Hydrogen bronze formation

Special mention must be given to the numerous studies concerning hydrogen bronze formation induced by spillover. In particular, tungsten and molybdenum oxide hydrogen bronzes have been thoroughly studied since Khoobiar s experiments (4). 

Similar, but cyclic mechanism with hydrogen bronze formation is given for rare earth platinum bronze electrodes [11]. It is known that Ru adsorbs OH species better than platinum [12] and hence presence of Ru in near vicinity of Pt enhances the oxidation of half combusted or partially oxidised poisons adsorbed on Pt as shown below... 

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). 

The kinetics and mechanism of the formation of molybdenum bronzes was studied in detail and published in a series of three papers by Erre et al. (86-88), who studied the (100) face of Mo03. Pt was added and the sample was exposed to H2 under very low partial pressures (10 8-10-5 Torr), forming monoclinic Hj 6Mo03. The kinetics unexpectedly exhibited three steps (i) activation of H2 by Pt with an increasing rate with time on stream (ii) a kinetic plateau where the rate remained constant and finally (iii) a decrease in the rate of bronze formation. The salient results were that once step (i) starts, Pt is not required for the insertion of hydrogen into the lattice,... 

The above discussion demonstrates the multifaceted nature of spillover. The interphase transport of an activated species onto a surface (and sometimes into the bulk) where it is unable to be formed without the activator can induce a variety of changes on, and reactions with, the surface. All the reactions of atomic hydrogen are found to be induced by spillover exchange, bronze formation, reduction, demethoxylation, and catalytic activation. An activated species is able to gain indirect access to the nonsorbing surface. 

The influence of spillover species on an acceptor phase can be in the extreme either subtle or profound. Many of the phenomena associated with hydrogen spillover are as subtle as the influences of type-2 hydrogen on the activity of ZnO (189) or as significant as bulk reduction, bronze formation, or catalytic activation. The effects may be similar to the exposure of a surface to a hydrogen plasma. 

Cyclic voltammogram(CVM) of pure platinum electrode in 0.5 M H2SO4 is shown in Fig.lA. The two peaks indicated by arrow marks correspond to two types of H2 adsorption with different strengths on platinum and both the peaks appear at positive potentials with respect to NHE [6], Cyclic voltammogram of WO3 shows two cathodic peaks corresponding to the formation of hydrogen bronzes (HxWOs) or reduced oxides (WOs. ) [7-9]. The first peak appears at a positive potential and the second peak appears at a negative potential with respect to NHE. Fig. IB represents the... 

An alternative model for the reduction of Ce(>2 was inferred by other authors (Fierro et al. 1987, Cunningham et al. 1990). This model suggests that reduction of CeC>2 occurs via formation of hydrogen bronzes, followed by the reduction step, as follows ... 

Although the linear relationship found for the reduction of carbonate-free CeC>2 by Bruce et al. (1996) shows a positive intercept for a surface area of Om2 g, consistent with a formation of hydrogen bronzes, it does not provide sufficient evidence for their formation. In fact, this relationship was derived from relatively high-surface-area CeC>2 and neither was a low-surface-area sample included in the calculations of the linear relationship nor were standard deviations given to ensure the reliability of the extension of the results to low-surface-area samples. It therefore seems reasonable to conclude that at present there is not enough evidence available to discriminate to what extent the formation of hydrogen bronzes may be a predominant pathway for CeC>2 reduction. This point would certainly be worth of further study. 

Detailed studies have also been performed on the formation of the tungsten hydrogen bronze H WOs (x < 0.6) and of reverse spillover with it and with HxMoOs. 

SchoIIhom R, Klein-Reesink F, Reimold R (1979) Formation, structure and topotactic exchange reactions of the layered hydrogen bronze FlxV308. J Chem Soc Chem Commun 398-399... 

The formation of silicon carbide, SiC (carborundum), is prevented by the addition of a little iron as much of the silicon is added to steel to increase its resistance to attack by acids, the presence of a trace of iron does not matter. (Addition of silicon to bronze is found to increase both the strength and the hardness of the bronze.) Silicon is also manufactured by the reaction between silicon tetrachloride and zinc at 1300 K and by the reduction of trichlorosilane with hydrogen. 

Another ak pollutant that can have very serious effects is hydrogen sulfide, which is largely responsible for the tarnishing of silver, but also has played a destmctive role in the discoloration of the natural patinas on ancient bronzes through the formation of copper sulfide. Moreover, a special vulnerabihty is created when two metals are in contact. The electromotive force can result in an accelerated corrosion, eg, in bronzes having kon mounting pins. 

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. 

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