes.

Thermochemical behavior and transport properties of Pr-substituted SrTiO3 as potential solid oxide fuel cell anode

Phase composition, structural stability, electrical properties, thermochemical expansion and redox behavior of Pr-substituted SrTiO3 were assessed for potential application in solid oxide fuel cell anodes. XRD analysis confirms formation of single-phase perovskite-like Sr1−xPrxTiO3±δ and Sr1−1.5xPrxTiO3±δ (x = 0.02–0.30) ceramics under both oxidizing and reducing conditions, although microstructural studies indicate minor precipitation of TiO2 in reduced Sr-deficient ceramics with high Pr content. XPS analysis in combination with XRD suggests that Pr cations substitute into strontium sublattice in mixed 4+/3+ oxidation state and are essentially insoluble in titanium sublattice. Reduction at elevated temperatures results in 2–3 orders of magnitude increase of n-type electronic conductivity with respect to oxidized materials. Thermogravimetric and electrical studies demonstrate however very slow reduction kinetics at temperatures below 1273 K associated with nearly frozen equilibrium in cation sublattice and low oxygen vacancy concentration in oxidized materials. Electrical behavior is discussed in conjunction with defect chemistry of donor-doped strontium titanate. All oxidized and reduced materials exhibit moderate thermal expansion coefficients, compatible with that of common solid electrolytes, and a small contribution of chemical expansion on reduction.

A Ferrimagnetic Zintl Phase Pr4MnSb9: Synthesis, Structure, and Physical Properties

A new valence precise Zintl phase, Pr4MnSb9, has been successfully synthesized by solid-state reaction at high temperature. The single-crystal X-ray diffraction data reveal its monoclinic symmetry in the space group C2/m (No. 12) with a = 24.12(2) Å, b = 4.203(3) Å, c = 15.67(2) Å, β = 98.05(1)°, and Z = 4. The structure is characterized by the covalent three-dimensional network constructed by two types of five-atom-wide Sb5(7-) ribbons that are joined by 6-fold coordinated Mn(3+) cations, through which the narrower three-atom-wide Sb3(5-) ribbons are attached as a tag, and interstitial Pr(3+) cations and single Sb(3-) anions locate within the tunnels. Its magnetic susceptibility and isothermal hysteresis suggest ferrimagnetic behavior. The electrical conductivity and Seebeck coefficient of the cold-pressed pellet suggest a semimetal feature that agrees with the spin-polarized calculation results using the tight-binding linear muffin-tin orbital (TB-LMTO) method.

Mechanism of CH4 Dry Reforming on Nanocrystalline Doped Ceria-Zirconia with Supported Pt, Ru, Ni, and Ni–Ru

Specificity of CH4 dry reforming mechanism for Me-supported doped ceria-zirconia catalysts with high oxygen mobility was elucidated using a combination of transient kinetic methods (TAP, SSITKA) with pulse microcalorimetry and in situ FTIRS. Steady-state reaction of CH4 dry reforming is described by a simple redox scheme with independent stages of CH4 and CO2 activation. This is provided by easy CO2 dissociation on reduced sites of oxide supports followed by a fast oxygen transfer along the surface/domain boundaries to metal sites where CH4 molecules are transformed to CO and H2. The rate-limiting stage is irreversible transformation of CH4 on metal sites, while CO2 transformation proceeds much faster being reversible for steady-state surface. The oxygen forms responsible for CH4 selective transformation into syngas correspond to strongly bound bridging oxygen species with heats of desorption ≈600–650 kJ/mol O2, most probably bound with pairs of Pr and/or Ce cations able to change their oxidation state. Ni + Ru clusters could be involved in CO2 activation via facilitating C–O bond breaking in the transition state, thus increasing the rate constant of the surface reoxidation by CO2. Strongly bound carbonates are spectators.

Structural characterization and catalytic evaluation of transition and rare earth metal doped ceria-based solid solutions for elemental mercury oxidation

