Hydrogen tungsten bronzes

High-temperature reduction of Na2Ti03 with hydrogen produces nonstoichiometric materials, Na jTi02 (jr = 0.20-0.25), called titanium bronzes by analogy with the better-known tungsten bronzes (p. 1016). They have a blue-black, metallic appearance with high electrical conductivity and are chemically inert (even hydrofluoric acid does not attack them). 

It was found in the 1960s that disperse platinum catalyst supported by certain oxides will in a number of cases be more active than a similar catalyst supported by carbon black or other carbon carrier. At platinum deposits on a mixed carrier of WO3 and carbon black, hydrogen oxidation is markedly accelerated in acidic solutions (Hobbs and Tseung, 1966). This could be due to a partial spillover of hydrogen from platinum to the oxide and formation of a tungsten bronze, H WOj (0 < a < 1), which according to certain data has fair catalytic properties. 

Upon reduction, WO3 forms mixed-valence (WjWj W ) tungsten bronzes, H WOj, which have different colors depending on the degree of reduction, x [see Eq. (33.1)]. Due to parasitic reaction, such as hydrogen evolution, and decreased... 

The hydrogen molybdenum and tungsten bronzes H MoOj and H WOj, where X = 0.6, may be prepared by electrochemical reduction of WO3 or M0O3. They violate the condition that the metal must be a hydride former, but they cannot be prepared by direct reaction of the oxide with Hj, and they differ from other compounds because the hydrogen prefers to bond with oxygen rather than the metal. 

The importance of the solvation of adsorbates is well exemplified in reports by Levy and Boudart. They investigated the formation of a hydrogen tungsten bronze, H0.35 WO3, from hydrogen catalysed by admixed platinum black, supported platinum, or supported palladium, in the presence of various substances of different proton affinity. In the absence of proton acceptors,... 

In hydrogen tungsten bronzes x values between 0.03 and 0.53 and y values from 0.015 to 0.3 can be found. They have quite different structures (tetragonal, orthorhombic, monocline, hexagonal, and cubic) and can also be detected in tungsten blue oxides. 

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