2453
Communications to the Editor
adsorption process. The author claims that the equation he derives can be useful to compare ionic adsorption on different metals. According to his formulation b
-AHf+ K
+ aM
where b is the metal-ion interaction parameter, AHf the heat of formation of the surface (actually, bulk) compound, K includes the electronic affinity to form the anion and the hydration energy of the latter, and aMis the metal work function. On passing from one metal to another, it results from eq 1
+
AMCMb = - A M ~ ~ A HAM^^@ ~
(2)
According to the author's formulation more positive values for b imply stronger specific adsorption of ions. Equation 1 has not been tested by its author on the ground that experimental data are lacking. However, there are results in the literature regarding specific adsorption of ions on metals other than Hg which can actually be useful to test the validity of eq 1. In the case of Ga, AHf per equivalent' of anion (C1-) is14 -41.8 kcal mol-' andll 9 = 4.25 eV. In the case of Hg, A H f = -27.5 kcal mol-I and a = 4.50 eV. Then, A H ~ ~ ~= A-14.3 H ~ kcal molp1 and AH^^^@ = -5.8 kcal mol-l, and consequently from eq 2, AHgGab = 8.5 kcal mol-l. Thus, eq 1 predicts stronger adsorption of C1- on Ga than on Hg, at variance with experimental findings.15 In terms of change in potential of zero charge upon adsorption of C1-, AEo=o= -24 mV for16 Hg and practically zero17 for Ga. In the case of Au, A H f = -8.4 kcal molv1 and 9 = 4.8 eV. Then, A H ~ ~ =~ 19.1 A Hkcal ~ mol-', AH^*^@ = 6.9 kcal mol-l and AHgAUb= -12.2 kcal mol-'. Thus, eq 1 predicts weaker adsorption of C1- on Au than on Hg, at variance with experimental findings. In fact, AE,=o = -24 mV for Hg and -270 mV forla Au. Reasons for the inadequateness of eq 1 are presumably to be sought in the fact that actually the adsorption process takes place a t a hydrated surface. As an ion is adsorbed, a number of water molecules is removed from the ~ u r f a c e . ~ Therefore, a term containing the difference in metal-water interaction should be added to eq 1 to account for this. Since water molecules are held with different strengths on different metal surface^,^^^^^ the adsorption of ions is influenced in a specific way by this energy which may turn out to be the major factor determining the adsorbability scale of an ion on different metals. If eq 1 is conceptually inadequate, it seems reasonable to be cautious about the quantitative aspects of the whole treatment given by the author. However, his correlations may retain a semiquantitative significance if it is assumed that in the case of the adsorbability scale of different ions on the same metal the hydration energy of the metal surface is always the same. Strictly, this is conceptually not correct in that areas covered by different ions on the surface may differ so that different numbers of water molecules may be desorbed and different desorption energies for water may be involved. Further, the use of AE,=o as a criterion for specific adsorption may probably be accepted in the case of a single metal, but extension to compare different metals should be made only in the light of the correct picture of the adsorption process. It would otherwise upon be impossible to explain the positive shift in U,=O adsorption of clod- on Ga without resorting21 to ion-adsorbed water interactions.
Acknowledgment. Financial support to this work by the
National Research Council (CNR, Rome) is gratefully acknowledged. Laboratory of Electrochemistry University of Milan Milan, ltaly
Sergio Trasatti
Received February 25# 1975: Revised Manuscript Received August 72, 1975
References and Notes A. K. Vijh, J. Pbys. Cbem., 78, 2240 (1974). D. C. Grahame, Cbem. Rev., 47, 441 (1947). S. Levine, G. M. Bell, and D. Caivert, Can. J. Cbem., 40, 518 (1962). T. N. Andersen and J. O'M. Bockris, Eloctrocbim. Acta, 9, 347 (1964). D. J. Barclay, J. Nectroanal. Cbem., 19, 318 (1968); 28, 443 (1970). W. Lorenz, 2.Pbys. Cbem., 219,421 (1962). J. W . SchuRze and K. J. Vetter, J. Electroanal. Cbem., 44, 63 (1973). W. M. H. Sachtler and P. van der Plank, Surface Sci., 18, 62 (1969). 2. Bastl, Surface Sci., 22, 465 (1970). L. Pauling, "The Nature of the Chemical Bond", Corneii University Press, Ithaca, N.Y., 1960. S. Trasatti, J. Cbem. Soc., Faraday Trans. 7, 68, 229 (1972). S. Trasatti, J. Eiectroanal. Cbem., 39, 163 (1972). G. C. Bond, RiC Rev., 3, 1 (1970). Natl. Bur. Stand. Circ., No. 500 (1952). A. N. Frumkin, N. S. Polyanovskaya, and N. B. Grigoryev, Dokl. Akad. Nauk SSSR, 157, 1455 (1964). D. C. Grahame and R. Parsons, J. Am. Cbem. Soc., 83, 1291 (1961). A. M. Morozov, N. 8. Grigoryev, and I. A. Bagotskaya, Nektrokhimiya, 2, 1235 (1966). J. Clavilier and N. Van Huong, C. R. Acad. Sci. Paris, 270, 982 (1970). S. Trasatti, J. Nectroanal. Cbem., 54, 437 (1974). S. Trasatti, J. Cbem. Soc., Faraday Trans. 1, 70, 1752 (1974). A. Frumkin, N. Polianovskaya, N. Grigoryev. and I. Bagotskaya, Electrocbim. Acta, 10, 793 (1965).
