Sonochemical synthesis of Ce(1-x)Fe(x)O(2-δ) (0 ≤ x ≤ 0.45) and Ce(0.65)Fe(0.33)Pd(0.02)O(2-δ) nanocrystallites: oxygen storage material, CO oxidation and water gas shift catalyst

Nanocrystalline Ce(1-x)Fe(x)O(2-δ) (0 ≤ x ≤ 0.45) and Ce(0.65)Fe(0.33)Pd(0.02)O(2-δ) of ~4 nm sizes were synthesized by a sonochemical method using diethyletriamine (DETA) as a complexing agent. Compounds were characterized by powder X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS) and transmission electron microscopy (TEM). Ce(1-x)Fe(x)O(2-δ) (0 ≤ x ≤ 0.45) and Ce(0.65)Fe(0.33)Pd(0.02)O(2-δ) crystallize in fluorite structure where Fe is in +3, Ce is in +4 and Pd is in +2 oxidation state. Due to substitution of smaller Fe(3+) ion in CeO(2), lattice oxygen is activated and 33% Fe substituted CeO(2)i.e. Ce(0.67)Fe(0.33)O(1.835) reversibly releases 0.31[O] up to 600 °C which is higher or comparable to the oxygen storage capacity of CeO(2)-ZrO(2) based solid solutions (Catal. Today 2002, 74, 225-234). Due to interaction of redox potentials of Pd(2+/0)(0.89 V) and Fe(3+/2+) (0.77 V) with Ce(4+/3+) (1.61 V), Pd ion accelerates the electron transfer from Fe(2+) to Ce(4+) in Ce(0.65)Fe(0.33)Pd(0.02)O(1.815), making it a high oxygen storage material as well as a highly active catalyst for CO oxidation and water gas shift reaction. The activation energy for CO oxidation with Ce(0.65)Fe(0.33)Pd(0.02)O(1.815) is found to be as low as 38 kJ mol(-1). Ce(0.67)Fe(0.33)O(1.835) and Ce(0.65)Fe(0.33)Pd(0.02)O(1.815) have also shown high activity for the water gas shift reaction. CO conversion to CO(2) is 100% H(2) specific with these catalysts and conversion rate was found to be as high 27.2 μmoles g(-1) s(-1) and the activation energy was found to be 46.4 kJ mol(-1) for Ce(0.65)Fe(0.33)Pd(0.02)O(1.815).     

Two-phase synthesis of colloidal annular-shaped Ce(x)La(1-x)CO3OH nanoarchitectures assembled from small particles and their thermal conversion to derived mixed oxides

Undoped and cerium doped LaCO(3)OH annular-shaped nanoarchitectures with high specific surface area have been fabricated via the thermolysis of Ce(x)La(1-x)(oleate)(3) (x = 0-20 mol %) complexes in a toluene-water system containing tert-butylamine/oleylamine. The products exhibit 400 nm-sized monodisperse annular-shaped nanoarchitectures, which are constituted of 3-5 nm-sized primary particles. A possible mechanism of the reaction of Ce(x)La(1-x)(oleate)(3) and tert-butylamine for the formation of annular-shaped Ce(x)La(1-x)CO(3)OH nanoarchitectures is proposed. The thermal conversion of Ce(x)La(1-x)CO(3)OH to Ce(x)La(1-x)(CO(3))O(2) at 600 °C, to Ce(x)La(1-x)(OH)(3) at 800 °C, final to (Ce(x)La(1-x))(2)O(3-δ) at 900 °C were employed, while the original morphology was essentially unchanged. The dopant concentration was varied from 5 to 20 of cerium ions per LaCO(3)OH nanoparticle. The X-ray diffraction (XRD) results reveal that the cerium dopant could enter easily into the LaCO(3)OH structural lattice, whereas copper could unlikely enter into their lattice because of their large ionic radius difference. The cerium oxidation state was controlled by changing doping concentration. The X-ray photoelectron spectroscopy (XPS) results reveal that only one Ce(3+) oxidation state is in the as-synthesized Ce(x)La(1-x)CO(3)OH samples with cerium concentration ranging from 5 to 20 mol %, whereas both 3+ and 4+ ones coexisted in 20 mol % Ce:LaCO(3)OH structure. Remarkable luminescence emission intensity enhancement of 1.5-9.0 times were observed for Ce(x)La(1-x)CO(3)OH samples with cerium concentration ranging from 5 to 20 mol %, after doping with an undoped LaCO(3)OH.     

