Metal oxide bronzes

Solid isopoly- and heteropolymetalate compounds have been synthesized. Within this group of materials, one can include the so-called metal oxide bronzes (typically tungsten bronzes). Starting from WO, electrochemical reduction processes yield intercalation materials with electrochromic properties (Grandqvist, 1999). 

Casting metals consisting of Sb or Bi covered by a thin hydroxide layer and membranes of transition metal oxide bronzes show a relatively high selectivity for hydrogen ions, as recently reviewed by Vonau and Guth (2006). Metal/metal oxide electrodes display non-Nernstian responses, but applications in food control and medicine have... 

Originally, emphasis was placed on the apparent similarities between the electronic and structural properties of the hydrogen-containing phases and those of the well known alkali-metal oxide bronzes in which alkali-metal ions reside in cavities in the parent-oxide matrix. This approach is justifiable in the case of the electronic properties, since the two classes of compound share a common redox insertion process... 

In either VPO or MoVTeNbO catalysts, V-sites are directly involved in the selective oxidative activation of paraffins, while the presence of a second element is generally required in order to facilitate the formation of a defined crystalline phase (i.e. vanadyl pyrophosphate or orthorhombic metal oxide bronze, respectively) (Figs. 24.2a and 24.2c), but also for facilitating the multifuctionality of the catalysts. However, although both catalytic systems are active for the oxidative activation in alkanes, different selectivities are achieved depending on the alkane feed. Thus, a... 

Only about 10 elements, ie, Cr, Ni, Zn, Sn, In, Ag, Cd, Au, Pb, and Rh, are commercially deposited from aqueous solutions, though alloy deposition such as Cu—Zn (brass), Cu—Sn (bronze), Pb—Sn (solder), Au—Co, Sn—Ni, and Ni—Fe (permalloy) raise this number somewhat. In addition, 10—15 other elements are electrodeposited ia small-scale specialty appHcations. Typically, electrodeposited materials are crystalline, but amorphous metal alloys may also be deposited. One such amorphous alloy is Ni—Cr—P. In some cases, chemical compounds can be electrodeposited at the cathode. For example, black chrome and black molybdenum electrodeposits, both metal oxide particles ia a metallic matrix, are used for decorative purposes and as selective solar thermal absorbers (19). 

Although the band model explains well various electronic properties of metal oxides, there are also systems where it fails, presumably because of neglecting electronic correlations within the solid. Therefore, J. B. Good-enough presented alternative criteria derived from the crystal structure, symmetry of orbitals and type of chemical bonding between metal and oxygen. This semiempirical model elucidates and predicts electrical properties of simple oxides and also of more complicated oxidic materials, such as bronzes, spinels, perowskites, etc. 

Here is Albertus description of tuchia which was Usually an impure sublimate of zinc oxide mixed often with more or less of other metallic oxides, dust from the flues or domes of bronze or brass furnaces. 

More recently the transformation of carbon-supported Pt colloids of approximately 1 nm diameter into Pt alloys has been reported, which seems to yield an even better catalyst, since the alloy particles—although coarser than the initial Pt particles—show improved catalytic activity and stability. It is not unlikely that from these alloys by in-situ oxidation transition-metal platinum bronzes like NLPt304 are formed, being the catalysts proper. 

The term insertion compound is used for solids where atoms or ions enter a three-dimensional framework without disrupting its essential structure. Many oxide bronzes are of this type, based on transition metal oxides with inserted alkali or other electropositive metals. 

Defects in perovskite oxides can be due to cation vacancies (A or B site), amon vacancies or anion excess. Cation-deficient oxides such as A,WOj give rise to oxide bronze structures, W03 itself representing the limiting case of the A-sile deficient oxide A-site vacancies are seldom ordered in these metallic systems. B-site vacancies are favoured in hexagonal perovskites and ordering of these vacancies gives rise to superstructures in some of the oxides. 

Both types of reactions involving the spillover of either H2 or 02 have been termed topochemical heterogeneous catalysis (62). Besides the catalyzed reduction of metal oxides either to metals or suboxides, the formation of new and specific reduced oxides, such as the well-known hydrogen bronzes of W, Mo, and V, have attracted considerable attention (7,66-68). In many cases the reduction of the corresponding oxides by spillover of H2 led to reduced compounds not otherwise obtainable (69). 

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