2682
Communications to the Editor
for ZnO and Ti02.3,4However, in remarkable contrast is the fact that ZnS shows no photoeffect on the isotopic equilibration as is observed on ZnO.lJO
-
'Do e!= 3.00 ._ + ._ a
5
0
2.00
"
.-Q
Acknowledgment. We thank professor M. Nagayama and Dr. H. Konno of Hokkaido University for the results of the ESCA experiments and valuable advice, and also Dr. S. Sat0 for his experimental assistance. This work was supported partly by Science Research Grant No.203001 from the Ministry of Education (Japan).
+
3
IO0
Y
the oxygen desorbing above -100 "C. A question which arises here is whether the ZnS surface is oxidized by contact with oxygen at liquid nitrogen temperature. To shed light on this problem, an attempt has been made to detect oxygen signals by the Vaccum Generator ESCA-3 instrument with Mg K a or A1 K a x-ray excitation. The binding energies of copper signals with respect to the Fermi level were C u ( 2 ~ =~ 932.5, ~ ~ ) Cu(3s) = 122.5, and C U ( L ~ M ~ , ~auger M ~ ,=~918.8 ) eV, and these values were quite in accord with reference data.5,6 Prior to setting up the sample in the ESCA apparatus, a sample of ZnS-I1 powder was subjected to evacuation at 450 "C for 1 h followed by treatment with H2S (48 Torr) for 1 h at the same temperature. After that, it was evacuated at 300 "C for 1 h, and was again treated with 75 Torr of H2S a t 450 "C for 1 h. In a separate experiment, it has been certified that the ZnS undergoing the above treatment is active for the equilibration of oxygen as has been shown in Figure 3. The ZnS-I1 sample treated as described above was mounted on a Au mesh plate and heated under evacuation for 2 h at 450 "C in the ESCA chamber. No oxygen signals were observed for this evacuated ZnS-I1 sample by either Mg K a or A1 K a . After this premeasurement by ESCA, the sample was cooled to about -150 "C and contacted with oxygen (1 Torr) for 30 min in the apparatus. The manipulator was then warmed to room temperature with the evacuation of oxygen and the sample was subjected to ESCA measurement. A weak O(1s) signal having binding energy of 532.0 eV was observed after an accumulation of 64 times. The binding energy observed in this experiment is undoubtedly higher by about 2 eV than that of Zn0,7 and is rather close to O(1s) of adsorbed C0.8 This signal was erased by bombarding with an electron beam for Auger analysis. Accordingly, it may be concluded that the ZnS surface does not change to an oxide upon contact with oxygen at temperatures as low as that of liquid nitrogen, and that the carbon impurity may change to carbon monoxide during the ESCA measurement, It has been known that SiOz, MoS2, and Moo3 show no catalytic activityll for such a unique equilibration reaction at low temperatures. Accordingly, ordinal condensed oxygen should be inactive for the exchange reaction, and the isotopic equilibration observed on ZnS surface at rather low temperature is undoubtedly the catalytic process. The fact that no oxygen signals have been detected by ESCA on the ZnS surface after the equilibration reaction may indicate a lack of oxygen dissociation on the surface, which strongly support the O4intermediate, as has been proposed The Journal of Physical Chemistry, Vol. 8 I , No. 26, 1977
References and Notes (1) T. I. Barry and F. S. Stone, Proc. R . SOC. London, Ser. A , 335, 124 (1960). (2) K. Hirota and M. Chono, J. Catai., 3, 196 (1964). (3) K. Tanaka, J . Res. Inst. Catai., Hokkaido Univ., 23, 171 (1975). (4) K. Tanaka and A. Kazusaka, Chern. Phys. Lett., 39, 536 (1976). (5) G. Johansson, J. Hedrnan, A. Berndtsson, M. Klasson, and R. Nilsson, J . Necfron Spectrosc., 2, 295 (1973). (6) G. Schon, J . Electron Spectrosc., 1, 377 (1973). (7) J. Haber, J. Stoch, and L. Ungier, J. Nectron Spectrosc., 9,459 (1976). (8) P. R. Norton, Surface Sci., 44, 624 (1974). (9) K. Tanaka, J. Phys. Chem., 78, 555 (1974). (10) K. Tanaka and K. Miyahara, J. Phys. Chem., 78, 2303 (1974). (11) K. Tanaka et al., unpublished data. Research Institute for Cata/ysis Hokkaido University Sapporo, Japan
Ken-lchi Tanaka" Akio Kazusaka Akiko Yamazaki Koshlro Miyahara
Received December 28, 1976; Revised Manuscript Received October 26, 1977
Calculation of the Thermodynamic Functions for the Specific Adsorption of Ions on Mercury at the Potential of Zero Charge Publication costs assisted by the Consejo Nacionai de Investigaciones Cieneficas y T6cnicas (Argentina)
