ALEXANDER A. KAMNW,*~ BORIS B. EZHOV,~ VENTSSLAV RIJSANOV$ ... SDepartment of Atomic Physics, Faculty of Physics, Sofia University, BG-1126 Sofia, Bulgaria ... Fe(III) to nhe are undoubted; however, their conclusions .... Wssbeuer spectrum of fmic hydroxo complexes edsorbed et jkkkel(II) hydroxide from.
Elecrrochimico AM, Vol.37. No. Printed in Great Britain.
3. pp. 46%475,
C013-4686/92 $5.00 + 0.00 Q 1991. Pergamon Press plc.
1992
MtiSSBAUER SPECTROSCOPIC STUDY OF FERRIC HYDROXO COMPLEXES ADSORBED AT NICKEL HYDROXIDES FROM STRONGLY ALKALINE ELECTROLYTES ALEXANDERA. KAMNW,*~ BORIS B. EZHOV,~ VENTSSLAV RIJSANOV$ and VENELINANGELOV~
tDepartment
of Physical Chemistry, Scientific Research Institute of Chemistry, Saratov State University, 410026 Saratov, U.S.S.R. SDepartment of Atomic Physics, Faculty of Physics, Sofia University, BG-1126 Sofia, Bulgaria (Received 19 March 1991; in revisedform
28 May 1991)
Abstract-Transmission Mossbauer spectra of soluble ferric hydroxo species fixed at’the surface of Ni(I1) hydroxide and higher Ni oxyhydroxides as a result of their adsorption from concentrated alkaline aqueous solutions at ambient temperature have been obtained. A relatively high adsorption rate and its reversibility have been confirmed; parameters of the Miissbauer spectra obtained provide evidence that the adsorbed ferric species retain their hydroxo coordination while its symmetry is being distorted, which is considered to proceed with partial loss of water molecules of the outer (second) coordination sphere. Specimen
treatment in water leads to destruction of the initial coordination, the bonding strength of the resulting ferric particles to the adsorbent surface being significantly increased. The role of the surface phase non-homogeneity of higher Ni oxyhydroxide has also been outlined. Key words:
nickel hydroxides, ferric hydroxo complexes, adsorption, alkaline electrolytes, M8ssbauer spectroscopy.
FHC) has been proposed; treatment of the data of stationary polarization measurements according to that model provided evidence that nhe surface coverage by the adsorbed FHCs can reach high values[ 131. An investigation has also been carried out on the behaviour of FHC (as well as of ferric and binary Ni-Fe hydroxides) in alkaline electrolytes by Massbauer spectroscopy[l4] which is a highly sensitive probe of the chemical environment of Mossbaueractive nuclides; particularly, of the nonradioactive j’Fe isotope. In the present work we have applied transmission Mtissbauer spectroscopy to the direct study of FHC adsorbability, adsorption rate and reversibility at Ni hydroxides, as well as of FHC coordination alterations during adsorption from highly alkaline electrolytes.
INTRODUCTION
The effects connected with the well-known detrimental influence of iron impurity on the nickel hydroxide electrode (nhe) of alkaline storage batteries[l-81 due primarily to a considerable decrease of anodic oxygen evolution (AOE) overpotential[l, 3,6,8-lo] appear beneficial for alkaline water electrolysers[9, lo]. The results of the latter studies indicated especially high electrocatalytical activity of the Ni-Fe hydroxide system towards the AOE reaction. For a better effect, Fe species must be at the surface of the nhe active material or in the form of a coprecipitated hydroxide system, the latter case being somewhat less efficient[6].$ The electrocatalytic activity is diminished considerably when the nickel hydroxide layer covers the iron hydroxide one[Y]. In view of this distinctly exhibited “superficial” effect of Fe admixture on the AOE kinetics at nhe, a study has recently been undertaken on the influence of ferric hydroxo complexes (FHC) in alkaline electrolyte on the AOE overpotential[l 11. On consideration of regulation mechanisms of oxygen evolution reactions on nhe[l2], an AOE reaction model in the presence of electrocatalytically active complexes (eg
*Author to whom correspondence should be addressed. 5 The experimental results of Mlynarek ef a/.[4 who added Fe(III) to nhe are undoubted; however, their conclusions about the formation of Fe,O, during nhe charging, as well as their consideration of the effect of iron admixture on nhe on the basis of the properties of this iron (II, III) oxide are obviously erroneous.
