Irreversibility of the systems (Pt or Au) O2, H2O/OH− in molten nitrates at low hydroxide concentrations. Potentiometric findings and mechanistic considerations
1972; Elsevier BV; Volume: 35; Issue: 1 Linguagem: Inglês
10.1016/s0022-0728(72)80313-9
ISSN2590-2954
Autores Tópico(s)Electrochemical Analysis and Applications
ResumoThe oxygen electrode systems (Pt or Au) O2, H2O/OH− potentiometrically reversible in molten nitrates at 500 K in the concentration range [OH−] 10−3–10−m, O2+2H2O+4e=4OH−;E=O2,H2O/OH−0+RTF1n[O2]14[H2O]12[OH−] have been found to behave irreversibly at lower hydroxide concentrations. In particular, up to [OH−]≅10−4m the two systems present a well-defined behaviour expressed by the equations: E(Pt)=const.+(RT/F)1n[O2]14/[OH−](a)E(Au)=const.+(RT/2F)1n{[O2]12/[OH−]}(b) while the range 10−4<[OH−]<10−3m is characterized by a region of poorly defined “mixed potentials”. Some mechanistic models, involving noble metal oxides in the role of surface catalyst, can be suggested on the basis of the experimental results. The reversible behaviour of the two systems has been associated with a reaction path involving the dissociation process: 2OH−=O2−+H2O(c) followed by fast steps such as: PtO=Pt2++12O2+2e(1)Pt2++O2−=PtO or PtO=O2−=PtO2+2ePtO2=12O2+PtO(2) The deviation from reversibility (which occurs for both systems at approximately the same value of [OH−]) is related to the progressive slowing down of step (c) as the hydroxide concentration decreases. The well-defined low-hydroxide-concentration behaviour of the two systems, summarized by eqns. (a) and (b), is tentatively associated with reaction models such as: PtO+2OH−→slowPt(OH)2+12O2+2ePt(OH)2→fastH2O+PtO and Au2O3+OH−+NO3−→slowAu2O2OH(+)...NO3(−)+12O2+2eOH−+Au2O2OH(+)...NO3(−)→fastH2O+NO3−+Au2O3
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