Sir: The specific adsorption of ions plays an important role in the structural theories of the electrical double layer and evaluation of the pertaining thermodynamic data is very useful in interpreting the electrochemical behavior of a metal-ionic solution interface. The electrostatic model for ionic specific adsorption1 permits an estimation of the thermodynamic data for the adsorption of single charged ions on mercury at the potential of zero charge (pzc) in a relatively simple manner. The model has also been extended by introducing some modifications to cover ionic specific adsorptions at solid electrode/solution interfaces and has been used to calculate the standard free energy change for the adsorption of halides and OH- ions on gold.2 This communication reports a revision of the calculation of the thermodynamic data related to the specific adsorption of ions on mercury strictly following the electrostatic model referred to in the 1iterature.l These calculations showed some numerical errors in the published results which appreciably alter the values of the standard free energy (AG,"), enthalpy ( m a d " ) , and entropy (As,") changes. Results at 25 "C are assembled in Table I. The subscripts o and r refer respectively to the original and corrected data. A straightforward comparison of results shows the following features: (i) The ( A G a d o ) r values are actually more negative than those originally reported. (ii) The results now indicate the likely adsorption of K+ ion. (iii) Either the anion or the cation adsorbability order follows their qualitative behavior. (iv) The (AGado)r values
Communications to the Editor
2683
TABLE I: Calculated Thermodynamic Data for Ionic Adsorption at the Mercury/Aqueous Solution Interfaceu Ionic species Na+ K+
cs+ Fc1Br1-
10
9
kcal/mol
( A Hado )r,
(Asad" )o I
(Asado )r*
16.00 0.90 -12.11 10.23 -13.73 -16.95 - 17.44
16.3 13.9 1.6 17.5 -2.1 -5.1 -11.0
20.07 11.57 2.24 16.91 - 2.03 -6.14 -7.07
kcal/mol
18.5 6.8 - 6.8 14.1 -9.6 -12.9 -16.4
The standard states for A S " and
A G O
eu
eu
( AGad'
)o)
kcal/mol
13.7 2.7 -7.3 8.9 -9.0 - 11.4 - 13.1
( A Gad' )r 9
kcal/mol
10.02 -2.55 -12.77 5.19 -13.12 -15.12 - 15.33
are the same as quoted in the literature.'
for Br- and I- ions are practically equal. (v) The discrepancy between (AGad0)*and that arising from the reported experimental value for the I- ion1p3is somewhat larger than that originally rep0rted.l In spite of the numerical differences already indicated which have to be considered when such data are quoted as reference, it should also be emphasized that the calculated magnitudes are very sensitive to the chosen hydration parameters and the degree of surface coverage by water molecules. Although the electrostatic model approach appears, in the authors' opinion, essentially correct, it involves however several approximations which are open to discussion, as it has been already recognized by Andersen and B0ckris.l One of them refers to employing experimental primary hydration numbers together with ion-water electrostatic interaction energies which were derived by Eley and Evans4 for a coordination number equal to 4. Thus AHhyd, the calculated hydration enthalpy changes, are in disagreement with the experimental results. Another critical point is the contribution to the total enthalpy change of the difference in the number of hydrogen bonds playing a part in the transfer of ions from the solution to the interface. This number should change depending on whether the process involves an anion or a cation, because of the different orientation of the water dipole in the primary hydration sheath. As to the structure of water at the metal surface, there is an incongruence between the number of degrees of freedom assigned to the water molecule to evaluate the various thermodynamic contributions and the localized adsorption model which apparently explains the adsorption characteristics of water on mercury at room temperature. Other comments on the electrostatic model have already
been indicated by Reeves6 such as the use of vibrational states for the adsorbed system as if it were a gas phase. Certainly the electrostatic model for the specific adsorption on ions can be improved with a broader perspective following the more recent advances made both in the structure of water and solutions as well as in the field of the electrical double layer. Therefore, it deserves a further revision as far as the underlying theories on which the model is based.
Acknowledgment. The Institute (INIFTA) is sponsored by the following institutions: Universidad Naciona! de La Plata, Consejo Nacional de Investigaciones Cientificas y TBcnicas, and Comisidn de Investigaciones Cientificas (Provincia de Buenos Aires). This work is partially supported by the Regional Program for the Scientific and Technological Development of the American State Organization. R.G.M. thanks the Organization of American States for the fellowship granted.
References and Notes (1) T. N. Andersen and J. O'M. Bockris, €/ectrocbim. Acta, 9, 347 (1969). (2) D. D. Bod& J . Pbys. Chem., 76, 2915 (1972). (3) W. Anderson and R. Parsons, Proc. Int. Congr. Surf. Act. 2nd, 3 , 45 (1957). (4) D. D. Eley and M. G. Evans, Trans. Faraday Soc., 34, 1043 (1938). (5) R. M. Reeves in "Modern Aspects of Electrochemistry", Vol. 9, J. O'M. Bockris and B. E. Conway, Ed., Plenum Press, New York, N.Y., 1974, Chapter 4.
Instituto-de Investigaciones Fisicoquimicas Tecricas y Aplicadas Divisi6n Electroquimica Sucursal 4 La Plata, Argentina
R. Gonrller Maroto D. Posafllas A. J. Arvla"
Received April 20, 1977
The Journal of Physical Chemistry, Vol. 81, No. 26, 1977