EXPERIMENTAL
The experiments were performed using finely powdered Ni(I1) hydroxide (chemical purity) and Ni oxyhydroxide taken from the surface of a sinteredplate nhe after electrochemical cycling; their BET specific surface area using nitrogen as the gas was found to be 45 and 33 m* g-l respectively. Concentrated aqueous solutions of NaOH and KOH (analytical purity) were used as electrolytes; the content carbonate alkali concentrations and ( d 1.0 mol.%) were analysed by titration. Commercially available 95.6% enriched metallic 57Fe (State Fund of Stable Isotopes, Moscow) dis-
469
A. A. KAMNWet al.
410
solved in a slight excess of nitric acid (cu. 30%) was used, the resulting ferric nitrate solution being diluted afterwards up to the desired concentration (0.05 M) by bidistilled water. The Fe-containing alkaline solution was prepared by slowly dropping the diluted ferric nitrate solution into an appropriate portion of vigorously stirred 17 M NaOH at cu. 100°C. Some more details concerning the materials used and preparation procedure of alkaline FHC-containing solutions are given in our previous communication[l4]. To perform the adsorption experiments, a weighed sample of Ni hydroxide was mixed with an appropriate volume of alkaline FHCcontaimng solution. After a certain period of time during which the suspension was periodically stirred the solid phase was separated from the electrolyte (its excess was removed by pressing the specimen between two sheets of filter paper), evenly distributed over the surface of a filter disk (2 cm3 and placed in a thin Plexiglas support of the same area which was then attached to the detector holder of the spectrometer (note that the samples under study during acquisition of their spec tra still remained impregnated with a small residual amount of the electrolyte, so their spectra thus obtained may well be regarded as quasi in situ). The Miissbauer spectra were recorded with a conventional constant-acceleration spectrometer combined with a multichannel analyser using a “Co(Pd) source. Computer statistical analysis included fitting the experimental data to a theoretical curve composed of Lorentzian-shaped lines superimposed on a baseline using a least squares fit. The standard program also calculated the values of isomer shift (IS), quadrupole splitting (QS), linewidth (full width at half maximum, FWHM) and integrated intensity. In particular cases the ratio of the total area of a spectrum to its baseline was also calculated (vi& infra). All IS values are given in mm s-l relative to sodium nitroprusside (SNP, for alpha-Fe, IS = 0.258 mm s-l); all experiments, except where otherwise stated, were performed at room temperature (295 f 3 K).