The catalytic behavior of various CeO2-based solid solutions, namely, Ce1−xTMxO2−δ (TM = Mn, Fe, or Zr) and Ce1−xRExO2−δ (RE = Pr, La, or Sm) was studied for the removal of elemental mercury (Hg0) from coal-derived flue gas by catalytic oxidation (Hg0 → Hg2+). The investigated catalysts were synthesized by a coprecipitation method and characterized by various techniques, namely, X-ray diffraction (XRD), Raman spectroscopy (RS), high-resolution electron microscopy (HREM), Brunauer–Emmett–Teller (BET) surface area, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR), and diffuse reflectance spectroscopy (UV-DRS). The XRD results confirmed the incorporation of Mn, Fe, Zr, La, Pr, and Sm cations into the CeO2 lattice and the formation of nanocrystalline solid solutions. The TEM measurements established the nanocrystalline nature of the solid solutions. The RS measurements suggested that the substitution process promotes the formation of oxygen vacancies, which hastens the diffusion rate of oxygen and improves the Hg oxidation. UV-vis DRS studies demonstrated the presence of the charge transfer transitions O2− → Ce3+ and O2− → Ce4+. The XPS and H2-TPR results suggested that the reduction of Ce4+ → Ce3+ is the foremost reason for the increase in oxygen vacancies, which are beneficial for Hg0 removal. The order of mercury oxidation activity over various doped catalysts is as follows: CM > CL > CZ > CF > CS > C > CP.

Fluctuation of surface composition and chemical states at the hetero-interface in composites comprised of a phase with perovskite structure and a phase related to the Ruddlesden–Popper family of compounds

Surfaces and interfaces in composite solids can possess particular compositions and properties, thereby governing the functions of materials. X-ray photoelectron spectroscopy (XPS) has been used for the first time to explore the cation rearrangement between the surface and bulk material of the crystallites in the two-phase composites formed in the La–Sr–Pr–Co–O system. The two-phase composites contain a perovskite phase (major fraction) and a layered perovskite-like phase (La,Sr,Pr)2CoO4±δ related to the Ruddlesden–Popper family of compounds. The difference between the surface and nominal composition was revealed for all the composites explored. The surface concentrations of Pr and La cations are lower compared to the nominal stoichiometry, suggesting their preferable dissolution into the volume of the crystallites. Both Pr3+ and Pr4+ cations coexist at the surface. The surface of all the composites is enriched in Sr cations that could exist as SrCO3, SrO2, and Sr(OH)2 individual surface phases as well as SrO on the surface of the perovskite phase. The second phase plays an important role in balancing the surface composition at the hetero-interface in the composites. The rise in the fraction of (La,Sr,Pr)2CoO4±δ in the composites changes the “surface Sr:bulk Sr” ratio, increasing the later contribution due to the stronger accommodation of Sr cations within its crystal lattice and decreasing the surface depletion of Co cations. In addition to Co3+ and Co4+ cations, a small fraction of Co2+ cations exists in the near-surface region of the composites. The surface of all the composites is strongly enriched in oxygen (by 11.8–13.3 at %). The O1s spectra of the composites contain contributions from the lattice oxygen related to the perovskite phase and layered (La,Sr,Pr)2CoO4±δ phase as well as different surface oxygen states. The effect of surface treatment on the evolution of the surface composition has been discussed.

Enhancement in CO oxidation activity of nanosized CexZr1−xO2 solid solutions by incorporation of additional dopants

The influence of Hf, Pr and Tb dopant cations on structural and catalytic properties of nanosized CexZr1−xO2 solid solutions has been investigated. A wide range of analytical techniques are utilized to characterize the synthesized materials, and the catalytic activity is evaluated for CO oxidation. XRD, Raman, SEM and TEM results suggested formation of dopant cation incorporated ceria–zirconia solid solutions with highly homogeneous morphology and lattice defects. The CO-TPR measurements revealed an enhanced reducibility of CexZr1−xO2 which is reflected in their better catalytic activity. The role of Tb4+/Tb3+ and Pr4+/Pr3+ redox couples in facilitating a higher activity has been addressed.

Thematlockite-type praseodymium(III) oxide bromide PrOBr

The crystal structure of the praseodymium(III) oxide bromide, PrOBr, can be best described with layers of agglomerated square anti­prisms [PrO4Br4]9−. These slabs are stacked along the c axis and linked via two different secondary contacts between Pr3+ and Br−. The Pr3+ cations occupy the Wyckoff site 2c with 4mm symmetry and carry four O2− anions as well as four primary Br− anions, yielding a coordination number of 8. While the Br− anions exhibit the same site symmetry as the Pr3+ cations, the oxide anions are located at the Wyckoff position 2a with site symmetry m2 and have four Pr3+ cations as neighbours, defining a tetra­hedron.

Pr16Mo21O56

The structure of hexa­deca­praseodymium henicosa­molybden­um hexa­penta­contaoxide, Pr16Mo21O56, is isotypic with other rare earth representatives of formula type RE 16Mo21O56 (RE = La, Ce, Nd). It is characterized by Mo10O18 iO8 a units (where i = inner and a = apical O atoms) containing biocta­hedral Mo10 clusters and octa­hedral MoO6 units that share some of their O atoms to form the Mo–O framework. The two independent Mo10 cluster units are centred at Wyckoff positions 2b and 2c and have point-group symmetry . The Mo atom of the MoO6 unit is likewise situated at an inversion centre (2d). The eight crystallographically different Pr3+ cations occupy irregular voids in the framework with coordination numbers to the O atoms varying between 8 and 11.

