Synthesis of Higher Chromium Oxides Using Ozone Gas

Ozone gas in an ambient pressure was blown to Cr₂O₃ powder. The higher oxides, CrO₃, Cr₂O₅, Cr₅O₁₂, and Cr₃O₈, could be synthesized above 373 K. Both CrO₃ and Cr₂O₅ were found by in situ X-ray diffraction measurements. It was known that they could be produced only at high pressure of oxygen and at high temperatures. The CrO₃ formed was melted at 470 K in an ozone atmosphere and decomposed above 500 K. Because of thermal decomposition of ozone and the higher oxides, no higher oxides were formed above 660 K. The phase formation was discussed using the kinetic stability diagram.     

Ionic Transport Numbers of Group IIa Oxides Under Low Oxygen Potentials

The ionic transport numbers of BeO, MgO, CaO, and SrO in single-crystal and polycrystalline samples were examined under low oxygen potentials between 900° and 1180°C. The oxygen potentials were fixed by using metal/metal-oxide electrodes in electrochemical cells with the samples of the alkaline earth oxides as solid electrolytes. The electromotive forces of these cells were compared with the emf values obtained from similar cells of high stability using high-purity thoria-based electrolytes. From the results it could be concluded that under comparable conditions of low oxygen pressure the electron transport number increases with increasing ionic radius of the group II element. Only BeO and MgO approach electrolytic behavior, and this occurs at relatively high oxygen pressures (>10^-10 atm).     

Electronically Conducting Doped Chromium Oxides

Samples of polycrystalline chromium oxide doped with Li₂O, MgO, NiO, and Ta₂O₅ were prepared by hot-pressing to test for suitability as current collector materials of the sulfur electrode in the sodium-sulfur cell. Density, grain size, dopant uniformity, and resistivity (to 350°C) were measured. X-ray photoelectron spectroscopy (XPS) tends to support the established model of Ni²⁺ on Cr³⁺ sites to explain the compositional dependence of electrical resistivity after air anneal. A higher level of Cr₂O₃ powder purity was required to obtain the low resistivities with the Li₂O dopant than with the MgO or NiO dopant. An observed increase in the bulk resistivity during several months of electrochemical cycling in sodium polysulfidelsulfur melts is attributed to loss of electronic carriers caused by equilibration of those carriers with the low oxygen partial pressure of the melt. Li₂O- and (to a lesser extent) MgO-doped Cr₂O₃ appear to be suitable as container coating materials.     

Equilibrium Oxygen Pressures of Mn-Zn Ferrites

The equilibrium oxygen pressures of Mn-Zn ferrites with various oxygen/metal ratios were obtained at 1000° to 1400°C. The method used is the dependence of the lattice constant on oxygen content obtained in Mn ferrites and the relation between oxygen content and the equilibrium oxygen pressure found in Mn and Ni ferrites. Previous results were compared with those of the present study.     

Oxygen Vacancy Motion in Perovskite Oxides

Using electron paramagnetic resonance, the motion of oxygen vacancies within the oxygen octahedron in perovskite BaTiO₃ is observed via the alignment of oxygen vacancyrelated defect dipoles induced by bias/heat combinations. The vacancy motion is found to have an activation energy of 0.91 eV, in excellent agreement with that predicted. It is found that the onset of resistance degradation is also concurrent with oxygen vacancy motion. This result spectroscopically demonstrates that oxygen vacancy migration in the lattice is likely responsible for the observed degradation.     

Catalytic Decomposition of Formic Acid on Metal Oxides

Studies on the decomposition of formic acid were initiated during the first decade of the present century by Mailhe and Sabatier [1], using metals and metal oxides. These authors were concerned primarily with the selectivity of their catalysts, i.e., their ability to favor one of the reactions possible for the decomposition of formic acid (Eq. 1).     

Decomposition of Ruthenium Oxides in Lead Borosilicate Glass

Behaviors of apparent phase changes of Ru-containing oxides in lead borosilicate glass at high temperature have been investigated using X-ray diffraction and transmission electron microscopy in application to thick-film resistors. During firing of thick films containing Ru oxide powder and lead borosilicate glass frits, apparent phase changes of Ru oxides have been found to occur both ways between ruthenium dioxide and lead ruthenate pyrochlore via decomposition of one phase in glass and subsequent formation of the other. The formation of pyrochlore occurs in a lead-rich form, Pb₂(Ru_{2− x} Pb _x )O_6.5, whereas the formation of RuO₂ is characterized by a platelike morphology instead of initial globular morphology. A general tendency is observed that RuO₂ is stable in low-PbO glass compositions and at high temperatures, while Pb₂(Ru_{2− x} Pb _x )O_6.5 is stable in high-PbO glass compositions and at low temperatures, with the implication that the stability of these phases is dictated by the chemical activity of PbO in the glass melt.     