RESULTS AND DISCUS!3ION
As was found earlier[l4], the monomeric FHC form in a concentrated alkaline matrix is characterized by a symmetrical Miissbauer singlet with the parameters IS = 0.68 f 0.02 mm/s and FWHM = 0.72 k 0.04 mm s-i (at T = 78 K), which indicates the existence of cubic symmetry for the FHC coordination polyhedron (presumably, [Fe(OH),]-[ 14,151). The formation of polymeric FHC forms under similar conditions, along with the characteristic broad absorption band centred at cu. 370 nm[lS], is accompanied by the appearance of a quadrupole-split doublet in the Miissbauer spectrum with the same parameters and QS = 0.56 &-0.04 mm s-‘[14]. Adsorbed ferric species t&d at the surface of an adsorbent (eg Ni hydroxides) might be expected to give a sufficient Miissbauer effect, and the parameters of the resulting spectra might allow the state and coordination environment changes of FHC after adsorption to be ascertained. Qn the contrary, complexes in solution (eg FHC) mobile due to the diffusion process are known to give no Miissbauer effect. Our calculations proved by a special blank experiment showed that the total amount of “Fe contained in the solution which could be soaked up by the filter disk used as a support for adsorbent samples was too small to give any appreciable resonant absorption even in the case of its complete fixing, assuming an ordinary recoilless fraction. Hence, all the spectra obtained in our adsorption experiments (vide hfra) correspond to ferric species fixed at the surface of the adsorbent used (Ni hydroxides). FHC remaining in the dissolved state and/or loosely bound to the adsorbent surface (ie mobile) in conformity with the principles of MBssbauer spectroscopy give either no Miissbauer effect at all or a negligible recoilless fraction at room temperature[l6, 11. Figure 1 shows a typical resonant absorption spectrum of ferric species adsorbed at Ni(I1) hydroxide from FHC-containing 15 M NaOH solution; calculated Mtissbauer parameters of the spectrum are
94
Fig. 1. Wssbeuer spectrum of fmic hydroxo complexes edsorbed et jkkkel(II) hydroxide from 15 Iv!NeOH solution containing 0.005 M ,nFe(III). Central pe&-e&&ted Lorentzians composing the resulting spectrum; the seme for Figs 2-6 (see Table 1, sample I).
0.60
Doublet Doublet
Doublet
Doublet Singlet
Same as for sample 1 except shorter storage time (25 min)
0.15 g of B-Ni(OH), stored for 16 h in 10 ml of the solution left after sample 4; separated from the electrolyte
0.12 g of active material from redoxcycled sintered-plate nickel hydroxide electrode; stored for 4 h in 2 ml of 15 M NaOH + 0.005 M j’Fe(III); separated from the electrolyte
Sample 3 stored in water (lOOmI, 16 h)
15 M NaOH + 0.005 M 57Fe(III) solution (ca. 7 mm thick) dried at 120°C in air (1 h)
6
7
8
0.50 -
0.60
0.79
0.40
0.50
5
120 -
6
4
1
Figure
39
50
85
62
-
-
loot
Relative intensity
*Confidence intervals, mms-I: 20.01 for IS; kO.02 for QS and FWHM. tcalculated from ratios of spectrum area to its baseline, the ratio value for sample 1 assumed to be 100%. #The values in parentheses represent relative fractions of different ferric hydroxide forms evaluated from the resonant absorption areas assuming a common recoilless fraction for both forms.
0.62
0.39
0.42
0.36
0.28 0.32
0.39
0.28
0.47
0.43
FWHM/ mm s-’
0.34
0.61
0.59
0.60
Doublet
1.69
0.68
Sample 1 stored for 15 h in 20 ml of 8 M KOH; separated from the electrolyte
0.64
0.60
Doublet I (87%)$ Doublet II (13%)$
/?-Ni(OH), containing 1 mol.% 57Fe(III) hydroxide (impregnated with ferric salt solution, precipitated in 8 M KOH, rinsed with water); air-dry
0.31
2
QSI IlUllS_’
0.60
IS/mm s-’ (USSNP)
Doublet
Multiplicity
0.15 g of /I-Ni(OH), stored for 16 h in 15mlof 15MNaOH+0.005Ms7Fe hydroxo complexes; separated from the solution
Preparation procedure
1
Sample
Table I. Miissbauer parameters for various samples containing ferric species*
472
A. A. KAWEV et al.
J Fig. 2. Miissbauer spectrum of ferric hydroxide added to the surface of A-nickel(I1) hydroxide crystdites by impregnation in 57Fe(III)nitrate solution with subsequent precipitation in 8 M KOH. Parameters for doublets I and II see in Table 1 (sample 2).