Dimensionality-driven spin-flop transition in quasi-one-dimensionalPrBa2Cu4O8

In the quasi-one-dimensional cuprate PrBa$_2$Cu$_4$O$_8$, the Pr cations order antiferromagnetically at 17 K in zero field. Through a combination of magnetic susceptibility, torque magnetometry, specific heat and interchain transport measurements, the anisotropic temperature-magnetic field phase diagram associated with this ordering has been mapped out. A low-temperature spin-flop transition in the Pr sub-lattice is found to occur at the same magnetic field strength and orientation as a dimensional crossover in the ground state of the metallic CuO chains. This coincidence suggests that the spin reorientation is driven by a change in the anisotropic Rudermann-Kittel-Kasuya-Yosida (RKKY) interaction induced by a corresponding change in effective dimensionality of the conduction electrons.

Role ofA-site cations in the metal-insulator transition inPr0.5Ca0.5CoO3−γ(γ≈0)

Remarkable structural changes take place during the metal to insulator first-order transition in ${\text{Pr}}_{0.50}{\text{Ca}}_{0.50}{\text{CoO}}_{3}$: a contraction of the unit-cell volume, the bending of the Co-O-Co bond angle, and a decrease in (Pr,Ca)-O bond distance. However, the Co-O bond length and the volume of the ${\text{CoO}}_{6}$ octahedra remain almost unaltered during the transition. In view of these data and the absence of transition in ${({\text{Nd}}_{2/3}{\text{La}}_{1/3})}_{0.5}{\text{Ca}}_{0.5}{\text{CoO}}_{3}$, we review the current spin-state transition picture, emphasizing that an active role is played by the $A$-site Pr cations that could be related with the effect of Pr in 123 superconducting cuprates.

Studies on the oxidation properties of nanopowder CeO 2-based solid solution catalysts for model soot combustion

The catalytic behavior of CeO 2-based solid solution catalysts (Ce 0.7Zr 0.3O 2, Ce 0.7Pr 0.3O 2− δ , and Ce 0.7Gd 0.3O 1.85) had been studied for soot combustion. These catalysts were characterized using X-ray diffraction (XRD), UV-visible, Raman, and H 2 temperature-programmed reduction (H 2-TPR) measurements. The Zr, Pr, and Gd cations replaced Ce cations in the CeO 2 lattice to form nanocrystalline CeO 2-based solid solutions. The crystallite size of Ce 0.7Zr 0.3O 2, Ce 0.7Pr 0.3O 2− δ , and Ce 0.7Gd 0.3O 1.85 is 7.3, 7.8, and 11.1 nm, respectively, which are much smaller than pure CeO 2 (36.2 nm). The substituting process can promote the formation of oxygen vacancies, which can hasten the diffusion rate of oxygen, then improve the combustion activity. According to the catalytic soot combustion results, the ranking in activity of these catalysts is Ce 0.7Zr 0.3O 2 > Ce 0.7Pr 0.3O 2− δ > Ce 0.7Gd 0.3O 1.85 ∼ CeO 2, which is well consistent with the reducibility sequence of the samples. The mass of catalyst plus soot mixture have effect on soot combustion: the T ig and T max decrease with the increasing of sample weight. Combustion activity of catalysts decreases slowly with cycled runs, which can be attributed to the activity lost of catalyst.

Matrix Infrared Spectroscopic and Theoretical Studies on the Reactions of Early Lanthanoid Atoms with Nitrous Oxide in Excess Argon

Reactions of laser-ablated early lanthanoid atoms (Ce, Pr, Nd, Sm, and Eu, except for radioactive Pm) with N 2O molecules in excess argon have been investigated using matrix-isolation infrared spectroscopy. Lanthanoid monoxide-dinitrogen complexes, OLn(N 2) (Ln = Ce, Pr, Nd, and Sm), are observed during sample deposition and identified on the basis of isotopic shifts, mixed isotopic splitting patterns, and CCl 4-doping experiments, whereas no new product is observed for Eu. The OLnNN (+) (Ln = Ce, Pr, Nd, and Sm) cations appear during sample deposition and increase visibly upon broadband irradiation (lambda > 250 nm) at the expense of the neutral OLn(N 2) complexes. Density functional theory calculations have been performed on the new products, which support identification of the OLn(N 2) and OLnNN (+) complexes from the matrix infrared spectra.