Proton insertion

On the basis of electrochemical, optical and RBS [90] investigations the underlying mechanism was found to be proton insertion during bleaching, which can be written as [91]... 

The question as to why the electrical and optical properties of the anodic film change drastically upon proton insertion could be answered in correlation with UPS investigations (Fig. 28). 

Acrylonitrile or methyl acrylate readily inserts into allylnickel bonds (example 34, Table HI). A trans double bond is formed by loss of a proton. Insertion of acetylene followed by oxidative elimination with allyl halides gives cis double bonds (example 32, Table III). Insertion of methyl propiolate, followed by proton uptake, leads to a trans double bond (example 33, Table III). Norbomene has been shown to insert stereoselectively cis.exo into an allylnickel bond (example 35, Table III). 

Zinc manganese batteries consist of Mn02, a proton insertion cathode (cf. Figure 15F), and a Zn anode of the solution type. Depending on the pH of the electrolyte solution, the Zn + cations dissolve in the electrolyte (similar to the mechanism shown in Figure 15B) or precipitate as Zn(OH)2 (cf. mechanism in Figure 15C). 

Nucleophilic attack occurs at C(2) of the diene. The 1,3-cyclohexadiene complex 66 is converted to the homoallyl anionic complex 67 by nucleophilic attack, and the 3-alkyl-1-cyclohexene 68 is obtained by protonation. Insertion of CO to 67 generates the acyl complex 69, and its protonation and reductive elimination afford the aldehyde 70 [20]. Reaction of the butadiene complex 56 with an anion derived from ester 71 under CO atmosphere generates the homoallyl complex 72 and then the acyl complex 73 by CO insertion. The cyclopentanone complex 74 is formed by intramolecular insertion of alkene, and the 3-substituted cyclopentanone 75 is obtained by reductive elimination. The intramolecular version, when applied to the 1,3-cyclohexadiene complex 76 bearing an ester chain at C(5), offers a good synthetic route to the bicyclo[3.3.1]nonane system 78 via intermediate 77 [21]. 

Metal oxides which undergo proton insertion reactions find extensive application in batteries and are currently being investigated as potential electrochromic materials. The properties of battery oxides, e.g. manganese dioxide [107-110] and nickel [111-114] have been extensively reviewed in the literature and will therefore not be discussed here. Rather, the properties of electrochemically grown, electrochromic oxide films will be described since this is a relatively new and interesting field. 

It has been established that the above cathode reaction is actually a proton transfer (or proton insertion/ deinsertion) reaction between Ni(OH)2 and NiOOH lattices. Eq. (1) can thus be rewritten as ... 

Another interesting situation arises when there is proton insertion within the solid film so that protonation of immobile redox centers accompanies electron transfer of the type A -I- H+ -i- e -> HA, described by Wu et al. (1992) for redox polymers. Considering mass balance of protons over an infinitesimal film thickness in the boundary region of the film in contact with the electrolyte solution gives the diffusion equation ... 

The reduction of Mn02, however, is a complex process involving proton insertion into the crystal lattice with formation of a scarcely conductive thin surface layer of MnOOH, further reduced to Mn- in solution, the film limiting the reduction rate of MnO2 (Bodoardo et al., 1994 Amarilla et al., 1994). These processes can be presented as ... 

Schober T, Schilling W, Wenzl H. Defect model of proton insertion into oxides. Solid State Ionics. 1996 86-88 653-8. 

Proton Insertion in Polycrystalline WO3 Studied with Electron Spectroscopy and Semi-empirical Calculations... 

Interestingly, cyclic voltammograms (CVs) of monoclinic films prepared by chemical vapour deposition (CVD) [10] showed clear peaks both in the cathodic and anodic sweep directions, indicating different electrochemical processes in specific voltage ranges. It was discussed [10] that these features could be due to from two sources adsorbed on the oxide surface and in the bulk of the electrolyte. Alternatively, at least two redox waves in the CV can be interpreted hy a two-step oxidation of WO3, from W to W and then further to a state. The existence of W" states during proton insertion in evaporated amorphous WO3 films for x > 0.3 has been discussed by Ottermann et al. [11]. 

In the present chapter, we have studied monoclinic polycrystalline WO3 films during proton insertion using electron spectroscopy. Several workers [9,11-15] have used electron spectroscopy to investigate both electrochromic tungsten oxides and tungsten bronzes. Features in the W 4f and valence level photoemission spectra show pronounced similarities for the different systems, but the detailed interpretations are still not clearly resolved. In particular, the relation between the observed spectral changes due to ion insertion and the electrochemical conditions has not been fully investigated. The purpose of the present study is, firstly, to connect observed... 

Inserting protons at - 0.10 V for 3 min (the position of the first redox peak in Fig. 1) turns the electrode green. The effect of proton insertion is clearly seen in the photoelectron spectrum, a low binding energy shoulder appears in the W 4f core level region (Fig. 2a). The new state, referred to as is shifted - 1.27 eV relative to and has a relative intensity of 0.22 with a full width half maximum (FWHM) of 0.91 eV. We also observe that the state increases in FWHM to 1.27 eV. The intensity of the state in the bandgap region has also increased. 

The main reaction involved with all these electrode materials is a proton insertion/deinsertion which supplies protons to the electrolyte when electrodes are anodically polarized and catches protons from the electrolyte when they are cathodically polarized. This protonic cathode-anode exchange can be pointed out in the following reactions. 

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