Metal-oxide phase

Another example of the use of neutron diffraction to understand the role of atomic vacancies in producing a superconducting metal oxide phase is work that has been performed on Bao Kq 4fii03. This work demonstrates that at the synthesis temperature (700° C), under the proper conditions, oxygen vacancies are created to allow the formation of the parent phase with bismuth largely in the +3 oxidation state. The presence of the vacancies allows the incorporation of potassium in the... 

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

Raman spectroscopy has been used for a long time in order to study supported and promoted metal catalysts and oxide catalysts [84] since many information can be obtained (1) identification of different metal oxide phases (2) structural transformations of metal oxide phases (3) location of the supported oxide on the oxide substrate and... 

Within the inverse model catalyst approach, the y/7-V309-Rh(l 11) nanostructures have been used to visualize surface processes in the STM with atomic-level precision [104]. The promoting effect of the V-oxide boundary regions on the oxidation of CO on Rh(l 1 1) has been established by STM and XPS by comparing the reaction on two differently prepared y/7-V309-Rh(l 11) inverse catalyst surfaces, which consist of large and small two-dimensional oxide islands and bare Rh areas in between [105]. A reduction of the V-oxide islands at their perimeter by CO has been observed, which has been suggested to be the reason for the promotion of the CO oxidation near the metal-oxide phase boundary. 

The introduction of such a layer can dramatically improve the fuel cell performance. For example, in the SOFC with bilayered anode shown in Figure 6.4, the area-specific polarization resistance for a full cell was reduced to 0.48 Hem2 at 800°C from a value of 1.07 Qcm2 with no anode functional layer [24], Use of an immiscible metal oxide phase (Sn()2) as a sacrificial pore former phase has also been demonstrated as a method to introduce different amounts of porosity in a bilayered anode support, and high electrochemical performance was reported for a cell produced from that anode support (0.54 W/cm2 at 650°C) [25], Use of a separate CFL and current collector layer to improve cathode performance has also been frequently reported (see for example reference [23]). 

A. Metal Oxide Phases IMPROVEMENTS IN DETECTORS IMPROVEMENTS IN HPLC THROUGHPUT... 

The vanadium oxide species is formed on the surface of the oxide support during the preparation of supported vanadium oxide catalysts. This is evident by the consumption of surface hydroxyls (OH) [5] and the structural transformation of the supported metal oxide phase that takes place during hydration-dehydration studies and chemisorption of reactant gas molecules [6]. Recently, a number of studies have shown that the structure of the surface vanadium oxide species depends on the specific conditions that they are observed under. For example, under ambient conditions the surface of the oxide supports possesses a thin layer of moisture which provides an aqueous environment of a certain pH at point of zero charge (pH at pzc) for the surface vanadium oxide species and controls the structure of the vanadium oxide phase [7]. Under reaction conditions (300-500 C), moisture desorbs from the surface of the oxide support and the vanadium oxide species is forced to directly interact with the oxide support which results in a different structure [8]. These structural... 

Recall that we can take vertical slices of the ternary phase diagrams, as well as isothermal (horizontal) slices. If we take, for example, a slice that begins at the tenarite composition (CuO) and extends across to the hematite composition (Fc203), we would end up with a pseudobinary phase diagram, which, when plotted on the appropriate temperature-composition axes, would look like Figure 2.22. Note that the compound CuFc204 is present, here labeled as spinel (see Section 1.2.2.3), but there is much more phase and temperature information available to us. This is, in fact, how many metal oxide phase diagrams are presented. The most stable forms of the... 

As indicated above, type B clusters can accommodate one or two additional electrons, beyond the six needed to form the three Mo—Mo bonds of the triangular array. This redox versatility is clearly manifest in mixed-metal oxide phases of molybdenum. Examples include... 

Pigment industry raw material for the production of chromium-containing stains and pigments based on mixed metal oxide phases... 