Comparison of the reactivity of different Pd-O species in CO oxidation

The reactivity of several Pd-O species toward CO oxidation was compared experimentally, making use of chemically, structurally and morphologically different model systems such as single-crystalline Pd(111) covered by adsorbed oxygen or a Pd(5)O(4) surface oxide layer, an oriented Pd(111) thin film on NiAl oxidized toward PdO(x) suboxide and silica-supported uniform Pd nanoparticles oxidized to PdO. The oxygen reactivity decreased with increasing oxidation state: O(ad) on metallic Pd(111) exhibited the highest reactivity and could be reduced within a few minutes already at 223 K, using low CO beam fluxes around 0.02 ML s(-1). The Pd(5)O(4) surface oxide on Pd(111) could be reacted by CO at a comparable rate above 330 K using the same low CO beam flux. The more deeply oxidized Pd(111) thin film supported on NiAl was already much less reactive, and reduction in 10(-6) mbar CO at T > 500 K led only to partial reduction toward PdO(x) suboxide, and the metallic state of Pd could not be re-established under these conditions. The fully oxidized PdO nanoparticles required even rougher reaction conditions such as 10 mbar CO for 15 min at 523 K in order to re-establish the metallic state. As a general explanation for the observed activity trends we propose kinetic long-range transport limitations for the formation of an extended, crystalline metal phase. These mass-transport limitations are not involved in the reduction of O(ad), and less demanding in case of the 2-D Pd(5)O(4) surface oxide conversion back to metallic Pd(111). They presumably become rate-limiting in the complex separation process from an extended 3-D bulk oxide state toward a well ordered 3-D metallic phase.     

Pd/La0.5Pb0.5Mn0.9Sn0.1O3对甲苷伯羟基的选择催化氧化

用柠檬酸络合法制备了掺杂钙钛矿型复合金属氧化物载体, 采用沉淀法负载活性组分钯得到催化剂Pd/La0.5Pb0.5Mn0.9Sn0.1O3, 用XRD, SEM和XPS对载体和催化剂结构进行了表征分析, 考察了催化剂和氧化工艺条件对甲基葡萄糖苷的伯羟基选择氧化合成葡萄糖醛酸及其内酯的催化活性的影响. 结果表明, 载体La0.5Pb0.5Mn0.9Sn0.1O3由于掺杂效应而具有较强的助氧化还原能力, 有利于实现活性组分Pd在催化过程中的氧化还原循环. 在Pd负载量为1%、温度70 ℃、pH值为9的条件下, 葡萄糖醛酸及内酯的总收率达到60%, 反应选择性良好.     

Magnetization, Mössbauer and isothermal dilatometric behavior of oxidized YBa(Co,Fe)4O(7+δ)

M?ssbauer spectroscopy and magnetization studies of YBaCo(4-x)Fe(x)O(7+δ) (x = 0-0.8) oxidized at 0.21 and 100 atm O(2), indicate an increasing role of penta-coordinated Co(3+) states when the oxygen content approaches 8-8.5 atoms per formula unit. Strong magnetic correlations are observed in YBaCo(4-x)Fe(x)O(8.5) from 2 K up to 55-70 K, whilst the average magnetic moment of Co(3+) is lower than that for δ ≤ 0.2, in correlation with the lower (57)Fe(3+) isomer shifts determined from M?ssbauer spectra. The hypothesis on dominant five-fold coordination of cobalt cations was validated by molecular dynamics modeling of YBaCo(4)O(8.5). The iron solubility limit in YBaCo(4-x)Fe(x)O(7+δ) corresponds to approximately x ≈ 0.7. The oxygen intercalation processes in YBaCo(4)O(7+δ) at 470-700 K, analyzed by X-ray diffraction, thermogravimetry and controlled-atmosphere dilatometry, lead to unusual volume expansion opposing to the cobalt cation radius variations. This behavior is associated with increasing cobalt coordination numbers and with rising local distortions and disorder in the crystal lattice on oxidation, predicted by the computer simulations. When the oxygen partial pressure increases from 4 × 10(-5) to 1 atm, the linear strain in YBaCo(4)O(7+δ) ceramics at 598 K is as high as 0.33%.     

