Bismuth redox reactions

Platinum catalysts were prepared by ion-exchange of activated charcoal. A powdered support was used for batch experiments (CECA SOS) and a granular form (Norit Rox 0.8) was employed in the continuous reactor. Oxidised sites on the surface of the support were created by treatment with aqueous sodium hypochlorite (3%) and ion-exchange of the associated protons with Pt(NH3)42+ ions was performed as described previously [13,14]. The palladium catalyst mentioned in section 3.1 was prepared by impregnation, as described in [8]. Bimetallic PtBi/C catalysts were prepared by two methods (1) bismuth was deposited onto a platinum catalyst, previously prepared by the exchange method outlined above, using the surface redox reaction ... 

Reduction of bismuth compounds could take place by reaction with polymer radicals propagating the depolymerization of polypropylene, either by electron transfer or ligand transfer which are typical redox reactions between alkyl radicals and metal compounds 59... 

This result is consistent with the observed effective poisoning of the CO oxidation reaction as reflected in the increased potential induced by bismuth in the cyclic voltammetry on the supported platinum electrodes (Figure 10a). The voltammetry of CO stripping on the supported catalysts indicates a similar behavior to that found on Pt(llO) in that bismuth results in a higher overpotential for CO oxidation. One must conclude that the morphology of the supported platinum catalyst results in facets more akin to the more open-packed Pt(l 10) surface than the Pt(lll) surface, a conclusion supported by comparison of the bismuth redox chemistry on the supported catalyst and the single-crystal surfaces [77]. 

Platinum catalysts were prepared by an ion-exchange method [16,17]. Oxidised sites on the surface of an activated carbon support (CECA SOS) were created by pre-treatment with sodium hypochlorite (3%) the associated protons were subsequently exchanged with Pt(NH3)4 " ions, in an aqueous ammonia solution, and reduction was carried out on the dry catalyst under a flow of hydrogen at 300°C. A surface redox reaction was subsequently employed to deposit the bismuth whereby the catalyst was suspended in a glucose solution, under an inert nitrogen atmosphere, and the required volume of a solution of BiONOs, dissolved in hydrochloric acid (IM), was added [18]. 

The value of m is dependent upon the pH of the solution. If m 0, then redox reaction involves the incorporation and expulsion of protons during the redox reaction and this can be measured by a pH-sensitive bismuth oxide ring electrode [18,19]. In the special case when the ring electrode is a potentiometric sensor with a low exchange current density, such as the bismuth oxide sensor for pH [18,19], the ring electrode reaction does not significantly perturb the radial concentration profile of the disc product across the ring, and a different collection efficiency, termed the detection efficiency. 

The exact form of the proton transients seen at the ring is determined by the degree of protonation of the L centers and their ability to act as a buffer, mopping up the protons produced in the redox reaction. This is determined by the pATaS for deprotonation of the various protonated forms of L. Quantitative analysis of the proton transients observed at the bismuth oxide ring electrode enabled these values to be determined as... 

This redox reaction (1) proceeds so rapidly that if a drop of 0.01 % bismuth solution is mixed on a spot plate with a drop of alkaline stannite solution, there is an immediate precipitation of metallic bismuth in the form of black flocks. When the same experiment is made with 1 % lead acetate solution, after 3-10 minutes standing there is a slight reduction to lead, which is revealed by a light brown coloration. This proves that the redox reaction ... 

The very sensitive and selective redox reaction between bismuth hydroxide and alkali stannite giving metallic bismuth (page 134) may be applied for the rapid and reliable detection of bismuth in alloys. The sample can be dissolved or it may be subjected to attack with bromine vapors. Bismuth, as well as other components of alloys, are thus converted into bromides. This step, as well as the transformation into Bi(OH)a and the reduction of the latter to dark, finely divided metal, can be carried out without visible damage to the specimen. 

This system typically uses sulfuric acid as the electrolyte with a proton exchange membrane. While a porous separator could be used, for high efficiency operation, ion-selective membranes are generally preferred as vanadium crossover leads to losses in coulombic efficiency. At present, Nafion is the membrane of choice as V(V) is a powerful oxidizing agent, which can attack cheaper hydrocarbon-based ion selective membranes [21]. The redox reactions of different vanadium species have displayed reversibility and high activity on carbon based electrodes. Moreover, Li et al. discovered the catalytic effects of bismuth nanoparticles on V(II)/V(III) [51] and of niobium oxide nanorods on both V(II)Af(lII) and V(IV)Af(V) [52], which have been shown to further enhance the energy efficiency of the VRB by more than 10 %. 

Bi is detected in alkaline medium by stannite anions. Therefore, bismuth stands in the solution as bismuth hydroxide Bi(OH)3. At pH= 14, the involved bismuth couple is Bi(OH)3/Bi(s) according to the half-redox reaction... 

Ammosov and Sazonov [21,22,24] demonstrated that for iron antimonates the initial selectivities are lower than for bismuth molybdates due to a higher rate of the parallel combustion reaction. It is proved that both selective oxidation and combustion occur by a redox mechanism. In another publication [23], the same authors report the kinetics of the butene and butadiene combustion reactions. 

As mentioned earlier, the multicomponent oxide catalysts currently commercialized contain bismuth, iron, and molybdenum, in addition to several other cations. Although few reports concerning multicomponent catalysts have appeared in the literature, there is agreement that iron affects several aspects of the catalyst system. Measurements on multicomponent catalysts by Wolfs et al. (109-111) showed that Fe3+ was partially reduced to Fe2+ after the catalytic reaction, indicating that Fe3+ ions are involved in the reaction mechanism. The observed Fe3+/Fe2+ redox couple was associated with the increased activity of the catalyst. 

In addition to the microscopic redox processes of bismuth ions and molybdenum ions, the combination of these conductivity measurements leads to the conclusion that the macroscopic, bulk conductivity properties of the bismuth molybdate catalyst affect the catalytic reaction. 

Many other powerful oxidants are used in redox titrations. Often a metal ion may be present in more than one oxidation state which must be oxidized or reduced into the desired oxidation state. For example, a salt solution of iron may contain both Fe2+ and Fe3+ ions. Peroxydisulfates, bismuthates, and peroxides are often used as auxiliary oxidizing reagents to convert the ion of interest into the higher oxidation state. The half-reactions for these oxidants are as follows ... 

Very little is known of the redox chemistry of bismuth. Ford-Smith and Habeeb found that Bi(V) oxidized a variety of substrates, including IrCl63, with a rate law that is independent of the identity or concentration of the substrate (125). It is not clear that the reactions involve Bi(IV). 

In addition to the capability of Raman spectroscopy in determining the coordination numbers and bond lengths of molybdate species in bismuth molybdate phases, Raman spectroscopy can also be used as an in situ probe for the bismuth molybdates under reaction conditions. In situ Raman studies have been earned out, for example, on the 6-Bi2Mo209 phase under redox conditions, where insights into the surface mechanism of the... 

The reductive homocoupling of chlorobenzene to biphenyl is efficiently catalyzed by a recyclable, heterogeneous trimetallic catalyst in the presence of PEG 400 in H2O (Scheme 14.81) [164]. Bismuth is believed to trap the surplus hydrides and retard undesired side-reactions. A ternary metal redox system, BiCl3-Al-NiCl2(bpy), mediates the reductive coupling of /flirornoslyrene [165]. 

---