In principle, the number of possible mixed metal oxide phases that can be produced from different substituent elements and oxide lattices is extremely large. Systematic investigations have greatly increased the knowledge about mixed metal oxide phases [3.74],... 

The first Raman spectra of bulk metal oxide catalysts were reported in 1971 by Leroy et al. (1971), who characterized the mixed metal oxide Fe2(MoC>4)3. In subsequent years, the Raman spectra of numerous pure and mixed bulk metal oxides were reported a summary in chronological order can be found in the 2002 review by Wachs (Wachs, 2002). Bulk metal oxide phases are readily observed by Raman spectroscopy, in both the unsupported and supported forms. Investigations of the effects of moisture on the molecular structures of supported transition metal oxides have provided insights into the structural dynamics of these catalysts. It is important to know the molecular states of a catalyst as they depend on the conditions, such as the reactive environment. 

Beyond providing bulk structural information about 3-D metal oxide phases, Raman spectroscopy can also provide information about the terminating (and thus 2-D) surface layers of bulk metal oxides. For example, surface Nb=O, V = O, and Mo=O functionalities were detected by Raman spectroscopy for bulk Nb2Os, and for vanadium-niobium, molybdenum-vanadium, molybdenum-niobium, and vanadium-antimony mixed oxide phases (Guerrero-Perez and Banares, 2004 Jehng and Wachs, 1991 Zhao et al., 2003). 

Bismuth oxide forms a number of complex mixed-metal phases with the divalent metal oxides of calcium, strontium, barium, lead, and cadmium, and these show a wide variety in composition. With transition metal oxides, mixed-metal oxide phases have been observed which are based upon a Perovskite-type lattice (10) containing layers of Bi202. It is notable that the high Tc superconducting materials which include bismuth also have this Perovskite-type of lattice with layers of copper oxide interleaved with bismuth oxide layers. 

Many of the mineralogically important transition-metal oxide phases contain more than one cation species, or more than one type of coordination site for the cations. Commonly, the cations are in more than one oxidation state. Examples include ilmenite (FeTiOj) and the family of minerals with the spinel-type crystal structure, including magnetite (Fe304), chromite... 

Two main preparation methods have been used to synthesize Mo/V/Te/(Nb)/0 catalysts active in propane ammoxidation (a) the dry-up and (b) the hydrothermal synthesis. The dry-up method involves mixing aqueous slurries of metal oxide precursors followed by a gradual evaporation of the combined aqueous slurry. Solvent evaporation leads to nucleation and growth of precursor metal oxide phases, which require further heat treatment to obtain active catalysts. 

Macroporous VPO Phases. - The macroscale templating of bulk mixed metal oxide phases in the presence of colloidal sphere arrays typically consists of three steps shown in Figure 18. First, the interstitial voids of the monodisperse sphere arrays are filled with metal oxide precursors. In the second step, the precursors condense and form a solid framework around the spheres. Finally, the spheres are removed by either calcination or solvent extraction leading to the formation of 3D ordered macroporous structures [137]. 

The XRD patterns reveal weakening of the broad signal attributed to sp carbon. According to them, the metal-containing nanoparticles are comprised of metal and metal oxide phases (in the case of Fe-, and Co-containing samples ... 

Even when oxidation reactions are performed at much lower temperature (500 K), Pd surface oxides (oxidation state <+2) may be present and influence the catalytic activity. A number of recent surface science investigations and theoretical smdies of CO oxidation on noble metal (Pt, Pd, Rh) single crystals have suggested not only the involvement of metal-oxide phases, but the oxides were even proposed to exhibit higher activity than the metals (e.g., [60-62]). It is important to clarify whether the formation of a surface oxide (e.g., Pd O ) just accompanies the transition from the inactive to the active state or whether the surface oxide is the cause of high catalytic activity. 

Sorption of arsenic to hydrous metal oxide phases (usually found as coatings on other mineral phases, or as clay-to-colloidal-sized particles) is an... 

Figure 8 The possible states of a supported metal oxide phase.

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