presented in Table 1 (sample 1). The IS value corresponds to the data reported for Fe(II1) [ 171.However, the QS is much lower than that for amorphous ferric hydroxide or for similar phases, as well as for colloid ferric hydroxide particles[ 16-201. As was observed earlier[l4], polymeric FHC forms also give a greater QS (0.56 mm s-i). According to a well-known effect[21,22], thin-layer distribution of ferric particles over the surface of a support usually results in a marked increase of QS as compared to a bulk phase spectrum, on account of a lower electric field symmetry at the nuclei of superflcial ferric atoms. A similar effect was observed also for radioactive ‘i9Sn adsorbed onto a large surface area platinum black electrode at low coverage values[23], as well as for highly dispersed ferric oxide particles (cu. 10 nm)[22] due to a relatively high fraction of supertlcial ferric atoms. Figure 2 represents a Miissbauer spectrum for Ni(I1) hydroxide with the addition of ferric hydroxide onto the surface of its crystallites (1 mol% Fe/Ni) being a superposition of two quadrupole doublets evidently corresponding to different states of ferric
species. As can be seen from Table 1 (sample 2), the QS value (0.64 mm s-‘) for the main intense doublet (87% of the total spectrum area) is characteristic of amorphous ferric hydroxide[lGO] or fine particles of goethite (a-FeQOH)[14], while QS for the second doublet (1.69 mm s-i) is much higher, in agreement with the above-mentioned. Thus, the unusually small QS value observed for ferric species adsorbed at M(H) hydroxide (Fig. 1, Table 1) should be interpreted in terms of partial distortions of the coordination sphere symmetry of adsorbed mononuclear FHC and consequent appearance of a quadrupole-split spectrum instead of the singlet characteristic of mononuclear FHC in solution[l4], the QS value remaining noticeably lower than that for structures containing ferric ions linked by oxy- and hydroxy-bridging bonds[24]. The result obtained also refutes an alternative interpretation based on crystallization of Fe(II1) ions from the solution forming a super&ial phase at the surface of M(H) hydroxide crystallites as crystallixation nuclei, which, in principle, might take place in view of an existing supersaturation of the alkaline
Fig. 3. MBssbauer spectrum of sample 1 (cf Fig. 1, Table 1) after storing for 15 h in 8 M KOH (Table 1, sample 3).
MBssbauer study
-3
-2
11
0
413
41
Fig. 4. Miissbauer spectrum of FHC adsorbed from 15 M NaOH solution containing 0.005 M 57Fe(III) at active material taken from sintered-plate nickel hydroxide electrode after its redox cycling (Table 1, sample 6).
electrolyte with FHC (relative to a phase stable under these conditions)[25,26]. In an effort to examine the reversibility of the FHC adsorption, as well as the influence of change of the electrolyte, sample 1 (Fig. 1, Table 1) was then stored in pure 8 M KOH. After separating it from the solution as indicated above, it was again placed in the detector holder retaining its geometry with respect to the y-ray beam pathway, and its Miissbauer spectrum was recorded (Fig. 3). Its Miissbauer parameters (Table 1, sample 3) varied negligibly; however, the integrated intensity of the spectrum and consequently, the total amount of Fe(II1) in the sample decreased by approximately 40%, which points out the reversibility of FHC adsorption, in full agreement with our earlier conclusion[ 11, 131. Comparing the Miissbauer parameters for Ni(I1) hydroxide which has been in contact with the NaOH + FHC solution for a much lower time (25 