Synthesis and characterisation of nu3-octahedral [Ni36Pd8(CO)48]6- and [Ni35Pt9(CO)48]6- clusters displaying unexpected surface segregation of Pt atoms and molecular and/or crystal substitutional Ni/Pd and Ni/Pt disorder

The synthesis and structure, as well as the chemical and electrochemical characterisation of two new nu(3)-octahedral bimetallic clusters with the general [Ni(44-x)M(x)(CO)(48)](6-) (M = Pd, x = 8; M = Pt, x = 9) formula is reported. The [Ni(35)Pt(9)(CO)(48)](6-) cluster was obtained in reasonable yields (56 % based on Pt) by reaction of [Ni(6)(CO)(12)](2-) with 1.1 equivalents of Pt(II) complexes, in ethyl acetate or THF as the solvent. The [Ni(36)Pd(8)(CO)(48)](6-) cluster was obtained from the related reaction with Pd(II) salts in THF, and was isolated only in low yields (5-10 % based on Pd), mainly because of insufficient differential solubility of its salts. The unit cell of the [NBu(4)](6)[Ni(35)Pt(9)(CO)(48)] salt contains a substitutionally Ni-Pt disordered [Ni(24)(Ni(14-x)Pt(x))Pt(6)(CO)(48)](6-) (x = 3) hexaanion. A combination of crystal and molecular disorder is necessary to explain the disordering observed for the Ni/Pt sites. The unit cell of the corresponding [Ni(36)Pd(8)(CO)(48)](6-) salt contains two independent [Ni(30)(Ni(8-x)Pd(x))Pd(6)(CO)(48)](6-) (x = 2) hexaanions. The two display similar substitutional Ni-Pd disorder, which probably arises only from crystal disorder. The structure of [Ni(36)Pd(8)(CO)(48)](6-) establishes the first similarity between the chemistry of Ni-Pd and Ni-Pt carbonyl clusters. A comparison of the chemical and electrochemical properties of [Ni(35)Pt(9)(CO)(48)](6-) with those of the related [Ni(38)Pt(6)(CO)(48)](6-) cluster shows that surface colouring of the latter with Pt atoms decreases redox as well as protonation propensity of the cluster. In contrast, substitution of all internal Pt and two surface Ni with Pd atoms preserves the protonation behaviour and is only detrimental with respect to its redox aptitude. A qualitative rationalisation of the different surface-site selectivity of Pt and Pd, based on distinctive interplays of M--M and M--CO bond energies, is suggested.     

Promoter effect of BaO on CO oxidation on PdO surfaces

The effect of bulk BaO promoter on CO oxidation activity of palladium oxide phase was studied by density functional calculations. A series of BaO(100) supported Pd(x)O(y) thin layer models were constructed, and energy profiles for CO oxidation on the films were calculated and compared with corresponding profiles for the most stable PdO bulk surfaces PdO(100) and PdO(101). The most stable of the thin films typically exhibit the same PdO(100) and PdO(101) surface planes; the PdO(100) dominates already with double layer thickness. The supporting promoter improves the CO oxidation activity of the Pd(x)O(y) phase via a direct electronic effect and introduced structural strain and corrugation. Changes in CO adsorption strength are reflected in oxidation energy barriers, and the promoting effect of even 0.3 eV can be seen locally. Easier oxygen vacancy formation may partially facilitate the reaction.     

In situ XPS studies of perovskite oxide surfaces under electrochemical polarization

An in situ XPS study of oxidation-reduction processes on three perovskite oxide electrode surfaces was carried out by incorporating the materials in an electrochemical cell mounted on a heated sample stage in an ultrahigh vacuum (UHV) chamber. Electrodes made of powdered LaCr(1-x)Ni(x)O(3-delta) (x = 0.4, 1) showed changes in the XPS features of all elements upon redox cycling between formal Ni3+ and Ni2+ oxidation stoichiometries, indicating the delocalized nature of the electronic states involved and strong mixing of O 2p to Ni 3d levels to form band states. The surface also showed changes in adsorption capacity for CO2 upon reduction as a result of increased nucleophilicity of surface oxygen. Another perovskite oxide, La(0.5)Sr(0.5)CoO(3-delta), laser deposited as highly oriented thin films on (100) oriented yttria-stabilized zirconia (YSZ), also showed evidence of both local and nonlocal effects in the XPS features upon redox cycling. In contrast to LaCr(1-x)Ni(x)O(3-delta), redox cycling mainly affected the XPS features of cobalt with little effect on oxygen. This signifies reduced participation of O 2p states in the conduction band of this material. Small changes in surface cation stoichiometry in this film were observed and attributed to mobility of the A-site Sr dopant under polarization.     