Decomposition of N2O on Perovskite-Related Oxides

Mixed metal oxides crystalizing in a perovskite-related structure have long been of interest to solid state chemists and physicists because of their technologically important physical properties. The ready availability of a family of isomorphic solids with controllable physical properties makes these oxides suitable for basic research in catalysis. These mixed metal oxides are more advantageous and are better catalytic materials than simple oxides because: (i) the crystal structure can accomodate various metal ions and can stabilize unusual and mixed valence states of active metal ion; (ii) appropriate formulation of these oxides leads to easy tailoring of many desirable properties such as valence state of transition metal ion, distance between active sites, binding energy, diffusion of oxygen in the lattice, magnetic and conducting properties of the solid; (iii) the catalytic activity can be correlated to solid state properties since many of their solid state properties are thoroughly understood; (iv) the surface of these oxides can be regenerated by suitable activation procedure.     

Catalytic Decomposition of Formic Acid on Metal Oxides

Studies on the decomposition of formic acid were initiated during the first decade of the present century by Mailhe and Sabatier [1], using metals and metal oxides. These authors were concerned primarily with the selectivity of their catalysts, i.e., their ability to favor one of the reactions possible for the decomposition of formic acid (Eq. 1).     

Decomposition of N2O on Perovskite-Related Oxides

Mixed metal oxides crystalizing in a perovskite-related structure have long been of interest to solid state chemists and physicists because of their technologically important physical properties. The ready availability of a family of isomorphic solids with controllable physical properties makes these oxides suitable for basic research in catalysis. These mixed metal oxides are more advantageous and are better catalytic materials than simple oxides because: (i) the crystal structure can accomodate various metal ions and can stabilize unusual and mixed valence states of active metal ion; (ii) appropriate formulation of these oxides leads to easy tailoring of many desirable properties such as valence state of transition metal ion, distance between active sites, binding energy, diffusion of oxygen in the lattice, magnetic and conducting properties of the solid; (iii) the catalytic activity can be correlated to solid state properties since many of their solid state properties are thoroughly understood; (iv) the surface of these oxides can be regenerated by suitable activation procedure.     

Chromium Copper Catalysts for LiClO4 Decomposition

Chromium copper (Cr Cu) catalysts are well‐known burning rate catalysts for solid propellants, which were used as energy source for rocket propulsion [1]. The present work reports the enhancement of lithium perchlorate (LiClO₄) by employing copper chromium as a catalyst. The LiClO₄ decomposition rate depends on the catalyst characteristics, such as chemical composition, specific surface, and crystalline structure. Scanning electron microscopy, Brunauer‐Emmett‐Teller, X‐ray diffraction, X‐ray photoelectron spectroscopy, and H₂‐temperature‐programmed reduction analyses were used to characterize Cr_xCu_(1−x)O_(1+0.5x) catalysts. The samples are prepared using the sol‐gel method with different mole ratios. Furthermore, the samples are tested to evaluate their effect on the LiClO₄ decomposition at various temperatures. The blank tests comparison shows that the Cr_xCu_(1−x)O_(1+0.5x) catalysts strongly enhance the LiClO₄ decomposition. Moreover, CuCr₂O₄ is formed in the Cr_xCu_(1−x)O_(1+0.5x) catalysts. The Cr Cu binary composite catalysts show smaller crystallites, larger surface area, and better catalytic performance than the pure CuO samples because of the interaction of Cr and Cu ions. This study proposes a hypothetical reaction mechanism for the LiClO₄ catalytic decomposition of the Cr_xCu_(1−x)O_(1+0.5x) catalysts.     

Oxygen in Catalysis on Transition Metal Oxides

Reactions of catalytic oxidation over oxide catalysts have been extensively studied in recent years following expansion of their applications in petrochemistry as well as their wide theoretical interest. Studies in this field have also contributed greatly to our understanding of the general mechanisms of catalyst action.     

Cu-基类钙钛矿复合氧化物催化NO_x分解

合成了La2-x Ba x CuO4系列催化剂,应用一系列仪器测试结果分析得到,掺杂Ba量为0.6时在850℃催化NO活性最高。TG-DTA分析表明在450℃焙烧后样品中的有机组分完全消失。XRD结果显示了当x=1钙钛矿型为主相。H2-TPR研究结果显示催化剂活性与中心离子有很大关系。     

Thermodynamic Properties of Nonstoichiometric Yttria-Stabilized Zirconia at Low Oxygen Pressures

Thermodynamic properties of 8-mol%-yttria-stabilized zirconia have been determined in the 810° to 1040°C temperature range at low Po₂. A high-temperature solid-state coulometric titration method was used. The mass action constant, K_ma, can be represented at low Po₂ as K_ma=0.677 exp [(–3.98 ±0.03 eV)/kT].     

