Nitrite, electrocatalytic reduction

Amperometric nitrite detection at 0 V was conducted with a PANl-MWCNT-Au electrode [52]. The electrode showed a large electrocatalytic activity towards reduction of nitrite compared to the bare Au, MWCNT-Au and PANl-Au electrodes on which the nitrite reduction response was almost undetectable. Na", K", Ca ", Mg2", AP", CP, NO3, CIO3, H2PO , HPO , C0 , S0 (50-fold excess to nitrite) and Zn ", Cd ", Ba ", and Br (10-fold excess), UA and AP did not interfere with the nitrite detection, whereas Fe ", Cu ", P, BrOj, IO3, and AA were interfering. 

Electrocatalytic nitric oxide reductions were noted in the previously described studies by Meyer s group on nitrite reduction by water-soluble porphyrins. As NO is a proposed intermediate, it was used independently as a substrate to verify the assigned potential for the ferrous nitfosyl to nitroxyl transformation. Bettelheim et al. also investigated NO reduction by Fe(TMPyP) immobilized onto a GCE with a Nafion coating that repelled anions like nitrite from the electrode surface. Differential pulse voltammetry of this system obtained three distinct reduction waves at potentials ca. 0.5, -0.6, and —0.8 V, assigned using the mechanistic scheme of Meyer " to sequential reductions of Ee (NO" ), Ee (NO), and Ee (NO ). 

A nitrite-sensitive material has been developed by Fabre et al. with a poly(iV-methylpyrrole) film incorporating a metal-substituted heteropolyanion [(H20)Fe XWn039]" (X = P, n = 4, or X = Si, n = 5) as a doping anion [38C1-382]. Such a film was electrochemically stable and exhibited an efficient elec-trocatalytic activity vis-a-vis the nitrite reduction. In contrast, poor results were obtained when PPy was used as the immobilization matrix [383, 384]. The key step of this electrocatalytic process was the formation of an iron-nitrosyl complex generated from the replacement of H2O initially coordinated to the iron center by an NO group, the reduction of which led to the catalytic conversion of NO2 into ammonium ions [385, 386]. The measured catalytic currents were linear with the nitrite concentration over the range 1 X 10 to 3 X 10 M [382]. Furthermore, anions such as NOJ,... 

The investigation of electrochemical, electrocatalytic, and catalytic reduction of nitrite and nitrate have attracted much... 

It was found that the electrocatalytic activity strongly depends on the nature of the electrode it decreases in the order Rh > Ru > Ir > Pd and Pt for the transition-metal electrodes and in the order Cu > Ag > Au for the coinage metals. It was concluded that the rate-determining step on Ru, Rh, Ir, Pt, Cu, and Ag is the reduction of nitrate to nitrite. It was assumed that chemisorbed nitric oxide is the key surface intermediate in the nitrate reduction. It was suggested that ammonia and hydroxylamine are the main products on transition-metal electrodes. This is in agreement with the known mechanism for NO reduction, which forms N2O or N2 only if NO is present in the solution. On Cu the production of gaseous NO was found, which was explained by the weaker binding of NO to Cu as compared to the transition metals. 

Fvaluation of the behaviors of Ni3P2Wig, Ni4Mn2P3W24, Ni6As3W24, and Fe6Ge2Wi8 in the electrocatalytic reduction of nitrate and nitrite 678... 

It is usual that the first reduction wave of commonly studied POMs he monoelec-tronic or, at the utmost, hielectronic [89]. As a consequence, energetically favorable catalytic processes that require larger numbers of electrons can only be accomplished at fairly negative potentials where the necessary number of charges is accumulated and delivered by the POM framework hence the search for strategic parameters that could favor apparently multiple electron uptake on the first wave of POMs. The present case deserves emphasis as a simple illustrative example that proved to be very beneficial in the electrocatalytic reduction of nitrite. 

Table 13 gathers the main electrocatalytic processes triggered by POMs dissolved in solution. The electrocatalytic reductions of dioxygen, hydrogen peroxide and of several NOx including nitrite, nitric oxide, and nitrate are selected for a more detailed description. 

As a whole, it was found that the electrocatalytic reduction of nitrate, nitrite, and nitric oxide proceeds efficiently from pH 3 to 7. 

Cooperative effect of Mn- and Fe-centers within Mn2Fe2P4W3o in the electrocatalytic reduction of nitrite In the preceding sections, it was well established that Fe(III)-substituted POMs catalyze the reduction of NO and/or N02 [1]. The first step in this process is the formation of a complex with the Fe(II) form of the POM and nitrogen oxide. 

Fig. 28 Cyclic voltammetry study of the electrocatalytic reduction of nitrite with a 2 10 M solution of Fe2Mn2P4 in a pH = 1 medium (0.5 M Na2S04 -I- H2SO4). The scan rate was 2 mV s, the working electrode was glassy carbon, and the reference electrode was SCE (taken from Ref 1).

The electrocatalytic behavior of Fe6Ge 2W18 toward the reduction of nitrite... 

Electrocatalytic Reduction of Nitrite to Ammonia in the Presence of Iron-Substituted Heteropolytungstates, XWnFe3 Os9 (416, 417)... 

The activation of the cobalt porphyrin center was assigned to synergistic and induced electronic effects (170,176). A reduced metalloporphyrin center, probably with the character of M P, is responsible for the electrocatalytic reduction of sulfite and nitrite ions by M(3-TRPy P), where M = Ni, Co, and Mn, since the free-base and... 

Butt and coworkers studied the voltammetry of the (Escherichia coli) decaheme class of nitrite reductases [244] (Figure 2.13). Well-defined although unstable catalytic multi-electron voltammetric reduction of nitrite by the enzyme immobihzed on bare Au(lll) electrode surfaces is notable. The decaheme nitrite reductase is a second case for single-molecule in situ STM of a redox metalloenzyme, but image interpretation is presently not at the level for CuNiR [227]. Molecular-scale structures can be observed on the Au(lll) electrode surface under conditions where the enzyme is electrocatalytically active, with both the natural dimer and surface-dissociated monomer enzyme structures identified. Molecular conductivities (in situ STM contrasts) of the enzyme and the active enzyme-substrate states are, however, not very distinctive. 

Doherty, A.P, and Vos, J. G. 1992. Electrocatalytic reduction of nitrite at an [OsfbipyfjfPVC) igCl] Cl-modified electrode. Journal of the Chemical Society Faraday Transactions 88, 2903-2907. 

---