A density functional theory study of self-regenerating catalysts LaFe(1-x)M(x)O(3-y) (M = Pd, Rh, Pt)

Periodic density functional theory was used to investigate the stability and electronic structures of precious-metal atoms in the vicinity of LaFe(1-x)M(x)O(3) (M = Pd, Rh, Pt) perovskite catalyst surfaces. It was found that the surface segregation of Pd and Pt is significantly stabilized by the introduction of O vacancies, whereas the solid-solution phase is favorable for Rh, suggesting an important role of O vacancies in the self-regeneration of Pd and Pt. On the basis of the results, we propose a possible scenario for the self-regeneration of the precious metal in the perovskite catalyst.     

Synthesis, structure, chemical stability, and electrical properties of Nb-, Zr-, and Nb-codoped BaCeO3 perovskites

We report the effect of donor-doped perovskite-type BaCeO(3) on the chemical stability in CO(2) and boiling H(2)O and electrical transport properties in various gas atmospheres that include ambient air, N(2), H(2), and wet and dry H(2). Formation of perovskite-like BaCe(1-x)Nb(x)O(3±δ) and BaCe(0.9-x)Zr(x)Nb(0.1)O(3±δ) (x = 0.1; 0.2) was confirmed using powder X-ray diffraction (XRD) and electron diffraction (ED). The lattice constant was found to decrease with increasing Nb in BaCe(1-x)Nb(x)O(3±δ), which is consistent with Shannon's ionic radius trend. Like BaCeO(3), BaCe(1-x)Nb(x)O(3±δ) was found to be chemically unstable in 50% CO(2) at 700 °C, while Zr doping for Ce improves the structural stability of BaCe(1-x)Nb(x)O(3±δ). AC impedance spectroscopy was used to estimate electrical conductivity, and it was found to vary with the atmospheric conditions and showed mixed ionic and electronic conduction in H(2)-containing atmosphere. Arrhenius-like behavior was observed for BaCe(0.9-x)Zr(x)Nb(0.1)O(3±δ) at 400-700 °C, while Zr-free BaCe(1-x)Nb(x)O(3±δ) exhibits non-Arrhenius behavior at the same temperature range. Among the perovskite-type oxides investigated in the present work, BaCe(0.8)Zr(0.1)Nb(0.1)O(3±δ) showed the highest bulk electrical conductivity of 1.3 × 10(-3) S cm(-1) in wet H(2) at 500 °C, which is comparable to CO(2) and H(2)O unstable high-temperature Y-doped BaCeO(3) proton conductors.     

Oxygen surface exchange kinetics of SrTi(1-x)Fe(x)O(3-δ) mixed conducting oxides

The oxygen surface exchange kinetics of mixed conducting perovskite oxides SrTi(1-x)Fe(x)O(3-δ) (x = 0, 0.01, 0.05, 0.35, 0.5) has been investigated as a function of temperature and oxygen partial pressure using the pulse-response (18)O-(16)O isotope exchange (PIE) technique. Arrhenius activation energies range from 140 kJ mol(-1) for x = 0 to 86 kJ mol(-1) for x = 0.5. Extrapolating the temperature dependence to the intermediate temperature range, 500-600 °C, indicates that the rate of oxygen exchange, in air, increases with increasing iron mole fraction, but saturates at the highest iron mole fraction for the given series. The observed behavior is concomitant with corresponding increases in both electronic and ionic conductivity with increasing x in SrTi(1-x)Fe(x)O(3-δ). Including literature data of related perovskite-type oxides Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-δ), La(0.6)Sr(0.4)Co(0.2)Fe(0.8)O(3-δ), La(0.6)Sr(0.4)CoO(3-δ), and Sm(0.5)Sr(0.5)CoO(3-δ), a linear relationship is observed in the log-log plot between oxygen exchange rate and oxide ionic conductivity with a slope fairly close to unity, suggesting that it is the magnitude of the oxide ionic conductivity that governs the rate of oxygen exchange in these solids. The distribution of oxygen isotopomers ((16)O(2), (16)O(18)O, (18)O(2)) in the effluent pulse can be interpreted on the basis of a two-step exchange mechanism for the isotopic exchange reaction. Accordingly, the observed power law dependence of the overall surface exchange rate on oxygen partial pressure turns out to be an apparent one, depending on the relative rates of both steps involved in the adopted two-step scheme. Supplementary research is, however, required to elucidate which of the two possible reaction schemes better reflects the actual kinetics of oxygen surface exchange on SrTi(1-x)Fe(x)O(3-δ).     

