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Techniques for anion adsorption investigation Vladimir D. Jovi ć Center for Multidisciplinar y Studies , Belgrade University, 11030 Belgrade, P.O.Box 33, Serbia. +. M. +. . +. +. +. +. +. +. 2. +. . +. +. S. Double layer structure and corresponding potential distribution.
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Techniquesfor anion adsorption investigationVladimir D. JovićCenter for Multidisciplinary Studies, Belgrade University,11030 Belgrade, P.O.Box 33, Serbia
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Double layer structure and corresponding potential distributionin the presence of specifically adsorbed anionsDifferential capacity measurementsDetermination of the potential of zero charge, Epzc(nonadsorbing electrolytes)[G. Quincke, Ann. Phys., 113 (1861) 513.] [G.Valette, A.Hamelin, J.Electroanal.Chem., 45(1973)301.]
Differential capacity measurementsNonadsorbing electrolyte with addition of adsorbing Cl ions [G.Valette, R.Parsons, J.Electroanal. Chem., 204 (1986) 291.]
Bromide adlayer observed in the potential region III (0.15 V) and underlying Au(111)(1x1) substrate (0.05 V) observed in the potential region II.[A.Cuesta, D.M.Kolb, Surf. Sci., 465 (2000) 311316]
In situ STM image of Pd(111) surface obtained at 0.3 V, just before hydrogen adsorption (sharp peak).[LiJun Wan et al., J.Electroanal.Chem., 484 (2000) 189193]
In situ xray determination of ordered structures during anion adsorption(it requires high energy electrons obtained from the National Synchrotron Light Source at Brookhaven National Laboratory, New York, USA)
EQMC and in situ stress measurements during anion and cation adsorption.UPD of Cu onto Au(111) and sulfate adsorption/desorption[O.E. Kongstein, U. Bertocci, G.R. Stafford, J. Electrochem. Soc., 152 (2005) C111C123]
Determination of the double layer capacities is based on either, differential capacity measurements (Cdiff vs. E) performed at a single frequency, or on impedance measurements performed in a broad range of frequencies and the analysis of impedance diagrams using the adsorption impedance theory. According to this theory, the capacitance spectrum, C(), calculated from the measured impedance spectrum, Z(), can be expressed by the equation
where Rs represents resistance of the solution, Cdl the double layer capacity, while Cad, Rad and adcorrespond to the capacity, resistance and Warburg coefficient of the adsorbate, respectively. From this equation it can be concluded that at high frequencies and low concentrations of adsorbate, the contribution of the second term becomes insignificant and the C() spectrum corresponds to the double layer capacity only. The Cdiff for such a case is given by the equation
where cad and Dad represent the concentration and diffusion coefficient of the adsorbing anions, respectively. All the above mentioned consideration is valid for systems where the double layer capacity behaves as an ‘ideal double layer’, without ‘frequency dispersion’ in the range of low frequencies, i.e. assuming homogeneous electrode surfaces. If this is not the case, constant phase element (CPE) must be introduced (ZCPE = Y0(j),;Y0 [1cm2s]).
For parallel connection of CPE and R can be expressed by two different equations
Cdl
Rs
CPE
Rs
(c)
Zads
Cad
Rad
CPE
Zads
(a)
(d)
Zads
(b)
Rs
Rad
Cad
Zw
Rad
Cad
Double layer capacity is represented by the parallel plate condenser (homogeneous charge distribution)
Double layer capacity is represented by the Constant Phase Element (nonhomogeneous charge distribution)
Model and equivalent circuit for anion adsorption onto real single crystals
Hence, considering all above mentioned it could be concluded that the equivalent circuit for anion adsorption onto real single crystal surfaces should be represented by two impedances, one corresponding to the process of anion adsorption onto heterogeneous part of the surface (monoatomic steps), Zadhe, and another one corresponding to the process of anion adsorption (formation of ordered structures) onto homogeneous part of the surface (flat terraces), Zadho. Such equivalent circuit is presented here
with Radhe and CPEdlhe corresponding to the charge transfer resistance and constant phase element on the heterogeneous part of the surface respectively and Radho and Cad corresponding to the charge transfer resistance and capacity on the homogeneous part of the surface respectively.
Commonly accepted procedure, particularly in the case of diffusion controlled anion adsorption, is based on the complexplane CIm vs. CRe capacitance presentation and its analysis. Using the values for Cdl = 60 F, Cad = 200 F, Radho = 50 and Radhe = 5000 and varying the value of from 1.00 to 0.85 a complexplane CIm vs. CRe capacity diagram presented in a following figure are obtained by simulation process.
E =  1.2 V
E =  1.1 V
E =  0.6 V
E =  0.1 V
From the presented results it is obvious that the most sensitive dependence for anion adsorption investigation is Cdiff vs. f() function;
Considering charges under Cad vs. E curves for the system Ag(111)/0.01M NaCl (29 C cm2) and Ag(100)/0.01M KBr (31 Ccm2) and assuming that the electrosorption valence corresponds to the formation of ordered adsorbed structures, it appears that =  0.4 and =  0.3 respectively, i.e. both adsorbed anions are partially discharged. Hence, this analysis clearly indicates that neither the charge under the CV, nor that under Cdiff vs. E curve recorded at a single frequency, can be considered as relevant for determining either the structure of adsorbed anions or the value of ;
Finally, it should be stated that the combination of cyclic voltammetry, in situ STM technique and Cdiff vs. E (f) curve analysis could be the best way for qualitative and quantitative interpretation of anion adsorption processes.