min, sample 4) with those for sample 1 (Table l), one can see that during this period 85% of the total amount of FHC corresponding to the adsorption equilibrium (assuming that it is obviously reached within 16 h, CJ sample 1) has been adsorbed. Hence the adsorption process should be regarded as proceeding rather rapidly. Repeated adsorption of FHC at a new portion of Ni(II) hydroxide (sample 5) from the electrolyte separated from sample 4 evidently leads to a new adsorption equilibrium; Miissbauer parameters for sample 5 are close to those for sample 4, the total Fe(II1) amount being decreased (Table l), as was expected. The results obtained indicate that during contact of Ni(I1) hydroxide with an alkaline solution containing dissolved ferric forms, the adsorption equilibrium is determined by the FHC concentration in the solution which, in its turn, is related in a certain way to the superficial concentration of adsorbed FHC, ie surface coverage, in agreement with our previous conclusion[ 131. Note that in our electrochemical investigations with regard to AOE[ll-131 we dealt only with oxidized Ni oxyhydroxides. In order to ascertain whether there is any principal difference between
FHC adsorption at Ni(I1) hydroxide and higher Ni oxyhydroxides, a Mcssbauer spectrum was obtained for FHC adsorbed at a portion of black powder of nhe active material after its electrochemical cycling (Fig. 4 and Table 1, sample 6). In principle, the spectrum is similar to those for Ni(I1) hydroxide (Figs 1 and 3 and Table 1). However, one can observe a slight but distinct increase of QS (exceeding the experimental uncertainty value) indicative of a higher distortion of the coordination symmetry of the adsorbed FHC due to an increase of the electric field gradient at Fe nuclei, which can be caused by an increase of the average Ni oxidation state. Resides that, there is a noticeable broadening of the resonant absorption lines of the quadrupole doublet (Table l), which may be explained in terms of surface nonhomogeneity of the adsorbent (eg because of phase composition non-homogeneity of the oxyhydroxide which is usual for oxidized nhe active material[27], particularly, in the presence of ferric compounds[8]). A contribution to this effect can also be made by the asymmetry of Ni-0 coordination in the charged nhe material as compared to the symmetrical Ni-0 coordination in Ni(I1) hydroxide revealed on the basis of EXAFS and XANES spectral evidence by O’Grady et aL[28]. Treatment of the adsorbed FHC with water was expected to result in decomposition of their hydroxo coordination sphere. In fact, after sample 3 (see Fig. 3 and Table 1) was stored in water, a Miissbauer spectrum of the resulting sample 7 (Fig. 5 and Table 1) shows a doublet with a greater QS (0.5 mm s-i). More than a twofold rise of intensity observed for the “hydrated” sample (note that its linewidth value which usually tends to grow with the increase of the intensity remained constant) can be explained by the fact that, in view of the Fe(II1) content being the same in samples 3 and 7, in the latter case, as a consequence of FHC hydrolysis, a new different structure is formed for which the probability of recoilless absorption characterized by the DebyeWaller factor v) [ 181 is correspondingly higher. The observed leap of f indicates that in sample 7 ferric particles are more firmly bonded to the surface of nickel hydroxide; on the contrary,
A. A. KUNEV et al.
474
-3
-2
-1
6
+1
+2
VELOCITY,
m&
Fig. 5. Mijssbauer spectrum of sample 3 (cJ Fig. 3, Table 1) after storing in water (Table 1, sample 7).