Rh(1-x)Pd(x) nanoparticle composition dependence in CO oxidation by oxygen: catalytic activity enhancement in bimetallic systems

Bimetallic 15 nm Rh(1-x)Pd(x) nanoparticle catalysts of five different compositions and supported on Si wafers have been synthesized, characterized using TEM, SEM, and XPS, and studied in CO oxidation by O(2) in two pressure regimes: atmospheric pressure and 100-200 mTorr. The RhPd bimetallic nanocrystals exhibited similar synergetic effect of increased reaction activity at both atmospheric (760 Torr) and moderate (100-200 mTorr) pressures compared with pure Pd or Rh. The magnitude of the effect depends on the relative pressures of the CO and O(2) reactant gases and the reaction temperature. The catalytic activity of the nanocrystals measured at moderate pressure is directly correlated to the APXPS studies, which were carried out in the same pressure. The APXPS studies suggest that the Pd-Rh interfaces are important for the enhanced activity of the bimetallic nanoparticles.     

Interactions of O(2) with Pd nanoparticles on alpha-Al(2)O(3)(0001) at low and high O(2) pressures

The interaction of O(2) with small Pd particles (2-10 nm) supported on an alpha-Al(2)O(3)(0001) single crystal under both ultrahigh vacuum (UHV) and high-pressure conditions has been studied by temperature-programmed desorption (TPD), temperature-programmed low-energy ion scattering (TP-LEIS), and X-ray photoelectron spectroscopy (XPS). A low O(2) exposure (30 L) at 500 K leads to surface oxygen adatoms on the Pd nanoparticles, which desorb in TPD as O(2) in a peak at approximately 880 K. Surface O adatoms on the smallest Pd particles move to subsurface sites starting at 400 K, and they almost all move subsurface by approximately 750 K, desorbing mainly at considerably higher temperature. The dominant oxygen species above 700 K is subsurface, implying that it is more stable than oxygen adatoms on Pd. Exposures of the Pd nanoparticles to 25 Torr O(2) at 373-473 K readily convert the Pd to a species whose Pd XPS peak shifts by the same amount as the binding energy difference between bulk Pd and bulk PdO. We attribute this to PdO nanoparticles (or a thin film of PdO on or under the Pd for the larger particles). The decomposition of the PdO on these nanoparticles to Pd in an equilibrium O(2) pressure of 10-7 Torr does not occur until approximately 750 K, or approximately 200 K higher than the equilibrium decomposition of bulk PdO. This is attributed to the higher energy of Pd nanoparticles compared to bulk Pd and, for the larger particles, to the adhesion energy of the PdO film to the Pd, both of which stabilize the PdO on these Pd nanoparticles relative to bulk PdO. This PdO-like film on the larger particles may be similar to the ordered oxide thin film previously reported to form on Pd(111) but may also reside at the alpha-Al(2)O(3) interface and be partially stabilized by adhesion to this interface.     

Synthesis of Fe(3)O(4)/PdO heterodimer nanocrystals in silica nanospheres and their controllable transformation into Fe(3)O(4)/Pd heterodimers and FePd nanocrystals

The thermal annealing of silica nanospheres encapsulating Fe(3)O(4) nanocrystals and Pd(2+) complexes led to the formation of heterodimers consisting of Fe(3)O(4) and PdO nanoparticles encapsulated in a silica shell, allowing for their controllable transformation into either Fe(3)O(4)/Pd heterodimers or FePd alloy nanocrystals through a solid state reduction process.     