Pathways for Carboxylic Acid Decomposition on Transition Metal Oxides

The concept of structure sensitivity is well established for reactions catalyzed by metals as it has been generally demonstrated by the use of supported metal catalysts exhibiting different particle size [l-71. The con-cept of structure sensitivity in catalysis by metal oxides is considerably less well developed than in catalysis by metals, in spite of the growing number of examples of such reactions. Characterization of oxide catalyst is generally more problematical than that of metal; it is difficult, for example, to titrate the active surface areas of supported oxides by chemisorption techniques. Carboxylic acid decomposition could be used as a probe to establish struc-tural dependence and selectivity on metal oxides. For example, in the case of formic acid decomposition, bimolecular decomposition of two adsorbed formates occurs on a surface with Ti4+ cation of fourfold oxygen coordi-znation while unimolecular decomposition occurs in the case of formates adsorbed on Ti4+ fivefold coordinated cations [8]. Similarly, acetic acid adsorbed on Ti02 with Ti0,(011) leads to the production of ketene by unimolecular dehydration whereas on the Ti02(l 14) faceted surface, bi-molecular reaction of surface acetate takes place to form acetone [9, 101. Thus understanding the pathway of decomposition of carboxylic acid is likely not only to throw light on the new catalyst search for ketene formation but also to establish the use of this molecular decomposition model as a probe for the study of the structural property of metal oxide catalysts.     

Diffusion of Oxygen in Selected Monocrystalline Rare Earth Oxides

The diffusion of oxygen in monocrystalline ScaO₃, YzO₃, Dy,O₂, HosO₃, Er₂O₃, Tm₂O₂, and Lu₂O₃ was measured by a thermobalance technique. The reoxidation of partially reduced crystals of these materials was treated as a diffusion-controlled process with a moving boundary. Large diffusion coefficients and fairly low activation energies were found for this process, in agreement with a proposed migration mechanism which involves the inherently defective anion sublattice of these materials.     

Pathways for Carboxylic Acid Decomposition on Transition Metal Oxides

The concept of structure sensitivity is well established for reactions catalyzed by metals as it has been generally demonstrated by the use of supported metal catalysts exhibiting different particle size [l-71. The con-cept of structure sensitivity in catalysis by metal oxides is considerably less well developed than in catalysis by metals, in spite of the growing number of examples of such reactions. Characterization of oxide catalyst is generally more problematical than that of metal; it is difficult, for example, to titrate the active surface areas of supported oxides by chemisorption techniques. Carboxylic acid decomposition could be used as a probe to establish struc-tural dependence and selectivity on metal oxides. For example, in the case of formic acid decomposition, bimolecular decomposition of two adsorbed formates occurs on a surface with Ti4+ cation of fourfold oxygen coordi-znation while unimolecular decomposition occurs in the case of formates adsorbed on Ti4+ fivefold coordinated cations [8]. Similarly, acetic acid adsorbed on Ti02 with Ti0,(011) leads to the production of ketene by unimolecular dehydration whereas on the Ti02(l 14) faceted surface, bi-molecular reaction of surface acetate takes place to form acetone [9, 101. Thus understanding the pathway of decomposition of carboxylic acid is likely not only to throw light on the new catalyst search for ketene formation but also to establish the use of this molecular decomposition model as a probe for the study of the structural property of metal oxide catalysts.     

Higher Oxidation State Responsible for Ozone Decomposition at Room Temperature over Manganese and Cobalt Oxides: Effect of Calcination Temperature

The heterogeneous catalytic decomposition of ozone was investigated over unsupported manganese and cobalt oxide at room temperature. All catalysts were characterized by X-ray diffraction (XRD), N₂ adsorption–desorption (Brunauer–Emmet–Teller method), H₂-temperature programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS). The catalytic activity test indicated that these oxides had a good activity on ozone conversion meanwhile the catalysts remained highly active over time under reaction conditions. The treated temperature of the catalyst had a significant impact on the performance of ozone abatement and the samples treated at lower temperature showed higher activity. The surface area decreased obviously when developing the calcination temperature and H₂-TPR results demonstrated that much higher oxidation state of metal ions and active oxygen species were maintained on the surface under low treated temperature. XPS analysis showed that there were higher oxidation states of metal ions (Mn⁴⁺ and Co³⁺) and adsorbed oxygen species on the surface of catalysts treated at lower temperature, both of which play a significant role in ozone decomposition. However, the activity of manganese oxide was higher than that of cobalt oxide and the possible reason for this phenomenon was discussed.     

Microstructure and Infrared Absorption of Biomorphic Chromium Oxides Templated by Wood Tissues

Biomorphic Cr₂O₃ was synthesized using two different wood tissues as templates and was subjected to X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption measurements to investigate the phases and microstructures. A hierarchical pore structure from the ordered pore arrays derived from wood tissues to the nanopores formed on ceramic struts is demonstrated. The microstructure of the ceramic struts changed with increasing calcination temperature. From Fourier transform infrared spectroscopy, characteristic IR absorption peaks were detected, and furthermore, an overall collapse of transmission curves with increasing calcination temperature was observed.