mobile (within certain limits) adsorbed FHC exhibit a smallerf’ value together with some Doppler broadening of resonant absorption lines. It is noteworthy that under different conditions (eg acid solutions of Fe salts) ferric ion adsorption at hydrated catiomtes (as compared with their air-dry sam$les) gave they value diminished by an order of magnitude[22] or total disappearance of the M&sbq&r effect at room temperature both for adsorbed Fe(II1) and Fe(H) ions[29] displaying their diffusional mobility. For all the samples with adsorbed FHC the IS values appeared to be within the narrow interval of 0.60 f 0.01 mm s-r (Table 1); Earlier[l4], however, we found the IS for FHC in 15 M NaOH solution to be, 0.68 mm s-‘. In. our ‘opinion, the observed IS decrease upon adsorption of FHCs may be attributed to a partial loss of water molecules forming their outer (ie second) coordination sphere (stability of a similar outer-sphere hydration shell was described, eg, for strongly alkaline solutions of Al and Ga hydroxo complexes[30]). In accordance with the partial isomer shift concept [31,32], chemical environment of Fe atoms ‘gives an
-3
-2
-1
0
additive contribution to the IS value specific for every type of ligands. In particular, the contribution to the IS due to water is +0.03 mm s-’ for every Hz0 molecule per Fe atom[24,33]. O’Grady[24] found that the passive film on iron dried at ambient temperature (for < 5 h) showed a decrease of IS corresponding to a reversible loss of two water molecules by each Fe atom in the lllm. If such loosely bound water which can be removed already at room temperature is sufficient for the observed effect, then a loss (possibly partial) of the outer coordination hydration shell might also be responsible for a decrease of the IS value. In order to validate this, we performed a more intense drying of 15 M NaOH solution containing 0.005 M FHC at 12O”C, and then a Mtissbauer spectrum was recorded (Fig. 6). In general agreement with our earlier observation[l4], a symmetrical singlet is detected in the spectrum; however, a more complete removal of water from the alkaline matrix results in a decrease of IS from 0.68[14] to 0.62mms-’ (Table 1, sample 8) which con6rms the above-stated conclusion. Some noticeable broadening of the line (Table 1) as compared with FWHM = 0.72 mm s-‘[14] may be attributed to
+1
VELO;fTY,
mri:d
Fig. 6. Mbssbauer spaztnim of 15 M NaOH + 0.005 M %(III) solution dried at 120°C for 1 h (Table 1, sample 8).
Miissbauer study
an increased non-uniformity ment due to non-uniform
of the FHC environdrying, the FHC cosymmetry being nevertheless
ordination polyhedron retained. Thus, it can be concluded that adsorption of FHC onto the Ni hydroxide surface proceeds with a partial loss of the outer-sphere hydration shell (eg of 2 or 3 water molecules per Fe atom, considering the observed IS decrease from 0.68[14] to 0.59-0.61 mm S-I (Table l), in accordance with[24,33]). This might contribute to the symmetry distortion of the adsorbed FHC revealed in the appearance of QS (uide supra) which still remains smaller than that for, eg polymeric FHC[ 141. CONCLUSIONS The results of our quasi in situ Massbauer investigations have shown that, in full agreement with the conclusions previously drawn on the basis of our electrochemical studies, soluble ferric forms in alkaline electrolytes can be reversibly adsorbed at nickel hydroxides. The adsorption proceeds at a relatively high rate leading to an adsorption equilibrium determined by FHC concentration both in solution and at the surface (ie surface coverage). Comparison of the Miissbauer parameters obtained (eg quadrupole splitting) with those found in literature provides evidence that the adsorbed FHCs retain their hydroxo coordination; its symmetry is revealed as being distorted by the appearance of a doublet with a relatively small QS (instead of a singlet for monuclear FHC). Hydrolysis of the adsorbed FHC results in a marked increase of QS reflecting destruction of the hydroxo coordination with further symmetry lowering, as well as in a twofold increase of the recoilless fraction (without line broadening) indicating the increased bonding strength of the resulting species to the Ni hydroxide surface. Comparison of the Massbauer parameters for FHCs adsorbed at /I-nickel(H) hydroxide with those at the oxidized phase shows that superficial nonhomogeneity (due to phase composition nonhomogeneity) in the latter case accounts for line broadening. Some increase of QS is also observed due probably to a higher average nickel oxidation state. Some decrease of the IS value observed for the adsorbed FHCs as compared to those in alkaline matrix can be explained in terms of a partial loss of water molecules from the second coordination sphere during adsorption which is confirmed by a similar IS decrease for FHC in alkaline matrix after complete drying at 120°C. Acknowledgemenr-The
authors would like to express their gratitude to Professor Ts. Bonchev (Department of Atomic Physics, Sofia University) for encouragement, ance and valuable discussions.
kind assist-
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