Nanosized (mu12-Pt)Pd164-xPtx(CO)72(PPh3)20 (x approximately 7) containing Pt-centered four-shell 165-atom Pd-Pt core with unprecedented intershell bridging carbonyl ligands: comparative analysis of icosahedral shell-growth patterns with geometrically related Pd145(CO)x(PEt3)30 (x approximately 60) containing capped three-shell Pd145 core

Presented herein are the preparation and crystallographic/microanalytical/magnetic/spectroscopic characterization of the Pt-centered four-shell 165-atom Pd-Pt cluster, (mu(12)-Pt)Pd(164-x)Pt(x)(CO)(72)(PPh(3))(20) (x approximately 7), 1, that replaces the geometrically related capped three-shell icosahedral Pd(145) cluster, Pd(145)(CO)(x)(PEt(3))(30) (x approximately 60), 2, as the largest crystallographically determined discrete transition metal cluster with direct metal-metal bonding. A detailed comparison of their shell-growth patterns gives rise to important stereochemical implications concerning completely unexpected structural dissimilarities as well as similarities and provides new insight concerning possible synthetic approaches for generation of multi-shell metal clusters. 1 was reproducibly prepared in small yields (<10%) from the reaction of Pd(10)(CO)(12)(PPh(3))(6) with Pt(CO)(2)(PPh(3))(2). Its 165-atom metal-core geometry and 20 PPh(3) and 72 CO ligands were established from a low-temperature (100 K) CCD X-ray diffraction study. The well-determined crystal structure is attributed largely to 1 possessing cubic T(h) (2/m3) site symmetry, which is the highest crystallographic subgroup of the noncrystallographic pseudo-icosahedral I(h) (2/m35) symmetry. The "full" four-shell Pd-Pt anatomy of 1 consists of: (a) shell 1 with the centered (mu(12)-Pt) atom encapsulated by the 12-atom icosahedral Pt(x)Pd(12-x) cage, x = 1.2(3); (b) shell 2 with the 42-atom nu(2) icosahedral Pt(x)Pd(42-x) cage, x = 3.5(5); (c) shell 3 with the anti-Mackay 60-atom semi-regular rhombicosidodecahedral Pt(x)Pd(60-x) cage, x = 2.2(6); (d) shell 4 with the 50-atom nu(2) pentagonal dodecahedral Pd(50) cage. The total number of crystallographically estimated Pt atoms, 8 +/- 3, which was obtained from least-squares (Pt(x)/Pd(1-x))-occupancy analysis of the X-ray data that conclusively revealed the central atom to be pure Pt (occupancy factor, x = 1.00(3)), is fortuitously in agreement with that of 7.6(7) found from an X-ray Pt/Pd microanalysis (WDS spectrometer) on three crystals of 1. Our utilization of this site-occupancy (Pt(x)Pd(1-x))-analysis for shells 1-3 originated from the microanalytical results; otherwise, the presumed metal-core composition would have been (mu(12)-Pt)Pd(164). [Alternatively, the (mu(12)-Pt)M(164) core-geometry of 1 may be viewed as a pseudo-Ih Pt-centered six-shell successive nu(1) polyhedral system, each with radially equivalent vertex atoms: Pt@M(12)(icosahedron)@M(30)(icosidodecahedron)@M(12)(icosahedron)@M(60)(rhombicosidodecahedron)@M(30)(icosidodecahedron)@M(20)(pentagonal dodecahedron)]. Completely surprising structural dissimilarities between 1 and 2 are: (1) to date 1 is only reproducibly isolated as a heterometallic Pd-Pt cluster with a central Pt instead of Pd atom; (2) the 50 atoms comprising the outer fourth nu(2) pentagonal dodecahedral shell in 1 are less than the 60 atoms of the inner third shell in 1, in contradistinction to shell-by-shell growth processes in all other known shell-based structures; (3) the 10 fewer PR3 ligands in 1 necessitate larger bulky PPh(3) ligands to protect the Pd-Pt core-geometry; (4) the 72 CO ligands consist of six bridging COs within each of the 12 pentagons in shell 4 that are coordinated to intershell metal atoms. SQUID magnetometry measurements showed a single-crystal sample of 1 to be diamagnetic over the entire temperature range of 10-300 K.     