Further Considerations on the Effectsof Anions on the Potentials of Zero Charge of Metals Publication costs assisted by the Hydro-Quebec hstitute of Research
Sir: In a recent note,l Trasatti has raised several points regarding our treatment2 of the effects of anions on the potentials of zero charge of metals. Trasatti's comments touch some important questions which need further analysis since the interpretations put forward by him' are not completely valid. It appears most logical to treat the m a i n points raised by Trasatti under the following sections. (1) Demetallization o f t h e Electrode Surface During Specific Adsorption. It is generally realized"-j that at least partial charge transfer from the anion to the metal occurs during contact adsorption of anions. It would then follow that a surface compound, which is partially covalent and partially ionic, is always formed during specific adsorption. The strongest recent evidence for this demetallization at the potential of zero charge has been presented by Trasatti himself who showed6 that there is a direct correlation between the surface potentials of water at the electrode-solution interfaces and the heats of formation of the respective b u l k metal oxides for sp metals. If a nonionic species such as water can interact strongly enough with the metal surface to suggest the formation of a surface compound approaching a bulk oxide, as indeed Trasatti's correlation" cited above indicates, it is clear that the tendency for a surface compound formation would be much more pronounced for anions, especially on the transition metals. It is obvious, therefore, that chemisorption of water (or other neutral species having a significant dipole moment) or anions can lead to a surface compound formation both on sp and transition metals. The Journal of Pbysicai Chemistry, Vol. 79, No. 22, 1975
Communications to the Editor
2454
TABLE I: Some Parameters Involved in the Contact Adsorption of Chloride Ions on Au, Hg,and Gaa Metal Au Hg Ga
6 110.2 103.8 98
-AHf 8.4 27.5 41.8
-(AH,, i-)aqc 40.2 40.2 40.2
3AHh)mb 0 1.84 7.36
b
bmod
78.4 91.1 79.6
78.4 89.26 72.36
-AE,,,ImV
270 24 0
All values except those for -AEP7,, are in kilocalories; 4, - A H f , -(AHI,.,~-)~~, -AEp,,, and b are as defined in our previous paper.* are values of the metal-water interaction e n e r g i e ~ . ~The b values are calculated by the use of all the parameters involved in eq 8 of our previous paper;2 b m u d are the modified values of b when metal-water interaction energies are substracted from the b values calculated by our original procedure.2 It may be noted that b and bmod values are nearly equal and do not resolve the inconsistencies pointed of by Trasatti,l as discussed in the text here.
* -(AHh),
(2) Critique of Our Quantitative Treatment. Trasatti's principal suggestion' for modifying our previous quantitative treatment2 of the specific adsorption of anions on metals is that the heat of hydration of the metal surface should be taken into account by incorporating an additional term representing the difference in the metal-water interactions for various metals. This suggestion appears to be logical since specific adsorption of anions is indeed a substitution reaction in which the anions replace the water molecules on the electrode surface. This additional term, however, is unnecessary when considering the adsorption of several anions on a given metal, e.g., Hg, since a constant small term (1.84 kcal based on Trasatti's work7) would be added to the calculated b values; hence all the graphs presented in our previous paper2 as well as the entire treatment on mercury stays valid. When considering the contact adsorption of a given anion on a series of metals, e.g., specific adsorption of chloride ions on Hg, Ga, and Au cited by Trasatti,' we have no objection to the addition of this term which takes into account the different metal-water interaction energies (equal to 0, 1.84, and 7.36 kcal for Au, Hg, and Ga, respectively') on these metals. It turns out, however, that even when one incorporates these metal-water interaction energies to eq 8 of our original paper2 (Le., eq 1 of Trasatti's critique'), the inconsistencies pointed out by Trasatti' are not resolved (see Table I here). I t would thus appear that, when consid-
The Journal of Physical Chemistry, Vol. 79, No. 22, 1975
ering specific adsorption of a given anion on several metals, our original treatment2 would have to be modified, but this modification on the lines suggested by Trasatti' appears inadequate to explain the data. One would need much more extensive experimental data than presently available on the departures of the potentials of zero charge caused by a given anionb) on a series of a large number of metals before attempting some further modifications of our viewpoint2 of the contact adsorption of anions on metals. It should be stressed, however, that for the specific adsorption of several anions on mercury, for which reliable extensive data are available, our treatment2 not only explains all the existing experimental data but is also consistent with the various theoretical viewpoints considered in the literature. Hydro-Quebec institute of Research Varennes, Quebec, Canada
Ashok K. Vljh
Received April 29, 1975
References and Notes (1) (2) (3) (4) (5) (6) (7)
S. Trasatti, J. fhys. Chem., preceding paper in this issue. A . K. Vijh, J. fhys. Chem., 78, 2240 (1974). W. Lorenz and G. Kruger, 2.fhys. Chem., 221, 231 (1962). W. Lorenz, 2.fhys. Chem., 224, 145 (1963). R. Parsons, Adv. Electrochem.Nectrochem. Eng., 7 , 177 (1970) S.Trasatti, J. Nectroanal. Chem., 54, 437 (1974). S.Trasatti. J. Electroansl. Chem., 33, 351 (1971).