Direct evidence of redox interaction between metal ion and support oxide in Ce(0.98)Pd(0.02)O(2-δ) by a combined electrochemical and XPS study

A combined electrochemical method and X-ray photo electron spectroscopy (XPS) has been utilized to understand the Pd(2+)/CeO(2) interaction in Ce(1-x)Pd(x)O(2-δ) (x = 0.02). A constant positive potential (chronoamperometry) is applied to Ce(0.98)Pd(0.02)O(2-δ) working electrode which causes Ce(4+) to reduce to Ce(3+) to the extent of ~35%, while Pd remains in the +2 oxidation state. Electrochemically cycling this electrode between 0.0-1.2 V reverts back to the original state of the catalyst. This reversibility is attributed to the reversible reduction of Ce(4+) to Ce(3+) state. CeO(2) electrode with no metal component reduces to CeO(2-y) (y~0.4) after applying 1.2 V which is not reversible and the original composition of CeO(2) cannot be brought back in any electrochemical condition. During the electro-catalytic oxygen evolution reaction at a constant 1.2 V for 1000 s, Ce(0.98)Pd(0.02)O(2-δ) reaches a steady state composition with Pd in the +2 states and Ce(4+): Ce(3+) in the ratio of 0.65:0.35. This composition can be denoted as Ce(4+)(0.63)Ce(3+)(0.35)Pd(0.02)O(2-δ-y) (y~0.17). When pure CeO(2) is put under similar electrochemical condition, it never reaches the steady state composition and reduces almost to 85%. Thus, Ce(0.98)Pd(0.02)O(2-δ) forms a stable electrode for the electro-oxidation of H(2)O to O(2) unlike CeO(2) due to the metal support interaction.     

Surface structure and reactivity of Pd(100) during CO oxidation near ambient pressures

The surface structure of Pd(100) during CO oxidation was measured using a combination of a flow reactor and in situ surface X-ray diffraction coupled to a large-area 2-dimensional detector. The surface structure was measured for P(O(2))/P(CO) ratios between 0.6 and 10 at a fixed total gas pressure of 200 mbar and a fixed CO pressure of 10 ± 1 mbar. In conjunction with the surface structure the reactivity of the surface was also determined. For all P(O(2))/P(CO) ratios the surface was found to oxidize above a certain temperature. Three different types of oxides were observed: the surface oxide, an epitaxial layer of bulk-like PdO, and a non-epitaxial layer of bulk-like PdO. As soon as an oxide was present the reactivity of the surface was found to be mass transfer limited by the flux of CO molecules reaching the surface.     

Study on the reversible electrode reaction of Na(1-x)Ni(0.5)Mn(0.5)O2 for a rechargeable sodium-ion battery

Layered NaNi(0.5)Mn(0.5)O(2) (space group: R ?3m), having an O3-type (α-NaFeO(2) type) structure according to the Delmas' notation, is prepared by a solid-state method. The electrochemical reactivity of NaNi(0.5)Mn(0.5)O(2) is examined in an aprotic sodium cell at room temperature. The NaNi(0.5)Mn(0.5)O(2) electrodes can deliver ca. 105-125 mAh g(-1) at rates of 240-4.8 mA g(-1) in the voltage range of 2.2-3.8 V and show 75% of the initial reversible capacity after 50 charge/discharge cycling tests. In the voltage range of 2.2-4.5 V, a higher reversible capacity of 185 mAh g(-1) is achieved; however, its reversibility is insufficient because of the significant expansion of interslab space by charging to 4.5 V versus sodium. The reversbility is improved by adding fluoroethylene carbonate into the electrolyte solution. The structural transition mechanism of Na(1-x)Ni(0.5)Mn(0.5)O(2) is also examined by an ex situ X-ray diffraction method combined with X-ray absorption spectroscopy (XAS). The staking sequence of the [Ni(0.5)Mn(0.5)]O(2) slabs changes progressively as sodium ions are extracted from the crystal lattice. It is observed that the original O3 phase transforms into the O'3, P3, P'3, and P3" phases during sodium extraction. XAS measurement proves that NaNi(0.5)Mn(0.5)O(2) consists of divalent nickel and tetravalent manganese ions. As sodium ions are extracted from the oxide to form Na(1-x)Ni(0.5)Mn(0.5)O(2), nickel ions are oxidized to the trivalent state, while the manganese ions are electrochemically inactive as the tetravalent state.     

氧化铈形貌对Pd纳米粒子分散的影响

氧化铈由于在氧化和还原气氛下具有快速Ce4+/Ce3+氧化还原循环作用,使其具有优异的储放氧能力,不仅可以分散和稳定金属粒子,还可在界面处与金属物种发生化学键合,并形成活性位点,因此已被广泛应用于多个催化反应体系,且表现出显著的形貌效应.通过对氧化铈形貌进行调控,使其暴露特定(111)、(110)和(100)晶面,已成为调节金属-氧化铈相互作用强度及金属物种电子、几何结构,提高催化性能的有效策略,但对其机制及活性位结构还没有清晰的认识.我们以氧化铈纳米粒子和纳米立方体为载体,其中氧化铈立方体平均尺寸为23 nm,主要暴露6个{100}晶面,边缘和截角暴露少量{110}及{111}晶面;球形氧化铈纳米粒子平均尺寸为11 nm,主要暴露{111}晶面;并进一步将2.0 wt％Pd物种分散在氧化铈立方体和球形纳米粒子上,通过扫描透射电子显微镜(STEM)和X射线光电子能谱(XPS)等研究了钯物种在氧化铈球形粒子和立方体上的原子结构和化学环境,进而分析了纳米结构氧化铈形貌对钯物种分散的影响.在球形氧化铈纳米粒子上主要形成了平均尺寸为2.0 nm的非晶态Pd纳米粒子以及极小的Pd物种,这主要是因为球形氧化铈纳米粒子上丰富的表面氧空位可通过Pd-CeO2强相互作用和Pd物种紧密键合.氧化铈立方体上的晶态Pd粒子尺寸为2.9 nm,金属与载体之间具有明显的界面,且Pd原子嵌入到氧化铈晶格中.同时,CO化学吸附测试也证明了氧化铈球形粒子上的钯分散度(70％)高于氧化铈立方体(52％).对于甲烷燃烧反应,主要涉及发生在金属粒子表面的PdO/Pd氧化还原循环,即Pd被O2氧化,PdO被CH4还原,富氧条件下决速步骤是PdO对CH4中C?H的活化,因此氧化铈立方体表面大尺寸的晶态Pd粒子被氧化后更容易被CH4还原,有利于PdO/Pd氧化还原循环,从而具有更高的活性和稳定性;然而在CO氧化反应中Pd/CeO2却呈现了相反的形貌效应,这是由于该反应遵循Mars-van Krevelen机理:CO吸附在金属Pd上,化学吸附的CO移动到钯-氧化铈界面,被氧化铈晶格氧氧化成CO2,产生的氧空位被表面氧补充,最后表面氧空位被气相氧补充;由于氧化铈球形粒子上的较小尺寸Pd具有更大的钯-氧化铈界面周长和更强的氧物种移动性,更易完成界面处的氧化还原循环,因此具有更高的CO氧化活性.     

Zr掺杂对PdO/Ce1-xPdxO2-δ催化剂CO和CH4催化氧化的影响

采用溶液燃烧法制备出PdO/PdO/Ce_1-xPd_xO_2-δ （PdO/CP）和PdO/Ce1-x-yPdxZryO₂-δ （PdO/CPZ）催化剂,通过硝酸处理去除催化剂表面的PdO物种得到对应的PdO/Ce_1-xPd_xO_2-δ （CP）和Ce1-x-yPdxZryO₂-δ （CPZ）催化剂。考究四种催化剂（PdO/CP、PdO/CPZ、CP、CPZ）对CO和CH₄的氧化活性,并计算得出表面PdO和Pdⁿ⁺物种的转化频率（TOF）。结果表明Zr的添加对PdO催化剂上CO和CH₄的催化氧化活性具有不同的影响。Zr的添加对PdO/CPZ和CPZ催化剂的CO催化活性具有明显的促进作用,前者归因于PdO/CPZ催化剂表面生成了更小颗粒的PdO粒子,后者归因于CPZ催化剂中含有更多的氧空位。对于CH₄的催化氧化,Pdⁿ⁺物种起到关键的作用。由于Zr的掺杂导致CeO₂的晶格中Pd物种的含量减少,致使PdO/CPZ催化剂和CPZ催化剂对CH₄氧化活性的降低。